What Is a Step Drill Bit?
A step drill bit — also called a step bit, stepped drill bit, or unibit — is a conical cutting tool with a series of progressively larger stepped diameters machined into a single shank. Instead of drilling one fixed hole size like a standard twist drill, each step represents a specific diameter. You drill until the step you need disappears into the material, and that's your hole.
The result is a single tool that replaces a range of individual drill bits. A 4–32mm step drill, for example, covers every standard metric size across that range. For tradespeople working with electrical enclosures, switchboard panels, conduit knockouts, and thin sheet metal, that's a significant practical advantage on-site.
Step drill bits come in two flute configurations. Straight flute (also called flat-sided) is the simpler design — lower cost, adequate for occasional use. Spiral flute is the professional choice: the helical groove evacuates chips efficiently, reduces heat build-up, and produces a cleaner hole. For regular trade or industrial use, spiral flute is worth the difference in price.
Step Drill Bit vs Twist Drill Bit: When to Use Which
Step drills excel in specific conditions. Outside those conditions, a standard twist drill is usually the better tool. Use the table below as a quick guide.
| Situation | Step Drill | Twist Drill |
|---|---|---|
| Thin sheet metal (<4mm) | ✅ Preferred | ⚠️ Workable but can tear |
| Multiple hole sizes needed | ✅ One tool covers many sizes | ❌ Need multiple bits |
| Clean, burr-minimised hole | ✅ Self-deburring action | ❌ Leaves burr |
| No centre punch available | ✅ Self-starting on thin sheet | ❌ Tends to wander |
| Thick plate steel (>4mm) | ❌ Not suitable | ✅ Preferred |
| Timber, structural members | ⚠️ Works but not optimal | ✅ Preferred |
| Masonry, concrete, tile | ❌ Not suitable | ✅ Use SDS/masonry bit |
| Deep holes | ❌ Not designed for depth | ✅ Preferred |
| Precision tolerance holes | ❌ Not accurate enough | ✅ Preferred |
Types of Step Drill Bits
Step drills are defined by three variables: material, flute type, and shank. Understanding these helps you select the right bit for the job and avoid premature wear or failure.
Material
HSS (High Speed Steel) is the standard material for most step drills. Suitable for mild steel, aluminium, copper, brass, plastic, and wood. Good general-purpose choice for tradespeople who aren't regularly drilling stainless.
HSS TiN Coated (Titanium Nitride) adds a gold-coloured surface hardening treatment that extends cutting life by 3–5× compared to uncoated HSS. The coating reduces friction and heat. Good choice for frequent use in steel sheet — Saber and Sutton both offer TiN coated options in the AIMS range.
HSS Cobalt (HSS-Co, M35 or M42) is alloyed with 5–8% cobalt, which significantly raises heat resistance. Recommended for stainless steel, hardened alloys, and situations where the bit runs hot. The Bordo 2602 cobalt spiral flute step drill is the heavy-duty option in the AIMS range.
Flute Type
Straight flute step drills are simpler in construction and lower in cost. Adequate for occasional use or light-duty applications.
Spiral flute (helical) step drills eject chips more efficiently, which means less heat build-up, longer bit life, and a cleaner hole. For regular trade or production use, spiral flute is the correct choice. All Sutton D504 and Saber 8035 step drills in the AIMS range use spiral flute geometry.
Shank Type
Round shank is standard — fits any drill chuck.
Hex shank (typically 1/4" hex) is designed for use with impact drivers. The Alpha ONSITE Plus Impact step drill uses a hex shank and is specifically engineered to handle the rotational force of an impact driver without the shank slipping. If you're drilling off a ladder or in confined spaces where a corded drill is impractical, a hex shank step drill with an impact driver is a very practical combination.
Step Drill Bit Sizes: How to Read Them
Step drill size notation shows the minimum and maximum diameter the bit covers. A Sutton D504 labelled 4–20mm starts at 4mm and increases in 2mm increments to 20mm — giving you 9 hole sizes in a single tool.
