Micrometer Guide: Types, How to Read & Use One Correctly

Q: What is a micrometer used for?

A micrometer measures small distances to high precision, typically 0.01 mm or 0.001 inch resolution. Workshop micrometers are used for shaft outside diameters, hole bore diameters, sheet thicknesses, gauge block calibration, and any precision dimensional measurement tighter than a caliper can deliver. Modern micrometers come in outside, inside, depth, and specialty variants (thread, blade, disc, tube, V-anvil) for different geometries.

Q: What's the difference between a micrometer and a vernier caliper?

A vernier caliper measures four dimensional types (outside, inside, depth, step) at 0.02–0.05 mm accuracy across a wide range (150–600 mm typical). A micrometer measures one dimensional type per instrument at 0.001–0.01 mm accuracy across a narrow range (25 mm typical per instrument). Calipers are faster, more versatile, and lower precision; micrometers are slower, single-purpose, and tighter tolerance. Quality workshops use both — calipers for general dimensional checks, micrometers for precision work.

Q: How accurate is a micrometer?

A workshop-grade outside micrometer typically delivers ±0.003 mm to ±0.005 mm accuracy across its 25 mm range when properly calibrated. Mitutoyo and Starrett benchmark instruments specify ±2 µm. Cheap import micrometers may claim 0.01 mm resolution but deliver only ±0.01–0.02 mm real accuracy. Accuracy degrades with temperature variation, dirty anvils, worn thimble screws, and incorrect operator technique.

Q: How do I read a metric micrometer?

Read in three steps. First, the main scale on the sleeve shows whole millimetres above and half-millimetres below the reference line — read the highest value visible. Second, the thimble shows hundredths of a millimetre on its rotating scale — read the line that aligns with the sleeve reference. Third, add the values. So 7 mm on the sleeve, 0.5 mm below the line visible, and 23 on the thimble equals 7.73 mm total.

Q: What is the ratchet stop and do I need to use it every time?

The ratchet stop is a small knurled cap at the end of the thimble that disengages when the measuring pressure reaches the calibrated force (typically 5–10 N). Using the ratchet stop on every measurement is the only way to apply consistent measuring force — without it, two operators measuring the same part will get readings 0.005–0.02 mm apart simply due to different grip pressure. Yes, use it every time.

Q: What does zeroing a micrometer mean?

Zeroing means verifying that the micrometer reads zero when its anvils are in firm contact with no workpiece between them (for outside mics), or when properly seated on a reference standard (for inside, depth, or bore micrometers). If it doesn't, the screw needs adjustment to align zero correctly. Zero-check the micrometer at the start of every measurement session — the cheapest insurance you'll buy all day.

Q: What's the difference between an inside micrometer and an outside micrometer?

An outside micrometer measures external dimensions — shaft outside diameter, sheet thickness, pin diameter — by closing two anvils onto the workpiece. An inside micrometer measures internal dimensions — bore inside diameter, slot width, internal groove — by extending two contacts apart inside the feature. The thimble and reading mechanism are identical between the two types; the geometry of what's being measured differs. Both are precision instruments specified to typically ±0.005 mm accuracy.

Q: What is a bore micrometer used for?

A bore micrometer (also called a 3-point internal micrometer or Holtest, after Mitutoyo's product family) measures internal bore diameters using three measuring contacts spaced 120° apart. The 3-point geometry self-centres in the bore, giving repeatable measurements where a 2-point inside micrometer would require rocking to find the maximum diameter. Used for cylinder bores, bearing housings, hydraulic bores, and any precision bore inspection where roundness needs to be assessed alongside diameter.

Q: Can I use a micrometer for automotive work?

Yes — automotive workshops use micrometers extensively for engine rebuilds (cylinder bore, crankshaft journal, piston pin, valve stem measurements), brake disc thickness, bearing clearance, and dimensional inspection of any replacement part. The Mitutoyo Series 293 digital outside micrometer is a common automotive workshop choice for its IP65 rating and quick mm/inch toggle. For engine machining specifically (line boring, cylinder honing), 3-point bore micrometers are standard.

Q: What micrometer resolution do I need?

For general engineering, fabrication, and trade workshop use, 0.01 mm resolution on an outside micrometer is adequate. For automotive engine work, precision toolmaking, gear cutting, mould-making, and quality inspection, 0.001 mm digital resolution is the standard — though the actual measurement accuracy remains ±0.003–0.005 mm regardless of LCD digits. Higher resolution on the readout doesn't improve mechanical accuracy; it just makes the reading easier to interpret and faster to capture for SPC data collection.

Jun 4, 2026 AIMS Industrial

A micrometer is the tool that separates a good workshop from a precise one. Where a vernier caliper gives you ±0.05mm on a good day, a quality micrometer reads reliably to 0.01mm — and with a vernier or digital model, to 0.001mm. That level of accuracy matters when you're checking brake disc thickness, crankshaft journal wear, piston clearance, or any machined component operating at tolerance.

This guide covers every type of micrometer used in Australian industry — outside, inside, bore, depth, and thread — plus exactly how to read one, zero it, and use it without introducing error. We've included the real-world mistakes tradespeople make (often temperature-related), a plain-language brand guide, common automotive and maintenance applications, and answers to the 15 questions Australian workshops ask most. AEO-first: every major People Also Ask question and forum topic from the precision measurement community is addressed here.

Micrometer vs Vernier Caliper: Which Do You Need?

Both tools measure dimensions. The difference is resolution, specialisation, and how much precision your application actually demands.

A vernier caliper handles outside diameter (OD), inside diameter (ID), depth, and step measurements in a single instrument — typically to ±0.02mm. For most general maintenance, that's sufficient: checking a shaft diameter, measuring fastener head sizes, fitting a bearing housing. See our Vernier Caliper Guide for a full breakdown of caliper types, digital vs analogue, and how to read one.

Reach for a micrometer when:

Tolerances are tight. Any application calling for ±0.01mm or better — piston ring gaps, crankshaft journals, spindle bearings, precision ground shafts — needs a mic.
You're measuring wear against a specification. A brake disc worn from 28mm needs to be measured to the nearest tenth of a millimetre, not estimated with a caliper jaw.
You're working to a drawing tolerance. Machined components with h6/h7/g6/f7 shaft fits require micrometer-level accuracy to confirm compliance.
You're doing quality inspection or calibration. Gauge blocks and setting standards require better resolution than a caliper can provide reliably.

If you're unsure which to buy first, buy a caliper — it's versatile and covers most workshop measurement tasks. Add a micrometer when you're consistently working to tolerances a caliper can't reliably hold.

Types of Micrometer: Which One Do You Need?

The word "micrometer" most often refers to the outside micrometer, but the family covers five distinct types, each designed for a different class of measurement.

Outside Micrometer

The most common type in any workshop. An anvil and spindle close around the outside of a workpiece, measuring external dimensions — shaft diameters, bar stock, rod thickness, pin diameters, disc thickness.

Sold in 25mm increments: 0–25mm, 25–50mm, 50–75mm, 75–100mm, and so on up to very large ranges. A 0–25mm outside micrometer covers the majority of small-dimension work. If you regularly measure across multiple size ranges, a set is more economical than buying individually. Dasqua outside micrometers are available in both metric and imperial at AIMS — see the full micrometer range for what's currently in stock.

Automotive note: Dasqua makes a dedicated Disc Brake Micrometer with a 7–33mm range and 0.01mm graduation — purpose-built for checking brake disc thickness against manufacturer minimum specifications. A practical addition to any workshop doing brake work.

When to use: Shaft OD, fastener shank diameter, bar stock verification, seal journal diameter, brake disc thickness, any external dimension.

Inside Micrometer (Internal Micrometer)

Measures internal dimensions — bore diameters, groove widths, slot widths. Where an outside micrometer closes onto a surface, an inside micrometer expands outward to contact the inside of a bore or slot.

Inside micrometers come in two main forms: tubular (a measuring head with extension rods for different ranges, starting from around 5–30mm in basic sets) and three-point bore micrometers (discussed separately below). Dasqua 3-point inside micrometer sets in the 20–50mm and 50–100mm ranges are stocked at AIMS.

When to use: Bearing housing bore, cylinder bore, internal groove width, sleeve ID, any internal dimension requiring micrometer accuracy.

Bore Micrometer (3-Point Inside Micrometer)

A specialist inside micrometer with three measuring points arranged radially, ensuring it automatically centres in the bore and reads true diameter — not a chord. Significantly more accurate than a two-point inside micrometer, particularly in bores that may not be perfectly round.

Bore micrometers require a setting standard — typically an outside micrometer set to the nominal bore size, or a precision ring gauge — to zero before use. That extra setup step is why they're used where the result genuinely matters: engine cylinder bores, hydraulic cylinder bores, precision bearing housings.

When to use: Engine rebuilds (cylinder bore measurement, main bearing housing), hydraulic systems, any precision bore where roundness needs to be assessed alongside diameter.

Depth Micrometer

Measures the depth of holes, slots, shoulders, and recesses. A flat base plate sits on the workpiece surface; the spindle extends down into the feature. The thimble and sleeve read exactly as on an outside micrometer, just in a different orientation.

Depth micrometers often come with interchangeable rods to cover different depth ranges — typically 0–25mm, 25–50mm, and 50–75mm from a set of three rods. Dasqua and Maxigear depth micrometers and digital depth gauges are available through AIMS in metric and imperial variants.

When to use: Keyway depth, counterbore depth, slot depth, shoulder height, step measurement, blind hole depth.

