Tap breaks

?Likely causes

• Chips jammed or cloggedAgreed by 2+ makers^{[1][2][3][4][5][6]}
• Tap drill hole is too small and cutting torque is excessiveAgreed by 2+ makers^[3][2][7]
• The tap hits the bottom of the tap drill hole (bottoming)Agreed by 2+ makers^[3][2][7][8]
• Tap wearAgreed by 2+ makers^[3][2]
• Cutting speed is highAgreed by 2+ makers^[3][2][7]
• Insufficient lubrication or unsuitable cutting fluidAgreed by 2+ makers^[3][4]
• Misalignment between the tap drill hole and the tap, or the hole is tiltedOne maker^[2][7]
• Tap drill hole has work hardenedOne maker^[2][9]
• On a horizontal machine, long chips from a spiral tap wrap around the toolAgreed by 2+ makers^[6][10]
• Tap does not suit the hole shape (spiral tap in a through hole, or point tap in a blind hole)Agreed by 2+ makers^[5][11]
• Uneven feed, or abrupt reversal in a blind holeOne maker^[2]

✓Fixes

Cutting conditions

• Reduce the cutting speedAgreed by 2+ makers^[3][2][7]

Tool

• Use point taps for through holes and spiral flute taps for blind holesAgreed by 2+ makers^[12][5][2]
• Consider fluteless (form or roll) taps, which produce no chipsOne maker^[1][2][7]
• On horizontal machines, use a low-helix spiral tap or a tap designed for horizontal machinesAgreed by 2+ makers^[6][10]
• Replace worn taps earlyAgreed by 2+ makers^[3][2]

Setup

• Make the tap drill hole diameter larger within tolerance (confirm by measuring)One maker^{[1][2][7][12]}
• Allow extra tap drill hole depth (effective thread length + lead chamfer length + about 1 pitch + drill point)Agreed by 2+ makers^[8][3][2]
• Use a tap holder with torque setting and a torque-overload stop functionAgreed by 2+ makers^[3][2][7]
• Eliminate misalignment and tilt relative to the tap drill holeOne maker^[2][7]

Coolant

• Supply plenty of suitable cutting fluid (for taps, prioritize lubricity; non-water-soluble is the standard), and for taps with coolant holes, supply it through the tapAgreed by 2+ makers^[3][4][12]

▦Notes by material

• Stainless steel: Work hardening of the tap drill hole increases the load. Drill the hole with a stainless steel drill, and avoid steps that are too fine or feeds that are too low^[9][13]
• Stainless steel (small diameter): If breakage continues at small diameters such as M2, switching to a thread mill (helical interpolation) is an option^[14]

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