Troubleshooting

While tapping nuts by bent shank taps, the following troubles are found frequently:

  1. 1. Accuracy of female threads
    1. Oversize pitch diameter (goes through NO-GO Gage)
    2. Undersize pitch diameter (does not go through GO Gage)
    3. Oversize minor diameter (goes through NO-GO Plain Gage)
    4. Over inclination bearing surface
  2. Finished Surface of female threads
    1. Thorn or rough thread
    2. Chattering on tapped thread
  3. Tap life
    1. Breakage
    2. Chipping
    3. 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:

  1. Condition of tap wear
  2. Change of tap color due to contact by nut blank or chips
  3. Condition of the hole before tapping (taper, hardened surface, scale, etc.)
  4. Inconsistent hardness of nut blanks
  5. Abnormal noise, vibration and heat
  6. 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.