CNC Positioning Repeatability — Why Dimensions Drift Apart

Short answer

Positioning repeatability is the ability of an axis to approach the same position repeatedly with a small spread. If repeatability is poor, successive parts differ in dimensions despite the same program — and the operator cannot “dial in” a correction because the error is different each time. This is one of the most common reasons why the CNC does not hold dimension.

Repeatability vs positioning error — important distinction

• Positioning error — systematic difference between commanded and reached position; usually describable by a curve and partly compensable in the CNC control.
• Repeatability — spread of results during repeated approaches to the same point; compensation with a fixed value helps little because the error is not constant.

A machine can have a large but repeatable positioning error (easier case) or a small mean error with poor repeatability (harder case). That is why measurement evaluates both parameters separately.

Typical symptoms

• part dimensions “float” within or outside tolerance, with no clear pattern,
• after zeroing and returning to the reference point the dimension is different than a moment ago,
• the result depends on the direction of the last approach (symptom related to backlash),
• the spread increases when the machine runs longer or when the temperature in the hall changes,
• measuring the part on the machine gives a different result than measuring it on a CMM or control standards.

Possible sources of the problem

• clearance in the ball screw–ball nut coupling or in the ball screw thrust bearing,
• wear of guideways and slides, excessive friction or motion resistance,
• servo drive settings too soft (vibration at stop),
• contaminated or damaged measuring system (linear scale, encoder),
• clearance in part clamping, vice, or tool mounting — not everything that looks like a machine problem is a machine problem,
• temperature gradients: warming ball screw, coolant, open hall doors.

What inspection and measurement can assess

As part of an independent inspection and accuracy assessment of a CNC machine tool, we can:

• perform a series of approaches to selected axis points and measure the spread of actual positions,
• evaluate the effect of approach direction on the result,
• check axis behaviour at the beginning and end of the travel range,
• distinguish a machine problem from a clamping or process problem (e.g. through measurements under different loads and after warm-up),
• describe the results in a technical report — with numbers, not opinions.

The measurement scope is always qualified individually: it depends on the machine, purpose, on-site conditions, and available equipment. Laser positioning measurement is carried out after agreeing the scope and availability.

What the result does not determine

A repeatability measurement describes the condition under test conditions — it does not guarantee production results in other conditions and does not replace repair. We do not perform formal ISO calibration or issue calibration certificates. The technical report is a basis for a service decision, not a certificate.

When service is needed immediately

• the spread has deteriorated suddenly,
• the machine had a collision or drive failure — check CNC machine assessment after collision,
• unusual sounds or vibrations appeared in the drive,
• the CNC control reports positioning or following errors.

FAQ

Does poor repeatability always mean a worn ball screw? No. Similar symptoms are caused by bearing clearance, servo settings, the measuring system, and even part clamping. That is why before deciding on part replacement it is worth measuring and describing the problem with numbers.

What is ‘good’ repeatability? That depends on the machine class, its age, and part requirements. Reference values are given by the manufacturer’s documentation and ISO 230 standards — a measurement report lets you compare the actual condition against them.

Want to know how your machine scores in repeatability measurement? Submit the machine for inspection — we will qualify the scope and prepare a proposal. Also see when to measure CNC accuracy.