Trouble Sources in Tapping

Trouble Sources in Tapping – Possible Causes

Tap Cuts Oversize

1. Drilled hole prior to tapping too small (tap being overworked).
2. Tap not in line with hole
3. Incorrect lead chamfer grinding. Length of chamfer not equal, on each land.
4. Unevenly spaced cutting edges
5. Wrong type of tap for material of application

Tap Produced Rough Threads

1. Tap dull or chipped
2. Incorrect rake angle (see rake angle chart)
3. Drilled hole prior to tapping too small
4. Speed unsuitable
5. Unsuitable lubricant or inadequate supply

Tap Chips on Cutting Edges

1. Cutting face angle too acute
2. Excessive relief on chamfer lead.
3. Tap hitting bottom of blind hole, creating shock to cutting edges
4. Drilled hole prior to tapping too small
5. Tap too hard for prevailing conditions

Tap Wears Rapidly

1. Lubricant unsuitable or inadequate supply
2. Speed to high
3. Drilled hole prior to tapping too small
4. Lead angle too short (Bottoming tap used where Plug would give better results).
5. Very abrasive material

Tapped Holes Vary in Size

1. Inconsistent work location
2. Excessive play in tap holder or machine spindle.
3. Tapping holes out of round or varying in size, this applies particularly to punched holes.

Tap Picks-up

1. Lubricant dirty or unsuitable, supply inadequate
2. Wrong design of tap for material (i.e. Lands too wide, insufficiently spaced for chips in flutes).
3. Insufficient thread relief
4. Insecure mounting of work piece

Tap Breaks

1. Drilled hole prior to tapping too small
2. Cutting edges worn or chipped
3. Flutes too wide (weak lands)
4. Flutes too deep (weak on web diameter)
5. Tap out of line with hole
6. Tap choked with chips (deep hole tapping)
7. Tap bottoming when blind hole tapping
8. Skin hardness in hole from drilling
9. Lack of lubricant


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