A CNC machine should have positioning accuracy and repeatability that sit comfortably inside the tightest tolerance on your parts. There is no single right figure. What matters is that the values are measured under ISO 230-2 on the machine you are buying, not copied from a brochure.
Accuracy and repeatability are different things. A machine can repeat the same wrong position very well, so you need both numbers.
What the terms mean
- Positioning accuracy is how close the axis gets to the commanded position, measured across the travel.
- Repeatability is how closely the axis returns to the same position over repeated approaches.
- Reversal value, or backlash, is the difference in position when the axis approaches from opposite directions.
- ISO 230-2 sets out how these values are measured and calculated, usually with a laser interferometer along each axis.
How to set your requirement
Start from your drawings. Find the tightest tolerances on features cut by the machine, then allow for tool wear, temperature, fixturing and the measuring method. The machine's own error should use only part of the tolerance band, leaving room for everything else.
If you hold tight tolerances, ask about thermal stability. Spindle and ball screw heat change positions as the machine warms up. Ask whether the machine has thermal compensation, and how long it needs to warm up before cutting accurately.
What drives the numbers
- Ball screw class and whether the screws are pre-tensioned.
- Guideway type and quality, and how well they are aligned during assembly.
- Whether the machine uses linear scales or relies on motor encoders.
- Whether the maker applies laser-based compensation in the controller after assembly.
- The rigidity of the castings and how well they were stress-relieved.
How to check before you accept
Ask for the ISO 230-2 report for the exact machine serial number, with the date, instrument and test conditions. Ask for the geometric test chart as well, covered in geometric accuracy tests.
Then cut a test part. A report shows the axes are good, but a measured part shows the machine can make your work. Plan both into the trial, as explained in witnessing a machine trial.
Keep the reports with your purchase records. If accuracy falls inside the warranty period, they give you a baseline to compare against when you or a service engineer re-measure the machine.
Common gaps in quotes
- Some quotes give a single "accuracy" figure without saying which standard, axis or travel length it refers to.
- Some quote repeatability only, which looks better but says nothing about how close the machine gets to the commanded position.
- Some quote values measured on a sample machine, not the one you are buying.
- Some quote values measured with linear scales fitted, when the machine you ordered has none.
- Ask the maker to correct each of these gaps in writing before you compare prices.
Next step
We help you set realistic accuracy requirements, check the reports and witness the test cut. To start, book a free sourcing review.