Step increments are typically 1mm or 2mm for metric bits. Each step is machined to a specific diameter, so you get exact sizes, not approximations. When drilling, stop as soon as the step you need has fully entered the material — the hole is the size of the step at the surface, not the step currently cutting.
| Size Range | Typical Increments | Steps | Best For |
|---|---|---|---|
| 4–12mm | 2mm | 5 | Small knockouts, cable glands, thin panel work |
| 4–20mm | 2mm | 9 | Electrical boxes, light switchboard work, conduit |
| 4–30mm | 2mm | 14 | Heavy switchboard, larger conduit, industrial enclosures |
| 4–32mm | 2mm | 15 | Full-range general purpose |
For Australian electricians: standard conduit hole sizes are 20mm, 25mm, and 32mm. A 4–32mm step drill covers all of these in a single tool, which is why it's the most common choice in the trade.
For a full drill bit size reference covering metric, imperial, and fractional sizes, see our Drill Bit Size Chart.
What Materials Can Step Drill Bits Cut?
Step drills are optimised for thin, sheet-form materials. Below is a material-by-material guide.
| Material | Suitable? | Grade Needed | Notes |
|---|---|---|---|
| Mild steel sheet (<4mm) | ✅ Yes | HSS or HSS TiN | Use cutting fluid. Spiral flute preferred. |
| Stainless steel sheet | ✅ Yes (with correct grade) | HSS Cobalt | Low speed, steady pressure, cutting fluid essential. Standard HSS will burn out quickly. |
| Aluminium sheet | ✅ Yes | HSS | Higher RPM. Use cutting fluid (WD-40 or equivalent) to prevent material loading the flutes. |
| Copper and brass | ✅ Yes | HSS | Medium speed. Cutting fluid recommended. Brass can grab — reduce feed pressure. |
| Plastic (PVC, ABS, polycarbonate) | ✅ Yes | HSS | Medium-low RPM. No cutting fluid needed. Light pressure prevents melting. |
| Plywood and MDF | ✅ Yes | HSS | Works well. Not the optimum tool but practical for single-step holes. |
| Solid hardwood | ⚠️ Acceptable | HSS | Workable but spade or auger bits are faster and cleaner. |
| Thick plate steel (>4mm) | ❌ No | — | Use hole saw or annular cutter. Step drills overheat and lose edge quickly in thick material. |
| Hardened or tool steel | ❌ No | — | Beyond the capability of any standard step drill. |
| Masonry, concrete, tile | ❌ No | — | Use SDS/hammer drill with masonry bit. |
| Cast iron | ❌ No | — | Brittle material — high risk of cracking. |
When NOT to Use a Step Drill Bit
Most guides focus on what step drills are good for. The situations where they are the wrong tool are equally important to understand.
Thick material (over 4mm / 3/16"). Step drills are designed for sheet metal. In thick plate, each step only partially engages the cut at any time, generating excessive heat and causing rapid edge wear. For plate steel over 4mm, use an annular cutter or hole saw.
Deep holes. The cone geometry means the bit is only cutting near the tip. Deep holes accumulate chips with no way out, causing binding and breakage.
Precision tolerance work. Step drills produce a hole that is within a millimetre of the nominal size — adequate for knockouts and cable glands, not acceptable for close-tolerance engineering fits. Use a reamer after drilling if precision is required.
High-production drilling. For repetitive, high-volume work, a specific-diameter twist drill in a drill press is faster, cooler, and more consistent. Step drills suit on-site, variable-size work — not production lines.
Impact drivers (standard round shank only). Never use a round-shank step drill in an impact driver — the shank will slip in the chuck under impact torque. Only hex-shank impact-rated step drills (such as the Alpha ONSITE Impact) are designed for impact driver use.