Thread Micrometer (Pitch Micrometer)

Measures the pitch diameter of external threads — the effective diameter at which the thread mates with its counterpart. Standard flat contact faces are replaced with a V-shaped anvil and a 60° conical spindle that seat into the thread flanks at the pitch line.

Thread micrometers are specialist tools, primarily found in machining environments where threads are being cut and need to be verified against a pitch diameter specification. They're not a general workshop item, but they're irreplaceable when you need them.

When to use: Verifying lathe-cut threads, production thread inspection, thread wear assessment on precision screwed components.

Digital vs Analogue Micrometers

Both read to 0.01mm — this is a question of preference, not accuracy. Digital micrometers display the reading on an LCD screen, eliminating scale interpretation and the parallax errors that go with it. They also allow absolute/incremental switching, zero-setting at any position, and inch/metric conversion at the press of a button. Dasqua digital outside micrometers with wireless data output are available at AIMS — useful for quality management environments where measurement data needs to be logged.

Analogue micrometers require you to read the sleeve and thimble scales — a learned skill that takes 10–15 minutes to develop, but once learned, is reliable. No battery, no display to fail in dirty environments, and many experienced tradespeople prefer the tactile feedback. For field service in harsh conditions, analogue is often the more durable choice.

For a busy workshop or anyone new to precision measurement: digital. For field work or environments where battery management is a concern: analogue. Either works; digital eliminates the most common reading errors.

Type	What it measures	Typical range	Common application
Outside micrometer	External OD / thickness	0–25mm in 25mm steps	Shafts, bar stock, disc thickness
Inside micrometer	Internal ID / groove width	5–30mm (with rods)	Bearing housings, sleeve IDs
Bore micrometer (3-pt)	Bore diameter + roundness	6–150mm (model-dependent)	Engine cylinders, hydraulic bores
Depth micrometer	Hole / slot / shoulder depth	0–75mm (with rods)	Keyways, counterbores, slots
Thread micrometer	Thread pitch diameter	0–25mm in 25mm steps	Lathe-cut thread verification
Disc brake micrometer	Brake disc thickness	7–33mm typical	Automotive brake service

How to Read a Metric Micrometer (Step-by-Step)

Reading a metric analogue micrometer involves three values from two scales. Once understood, it takes seconds. The same method applies to outside, depth, and thread micrometers.

Understanding the Scales

The sleeve is the fixed barrel on the frame. It carries two scales along the datum line: the main scale above the line reads in 1mm increments; the lower scale below the line has 0.5mm marks that become visible between main-scale graduations as the thimble rotates. Together, the sleeve reads to 0.5mm resolution.

The thimble rotates around the sleeve. It has 50 equally spaced graduations around its circumference. Because one full rotation moves the spindle 0.5mm, each thimble division = 0.5 ÷ 50 = 0.01mm.

On micrometers with 0.001mm resolution, a vernier scale — 10 additional graduations on the sleeve — refines the thimble reading to the nearest 0.001mm by identifying which vernier line best aligns with a thimble graduation.

Reading the Measurement (Worked Example)

Read the sleeve main scale. Count the highest visible graduation above the datum line. Each mark = 1mm. Example: 7mm exposed.
Check the lower 0.5mm scale. If a lower-scale mark is visible between the last main graduation and the thimble edge, add 0.5mm. Example: visible → running total = 7.5mm.
Read the thimble graduation. Find which thimble line aligns with the sleeve datum. Each division = 0.01mm. Example: graduation 28 aligns → add 0.28mm.
Add the three values. 7mm + 0.5mm + 0.28mm = 7.78mm.
Add vernier correction (if present). On 0.001mm micrometers, identify which vernier graduation aligns with a thimble graduation, and add that many thousandths of a millimetre.

Most common reading error: Missing the 0.5mm lower-scale mark. This produces a reading 0.5mm too low — one of the most frequent mistakes new users make. Always check the lower scale before reading the thimble.

Resolution Options: 0.01mm vs 0.001mm

Resolution	Use case	Who needs it
0.01mm (standard)	General precision work, maintenance, automotive	Most workshops, tradespeople
0.001mm (vernier or digital)	Precision machining, calibration, tight-tolerance components	Toolmakers, machinists, calibration technicians

For workshop maintenance, automotive, and most industrial inspection work, 0.01mm resolution is sufficient and the right choice. If you're working to tolerances tighter than ±0.02mm — fine-pitch thread inspection, precision bore fitting, gauge-block verification — you need 0.001mm resolution. When in doubt, 0.01mm is correct for 95% of Australian trade and maintenance work.

How to Zero a Micrometer (And Why It Matters)

A micrometer that reads 0.02mm when fully closed is introducing a systematic error into every single measurement. Zeroing takes 30 seconds and eliminates it entirely.

Zeroing a 0–25mm Outside Micrometer

Clean both measuring faces — anvil and spindle — with a lint-free cloth.
Close the micrometer using the ratchet stop (not the thimble directly) until it clicks and slips.
Check the reading. The thimble zero graduation should align exactly with the sleeve datum line, and the sleeve zero graduation should be visible at the thimble edge.
If it doesn't read zero: use the C-spanner (adjusting wrench) supplied with the instrument to rotate the sleeve until zero is achieved. On digital micrometers, press the zero/origin button.

Zeroing Larger Range Micrometers

Outside micrometers with ranges starting above 0mm — e.g. 25–50mm, 50–75mm — are zeroed using the setting standard supplied with the instrument: a precision-ground hardened reference bar of the minimum size in the range. Place the standard between the faces, close using the ratchet stop, then zero as above. Never close a 25–50mm micrometer fully without a standard — you'll damage the measuring faces.

Inside and Bore Micrometers

Inside micrometers are zeroed against a known reference: an outside micrometer pre-set to the nominal bore size, or a precision ring gauge of the correct diameter. Bore micrometers follow the same principle — set the reference, then zero the bore mic against it before measuring. This extra step is why bore measurement takes longer but is far more accurate than a two-point inside measurement.

When to Adjust vs When to Recalibrate

Zeroing corrects for minor drift — this is normal workshop practice. If your micrometer requires adjustment of more than 0.02–0.03mm, or returns to error quickly after zeroing, something is wrong: worn spindle thread, damaged measuring faces, or a distorted frame. Send it for professional service.

For regulated environments — NATA-accredited laboratories, defence, aerospace, certified quality management systems — instruments require periodic calibration against traceable standards, not just zeroing. NMI Australia (National Measurement Institute) maintains the national measurement standards to which NATA-accredited calibration laboratories trace their references. Annual calibration is a typical interval; high-use environments may require more frequent recertification.

How to Use a Micrometer Correctly

Technique matters as much as the instrument. The most common measurement errors in workshops aren't equipment failures — they're technique failures that would produce wrong results on any brand of micrometer.

Use the Ratchet Stop — Every Single Time

The ratchet stop (or friction thimble) is the knurled cap at the end of the thimble. It limits closing force to a controlled, repeatable amount — typically 5–10N — and clicks and slips when that force is reached. (The knurled finish on the cap is there for grip with oily or gloved fingers — see our Knurling Guide for the patterns and pitches used on precision-instrument controls.)

Never close a micrometer by gripping the thimble directly. Hand pressure varies with grip strength, hand temperature, and fatigue. Two measurements of the same dimension — one with hand pressure, one with the ratchet — can differ by 0.02–0.05mm. That's enough to make or break a tolerance call. Use the ratchet stop every time, without exception.

Temperature: The Most Underrated Error Source

Steel expands at approximately 11.7 × 10⁻⁶ m/m/°C. A 100mm steel shaft measured at 30°C is approximately 0.012mm longer than the same shaft at the ISO 1 standard measurement temperature of 20°C. In a Sydney summer workshop or measuring a part fresh off a lathe, this error is real and measurable.

Allow workpieces to reach room temperature before measuring. For 0.01mm resolution work, leave parts on the bench for at least 15–30 minutes after machining, welding, or extended handling. Don't hold the instrument frame with bare hands for extended periods — your hands warm the frame and cause it to expand. Handle by the insulated grips where fitted, or hold by the frame briefly before measuring.

In Australian conditions — cold Highlands workshops in winter, hot Far North Queensland factory floors in summer — this matters more than most people realise. Measurement at non-standard temperatures produces systematically wrong results that no amount of precision equipment can correct.

Clean the Measuring Faces Before Every Use

Even a thin layer of swarf, coolant film, or grit on the anvil or spindle face adds error to every reading. Before measuring, wipe both faces with a lint-free cloth. When measuring oily workpieces, clean the measurement surface on the part first. This takes 10 seconds and eliminates one of the most common sources of creeping error.

Avoiding Parallax Error

On analogue micrometers, reading the scale from an angle rather than directly perpendicular introduces parallax error — the scale appears to read differently depending on your eye position. Position your eye directly in line with the datum when reading. Digital micrometers eliminate this entirely, which is one of their practical advantages for users who are still building measurement habits.

Seating the Anvil Correctly on Round Workpieces

For round workpieces, seat the anvil flat against the part first, then bring the spindle in using the ratchet stop. Rock the micrometer gently during closing — the maximum reading (tightest contact) is the true diameter. A common error is measuring a chord rather than the true diameter by letting the part sit off-centre on the anvil. Take 2–3 readings at the same location — if they're consistent, your seating technique is correct.

