Understanding Basic Diameters in Tapping Drill Sizes

Basic diameters in threading

When discussing tapping drill sizes and thread dimensions, we primarily deal with three basic diameters:

• Major diameter: This is the largest diameter of the thread, measured across the outer peaks of the thread profile
• Minor diameter: This is the smallest diameter of the thread, measured across the roots of the thread profile.
• Effective diameter: This is an intermediate diameter, often used in thread calculations, and is typically defined as the average of the major and minor diameters. However, its precise definition can vary depending on the specific thread standard.

Relationship to tapping drill size

The tapping drill size is closely related to the minor diameter of the thread. When drilling a hole for tapping, the drill bit should create a hole slightly larger than the minor diameter to accommodate the thread formation.

• For metric threads: A common rule of thumb is to subtract the thread pitch from the major diameter to estimate the tapping drill size. However, this is a simplified approximation, and actual tapping drill sizes are often listed in tap drill charts for specific thread standards.
• For inch threads: The tapping drill size is often calculated as a percentage of the major diameter, with different percentages used for coarse and fine threads.

Importance of accurate tapping drill size

Selecting the correct tapping drill size is crucial for achieving proper thread formation and ensuring the strength and durability of the threaded joint. An undersized hole can lead to thread stripping, while an oversized hole can result in a weak thread.

Notes:

• The specific calculations and factors influencing tapping drill size can vary depending on the thread standard (eg metric, UNC, UNF etc), thread class, material and desired thread depth.
• It's essential to refer to appropriate tap drill charts and engineering standards for accurate and reliable results.