How to Use a Step Drill Bit: Technique and Speed Guide
The most common reason step drill bits fail prematurely is incorrect speed. Running too fast generates heat that destroys the cutting edge — sometimes within a single hole. The table below gives recommended speeds by material.
| Material | Recommended RPM | Cutting Fluid? |
|---|---|---|
| Mild steel (thin sheet) | 500–900 RPM | Yes — cutting oil or Trefolex |
| Stainless steel | 200–400 RPM | Yes — essential |
| Aluminium | 1,500–3,000 RPM | Yes — WD-40 or cutting oil |
| Copper and brass | 1,000–2,000 RPM | Yes — cutting oil |
| Plastic (PVC, ABS, polycarbonate) | 500–1,500 RPM | No |
| Plywood / MDF | 1,000–2,000 RPM | No |
Note: Use the lower end of the RPM range for larger step diameters; higher RPM for smaller diameters. When in doubt, go slower — you can always speed up, but you cannot undo a burnt edge.
Step-by-Step Technique
- Secure the workpiece. Movement during drilling causes chatter, grab, and off-centre holes. Clamp or vice-grip thin sheet — never hold it by hand when step drilling.
- Mark the hole location. For thin sheet metal, a stepped drill is self-starting and a scribed mark is sufficient. For harder materials, use a centre punch to prevent the tip wandering on entry.
- Set the correct speed. Refer to the table above. If your drill has a speed selector, set it before you start. Most cordless drills have high and low range — use low range for steel.
- Apply cutting fluid. A small amount of cutting oil on the tip before you start is more effective than applying it mid-hole. For through-work, a drop or two is enough.
- Start with light, steady pressure. Let the bit establish the cut before increasing feed pressure. Heavy pressure at entry causes the bit to wander and the first step to load up with chips.
- Watch the step, not the drill. Stop drilling as soon as the target step has fully passed through the material. The hole diameter equals the step that just cleared the surface.
- Ease off before breakthrough. Reduce pressure just before the bit exits the back of the material to avoid tearing or burring the exit hole.
Cutting Fluid: Do You Need It?
For any metal drilling — yes. Cutting fluid reduces friction, carries heat away from the cutting edge, improves surface finish, and significantly extends bit life. Even a cheap step drill lasts much longer with cutting fluid than an expensive one run dry.
Common options for step drill use:
- Cutting oil / neat oil — Best performance for steel and stainless. Trefolex paste or Tap Magic liquid are well-regarded Australian trade options.
- WD-40 — Adequate for aluminium and light steel use. Not ideal for stainless or heavy-duty work, but widely available on-site.
- CRC cutting fluid spray — Convenient for on-site use. Apply before drilling and reapply if the bit runs hot.
For plastic: no cutting fluid needed, but keep RPM moderate and use light, consistent pressure to prevent the material from melting onto the cutting edges.
For a full guide to cutting fluids by material and application, see our Cutting Fluids & Cutting Oils Guide.
Diagnosing Common Step Drill Problems
| Problem | Likely Cause | Fix |
|---|---|---|
| Chatter or vibration during drilling | RPM too high; workpiece not secured; bit dull | Reduce speed; clamp workpiece firmly; replace bit if edge is gone |
| Bit grabs and jerks on breakthrough | Too much feed pressure at exit; workpiece not clamped | Ease off pressure before breakthrough; clamp the workpiece |
| Overheating / smoke / discolouration | Speed too high; no cutting fluid; bit dull | Reduce RPM; add cutting fluid; replace bit |
| Hole is not round or has a raised lip | Worn or chipped cutting edge | Replace the bit |
| Bit won't start on the mark | No centre punch; surface too hard | Punch mark; check material suitability |
| Bit loading up with material (aluminium, plastic) | Chips not clearing; speed too low for aluminium, too high for plastic | Add cutting fluid for aluminium; reduce speed for plastic; clear flutes regularly |
Do Step Drill Bits Self-Deburr?