Common Measurement Mistakes and How to Fix Them

Mistake	Effect on reading	Fix
Not zeroing before use	Systematic error in every reading	Zero the instrument at the start of every session
Closing with hand pressure (not ratchet)	Variable; typically reads low	Always use the ratchet stop
Measuring a warm or recently machined part	Reads high; part contracts on cooling	Allow part to reach ambient temperature first
Dirty measuring faces	Adds grit/film thickness to reading	Wipe both faces with lint-free cloth before measuring
Missing the 0.5mm lower sleeve mark	Reading 0.5mm too low	Always check lower sleeve scale before reading thimble
Misreading thimble graduation (off by 1–2 divisions)	0.01–0.02mm error	Read perpendicularly; switch to digital if recurring
Part not seated centrally on anvil	Reads smaller than true diameter (chord, not diameter)	Rock gently during measurement; maximum reading is true OD
Holding frame bare-handed for extended periods	Frame warms and expands; reading drifts	Handle by insulated grip; minimise contact time with frame

Buying a Micrometer: What to Look For

The right micrometer depends on the range you need, the resolution required, whether you want digital or analogue, and how many sizes you need to cover.

Range and Sets

For a general workshop starting out, a 0–25mm outside micrometer covers the majority of small-dimension work. If you regularly measure across multiple size ranges — shaft diameters from 10mm to 75mm, for example — a three-piece set (0–25, 25–50, 50–75mm) is more economical than buying individually and covers the breadth of typical maintenance work.

For large-range applications — measuring large-diameter shafts, heavy machinery components, structural parts — Dasqua offers outside micrometers up to 1000mm range. These are specialist instruments for heavy industry and engineering environments.

View the full AIMS micrometer collection for current stock and pricing across Dasqua and Maxigear ranges.

0.01mm vs 0.001mm: Which Resolution?

Most workshop applications need 0.01mm. Unless you're doing precision grinding, toolmaking, or working to tolerances finer than ±0.02mm, 0.001mm resolution is more instrument than you need. Standard 0.01mm digital models are the practical sweet spot for the vast majority of buyers.

Brand Guide for Australian Buyers

Mitutoyo is the global reference standard. Japanese-manufactured, consistent accuracy, excellent long-term reliability, and widely used by Australian calibration laboratories as working standards. For NATA-accredited work, defence, aerospace, or any environment where instrument traceability and documentation matters, Mitutoyo is the professional specification. Available through AIMS — contact us to discuss Mitutoyo requirements and pricing.

Dasqua offers reliable instruments at a price point that suits production environments, multi-instrument workshops, or applications where Mitutoyo investment isn't justified by the measurement frequency or tolerance requirements. Dasqua instruments are factory-calibrated, built to ISO standards, and include calibration certificates. The digital range with wireless data output is useful for quality management environments where measurement data needs to be logged electronically. Dasqua is AIMS's core in-stock micrometer brand — available in outside (metric + imperial), depth, disc brake, and 3-point inside configurations.

Maxigear digital depth gauges are also stocked at AIMS — suited to depth and step measurement applications where a full depth micrometer set isn't required.

New vs Used

Used micrometers — particularly Mitutoyo instruments — can be excellent value from industrial auctions and secondhand tool markets. Before buying used: check anvil and spindle faces for pitting, flat spots, or scoring (sign of dropping or abuse); verify the instrument zeros and holds zero through repeated open/close cycles; confirm the thimble moves smoothly through its full range without binding or play; confirm the ratchet stop functions correctly. A used Mitutoyo in good condition is often a better buy than a new budget instrument — the Mitutoyo will outlast the workshop. Avoid anything with visible frame damage or damaged measuring faces.

Metric vs Imperial

Australia is metric. Buy metric unless you're servicing equipment built to US imperial specifications — old American agricultural gear, vintage machinery, some imported plant — where fasteners and components may be sized in inches. Dasqua stocks imperial variants across several ranges at AIMS for this specific use case.

Micrometer Use in Automotive and Industrial Maintenance

Micrometers appear throughout automotive and heavy maintenance work. Knowing where to use them — and what tolerance you're measuring to — is as important as knowing how to read them.

Automotive Applications

Brake disc thickness. Minimum disc thickness specifications are cast into the disc hub or listed in the vehicle service manual. A disc worn below minimum is a safety defect — and it needs to be measured, not estimated. Dasqua's purpose-built Disc Brake Micrometer (7–33mm range, 0.01mm graduation) is designed for this task. Measure at multiple points around the circumference to check for disc thickness variation (DTV), which causes pedal pulsation and premature pad wear.

Crankshaft journal diameters. Main bearing and big-end journal diameters are measured against the manufacturer's wear limits and out-of-round limits. Requires a 25–50mm or 50–75mm outside micrometer depending on engine size. Measure at two perpendicular diameters to check for ovality — a journal that's round in one plane but oval in another will cause rapid bearing wear even if the average diameter is within spec.

Piston-to-wall clearance. Measure piston skirt OD with an outside micrometer and cylinder bore ID with a bore micrometer. The difference is piston-to-wall clearance. Correct clearance is critical for oil consumption, ring seal, blowby, and engine longevity. Insufficient clearance causes seizure; excessive clearance causes oil consumption and noise.

Valve stem diameter. Check valve stems for wear and out-of-round against the manufacturer's specification. Typically a 0–25mm outside micrometer. Worn or oval valve stems cause poor sealing, increased oil consumption through guides, and erratic valve timing.

Industrial Maintenance Applications

Bearing journal wear. Shaft diameters at bearing seats wear over time, particularly in high-load or poorly lubricated applications. Measuring shaft diameter at the bearing seat and comparing to the original specification determines whether the shaft is within tolerance or needs grinding, metal spray, or replacement.

Seal journal condition. Lip seal performance depends on a smooth, correctly dimensioned journal. Check diameter before fitting a new seal — an undersized journal from wear will allow the lip to collapse inward, causing premature leakage. An oversized journal (from a previous metal spray repair) will overstress the lip.

Hydraulic rod wear. Hydraulic cylinder rods wear at the seal contact zone, producing a reduced-diameter groove at the seal face. This wear allows the seal lip to bottom out in the groove, causing premature failure. A bore micrometer or outside mic confirms whether the rod is within tolerance for resealing or needs chrome plating / replacement.

Hydraulic cylinder bore wear. Bore micrometers (3-point) are used to check hydraulic cylinder bore wear and out-of-round. Worn or oval bores cause bypassing, heat generation, and shortened seal life even with new seals fitted.

If you're unsure which micrometer suits your specific maintenance application, contact the AIMS team — we can advise on the right instrument and range for your equipment. Call us on (02) 9773 0122.

Inside, Bore & 3-Point Internal Micrometers: The Mitutoyo Series Decoded

The internal micrometer family is broader than most workshops realise. Mitutoyo — the global benchmark for precision measurement — produces five distinct inside-micrometer product families, each engineered for a different bore-measurement problem. Choosing the wrong family is one of the most common causes of "the bore looks right but the bearing won't seat" rework. This section is the deep dive for engineers, fitters and quality inspectors who need to specify the right Mitutoyo Series for the job.

AIMS is an authorised supply channel for the full Mitutoyo small-tool range. While our online catalogue currently shows the Dasqua and Maxigear inside-micrometer sets, we can quote and supply the full Mitutoyo Series 145, 343, 368, 468 and 568 ranges — including interchangeable-head sets, complete-unit sets, extension rods and Series 177 setting rings — through the authorised Mitutoyo Australian distributor network. Contact us for a Mitutoyo quote or call (02) 9773 0122.

The five Mitutoyo inside-micrometer families at a glance

Mitutoyo Series	Type	Sensing	Range	Resolution	Best for
Series 133 / 137 / 145 (Tubular)	Two-point, single rod or extension-rod	Mechanical / vernier	50–1000+ mm	0.01 mm	Large bores, deep IDs, pipe and hydraulic cylinder work
Series 343 / 145 (Caliper-jaw)	Two-point opposing jaws	Mechanical or digital	5–200 mm	0.01 mm	Small IDs, slot widths, narrow grooves below 6 mm
Series 368 (Mechanical Holtest)	Three-point self-centering	Mechanical / vernier	2–300 mm	0.001 / 0.005 mm	Engine bores, precision bearing housings, no batteries
Series 468 (Digimatic Holtest)	Three-point self-centering	Digital LCD, IP65	6–300 mm	0.001 mm	SPC environments, digital data capture, harsh shop floors
Series 568 (Borematic)	Three-point with snap-open lever	Digital ABSOLUTE encoder	6–100 mm	0.001 mm	Production-line bore checking, go/no-go judgement

Mitutoyo Series 145 / 133 / 137 — Tubular inside micrometer (extension-rod type)

The classic tubular inside micrometer is a two-point measuring head with a fixed contact at one end and a micrometer-head spindle at the other. Extension rods screw onto the body to extend its reach — standard sets cover 50–200 mm, 50–500 mm, 50–1000 mm and even 50–2000 mm ranges. Series 133 is the single-rod variant, Series 137 and Series 145 are the multi-rod extension types.

Tubular micrometers are the go-to tool when the bore is too large for a Holtest (above ~300 mm) or too deep for a caliper-jaw inside mic. Hydraulic cylinder bores on excavators, large engine main bearing housings on heavy diesel, pipework internal diameters — this is where tubular wins. Accuracy is modest by Holtest standards (typically ±5 µm at the smallest range, rising to ±10–15 µm once extension rods are added) because each rod junction introduces a small error stack. Compensate by using the shortest possible rod combination for your target bore.

Practitioner note: the tubular mic is a two-point measuring tool, which means it measures a chord across the bore unless you "rock" it to find the maximum reading. With practice, the rocking technique gives genuinely useful accuracy on round bores; on out-of-round bores, swap to a Holtest 3-point.

