Troubleshooting
While tapping nuts by bent shank taps, the following troubles are found frequently:
- 1. Accuracy of female threads
- Oversize pitch diameter (goes through NO-GO Gage)
- Undersize pitch diameter (does not go through GO Gage)
- Oversize minor diameter (goes through NO-GO Plain Gage)
- Over inclination bearing surface
- Finished Surface of female threads
- Thorn or rough thread
- Chattering on tapped thread
- Tap life
- Breakage
- Chipping
- Wear
These troubles are, theoretically, caused by incorrect tap and improper cutting conditions (nut material, cutting speed, cutting oil, tapping machine, etc.), but they are related each other in a complicated way and in some cases it is quite difficult to specify single or simple cause and to take effective counter-measures.
It is possible and recommended, however, to narrow the causes down by observing the following condition in detail:
- Condition of tap wear
- Change of tap color due to contact by nut blank or chips
- Condition of the hole before tapping (taper, hardened surface, scale, etc.)
- Inconsistent hardness of nut blanks
- Abnormal noise, vibration and heat
- Volume and cleanness of cutting oil and oil supply frequency
The following are examples of causes of troubles and countermeasures and can be utilized to
shoot troubles:
1. a) Oversize Pitch Dia. (goes through NO-GO Gage)
Incorrect pitch diameter of tap:
- Use correct pitch diameter tap.
- (Use smaller pitch diameter tap.)
Run-out of tap shank axis:
- Correct bending and run-out of tap shank.
- Use larger diameter shank.
- Adjust clearance between the guide of the machine and a nut blank.
Improper biting of tap:
- Enlarge rake angle of tap.
- Increase indicator drop for chamfer relief.
- Make the point diameter of chamfer to fit the inner hole of the nut.
- Use a combined tap.
- Increase the pressure of the pushrod of machine.
Built-up edge and galling:
- Use coated tap.
- Correct cutting speed (slow down).
- Increase cutting oil volume.
- Put addition agent into cutting oil.
- Change the oil type.
- Improve the way of oil supply.
Over cutting:
- Use tap with no thread relief.
- Keep margin portion.
- Decrease rake angle.
- Decrease chamfer relief angle.
- Decrease the pressure of pushrod of the machine.
Wear of NO-GO Gage:
- Replace with a new gage.
1. b) Undersize Pitch Diameter (do not go through GO Gage)
Incorrect pitch diameter:
- Use accurate tolerance pitch diameter. (Use larger pitch diameter tap.)
Insufficient cutting:
- Increase rake angle and use better performance tap.
Flaws and dents of nut thread:
- Eliminate incomplete threads on the side of the shank.
- Increase the radius (R) of bent shank and decrease shank diameter to improve flow of tapped nuts through the shank.
- Adjust the cycle of pushrod to rpm.
Chips remaining:
- Use the tap which discharges chips well.
- Increase cutting oil volume.
- Increase oil supply pressure.
The roundness of female threads is bad:
- Decrease chamfer relief angle of the tap.
Too hard nut blanks:
- Use wear resistance taps.
- Anneal nut blanks.
- Employ suitable forming method to decrease work hardening of nut blanks.
- Avoid irregular hardness from quenching.
Tap wear:
- Use wear resistance taps.
- Replace the tap earlier.
- Regrind flutes of the tap.
- Increase cutting oil volume.
- Put addition agent into cutting oil.
- Use different cutting oil.
Dented GO Gage:
- Remove the dent.
- Replace with a new gage.
1. c) Oversize internal thread
Oversize of mouth taper of hole before tapping:
- Improve forming method of nut blanks.
- Increase the chamfer angle of the tap.
Improper biting:
- Increase the pressure of the pushrod of the tapping machine.
- Increase rake angle.
- Increase chamfer relief angle of the tap.
- Align the point diameter of chamfer with the inner diameter of the nut.
- Use combined taps.
- Change the tap.
- Regrind the flutes.
1. d) Minor diameter too small
Tap wear:
- Use wear resistant taps.
- Change the tap more often.
- Regrind the flutes.
- Increase the amount of cutting oil.
- Put addition agent into oil.
- Change the type of the oil.
Poor performance of tap:
- Increase rake angle.
- Increase chamfer relief angle of the tap.
- Change the tap.
- Regrind the flutes.
1. e) Large inclination of bearing surface
Improper adjustment of machine:
- Adjust guide, etc. of the machine to avoid inclination.
- Adjust the width and thickness of the chute.
- Revise or change the point diameter of pushrod.
Cutting quality is poor:
- Use a pilot tap.
- Make point diameter smaller.
2. a) Thorn or Rough Thread
Poor cutting quality of tap:
- Increase of rake angle of the tap.
- Increase chamfer relief angle.
- Change the tap.
- Regrind the flutes
Galling:
- Use coated taps.
- Apply suitable cutting speed (slow down).
- Increase cutting oil volume.
- Put addition agent in cutting oil.
- Use different cutting oil.
- Improve the way of cutting oil supply.
Tap wear:
- Use wear resistance taps.
- Replace a tap earlier.
- Regrind tap flutes.
- Increase cutting oil volume.
- Put addition agent in cutting oil.
- Use different cutting oil.
Chip packing:
- Use tap flutes which are designed to cutting conditions given.
- Reduce a number of flutes (especially for high nuts).
2. b) Chattering
Over cutting:
- Decrease rake angle or chamfer relief angle of the tap.
- Use a tap with no thread relief.
- Keep margin portion.
Improper adjustment of machine:
- Improve stiffness of the machine.
- Use a different machine.
3. a) Chipping
Improper choice of tap:
- Use taps which suit cutting conditions.
Galling:
- Use coated taps.
- Apply proper cutting speed (slow down).
- Increase cutting oil volume.
- Put addition agent in cutting oil.
- Improve cutting oil supply.
Improper adjustment of machine:
- Avoid empty tapping. (Arrange consistent feeding from the chute.)
3. b) Breakage
Chip packing:
- Increase volume of the flutes.
- Reduce the number of flutes.
- Use spiral taps or similar taps that eject chips forward.
Prepared hole (Inner thread) is small:
- If possible, make the hole (of inner thread) bigger.
Improper adjustments of machine:
- Avoid tapping two pcs at the same time.
3. c) Wear
Improper choice of a tap:
- Use the taps which suit materials, surface treatments and other cutting conditions.
Nut blanks are too hard:
- Use wear resistance taps.
- Anneal nut blanks.
- Examine suitable forming method to decrease hardening of nut blanks.
- Avoid irregularity of hardness from quenching.
A scale on the hole of nut blank (hot forging):
- Remove a scale by shot blasting before tapping.
Heat:
- Increase cutting oil volume.
- Put addition agent in cutting oil.
- Use different cutting oil.
- Improve cutting oil supply method.
Improper adjustment of machine:
- Avoid empty tapping of the guide by preventing hitting the pushrod.