Partly — and this is worth understanding clearly. When you drill to a step and pull the bit back through the hole, the trailing edge of the next step passes through the entry hole and removes the raised burr. This is the "self-deburring" action you'll see in product descriptions.
However, there are two important caveats. First, the deburring action only occurs at the entry side of the hole — the exit side will still have a burr in most cases. Second, allowing the next step to pass through the hole means that step is now slightly enlarging it. If you need a precise 12mm hole, letting the 14mm step pass through to deburr it will make the hole 14mm.
For clean edge finishing — particularly in electrical applications where wiring insulation must not be damaged — use a purpose-made deburring tool after drilling. This gives a controlled chamfer on both sides of the hole without any risk of enlarging it.
Step Drill Bit vs Hole Saw: Which Should You Use?
| Step Drill Bit | Hole Saw | |
|---|---|---|
| Material thickness | Up to ~4mm | 4mm and above |
| Hole diameter | Up to ~32–35mm | Anything from 14mm to 150mm+ |
| Multiple sizes | One tool, many sizes | One saw per size |
| Speed of use | Fast | Slower; more setup |
| Portability | Excellent — one bit | Bulkier; requires arbor |
| Hole quality | Clean in thin sheet | Can be rough; requires pilot drill |
| Best application | Electrician knockouts, thin enclosures, conduit holes | Thick panels, wall plates, structural members |
The rule of thumb: if the material is thinner than 4mm and the hole is smaller than 32mm, a step drill is almost always faster and cleaner. If either condition isn't met, reach for the hole saw.
Choosing the Right Step Drill Bit for the Job
AIMS stocks step drills from four manufacturers across different grades and configurations. Here's how to match the right tool to your application.
| Application | Recommended Grade | AIMS Options |
|---|---|---|
| General electrical / plumbing sheet work | HSS TiN Coated, Spiral Flute | Sutton D504, Saber 8035 |
| Heavy switchboard / enclosure work (daily use) | HSS TiN Coated, Spiral Flute — full range set | Sutton D504SET3 (4–12, 4–20, 4–30mm) |
| Stainless steel sheet and harder alloys | HSS Cobalt, Spiral Flute | Bordo 2602 Cobalt |
| Impact driver (off-ladder, confined spaces) | Hex shank, Impact-rated | Alpha ONSITE Plus Impact |
| Occasional / light DIY use | HSS Straight Flute | Bordo 2600, Saber HSS Straight Flute |
| Full metric range in one tool | HSS TiN Coated, 4–32mm | Saber 8035-M3 (4–32mm) |
Set vs individual bits: If you're a sparkie or plumber doing regular knockout and conduit work, a set covering 4–12mm, 4–20mm, and 4–30mm (such as the Sutton D504SET3) is the practical choice — one purchase covers every job. If you only occasionally need a specific size range, buy the individual bit that covers it.
One bit. Multiple holes.
Shop HSS & cobalt step drill bits for sheet metal, plastic & thin steel
From standard HSS step bits for mild steel and plastic to cobalt step bits for stainless — AIMS Industrial stocks step drill bits across a full range of diameter steps, ready to ship Australia-wide.
Frequently Asked Questions
What is a step drill bit?
A step drill bit is a conical cutting tool with a series of stepped diameters machined into a single shank. Each step is a specific hole size. You drill until the step matching your required diameter passes through the material — one tool covers multiple hole sizes.
What is a step drill bit used for?
Step drill bits are primarily used to drill clean, accurately-sized holes in thin sheet materials — electrical enclosures, switchboard panels, steel sheet, aluminium, copper, plastic, and thin plywood. They are the go-to tool for electricians, plumbers, HVAC technicians, and metalworkers who regularly need multiple hole sizes without changing bits.
What is a unibit?
Unibit is a brand name — originally trademarked by Irwin Tools — that has become a generic term for step drill bits in some markets. In Australia, the terms "step drill" and "step bit" are more common in trade usage. They refer to the same tool.
Can you use step drill bits on wood?