Mitutoyo Series 343 — Caliper-jaw inside micrometer (the small-bore specialist)

For internal dimensions below 6 mm, the Holtest geometry doesn't fit. The Mitutoyo caliper-jaw inside micrometer (Series 343 digital, or mechanical equivalents) uses two opposing measuring jaws driven by a micrometer thimble — same principle as an inside micrometer, but configured to access very small IDs, narrow grooves, slot widths and other features a 3-point can't enter. Standard ranges run 5–30 mm, 25–50 mm, 50–75 mm and 75–100 mm.

This family also wins on internal groove width measurement, which a 3-point Holtest cannot do at all (the three anvils need to register on a continuous cylindrical surface). Sealing ring grooves, snap-ring grooves, internal keyway widths — caliper-jaw inside mic is the right tool.

Mitutoyo Series 368 — Mechanical Holtest (3-point, vernier scale)

The Series 368 is the original Holtest design that built Mitutoyo's reputation for bore measurement. Three measuring anvils arranged at 120° expand outward from the centre body, automatically self-centering in the bore. The vernier scale reads to 0.001 mm on small heads (up to 12 mm) and 0.005 mm on larger heads. Two-point heads are used for the 2–6 mm range; three-point heads cover 6–300 mm in interchangeable sets.

The Series 368 catalogue covers metric ranges 2–3, 3–6, 6–12, 12–20, 20–30, 30–50, 50–63, 62–75, 75–88, 87–100, 100–125, 125–150, 150–175, 175–200, 200–225, 225–250, 250–275 and 275–300 mm. Imperial sets cover 0.08–12 inches in equivalent step ranges. Anvils on heads above 6 mm are titanium-nitride coated alloy steel for wear resistance; heads of 6 mm and below use solid carbide.

Series 368 published accuracy specification (Bulletin 2230)

Measuring range	Accuracy (max diff)
2–6 mm (two-point)	±2 µm
6–12 mm	±2 µm
12–50 mm	±3 µm
50–100 mm	±4 µm
100–150 mm	±5 µm
150–300 mm	±6 µm

When to specify the Series 368 over the digital options: reference-grade work where battery dependence is unacceptable, calibration-room measurement, low-volume precision inspection, environments where data capture is not required, or any application where the operator strongly prefers analog feel for "approach to size" tactile feedback. The Series 368 also doesn't need batteries — ever — which matters in remote field service.

Mitutoyo Series 468 — Digimatic Holtest (3-point, digital LCD, IP65)

The Series 468 takes the mechanical Holtest geometry and adds an electromagnetic-induction rotary encoder for digital readout. Same three-point self-centering anvils, same TiN-coated measuring pins, but with LCD display reading to 0.001 mm, ABS/INC measuring modes, two-point preset for datum management, data output for SPC, and IP65 protection against splash and dust. Battery life is approximately 1.2 years under normal workshop use on a single SR44 cell.

Ranges mirror the Series 368: 6–12 mm through 275–300 mm in matched interchangeable-head sets. The function lock key prevents accidental datum-point changes — a small detail that matters enormously in production environments where multiple operators share the instrument.

Series 468 published accuracy specification (Bulletin 2230)

Measuring range	Accuracy (max diff)
6–12 mm	±2 µm
12–50 mm	±3 µm (max diff. 2–3 µm)
50–100 mm	±4 µm
100–200 mm	±5 µm
200–300 mm	±6 µm

When the Series 468 is the right call: production quality control with SPC data capture, environments with splash, oil mist or coolant, shared-tool workshops where digital readout eliminates parallax reading errors between operators, or any setting where data output to U-WAVE wireless transmitters or USB Input Tools is part of the inspection workflow.

Mitutoyo Series 568 — Borematic (ABSOLUTE 3-point with snap-open lever)

The Series 568 Borematic is the production-line specialist. The principal difference from the Series 468: a spring-loaded lever mechanism that snaps the three anvils open and shut for fast bore-to-bore checking, plus an electrostatic capacitance ABSOLUTE encoder that eliminates zero-reset between power cycles. Functions include go/no-go judgement with user-defined upper and lower limits, GO/NO-GO zoom display, 2-point preset, data hold, error alarm, 330° rotary display, inch/mm conversion and SPC data output.

Borematic ranges cover 6–100 mm in tighter step intervals than the Series 368/468 (the catalogue lists 12 individual range cells in the 6–100 mm span). Battery life is approximately 5,000 hours of continuous use on a single SR44 cell. Titanium-coated measuring pins allow measurement to the bottom of blind holes; extension rods (100 mm or 150 mm) handle deep IDs.

Series 568 published accuracy specification (Bulletin 2230)

Measuring range	Accuracy (within)
6–12 mm	±5 µm
12–20 mm	±5 µm
20–100 mm	±6 µm

The honest call on Borematic vs Digimatic Holtest: the Series 468 Digimatic Holtest is mechanically a tighter instrument by a microgram or two on each range cell — the published accuracy figures (±2 µm vs ±5 µm at 6–12 mm) reflect the fact that the spring-loaded snap-open lever introduces a small repeatability variation versus the constant-force ratchet on the Series 468. Borematic earns its place on production lines where the operator throughput gain — potentially 30–40% more bores measured per hour — outweighs the marginal accuracy difference. For one-off precision measurement, the Series 468 is the better engineering choice.

Setting a Holtest or Borematic — the calibration step that catches people out

All three Holtest families (Series 368, 468 and 568) require a setting ring to establish their zero point. Mitutoyo's Series 177 Setting Rings are the matched accessory; full sets are included with the Complete-Unit gage sets above. The setting procedure is straightforward in principle — insert the Holtest head into the setting ring, square it up, take a reading, adjust the zero — but two practitioner traps catch people repeatedly.

Trap 1: the setting ring is not exactly nominal size

Each Mitutoyo Series 177 setting ring is supplied with an inspection certificate showing the actual marked diameter. Per Bulletin 2230, the marked value may differ from the nominal size by:

±10 µm for ø1–100 mm setting rings
±20 µm for ø125–300 mm setting rings
±0.0004" for 0.1"–1.8" diameter setting rings
±0.0008" for 2"–12" diameter setting rings

The cylindricity of the setting rings themselves is held to 1.0 µm for rings 1–60 mm, rising to 3.0 µm for rings 250–300 mm. The marked-diameter accuracy is ±1.5 µm for small rings and ±2.5 µm for 50 mm+ rings.

The practitioner technique: when zeroing the Holtest, set it to the marked actual diameter from the calibration sticker, not the nominal ring size. A "ø25 mm" ring might actually be 25.008 mm — you set the Holtest to read 25.008, not 25.000. This is the single most common mistake among machinists new to 3-point bore measurement, and on precision work it accounts for almost all of the measurement bias people blame on the tool itself.

Trap 2: re-calibrate when interchanging Holtest heads

Interchangeable-head sets (Series 368/468/568 ending in -971 through -980) share one display body and several measuring heads. As confirmed in Practical Machinist discussion: "one readout can handle a wide range of different size heads, you need to re-calibrate with a master ring gauge each time you interchange. Additionally, it doesn't hurt to check with a setting ring every time you use it." Each head has its own characteristic offset which must be re-zeroed every time it is screwed onto the display unit. Skipping this step is the second most common cause of "the Mitutoyo is reading wrong" complaints.

Ceramic setting rings — when to upgrade

Mitutoyo also offers ceramic setting rings (177-418, 177-420, 177-423 etc.) with extreme wear-resistance and no anti-corrosion treatment requirement. For workshops doing daily Holtest setting, ceramic rings last effectively forever and don't suffer corrosion in damp environments — worth specifying if the budget permits.

Two-point vs three-point: chord versus true diameter

The fundamental reason engine builders insist on a 3-point Holtest for cylinder bore measurement — not a tubular inside mic, not a telescoping gauge with an outside mic — comes down to geometry. A two-point inside micrometer measures a chord across the bore. If the bore is perfectly round, the chord across two opposing points equals the diameter and you get the right answer. But on a bore that is out-of-round — even by 0.005 mm — a two-point reading depends entirely on the orientation of the tool inside the bore. You can rotate it and get five different readings.

A 3-point Holtest at 120° self-centres regardless of bore orientation, and reads the inscribed circle of the three contact points. On an out-of-round bore, this gives you a single repeatable reading that corresponds to the largest cylinder that will fit through the bore — which is exactly what matters for fit calculations with bearings, sleeves, pistons or any cylindrical mating part.

However: the 3-point Holtest only gives you the inscribed-circle diameter. It does not tell you how out-of-round the bore is. To assess roundness, you need a dial bore gauge with a dial indicator (Mitutoyo Series 511 Dial Bore Gages), rotated through the bore to map the variation. Holtest and dial bore gauge are complementary instruments — not substitutes. See our Bore Gauge Types Guide for the full instrument-family decision matrix.

The anvil-tip vs anvil-body measurement trap

Per Mitutoyo's own published guidance: "When Holtest is used, the measured value differs between measurement across the anvil and the measurement only at the tip of the anvil due to the product mechanism. Adjust the start point under the same condition before measurement. When you use the tip of the anvil for measurement, adjust the start point for using the tip of the anvil."

Translated for the shop floor: the Holtest measuring pins are slightly conical, so contact at the tip of the pin gives a marginally different reading than contact along the body of the pin. If you set the zero with the pins seated against the setting ring at their bodies, then measure a blind-hole bottom with the pin tips, your reading is off by the cone-angle difference. The discipline is to zero the Holtest using the same anvil-contact condition you will use for the work measurement. This is one of the most common reasons fresh graduate machinists distrust their Holtest readings.

