Threads and Tapping Basics (Metric Threads & Tap Drill Chart)

Tapping cuts an internal thread into a pilot hole. Choosing the right pilot (tap drill) diameter and tap type is key to thread accuracy and tool life. This article compiles the basics of threads and tapping.

Thread basics

• Metric thread (M): the most common. "M6" means a 6mm nominal diameter
• Pitch: distance between thread crests. Coarse (standard) and fine exist
• Designation: M6×1.0 (diameter × pitch). Fine is written like M8×1.0
• Pipe threads (G, R): for piping (tapered/parallel)

Tap drill chart (metric coarse)

Tap drill diameter ≒ nominal diameter − pitch.

Thread	Pitch	Tap drill (approx.)
M2	0.4	1.6mm
M3	0.5	2.5mm
M4	0.7	3.3mm
M5	0.8	4.2mm
M6	1.0	5.0mm
M8	1.25	6.8mm
M10	1.5	8.5mm
M12	1.75	10.2mm

Through vs blind holes

• Through hole: passes through. A taper (through) tap with a long lead is easy to use
• Blind hole: has a bottom. A spiral tap threads near the bottom and ejects chips upward

Tap types

Type	Suited hole	Traits
Hand tap (taper, plug, bottoming)	Hand work, through	A set of three, staged
Spiral tap	Blind hole	Ejects chips upward
Gun (point) tap	Through hole	Pushes chips forward

Tapping tips

• Keep the correct pilot diameter (too big = incomplete threads, too small = breakage)
• Use cutting oil
• Keep the tap square (tilting causes defects/breakage)
• For blind holes, mind chip evacuation (spiral taps help)

FAQ

Q. How to find the pilot diameter? A. Estimate with "diameter − pitch" (e.g., M6×1.0 → 6−1 = 5mm). Confirm with a chart or drill table.

Q. Why do taps break? A. Pilot too small, clogged chips, tilt, or insufficient cutting oil. Use spiral taps for blind holes to be safe.

Q. Coarse vs fine pitch? A. Fine has a smaller pitch, resists loosening, and suits fine adjustment. Coarse is the most common.

Conclusion

Threading comes down to "the correct pilot diameter × a tap suited to the hole type." Keeping a chart handy gives peace of mind.

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