The Geometric Accuracy Report: Your Best Protection Before Shipment

Of all the documents in a machine tool purchase, the one that protects you most is a few pages of measurement data: the geometric accuracy report of the machine you are actually buying. Too many buyers accept a brochure number, then discover at acceptance that their parts do not agree with it. Here is what the report contains and how to use it.

What the report measures

The ISO 10791 series defines the geometric and positioning tests for machining centers. In practice two instruments do most of the work. A laser interferometer measures linear positioning accuracy and repeatability along each axis, comparing commanded positions against measured ones over the full travel. A ballbar draws programmed circles in different planes; deviations from a perfect circle expose squareness errors between axes, backlash, scale mismatch and servo tuning problems that a simple linear test will miss.

A proper report shows the raw plots and the derived values, with the test conditions, usually ambient temperature and warm-up state, noted on the page.

Accuracy classes in plain numbers

Most standard production VMCs position within about plus or minus 0.01-0.02 mm. Precision-grade machines reach roughly 0.005 mm. Neither number means anything unless it was measured on the machine being shipped, after final assembly and scraping, not on a prototype or quoted from a specification sheet.

Why factory verification matters

Accuracy is built in before the laser ever switches on. It starts with castings that have been properly aged, which is why we use HT300 resin-sand castings with double ageing treatment at VITOP. It continues with hand-scraped and matched guideways, where a fitter removes high spots until the geometry is true rather than relying on machined surfaces alone. The laser interferometry and ballbar tests at the end confirm the whole chain: castings, guideways, drives and assembly.

If a supplier cannot show you this data per machine, you are buying a brochure, not a measurement.

What to demand contractually

• The report of your serial-numbered machine, attached to the proforma invoice or the acceptance certificate, not a sample report.
• The test standard and conditions: ISO 10791 series, with temperature and warm-up noted.
• Agreed limit values for positioning accuracy and repeatability, written into the contract, with a defined remedy if the machine misses them.
• Ballbar plots in at least the XY, XZ and YZ planes.

Common mistakes that cost buyers money

• Accepting a brochure tolerance. The sheet shows what the model family achieved once. You need the measured values of the machine with your serial number.
• Skipping the test conditions. A positioning figure measured on a warm machine in a 20-degree metrology room tells you little about a 33-degree shop floor in the tropics. Ask how the values were taken.
• Ignoring repeatability. Positioning accuracy is the best single pass; repeatability is what your parts see every cycle. Both belong in the contract.
• Paying before the report arrives. Link a payment milestone to delivery of the report, and the incentive lines up neatly.

Checking again on arrival

Transport, foundation and levelling all influence final geometry. Re-level the machine on its foundation, let it reach thermal equilibrium, and if possible re-run a ballbar test during commissioning. Compare against the factory report. Small deviations point to levelling; large ones justify a claim while the machine is still the supplier's responsibility.

One honest report is worth more than a page of marketing tolerances. Ask for it early, in writing, and make it part of the payment milestones.

Talk to VITOP Machinery

Planning a machine purchase or need help matching a model to your parts? Send your drawings or questions to vitopmachinery@163.com, or message us directly on WhatsApp: +86 183 1908 0212. You can also browse all machines at www.vitopmachinery.com. We reply within one business day.