Counterfeit Mitutoyo — how to spot the fake

Mitutoyo is the most-counterfeited precision-tool brand in the world. Counterfeits show up on online marketplaces with realistic-looking packaging and serial numbers, often at 30–50% below genuine pricing. The five-check spot-the-fake procedure:

Engraving depth. Genuine Mitutoyo serial numbers and model markings are laser-engraved with crisp edges and consistent depth. Counterfeits use shallow stamped or printed markings that show wear or inconsistency.
Ratchet feel. The constant-force ratchet on a genuine Mitutoyo gives a precise three-click feel before disengaging. Counterfeits typically have a vaguer, smoother slip.
Anvil colour and TiN finish. The TiN coating on genuine Mitutoyo measuring pins is a consistent gold-bronze colour with even reflectivity. Counterfeits show patchy colour or yellow tint deviations.
Country-of-origin stamp. Genuine Mitutoyo inside-micrometer products are stamped "Made in Japan" with deep engraving in a consistent location for that series. Counterfeit pieces often substitute generic markings or have stamps in the wrong location.
Serial number quality. Genuine Mitutoyo serials are sequential, traceable through the Mitutoyo dealer network, and the inspection certificate matches. If the serial doesn't validate through an authorised distributor, treat it as fake.

AEO note for search engines: counterfeit Mitutoyo products are also marketed under deliberate misspellings — Mitutogo, MITU-tyo, Mituttoyo, Mito_tuyo — designed to bypass keyword filters on online marketplaces. Genuine Mitutoyo is always spelled Mitutoyo, capitalised, with no hyphens.

How to specify the right Mitutoyo inside micrometer for your application

Your application	Recommended Mitutoyo series	Why
Engine cylinder bore measurement	Series 468 or 568 (3-point)	Self-centering + digital data + IP65 splash protection
Hydraulic cylinder bore (large)	Series 145 / 137 (Tubular)	Extension rods cover 50–2000 mm range
Precision bearing housing (production)	Series 468 Digimatic Holtest	Tightest accuracy spec, SPC data capture
Production-line bore checking	Series 568 Borematic	Snap-open lever + go/no-go judgement
Internal groove or slot width	Series 343 (Caliper-jaw)	3-point geometry cannot register on a groove face
Small bore < 6 mm	Series 368 (2-point heads)	3-point geometry won't fit below 6 mm
Reference / calibration room	Series 368 Mechanical Holtest	No batteries, no electronics to fail, traceable to inspection cert
Field service in remote location	Series 368 (mechanical)	No battery dependence
Bores with significant out-of-round	Series 511 Dial Bore Gauge + 3-point Holtest	Roundness assessment needs continuous rotation, not single point

Mitutoyo inside micrometer supply through AIMS

AIMS is an authorised supply channel for the full Mitutoyo small-tool range in Australia. Our current online catalogue lists Dasqua 3-point inside micrometer sets (6–12 mm, 12–20 mm, 20–50 mm and 50–100 mm) and Maxigear inside micrometer caliper-type and 3-point sets — ready stock for everyday workshop use. For Mitutoyo Series 145, 343, 368, 468 and 568 Holtest and Borematic products — including individual gages, interchangeable-head sets, complete-unit sets, Series 177 setting rings and extension rods — we quote and supply offline through the authorised Mitutoyo distributor network.

Browse the AIMS micrometer range for currently-stocked product, or contact us for a Mitutoyo quote on any Series 145, 343, 368, 468 or 568 specification. Bulk-buy pricing is available for production environments specifying multiple Holtest heads or Borematic units.

Thread Micrometers: The Mitutoyo Series 126 & 326 + the 3-Wire Method

Pitch diameter is the most safety-critical dimension on any external thread. It controls how the load is distributed across the mating thread flanks — get it wrong and the joint strips under load, regardless of whether the major and minor diameters are within tolerance. Two methods measure pitch diameter on external threads: a dedicated thread micrometer (fast, ±4 µm typical, the production standard) or the 3-wire method (slower, achievable ±1 µm with quality wires, the calibration and audit standard). This section covers both, the Mitutoyo Series 126 and 326 thread mic families, and the critical 55°-vs-60° anvil distinction that catches out users who switch between Metric/UN threads and Whitworth/BSP threads.

AIMS is an authorised supply channel for the full Mitutoyo thread micrometer range. We can quote and supply Mitutoyo Series 126 (mechanical interchangeable V-anvil) and Series 326 (Digimatic ABSOLUTE digital), including 55° and 60° anvil sets and the 126-800 6-piece anvil kit, through the authorised Mitutoyo Australian distributor network. Contact us for a Mitutoyo quote or call (02) 9773 0122.

Mitutoyo Series 126 — interchangeable V-anvil thread micrometer

The Series 126 is the production-standard mechanical thread micrometer. The frame and thimble are similar to a standard outside micrometer, but the contact geometry is different — one anvil is a V-shape that seats into two adjacent thread flanks, and the spindle has a 60° (or 55°) conical point that seats into the opposing flank. When closed onto a thread, the V-anvil and conical spindle contact the thread at the pitch line, and the micrometer reads pitch diameter directly with no calculation needed.

Mitutoyo Series 126 specifications:

Range: 0–25 mm (with extension rods, up to 0–150 mm)
Resolution: 0.01 mm (analog vernier reading)
Accuracy: ±4 µm
Interchangeable anvil sets: 60° anvils (Metric, UN, UNC, UNF threads), 55° anvils (Whitworth, BSW, BSF, BSP threads)
Series 126-800 (6-piece kit): covers both 55° and 60° angles plus the full pitch range a typical workshop encounters — the AU industrial standard buy
Ratchet stop: mandatory — the V-anvil contact creates non-uniform pressure that needs the ratchet to normalise

Mitutoyo Series 326 — Digimatic ABSOLUTE digital thread micrometer

The Series 326 is the digital version of the 126. Same V-anvil and conical spindle geometry, same accuracy, but with Mitutoyo's capacitive ABSOLUTE encoder driving a 0.001 mm LCD readout and supporting Digimatic SPC data output via U-WAVE for production QC workflows. The accuracy spec doesn't change — both 126 and 326 are ±4 µm — but the digital readout removes vernier-reading interpretation error and integrates with MeasurLink statistical process control.

Choose Series 326 for inspection departments, SPC-controlled production, and any workflow that captures data to a quality management system. Choose Series 126 for general workshop and toolroom use where battery-free reliability and lower per-instrument cost matter more than data integration.

The 3-wire method — the calibration-grade alternative

The 3-wire method measures pitch diameter using three precision wires of known diameter and a standard outside micrometer. Two wires sit in adjacent thread grooves on one flank; the third wire sits in the opposing groove. The micrometer measures the dimension across the outer surfaces of the wires, and a formula derives the pitch diameter from that measurement.

The core formula for 60° threads (Metric, UN, UNC, UNF):

PD = M − (3 × W) + (0.866025 × P)

Where:

PD = pitch diameter (the answer)
M = micrometer measurement over the three wires
W = wire diameter (use the same diameter for all three wires)
P = thread pitch (distance between thread peaks)
0.866025 = the trigonometric constant for 60° thread angles (cot 30°)

For 55° threads (Whitworth, BSW, BSF, BSP) the formula changes:

PD = M − (3.1657 × W) + (0.9605 × P)

Optimal wire diameter for any thread: W_best = P / (2 × cos α), where α is half the thread angle (30° for 60° threads, 27.5° for 55° threads). For standard 60° threads this simplifies to W_best ≈ 0.57735 × P. Using the optimal wire diameter ensures the wires seat exactly at the pitch line — the position where pitch errors and small flank-angle errors cancel out, giving the most accurate pitch diameter reading.

Quality 3-wire sets (Mitutoyo, Starrett, Brown & Sharpe) come with precision-ground wires in standardised sizes for common thread pitches. Workshop-grade sets achieve ±1 µm pitch diameter accuracy when used with a calibrated outside micrometer — significantly tighter than the ±4 µm of a Series 126 thread mic.

Thread mic vs 3-wire — decision matrix

Factor	Series 126 / 326 thread micrometer	3-wire method
Speed per measurement	Fast — ~30 seconds, no math	Slow — ~2–3 minutes including formula calculation
Accuracy (typical)	±4 µm	±1 µm achievable with quality wires + master mic
Calculation required	No — reads pitch diameter directly	Yes — formula varies by thread angle
Anvil/wire change for different pitches	Anvil set covers a range; some pitches need a swap	Different wire diameter optimal for each pitch
Best for	Production QC, in-process verification, on-machine checks during lathe threading	Calibration of thread mics, audit-grade inspection, toolroom precision verification, dimensional traceability to gauge blocks
Cost	Higher capital — full Series 126-800 set is significant capex	Lower capital — just a wire set + existing outside mic
Operator skill	Lower — direct reading	Higher — wire seating, math, formula choice

⚠️ The #1 thread mic error — 55° vs 60° anvil mismatch. The anvil V-angle must match the thread profile angle exactly. 60° anvils work on Metric, UN, UNC, UNF threads (all 60° forms). 55° anvils work on Whitworth, BSW, BSF, BSP threads. Using a 60° anvil on a 55° Whitworth thread (or vice versa) contacts the thread flanks at the wrong position relative to the pitch line and gives a reading several thousandths of a millimetre off — enough to fail a thread that's actually within tolerance, or pass a thread that's actually out. AU workshops that work across both thread families (common in plumbing, gas fitting and BSP-port hydraulic work alongside Metric machining) need both anvil sets. The Mitutoyo Series 126-800 6-piece kit covers both angles in one purchase — the right buy if the workshop sees both thread families.

