A geometric accuracy test checks how well a machine's main parts are aligned with each other. It measures straightness, flatness, squareness, parallelism and spindle run-out, with the machine unloaded or running slowly.
ISO 230-1 describes the test methods. Separate acceptance standards for each machine type list which tests to run and the permitted deviations.
What gets measured
- Straightness of each axis movement, in the two directions across the travel.
- Flatness of the table or bed, and parallelism of the table to its movement.
- Squareness between axes, such as the cross-slide to the spindle axis on a lathe, or the X axis to the Y axis on a machining centre.
- Spindle run-out, both radial and axial, measured at the spindle nose and at a distance on a test bar.
- Alignment of the tailstock with the spindle on a lathe.
Which standard applies
Acceptance conditions are set by machine type. Examples include ISO 1708 for conventional lathes, ISO 13041 for CNC turning machines and ISO 10791 for machining centres. Indian makers may also refer to Indian Standards for the same machines. Ask the maker which standard its test chart follows, and which edition.
The test chart should list each test, the permitted value from the standard, and the measured value. A chart that shows only ticks or "OK" is not a test record.
Geometric tests and positioning tests
A geometric test checks alignment. It does not show how accurately the axes reach programmed positions. That is measured separately under ISO 230-2, as explained in CNC accuracy and repeatability. For a CNC machine you should get both reports.
Geometric tests use dial indicators, test bars, squares, straightedges and precision levels. Ask what instruments are used and when they were last calibrated. Uncalibrated tools make the chart meaningless.
Repeat the test after installation
Transport and installation can change alignment. A long lathe bed or a heavy machining centre on a poor foundation will not hold its factory values. Agree in the contract that key geometric tests will be repeated after levelling at your site, and who carries them out.
At the factory, we suggest you witness a sample of the tests yourself or through an inspector. A few readings taken in front of you show whether the chart matches the machine.
Why this test matters to you
Poor alignment shows up in your parts. A lathe with a misaligned tailstock turns a taper instead of a parallel shaft. A milling machine with an out-of-square table cuts angled faces. A spindle with high run-out shortens tool life and spoils surface finish.
Many of these faults are hard to correct once the machine is in your plant. Some come from the castings, the scraping of slideways or the assembly itself. Finding them at the factory gives the maker the chance to fix them before you have paid in full.
Next step
We check test charts against the right standard, witness sample measurements and keep the record with the shipment file. To start, book a free sourcing review.