Thread is loose (internal thread oversized; no-go gauge goes in)

?Likely causes

• Tap feed and thread lead do not match (feeding too far or too slow thins the thread)Agreed by 2+ makers^{[1][2][3][4][5]}
• Wrong tap accuracy class selectedAgreed by 2+ makers^[4][5][6]
• Welding to the tapOne maker^[5][6]
• Chip cloggingOne maker^[5][6]
• Axis runout, or misalignment with the tap drill holeOne maker^[5][6]
• Poor regrinding (flute indexing, grinding burrs)One maker^[5][6]
• Cutting speed is too slowOne maker^[4]

✓Fixes

Cutting conditions

• If it still feeds too far, adjust the feed to 80-90% of the thread leadOne maker^[3]

Tool

• Use a tap with a smaller accuracy classAgreed by 2+ makers^[4][5]
• Prevent welding with a surface-treated tap (TiN, etc.)One maker^[5][6]

Setup

• On machines with a synchronized feed mechanism, use synchronized feed with a rigid (non-floating) holderAgreed by 2+ makers^[2][3][1]
• Without synchronized feed, strengthen or adjust the spring of the floating holderAgreed by 2+ makers^[3][1]
• Chamfer the tap drill hole entrance to prevent misalignmentOne maker^[5]

Coolant

• Use a cutting fluid with high anti-welding performanceOne maker^[5][6]

Where sources differ: Whether thread flank thinning is occurring can be judged from abnormal cutting marks on the leading and trailing flanks of the tap's flute faces (Yamawa).

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