Thread mic and the Go/No-Go gauge — complementary, not substitutes

Thread micrometers and thread gauges measure different things. Go/No-Go thread gauges (covered comprehensively in our Thread Gauge & Pitch Gauge Guide) verify whether a thread falls within tolerance — they're a pass/fail test, fast for production QC, but they don't tell you the actual pitch diameter value. Thread micrometers measure the actual pitch diameter value — slower per check, but they tell you exactly where in the tolerance band the thread sits and how much rework room you have if it fails.

The production workflow that combines both: Go/No-Go gauges for the production QC pass/fail check (fast, every thread); thread micrometer (or 3-wire) for the first-article inspection on a new setup and for failure investigation when a thread fails the Go/No-Go (so you know whether to adjust the lathe tool, change the threading insert, or scrap the part).

Common thread micrometer errors

Wrong anvil angle — 60° on 55° threads or vice versa. The #1 error. See callout above.
Wrong wire diameter on 3-wire — wires must seat at the pitch line. Using arbitrary wires (not the optimal W_best diameter) gives a wrong reading.
Skipping the ratchet stop — V-anvil + thread flank creates non-uniform contact pressure; without the ratchet, two operators measuring the same thread will get different readings. Mandatory on thread mics.
Internal thread confusion — thread micrometers measure external threads only. Internal pitch diameter requires a thread plug gauge or a specialty internal pitch mic (much rarer instrument class).
Substituting Go/No-Go gauges for dimensional measurement — gauges verify tolerance, not value. A thread that passes Go/No-Go is acceptable; it doesn't mean the pitch diameter is on nominal.

AIMS thread micrometer supply

AIMS is bringing the Mitutoyo thread micrometer range into stock through our authorised distributor channel — Series 126 (interchangeable V-anvil mechanical production tool) and Series 326 (Digimatic ABSOLUTE digital with SPC integration). The 6-piece 126-800 anvil kit is the AU industrial standard purchase for workshops working across both 55° and 60° thread families. Thread micrometers are capital equipment and we quote on a per-specification basis — pricing, demonstration arrangements, lead times and application advice handled by our team.

For Mitutoyo thread micrometer pricing, application advice and quote requests, contact our team on (02) 9773 0122 or via aimsindustrial.com.au/contact-us.

Setting Rings & Master Rings: How Holtest, Inside Mics and Dial Bore Gauges Get Zeroed

Inside-measuring instruments — 3-point Holtest, 2-point inside mics, dial bore gauges, plug gauges — don't measure absolute dimensions. They measure deviation from a known reference. The known reference is a setting ring: a hardened, ground and lapped steel ring with a precision-engraved actual diameter value to 0.001 mm. The setting ring establishes the instrument's zero. Every subsequent reading is "how far off the master is this part?" Without a setting ring, none of these instruments can be trusted.

This section covers setting ring fundamentals, the Mitutoyo Series 177 range, the procedure for zeroing each instrument type, and why you can't substitute a gauge block stack or a "good bolt" no matter how tempting it looks.

Master ring vs setting ring vs ring gauge — the terminology decoded

The three terms get used interchangeably but they describe different jobs:

Term	Job	Accuracy class
Master ring	Calibration laboratory reference — used by NATA-accredited labs to certify other rings and instruments	Higher grade, ±0.5 µm typical
Setting ring	Workshop master — used to zero inside-measuring instruments before production measurement	±1.0 to ±1.5 µm typical
Ring gauge (go/no-go)	Production pass/fail check — workpiece either fits or doesn't, no reading taken	ISO 1502 H6/H7 thread or H6 plain

Physically, a master ring and a setting ring often look identical — both are hardened ground rings with an engraved diameter. The difference is paperwork: a master ring carries a calibration certificate stating its actual diameter to higher accuracy, and it gets used only in controlled metrology room conditions. A setting ring is the same ring at workshop tolerance class, used on the shop floor to zero an instrument before measurement.

Confusingly, a "ring gauge" in the AS 1722 / ISO 1502 thread-gauging sense is a completely different tool — a go/no-go threaded ring that the threaded part either screws into or doesn't. That kind of ring gauge gets no reading and has no engraved value. It's a pass/fail instrument, not a measuring reference. Thread ring gauges are covered separately in our contact our team for current sizes and pricing.

Size range: 1 mm to 300 mm inside diameter, with the most-stocked sizes spanning 5–100 mm (matching the inside mic / Holtest working range that needs the most frequent zeroing)
Accuracy class: ±0.5 µm for sizes up to 25 mm, ±1.0 µm for 25–100 mm, ±1.5 µm above 100 mm
Engraved actual value: Each ring is individually calibrated and the actual diameter is engraved on the ring face to 0.001 mm (not the nominal — the real measured value, which is the number you set the instrument to)
Material: Hardened tool steel, precision-ground bore, lapped to mirror finish, dimensionally stabilised for long-term stability
Certification: NIST and JCSS traceable certificates available — the AU production QC standard for critical work, mandatory for aerospace, defence and pharma calibration chains
Storage: Each ring ships in a fitted wooden case with light protective oil film

Quick example of how the engraved-actual-value system works: a Mitutoyo Series 177 setting ring with nominal 25 mm might be engraved 24.9994 mm. That number — 24.9994 mm — is what you set the inside mic or Holtest to read when the contacts seat in the ring. The 0.0006 mm offset from nominal is built into every subsequent measurement. The ring isn't "off"; the engraved value is the truth, and the instrument is being zeroed to truth.

How to zero a 3-point Holtest using a Mitutoyo Series 177 setting ring

The procedure looks fussy. Every step matters. A Holtest reads ±2 µm only if zeroed correctly; a sloppy zero turns a ±2 µm instrument into a ±10 µm guess.

Temperature stabilise. Bring the setting ring, the Holtest, and the workpiece all to 20°C reference temperature. Minimum 30 minutes equalisation in the metrology room or temperature-controlled workshop area. Never use a setting ring straight out of a cold ute or hot from sun exposure — the steel expands 0.012 mm per metre per °C, enough to throw the zero by several micrometres on a 100 mm ring.
Clean. Wipe the setting ring bore, the Holtest contacts, and your hands with a lint-free cloth lightly damped with mineral spirits or instrument cleaner. Fingerprint chloride is the #1 long-term corrosion cause on setting rings — leave none.
Insert the Holtest contacts. Gentle insertion, no forcing. The 3 contacts spaced 120° apart self-centre in the ring. If you feel resistance, withdraw and try again — forcing damages the ring bore and the Holtest contacts.
Rotate the Holtest 360° in the ring. Look for the true reading — the position where the thimble reading is consistent through rotation. A Holtest that fluctuates as it rotates isn't seated correctly or the ring has been damaged.
Lock the thimble at the engraved value. If the engraved value is 24.9994 mm, set the thimble to read exactly 24.9994 mm. Don't round to nominal. The engraved value is the truth.
Repeat the cycle three times. Insert, rotate, read, withdraw — three full cycles. The three readings should agree within 1 µm. If they don't, the ring is contaminated, the Holtest is damaged, or the temperature hasn't stabilised. Diagnose before measuring any part.
Now you can measure your workpiece. Every subsequent reading is the workpiece deviation from the engraved master value. If the Holtest reads 25.0026 mm on the workpiece, the actual bore is the engraved-master + delta = 24.9994 + (25.0026 − 24.9994) = 25.0026 mm. The instrument has done the maths automatically.

How to zero a dial bore gauge using a setting ring

Dial bore gauges measure differently from Holtest instruments — they read deviation from a reference set on the dial, not absolute diameter. The setting ring procedure is similar but the zeroing step is different: instead of setting a thimble to the engraved value, you set the dial to zero (or to the engraved value, depending on dial design).

Temperature stabilise as above — 30 minutes minimum.
Clean ring, gauge, hands.
Insert the bore gauge into the setting ring. Rock the gauge through the bore axis — the dial swings through a minimum reading as the gauge passes through true diameter. Note that minimum reading.
Set the dial to zero at the rocking minimum (on dials calibrated to read zero at setting reference) or set the dial to the engraved master value (on dials calibrated to read absolute). Workshop convention varies — check your gauge's instruction sheet.
Withdraw the gauge.
Re-insert and re-rock. The dial should return to zero (or the engraved value) at the rocking minimum, repeatable within 1 div on the dial. If not, repeat the zero set.
Measure your workpiece. Insert into the bore, rock through the axis, note the rocking minimum on the dial. Apply the dial reading as a delta from the setting ring's known diameter. Workpiece diameter = engraved master + dial reading.