Yes. Step drill bits cut through plywood, MDF, and timber effectively. They're not the optimum tool — spade bits and auger bits are faster and produce cleaner holes in solid timber — but on-site, a step drill is a practical alternative when a single tool needs to cover both metal and timber work.
Can you use step drill bits on plastic?
Yes, with care. Use a medium-low RPM (500–1,500 RPM depending on material), light and steady pressure, and avoid forcing the bit. High speed causes plastics to melt and load the flutes. Polycarbonate and acrylic are best drilled slowly; PVC and ABS are more forgiving.
Can you use a step drill bit on stainless steel?
Yes, but standard HSS is not adequate for stainless. Use an HSS Cobalt step drill (M35 or M42 grade), run it slowly (200–400 RPM), use cutting fluid throughout, and apply steady rather than heavy feed pressure. Stainless work-hardens when drilled incorrectly — if you pause mid-hole, the material can harden around the bit. Drill through in a continuous pass.
What is the difference between HSS and cobalt step drill bits?
HSS (High Speed Steel) is the standard material — suitable for mild steel, aluminium, copper, brass, plastic, and wood. HSS Cobalt adds 5–8% cobalt to the alloy, significantly raising heat resistance. Cobalt is the correct choice for stainless steel, harder alloys, and any application where the bit runs hot. It is also more brittle than standard HSS — avoid heavy lateral pressure or impact.
What speed (RPM) should I use for step drills on steel?
For mild steel sheet, use 500–900 RPM. For stainless steel, reduce to 200–400 RPM. Running too fast is the most common cause of premature wear and burnt cutting edges. If you see discolouration on the bit or the chips turn blue/black, your speed is too high. Always use cutting fluid when drilling steel.
Why does my step drill bit chatter?
Chatter — vibration and stuttering during drilling — is almost always caused by one of three things: RPM is too high, the workpiece is not secured, or the bit is worn. Clamp the material firmly, reduce speed, and check the cutting edge. A step drill with a damaged cutting edge will never run smoothly regardless of technique.
Do I need cutting fluid with a step drill bit?
For any metal drilling — yes. Cutting fluid reduces friction, draws heat away from the cutting edge, and significantly extends bit life. For steel and stainless, use a dedicated cutting oil or compound. For aluminium, WD-40 is adequate. For plastic and wood, no cutting fluid is needed.
Do step drill bits self-deburr?
Partially. The trailing edge of the next step can remove the burr from the entry side of the hole as the bit is withdrawn. However, this also slightly enlarges the hole — the next step diameter, not the target step, is what contacts the entry edge. For critical electrical applications where burrs could damage wire insulation, use a dedicated deburring tool for a controlled chamfer on both entry and exit faces.
What is the maximum material thickness for a step drill bit?
Step drill bits are designed for sheet materials up to approximately 3–4mm (about 12–16 gauge steel). In thicker material, the bit generates excessive heat, wears rapidly, and produces a poor-quality hole. For material over 4mm, use an annular cutter or hole saw instead.
Step drill bit vs hole saw — which should I use?
Use a step drill for thin material (under 4mm) and smaller holes (up to ~32mm). Use a hole saw for thicker material, larger diameter holes, or where you need a plug of material removed. For standard electrical conduit and knockout work in thin enclosures, a step drill is faster and more versatile.
Can I use a step drill bit in an impact driver?
Only if the bit is specifically rated for impact use and has a hex shank. Standard round-shank step drills are not designed for the rotational force of an impact driver and will slip or be damaged. The Alpha ONSITE Plus Impact step drill has a 1/4" hex shank and impact-rated construction for safe use with impact drivers.
What size step drill bit do I need for conduit work?
Australian standard electrical conduit sizes are 20mm, 25mm, and 32mm. A 4–32mm step drill covers all three in a single tool, making it the most practical choice for switchboard and conduit installation work. If you're only running 20mm conduit, a 4–20mm bit is sufficient and slightly more manageable on-site.
