CNC Axis Positioning Error — Symptoms, Causes and Measurement

What is a CNC axis positioning error?

A positioning error is the difference between the commanded position in the program (or on the handwheel) and the position where the axis actually stops. The CNC control “thinks” the axis is at coordinate 250.000 mm, while in reality the table or saddle has stopped, for example, 15–30 µm short of or past it. The problem affects both linear axes (X, Y, Z) and rotary axes.

Important distinction: positioning error tells you where the axis ended up, while positioning repeatability tells you how steadily it hits the same spot on repeated approaches. These are two different parameters, and a machine can have a problem with one, the other, or both at the same time.

Typical symptoms

• part dimensions are systematically shifted by a constant value or increase with distance from the zero point,
• holes drilled at two ends of a long part do not match the drawing dimension,
• the dimension is correct in one area of the table and wrong in another,
• after a tool change or fixture change the part “shifts” by tenths of a millimetre,
• the error depends on the approach direction — different dimensions when approaching from the left than from the right (this often indicates backlash).

Possible sources of the problem

• Ball screw pitch error — wear or non-uniform thread; the error changes periodically along the axis.
• Wear of the ball nut or thrust bearing — clearance appears and the position drifts.
• Guideway errors — worn guideways change the path of motion and overload the drive.
• Thermal expansion — ball screws and the machine body lengthen with temperature; the error grows as the temperature changes.
• Incorrect or outdated compensation tables in the CNC control — for example after replacing a ball screw or servo drive.
• Position measuring system (encoder, linear scale) — contamination or damage to a section of the scale produces local, repeatable errors.

What inspection and measurement can assess

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

• measure positioning deviations along selected axes at agreed points,
• evaluate axis behaviour on approaches from two directions (separating positioning error from backlash),
• check the effect of temperature and machine warm-up on the result,
• compare the CNC control read-out with the actual axis position,
• describe the results in a technical report that can be used in talks with the service department, seller, or when planning a repair.

Laser positioning measurement is carried out after agreeing the scope and availability. The scope depends on the machine, the purpose of the measurement, on-site conditions, and available equipment — that is why every enquiry starts with qualifying the measurement scope.

What the result does not determine

The measurement shows what the error is and where it occurs, but it does not by itself point unambiguously to the cause (for example whether the ball screw, ball nut, or compensation is at fault) and does not guarantee that the machine will achieve a specific accuracy after repair. We do not perform formal ISO calibration or issue calibration certificates — we deliver a technical report describing the measured condition.

When service is needed immediately

• the error increases day by day or appeared suddenly,
• the machine had a collision — see CNC machine assessment after collision,
• knocking or resistance is audible in the axis drive,
• the axis loses steps or the control reports following errors.

FAQ

Can positioning error be corrected in the CNC control? Many controls have positioning error compensation tables, but entering corrections only makes sense after measurement and on a mechanically stable machine. The decision on compensation is best discussed with the service provider or technology partner — based on the measurement report.

Is one measurement enough? It depends on the question. A single axis measurement answers “what is the error now”. If you suspect temperature influence or wear, repeat the measurement after the machine has warmed up.

If you want to find out what can be measured on your machine, submit the machine for inspection — we will qualify the measurement scope and prepare a proposal. You can read more about the method in the article what is CNC laser calibration.