Why you can't substitute a gauge block stack or "a good bolt"

The substitution temptation is real. A setting ring is a capital purchase — hundreds of dollars for a single 25 mm ring, more for larger sizes — and a gauge block stack costs a fraction of that, while a "known good" bolt or shaft costs nothing. The substitution doesn't work, and the reason is geometric:

A gauge block stack measures flat-to-flat. The Holtest, inside mic, and dial bore gauge all measure on an arc — three contacts on a circumference (Holtest), two contacts on a chord (inside mic), or two contacts plus a centralising shoe on a curved bore (dial bore gauge). The arc geometry of those instruments needs a true cylindrical reference at the target diameter. A flat-to-flat gauge block stack has zero arc and reads several µm off when used as a substitute reference.
A "known good" bolt or shaft isn't known good. Even a freshly machined precision shaft is only as accurate as the lathe that made it — typically ±0.01 mm at best, often worse. Setting a Holtest to a shaft of "known" diameter introduces the shaft's manufacturing error into every subsequent measurement. The shaft has no engraved actual value, no calibration certificate, no traceability, and no concentricity guarantee.
Surface finish matters. A setting ring is lapped to a mirror finish (Ra ≤ 0.1 µm). The instrument contacts seat against a uniform polished surface and the reading is repeatable to 1 µm. A turned shaft surface (Ra 1–3 µm) creates contact-point variation that scatters readings by 5–10 µm — enough to make the instrument's specification accuracy irrelevant.

The substitution argument also fails on traceability. NATA-accredited inspection, AS 1210 pressure vessel work, AS 4291 fastener inspection, and any aerospace or defence work require a documented calibration chain back to the National Measurement Institute. A setting ring with a NATA certificate is in the chain; a gauge block stack used as a substitute reference isn't. Failed audits and rejected first-article inspections trace back to substituted references more often than any other single cause.

Temperature stabilisation — the silent accuracy killer

Mitutoyo specifies setting ring accuracy at 20°C. The thermal expansion coefficient of hardened tool steel is approximately 11.5 × 10⁻⁶ per °C. On a 100 mm ring at 30°C workshop temperature, the ring is 11.5 µm larger than its engraved value. On a 25 mm ring under the same conditions, the deviation is 2.9 µm — at the same order as the instrument's specification accuracy, completely swamping the precision you're trying to capture.

The practical rule: 30 minutes minimum equalisation in the metrology room or temperature-controlled measurement area before zeroing or measuring. For critical work, 1 hour. The setting ring, the instrument, and the workpiece must all be at the same temperature before the zero is set. The "warm ute, cold workshop" trap — bringing tools in from a sun-exposed vehicle and using them immediately — is documented across Practical Machinist, Hobby-Machinist and UK metrology forums as the #1 inside-measurement accuracy failure.

Storage, care and recalibration

Wooden case storage — fitted wooden cases ship with each Mitutoyo Series 177 ring and aren't optional. Loose drawer storage causes ring-to-ring contact wear that degrades the bore and the engraved value over years.
Light oil film — apply a thin film of light machine oil or rust preventative on all surfaces before returning to the case. Setting rings live for decades with proper care; they corrode within months without it.
No fingerprints — sweat chloride at workshop concentrations causes pitting corrosion in hardened tool steel that's irreversible. Handle with cotton or nitrile gloves for long-term integrity; at minimum wipe with light oil after every use.
Recalibration interval — annual NATA-accredited recalibration is the AU production QC standard. Critical work (aerospace, defence, pharma, AS 1210 pressure vessels) may require 6-monthly. Occasional workshop use can extend to 2–3 years between calibrations. Any drop, dent, or suspected damage triggers immediate recalibration regardless of interval.
Drift indication — if a setting ring's engraved value drifts by more than its tolerance class (±0.5 to ±1.5 µm depending on size) between calibrations, the ring is failing and needs replacement. Drift this large typically indicates corrosion of the bore, impact damage, or thermal cycling beyond design limits.

AIMS setting ring supply

AIMS is bringing the Mitutoyo Series 177 setting ring range into stock through our authorised distributor channel — full size range 1 mm to 300 mm with NIST traceable certification available. Setting rings are capital equipment and we quote on a per-specification basis: nominal size, accuracy class, certification level, and quantity all affect pricing. Application advice and demonstration arrangements handled by our team.

For Mitutoyo Series 177 setting ring pricing and quote requests, contact our team on (02) 9773 0122 or via aimsindustrial.com.au/contact-us. We also support the bore gauge and Holtest workflow — see our Bore Gauge Types Guide for the dial bore gauge and 3-point Holtest deep-dive.

Micrometer Care and Maintenance

A micrometer is a precision instrument — not a robust workshop tool. Stored and maintained correctly, a quality micrometer lasts decades. Left rolling around a toolbox, it'll drift from zero and need replacement within a few years.

Storage

Always store in the supplied case, not loose in a toolbox where it can be knocked or dropped. Leave the measuring faces slightly apart — don't close them tight in storage, as thermal expansion in a warm environment can exert stress on the frame over time. A light wipe of instrument oil (ISO VG 10 or light machine oil) on the spindle thread prevents corrosion in humid environments, particularly in coastal locations. Wipe off any excess — pooled oil attracts swarf and dust.

Cleaning

After use, wipe measuring faces and spindle with a lint-free cloth. For oily instruments, isopropyl alcohol (IPA) on the cloth dissolves oil and cutting fluid residue effectively. Don't use compressed air on digital micrometers — it can drive fine swarf and coolant mist into the electronics and display. On analogue models, a brief burst of dry compressed air to clear chips from the frame is acceptable, but avoid directing it into the thimble mechanism.

Periodic Zero Checks

Zero at the start of every measuring session — not just when you think of it. If your micrometer consistently drifts from zero between sessions, it needs service: a sign of wear in the spindle thread, contamination in the mechanism, or a damaged measuring face.

Calibration Intervals

For regulated environments — NATA-accredited laboratories, defence, certified quality management systems — instruments require periodic formal calibration against traceable standards, with documented calibration certificates. Annual calibration is a common interval; high-use or critical-measurement environments may require six-monthly or quarterly recertification. Certificates should reference traceability to NMI Australia.

For general workshop use, an annual zero check against a known gauge block or precision pin, and verification that the instrument reads correctly across its range, is adequate. The key trigger for immediate calibration is any drop, impact, or unexpected reading — never continue using a micrometer that's been dropped without verifying it first.

What to Do If Your Micrometer Is Dropped

Don't trust it until verified. Check zero, then measure a known standard (a gauge block or precision pin of known size) and compare to the expected reading. If the discrepancy exceeds 0.01mm, the instrument needs adjustment or professional service before further use. A dropped micrometer on a concrete floor generates enough shock to distort the frame and shift the zero — subtle enough to miss casually, significant enough to cause real measurement error.

Frequently Asked Questions

What is a micrometer used for?

A micrometer is a precision measuring instrument used to measure external or internal dimensions, depths, and bore diameters to an accuracy of 0.01mm (or 0.001mm on higher-resolution models). Common applications include shaft diameter measurement, brake disc thickness, piston clearance, crankshaft journal wear, bore diameter, and any machined component where dimensional tolerance needs to be verified precisely.

What's the difference between a micrometer and a vernier caliper?

Both are precision measurement tools, but a micrometer is more accurate and more specialised. A vernier caliper measures OD, ID, depth, and step in one instrument to ±0.02mm — it's the versatile general-purpose tool. A micrometer measures one class of dimension (e.g. outside diameter) to ±0.01mm or better. Use a caliper for most maintenance work; use a micrometer when tolerances are tight or you need to verify against a specification. See our Vernier Caliper Guide for a full comparison.

How accurate is a micrometer?

A standard micrometer reads to 0.01mm (10 microns). Higher-specification models with a vernier scale or digital display read to 0.001mm (1 micron). Actual measurement uncertainty in practice depends on technique, temperature, instrument condition, and how well it's zeroed. A correctly zeroed, well-maintained 0.01mm micrometer in good hands is typically accurate to ±0.005–0.010mm in workshop conditions.

How do I read a metric micrometer?

Add three values: (1) the highest visible main-scale graduation above the datum line on the sleeve (×1mm each); (2) the 0.5mm mark if it's visible below the datum between the last main graduation and the thimble edge (add 0.5mm); and (3) the thimble graduation that aligns with the sleeve datum line (×0.01mm each). Sum all three. For 0.001mm micrometers, also add the vernier graduation alignment (×0.001mm). See the full step-by-step section above with a worked example.

What is the ratchet stop and do I need to use it every time?

Yes — every single time. The ratchet stop is the knurled cap at the end of the thimble that limits closing force to a controlled, repeatable amount. Without it, closing force varies with your grip and fatigue, producing readings that can differ by 0.02–0.05mm from the true dimension. Using hand pressure instead of the ratchet stop is one of the most common causes of micrometer error in workshops.

What does zeroing a micrometer mean?

Zeroing means confirming — or adjusting — the instrument so it reads exactly 0.00mm against its reference (fully closed for a 0–25mm mic; against its setting standard for larger ranges). If the instrument reads 0.02mm when it should read zero, every measurement is 0.02mm in error. Zero at the start of every measuring session. Use the C-spanner supplied with the instrument to adjust the sleeve if needed; press the zero button on digital models.

What's the difference between an inside micrometer and an outside micrometer?

An outside micrometer has two opposing faces that close around the outside of a workpiece. An inside micrometer has faces that expand outward to measure the internal diameter of a bore or groove. They're mechanically different instruments designed for opposite measurement directions. For bore measurement where accuracy is critical, a three-point bore micrometer (which self-centres in the bore) is more accurate than a two-point inside micrometer.

What is a bore micrometer used for?

A bore micrometer (three-point inside micrometer) measures the internal diameter of cylindrical bores. Three contact points arranged radially ensure it automatically centres in the bore and reads true diameter — not a chord. This makes it the correct tool for engine cylinder bore measurement, hydraulic cylinder bore inspection, and any precision bore where both diameter and roundness need to be assessed. It requires a setting standard (outside mic or ring gauge) to zero before use.

Can I use a micrometer for automotive work?

