This document explains some of the most common causes for tap breakage.

• The RPM may be too high:
  • Set the RPM to a maximum of 3000, or 150 inches per minute feed, whichever comes first.
  • The DT-1 can tap at up to 5000 RPM.
• If you are using a tension-compression holder:
  • Use 90% of the actual feed calculated for the thread to allow constant pressure on the tap as it enters and exits the hole.
  • Use 90% of the actual feed calculated for the thread if the machine does not have the Rigid Tap option.
• Check the feedrate:
  • The speed and feedrate must be precise to maintain the correct synchronization between the tool and the Z-axis.
• Keep the R-plane no shorter than 0.200” ( 5 mm) to give the machine some space to synchronize and accelerate.
• If tapping small holes:
  • Put the machine to high gear (M42).
  • Tapping small holes in high gear speeds up the tapping process.
  • The high gear has less mass in the drive train to start and stop.

• Spiral flute taps:
  • Spiral flute taps are designed to pull chips out of the hole and away from the cutting area.
  • They work well when tapping a blind hole.
• Spiral point taps:
  • Spiral point taps are designed to push chips forward and into the hole away from the cutting area.
  • They work well when tapping a through hole.
  • They do not work well in blind holes because the tapping chips can pack into the bottom of the hole and cause the tap to bottom.
• Tap run out:
  • Make sure the run out is correct for the tap.
  • Some taps break if the TIR is more than 0.001" (.03 mm).
• Make sure the tap is designed for use on CNC equipment:
  • Spiral flute taps work well and will eject chips from the working area.
  • Roll taps are stronger than cut taps and are recommended for small diameter tapping.
  • Hand taps are not designed for use on CNC machines.
• Check the tap holding:
  • If the tap holder is a collet, make sure the tap does not slip.
  • If the tap holder is a collet, make sure the tap is not running out.
  • Taps must be held with a positive-drive device.
  • Make sure the collet is specifically designed to hold taps.

• Hard Materials:
  • If the material is very hard, do not use coolant.
  • Clear the chips out of the holes and use a cutting fluid.
• Soft Materials:
  • Soft, gummy materials require a consistent cooling mix.
  • They may also require special tapping compounds.
  • Check the coolant concentrations with a refractometer.
  • Always follow the manufacturer's mixing directions or use tapping solutions.

• Check the hole depth:
  • Blind holes need be drilled at least 0.050” deeper than the tap is programmed to go.
  • The hole will be shallower than what is programmed because of the tip angle on the drill.
  • Setting an offset for the tip of the tool will make the hole shallow by the amount lost by the tip angle.
  • The program can compensate for this by drilling slightly deeper. This gives you the diameter required to the depth required.
• Spot drill, then tap:
  • Spot drill a pilot hole before tapping.
  • This will insure that the drill is located accurately.
  • Drill the pilot hole slightly greater than the major diameter of the tap size.
• Make sure the tap drill is the correct size:
  • For 75% of effective thread use this formula:
    • D - 1/N
    • D = major diameter of the tap
    • N = number of threads per inch
• Hole dimensions:
  • A tapped hole with 75% of thread depth has only 5% less strength than 100% thread and takes only 1/3 of the cutting force of a 100% thread.
  • One and one half times the major diameter is the usual depth requirement for a tapped hole.
  • The strength of a threaded connection does not increase if the contact length is more than 1.5 times the diameter of the fastener.
  • Chances of breaking the tap increase dramatically when tapping more than one-and-a-half times the major diameter 

• Z Axis Overshoot and Drift:
  • Use an indicator and a simple, repeating incremental tapping program to test drift.
  • If the Z-Axis overshoots/drifts farther than what is programmed in the G84:
  • Parameter 120, SPINDLE ACC/DECEL adjusts the amount the Z axis can overshoot.
  • Parameter 144, RIG TAP FINISH DIST also adjusts the amount the Z axis can overshoot.
• Rigid Tapping:
  • If the customer has rigid tapping, make sure Parameter 57:4 RIGID TAP is ON.
• Peck Tapping:
  • If the customer is tapping the same hole more than once (peck tap), make sure Setting 133 REPEAT RIGID TAP is ON.
  • Prior to mill version 12.09 REPEAT RIGID TAP was turned on with Parameter 57:6.
• Tap Retract:
  • Setting 130, TAP RETRACT SPEED, controls the speed at which the tap retracts.
  • Reduce the value in this setting if cross-threading or tearing occurs.
• Spindle Encoder:
  • Inspect the spindle encoder. For more information, please refer to the Spindle Non-Contact Encoder (NCE) - Troubleshooting Guide. 

• Tapping Compound:
  • If coolant may be the cause of the problem, program a series of holes and brush a tapping compound or tapping oil on the tap instead of coolant.
  • Tap Magic makes several different types of high quality tapping fluid.
• Coolant Condition:
  • Always follow the coolant manufacturer's recommendations for mixing coolant.
  • Check the coolant concentrations with a refractometer.
  • Make sure the water has a low mineral content. Minerals in the water break down the quality of the coolant and reduce the coolant's effectiveness.