Thread will not go through (internal thread undersized; go gauge does not go in)

?Likely causes

• Workpiece material springs back elastically and the internal thread shrinks (copper alloys, aluminum alloys, cast iron, thin walls, extruded holes)Agreed by 2+ makers^[1][2][3]
• Tap wearAgreed by 2+ makers^[3][1]
• Tap drill hole is small (inner diameter is reduced)Agreed by 2+ makers^[3][2]
• The thread mouth is damaged when backing out in reverseOne maker^[1][2]
• Whisker-like chips remain in the internal threadOne maker^[1][2]
• Pressure when withdrawing is excessiveOne maker^[3]

✓Fixes

Cutting conditions

• Use a proper reverse speed when backing out so the thread mouth is not damagedOne maker^[1][2]

Tool

• Use an oversize (higher accuracy class) tapAgreed by 2+ makers^[3][1][2]
• Replace worn tapsAgreed by 2+ makers^[3][1]
• For form taps, use a smaller accuracy class (inner diameter shrinks)One maker^[2]

Setup

• Increase the tap drill hole diameter to the proper valueAgreed by 2+ makers^[3][2]
• Remove all chips completely before gauge inspectionOne maker^[1][2]

▦Notes by material

• Aluminum: Aluminum tends to weld to thread gauges, and a gauge with welded material cannot guarantee accuracy^[4]

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