Yes — micrometers are standard equipment for any precision automotive work. Common applications include brake disc thickness (Dasqua makes a dedicated disc brake micrometer for this), crankshaft journal diameter, piston-to-wall clearance, and valve stem diameter. A 0–25mm and 25–50mm outside micrometer pair covers most engine measurement tasks; a bore micrometer or dial bore gauge is needed for cylinder bore measurement. For full coverage of all bore gauge types — telescopic, small hole, dial bore — and how to choose between them, see our Bore Gauge Types Guide.

What micrometer resolution do I need?

For most workshop maintenance and automotive work, 0.01mm resolution is sufficient. Only specify 0.001mm if you're doing precision grinding, toolmaking, or working to tolerances finer than ±0.02mm. Standard digital outside micrometers at 0.01mm resolution cover 95% of Australian trade and maintenance applications — easy to read, no scale interpretation required, and accurate enough for the work.

Digital or analogue micrometer — which should I buy?

Digital for most buyers — no parallax error, no scale interpretation, instant reading, inch/metric switching, and zero-at-any-position capability. Analogue is preferred by some experienced users who value the tactile feedback and don't want batteries in field conditions. Both are accurate; digital eliminates the most common reading errors for occasional or new users. Dasqua digital outside micrometers are available at AIMS in standard and wireless-output variants.

How often should a micrometer be calibrated?

In regulated environments (NATA labs, certified quality systems, defence), annually or per your quality system's calibration schedule. For general workshop use: zero and verify against a known standard at the start of every session; send for professional calibration if dropped or if readings appear inconsistent. Any instrument used for certified work should have a calibration certificate traceable to NMI Australia.

Can I buy a used micrometer?

Yes — used Mitutoyo instruments in particular can be excellent value. Before buying used: check anvil and spindle faces for pitting, flat spots, or surface damage; verify the instrument zeros and holds zero; confirm the thimble rotates smoothly with no binding; confirm the ratchet stop functions. A used Mitutoyo in good condition will outperform a new budget instrument. Avoid anything with visible frame damage or damaged measuring faces.

Does temperature affect micrometer readings?

Yes — significantly for precision work. Steel expands at approximately 11.7 × 10⁻⁶ m/m/°C. A 100mm shaft measured at 30°C is about 0.012mm longer than at the ISO 1 standard measurement temperature of 20°C. For 0.01mm resolution work, always allow workpieces to reach ambient temperature (15–30 minutes minimum after machining or extended handling) before measuring. In Australian workshop conditions — particularly in summer or near heat sources — thermal expansion is a real and measurable source of error.

How do I store a micrometer correctly?

Store in the supplied case with the measuring faces slightly apart (not closed tight). Apply a light wipe of instrument oil to the spindle to prevent corrosion, particularly in humid or coastal environments. Don't store loose in a toolbox — a dropped micrometer needs to be verified before further use. Keep away from extreme temperatures and magnets (relevant for digital models). Cases should be kept closed to prevent dust contamination of the mechanism.

How is a depth micrometer different from a depth gauge?

A depth gauge uses a sliding scale (vernier, dial, or digital) on a beam with a depth rod that drops into the feature being measured — fast, versatile, typically ±0.02–0.05 mm accuracy, wider range per instrument (150–600 mm common). A depth micrometer uses a screw thimble (same mechanism as an outside micrometer, but oriented to measure depth) with a precision-ground base — significantly tighter accuracy (around ±0.005 mm), but slower per measurement, narrower range per instrument (typically 0–25 mm per rod, with interchangeable rod sets extending to 0–150 mm or 0–300 mm), and more expensive. The decision rule: use a depth gauge for general workshop depth checks where 0.02–0.05 mm is plenty. Step up to a depth micrometer when tighter tolerance is required — tooling inspection, gear cutting setup, mould-making, precision blind-hole or counterbore depth measurement. See our Depth Gauge Guide for the broader depth-measurement landscape.

Why does a depth micrometer read in reverse?

A depth micrometer thimble has its numbers placed in the OPPOSITE order compared to an outside micrometer. The reason is geometric: the spindle extends DOWN into the workpiece as the thimble rotates, so the deeper the spindle goes, the larger the reading needs to be — and the scale direction has to reverse to track that. Mechanically you read it the same way as an outside micrometer (full sleeve graduations plus thimble graduations), but the scale direction is reversed. This reverse-reading characteristic is the #1 source of measurement errors for users switching from outside micrometers to depth micrometers, particularly for partial readings between marked graduations. The Practical Machinist consensus is "don't over-think it, but be aware of the reversal" — once you've handled a depth mic for an afternoon the reversal becomes natural, but the first dozen measurements warrant extra care.

How do I calibrate a depth micrometer with interchangeable rods?

Calibrate the smallest rod first against a gauge block of known dimension on a clean reference surface plate. The smallest rod establishes the zero offset for the entire rod set — all other rods inherit this offset, so getting the small rod right is critical. Clean both seating surfaces before each calibration check by running a piece of paper between them — grease, dirt and grinding dust at the rod adjustment cap shift the zero and are the most common cause of rod-set calibration drift. Use a gauge block of appropriate length for each rod (a 100 mm rod needs a 100 mm reference, not a shorter gauge block — the rod-specific error compounds with length). Test at intermediate thimble positions, not just full turns, to verify scale accuracy across the head's travel. Use only the supplied spanner or adjustment wrench to rotate the sleeve until zero aligns — unauthorised tools or excess force damage the precision sleeve mechanism. Annual NATA-accredited calibration is the AU production QC standard for critical work.

What is a thread micrometer used for?

A thread micrometer measures the pitch diameter of external threads — the most safety-critical thread dimension, controlling how load distributes across mating thread flanks. The instrument has a V-shaped anvil that seats into two adjacent thread grooves and a 60° (or 55°) conical spindle that seats into the opposing groove; together they contact the thread at the pitch line, giving a direct pitch diameter reading without calculation. Used for verifying lathe-cut threads, production QC on threaded fasteners, thread wear assessment on precision screwed components, and first-article inspection on new threading setups. Doesn't replace Go/No-Go thread gauges (which verify tolerance pass/fail) — the two are complementary: gauges for production speed, thread mic for dimensional value and rework decisions.

What's the difference between a thread micrometer and the 3-wire method?

Both measure pitch diameter on external threads. A thread micrometer has a V-anvil and conical spindle that seat at the pitch line, giving a direct reading without calculation — fast, ±4 µm typical accuracy, the production standard. The 3-wire method uses three precision wires of known diameter placed in the thread grooves, with a standard outside micrometer measuring across the wires and a formula deriving pitch diameter (PD = M − 3W + 0.866025P for 60° threads). The 3-wire is slower (requires calculation, optimal wire diameter selection per thread pitch) but achieves ±1 µm accuracy with quality wires — the calibration and audit standard. Thread mic for production; 3-wire for toolroom critical work, calibrating other thread mics, and any dimensional traceability requirement.

Why are 55° and 60° thread mic anvils not interchangeable?

The V-anvil angle must match the thread profile angle exactly to seat correctly at the pitch line. 60° anvils work on Metric, UN, UNC and UNF threads (all 60° thread forms). 55° anvils work on Whitworth, BSW, BSF and BSP threads (all 55° forms). Using a 60° anvil on a 55° Whitworth thread (or vice versa) makes the anvil contact the thread flanks at the wrong position relative to the pitch line — typically reads several thousandths of a millimetre off, enough to fail a thread that's actually within tolerance or pass a thread that's actually out. AU workshops working across both thread families — common in plumbing/gas fitting (BSP) alongside metric machining — need both anvil sets. The Mitutoyo Series 126-800 6-piece kit covers both 55° and 60° angles in one purchase.

What is the difference between a master ring and a setting ring?

A master ring is a calibration laboratory reference of certified higher-grade accuracy (±0.5 µm typical), used by NATA-accredited labs to certify other rings and instruments under controlled metrology room conditions. A setting ring is the workshop master used on the shop floor to zero inside-measuring instruments before production measurement (±1.0 to ±1.5 µm typical). Physically they often look identical — both are hardened ground rings with an engraved diameter value — but the master ring carries a calibration certificate stating its actual diameter to higher accuracy. The Mitutoyo Series 177 range covers both grades depending on the certification level you specify at purchase.

Can I use a gauge block stack instead of a setting ring to zero a bore gauge?

No. Gauge block stacks measure flat-to-flat — they have zero arc curvature. A Holtest, inside micrometer or dial bore gauge measures on an arc — three contacts on a circumference (Holtest), two contacts on a chord (inside mic), or two contacts plus a centralising shoe on a curved bore (dial bore gauge). The arc geometry of those instruments needs a true cylindrical reference at the target diameter. Using a gauge block stack as a substitute reference introduces several µm of geometric error, completely swamping the instrument's specification accuracy. The substitution also fails on traceability: NATA-accredited inspection, AS 1210 pressure vessel work and aerospace/defence calibration chains all require documented traceability that a substituted reference can't provide.

How often should setting rings be recalibrated?

Annual NATA-accredited recalibration is the AU production QC standard. Critical work — aerospace, defence, pharmaceutical, AS 1210 pressure vessels — may require 6-monthly. Occasional workshop use can extend to 2–3 years between calibrations. Any drop, dent, or suspected damage triggers immediate recalibration regardless of interval. Between calibrations, check the engraved value hasn't drifted by more than the ring's tolerance class — drift this large typically indicates corrosion of the bore, impact damage, or thermal cycling beyond design limits and means the ring needs replacement. Mitutoyo Series 177 rings ship with NIST and JCSS traceable certificates as the calibration baseline.