Lathe Does Not Hold the Programmed Part Size

The Workpiece Moves in the Chuck

If your workpiece moves in the workholding during  the cut, you will have accuracy issues, difficulty holding tolerances and chatter issues. 

Incorrectly bored chuck jaws can let the workpiece move. The soft jaws should be machined to match the nominal size of the part being held. 

Be sure to use a chuck jaw support slug (1) or adjustable boring ring (2) in the center travel of the jaws, before you machine the jaws. If the Jaws are too close to the top of the stroke you will have diffucalty loading and unloading your workpiece in the jaws, if the jaws are too close to the bottom of the stroke the full clamping force will not achieved.   

Use a 0.001" (0.03 mm) feeler gauge to check for gaps between the workpiece and the chuck jaws. Check both the front and the back. The jaws may have deflected from the clamp force, in some cases you may need to machine a slight taper in the jaws to compensate for the jaw's deflection allowing for full jaw contact with the workpeice . 

Make sure that the chuck jaws are tight, and that the screws do not bottom out on the T-nuts or T-slot. The T-nuts should not extend outside the T-slots. 

When OD clamping parts, the centrifugal forces generated at high RPMs will reduce the clamp force and may allow the part to move. Use this chart to determine the if you need to increase the clamp force on the chuck or reduce the max RPM during your program. You can find this chart on a sticker near the hydraulically pump on your lathe. 

Refer to the How to Properly Cut Lathe Soft Jaws — Part 1: Fundamentals and OD Gripping and How to Properly Cut Lathe Soft Jaws — Part 2: ID Gripping, Re-cutting, and Adding a Taper videos for more details on cutting soft jaws.

Note: Be sure to clean and deburr you workpiece before you clamp it into your workholding device. A dirty surface, chips, or burrs can let the workpiece move during the cut.

The Machine is Not Level - Lathe

To operate correctly, the machine must be level. An out-of-level machine can have problems such as poor surface finish, tapered parts, accuracy and repeatability issues, out-of-round circular motion, and out-of-true linear motion.

Watch the Lathe Leveling video to learn how to properly level your Haas lathe, or contact your Haas Factory Outlet to have your machine’s level checked.

Excessive Backlash - Lathe

Excessive backlash or lost motion in a machine axis can cause accuracy errors. Haas machines use mechanical adjustments and Electronic Backlash Compensation together to adjust for backlash and keep the machine accurate.  

Mechanical Backlash is lost motion caused by excessive gaps or clearances between parts in the axis drive system. The Electronic Backlash Compensation uses parameters to compensate for the small amount of mechanical backlash when an axis changes direction.   

You can use the Lathe - Axis Ballscrew - Backlash Test procedure to inspect the backlash on your machine.

If you find that your machine has excessive backlash, inspect your machine for: 

  • A damaged ballscrew coupling
  • Loose screws on the ballscrew, nut or the nut housing
  • A loose lock nut on the end of a ballscrew
  • A waycover that does not move freely.
  • Excessive wear on a ballscrew or nut
Run a ball bar plot to check for issues with the Electronic Backlash Compensation parameters. 


Electronic Thermal Compensation - Lathe

Electronic Thermal Compensation (ETC) uses a proprietary algorithm to compensate for the expansion and contraction in linear axes caused by the heating and cooling of the ballscrews. The ETC algorithm uses a model of the ballscrew, and it estimates expansion or contraction based on the distance traveled and the torque applied to the motor. Heat is represented by a thermal coefficient of expansion, and the axis distance is multiplied by the coefficient to get the amount of correction needed. A real-time clock monitors in-motion time and non-motion time (e.g., lunch, breaks), and compensates accordingly.

Remember that ETC does not correct for thermal growth due to changes in ambient temperature, growth due to part expansion, or growth due to spindle expansion/contraction.

The X, Y, and Z Screw Thermal Comp % settings let you adjust adjust the existing screw thermal compensation by -30% to +30%. If these values are incorrect, you can have accuracy errors.

Start each setting with a value of zero, and then adjust as needed.

These are the settings:
Setting 158 - X Screw Thermal Comp % (-30 to 30)
Setting 159 - Y Screw Thermal Comp % (-30 to 30)
Setting 160 - Z Screw Thermal Comp % (-30 to 30)

If the parameter values used in the ETC algorithm are incorrect, you can have accuracy errors.

If you suspect the ETC parameters are incorrect, contact your Haas Factory Outlet to have the machine parameters checked and corrected if needed.

Thermal Sensors
On Haas lathes, ETC uses a thermal sensor on the X-Axis ballscrew to help ETC compensate the axis. If the thermal sensor on the ballscrew is faulty, this compensation can be incorrect. This causes accuracy errors.

To check for a faulty sensor, find the axis thermal sensor status on the DIAGNOSTICS screen. If the temperature status does not stay constant, the thermal sensor should be replaced.


Thermal Growth During Warm Up Period - Lathe

Most thermal distortion effects occur during a "warm-up period," when you use the machine for the first time after it has stood idle for many hours. The Haas control has a feature to compensate for thermal growth during this time.

There are (4) settings that control thermal growth compensation:
Setting 109 specifies the duration of the warm-up period, in minutes.
Settings 110, 111, and 112 specify the amount of compensation that the control applies during the warm-up period.

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.