Lathe Thread Pitch Error Troubleshooting

The Feedrate is Incorrect for the Threading Operation

The programmed feedrate must be correct for the thread. Given in inches per revolution, your feedrate should be equal to 1 divided by the number of threads per inch. For example, for a 1"-14 TPI thread, your feedrate should be 0.0714 (1/14).

Other example feedrates:

Threads Per Inch (TPI) Thread Lead (1/TPI) (Feedrate in in/rev) Calculated Spindle Speed (TPI x Desired IPM (NTE 125 IPM))
7 0.142857 875 RPM
8 0.125 1000 RPM
9 0.111111 1125 RPM
10 0.1 1250 RPM
11 0.090909 1375 RPM
12 0.083333 1500 RPM
13 0.076823 1625 RPM

Corrective Action:

Use a feedrate that matches your desired threads-per-inch. Do not exceed the machine's maximum feedrate. Haas recommends a feedrate no higher than 125 IPM; however, if the calculated IPM is equal to or less than 125 IPM and you still see thread pitch errors, lower the RPM until the pitch error is within tolerance.

The Commanded Feed Rate is Too High

The spindle and the Z Axis must be synchronized for threading operations. If the commanded feed rate is too high, the spindle and Z axis will not remain synchronized. This results in thread pitch errors.

Corrective Action:

Haas recommends a maximum feed rate of 150 inches per min while threading .

The Threading Start Point is Too Close to the Workpiece

If the thread pitch error happens at the start of the thread, then the start point of the threading operation may be too close to the workpiece.

Corrective Action:

 

 

Move the start point of the thread [1] to a distance equal to or greater than 3 times the thread pitch [2].

Incorrect Chuck Clamp Force

Thread pitch errors occur if the workpiece moves in the chuck.

Corrective Action:

Refer to the clamp force chart decal on the machine enclosure. Select the correct clamp force for the application and adjust the chuck pressure.

Note that high spindle speeds can cause the chuck to lose clamp force. You may need to reduce the spindle speed (and adjust the threading operation to match) if you cannot increase the chuck pressure; for example, in the case where a higher pressure would crush a delicate part.

Incorrectly Bored Chuck Soft Jaws

If the chuck jaws are not bored correctly, problems like these are possible:

  • The workpiece can move in the jaws.
  • The workpiece can be non-concentric to the spindle centerline.
  • The clamp force can deform both the workpiece and the chuck jaws.

Even a correctly bored set of soft jaws can experience wear and start to cause problems with lathe applications.

Corrective Action:

Re-bore the soft jaws. Follow the instructions in the videos linked below. Be sure to use a chuck jaw support (ID or OD adjustable boring ring or a piece of material) to correctly bore the jaws.

Test the jaws with a 0.001" (0.03 mm) feeler gauge to make sure that there are no gaps, in the front or the back, between the workpiece and the chuck jaws.

Soft Jaw Boring Videos

How to Properly Cut Lathe Soft Jaws — Part 1: Fundamentals and OD Gripping

How to Properly Cut Lathe Soft Jaws — Part 2: ID Gripping, Re-cutting, and Adding a Taper

Be aware: Many service and repair procedures should be done only by authorized personnel. The service technicians at your Haas Factory Outlet (HFO) have the training, experience, and are certified to do these tasks safely and correctly. You should not do machine repair or service procedures unless you are qualified and knowledgeable about the processes.

Danger: Some service procedures can be dangerous or life-threatening. DO NOT attempt a procedure that you do not completely understand. Contact your Haas Factory Outlet (HFO) and schedule a service technician visit if you have any doubts about doing a procedure.