Backlash in a CNC machine — symptoms, causes, assessment

Short answer

Backlash is lost motion in the axis kinematics that appears when the direction of travel changes. The CNC control sends a positioning command, e.g. 0.02 mm in the opposite direction, but the table or saddle starts moving only after the lost motion in the ball screw, ball nut, bearing or coupling is taken up. Result: the same dimension produced with an approach from the left and from the right differs by the backlash value.

Typical symptoms

• circles come out oval with characteristic “flat” spots at direction changes (visible on the part or in a circularity test),
• a dimension corrected by hundredths of a millimetre does “not respond” or responds in steps,
• roughing and finishing cuts from different directions give different results on the same surface,
• on a lathe: diameters turned in conventional feed differ from those turned in climb feed,
• the problem increases with machine runtime and is larger on heavily used sections of the axis.

Possible sources of the problem

• ball nut wear — the most common cause in axes operating in one area,
• axial play of the ball screw in the thrust bearing or a worn bearing,
• backlash in the gear or belt of the axis drive (machines with indirect drive),
• incorrect backlash compensation value in the CNC control parameters — e.g. after replacing the ball screw or servo drive,
• backlash in the coupling between the motor and the ball screw.

What inspection and measurement can assess

Within an independent inspection and accuracy assessment of a CNC machine, it is possible to:

• measure the backlash value at selected points of the axis — with approaches from two directions and reading of the actual displacement,
• check whether the backlash is constant along the axis or larger on heavily used sections (indicates ball nut wear),
• assess whether the current backlash compensation in the control matches reality,
• separate backlash from problems with positioning repeatability and positioning error,
• describe the results in a technical report — useful when talking to the service department or during machine acceptance after repair.

The scope of measurement is qualified individually — it depends on the machine, purpose, on-site conditions and available equipment.

What the result does not decide

Backlash measurement alone does not answer whether screw regeneration, ball nut replacement or just parameter correction is worthwhile — that is a service decision, and the report provides numbers for it. We do not guarantee repair results and do not perform formal ISO calibration, calibration to a reference standard or issue calibration certificates.

When immediate service is needed

• backlash suddenly increased after a collision or axis overload — see machine after collision,
• a distinct knock is heard when the direction changes,
• the control reports following errors during braking and starting,
• the backlash is so large that maintaining the basic part tolerance is impossible.

FAQ

Can backlash be “zeroed” with a parameter? The backlash compensation parameter in the control masks the problem up to a certain value, but it does not fix the mechanics. With large or uneven backlash, compensation works poorly — first measure how much backlash there really is and where.

What backlash is “acceptable”? That depends on the machine class and part requirements. Small backlash in a roughing machine may be acceptable; in a machine for precision moulds it is not. The measurement report lets you compare the actual condition against the manufacturer documentation.

Seeing symptoms suggesting backlash? Report the machine for inspection — we will qualify the measurement scope and prepare a proposal. If the issue concerns dimensions in general, also read: CNC does not hold dimension.