Guide to Metric Tap Drill Sizes for Precision Threading

Selecting the correct tap drill size is crucial for successful threading operations. While tapping may seem straightforward, the diameter of the pilot hole significantly impacts thread quality. An oversized hole compromises thread strength, while an undersized one makes tapping difficult and risks damaging the tap. This comprehensive guide provides quick-reference metric thread drill sizes to help achieve perfect threads.

Tapping, the process of cutting internal threads in metal or other materials, depends heavily on proper pilot hole preparation. The ideal drill diameter ensures sufficient thread engagement strength while minimizing tapping resistance and extending tap life.

Standard calculations base drill sizes on thread pitch and desired thread depth. The recommended sizes in this guide assume 75% thread depth—a common standard suitable for most applications. For higher strength requirements, slightly reduce the drill size, though this increases tapping difficulty. Conversely, softer materials or less critical applications may permit slightly larger holes.

The following table lists standard metric thread sizes with corresponding drill diameters in both metric and imperial measurements (where applicable):

• Material Selection: Harder materials may require slightly larger drill sizes to reduce tapping difficulty.
• Tap Type: Spiral flute taps work best for through holes, while straight flute taps suit blind holes.
• Lubrication: Always use appropriate cutting fluid to reduce friction and extend tool life.
• Precision: Maintain hole perpendicularity and positional accuracy to prevent thread misalignment.
• Safety: Wear protective eyewear to shield against metal chips during operations.

Mastering proper drill sizing is fundamental to quality threading. This reference guide provides essential data for common metric threads, while the operational tips help optimize results. As with all machining processes, practical experience remains invaluable—consistent practice and careful observation will develop true threading expertise.