“±0.1 mm” appears in many CNC cutting machine brochures. It is a useful number, but it is not a complete answer. The tolerance of a finished part depends on the machine, the material, the tool and the way the material is held.
The buyer needs to separate machine positioning accuracy, repeatability and part tolerance.
Machine accuracy
Machine accuracy describes how closely the cutting head reaches the commanded position under defined conditions. It is influenced by the frame, guide rails, rack, servo system, calibration and compensation.
Repeatability
Repeatability describes how closely the machine returns to the same position over repeated cycles. A machine can be reasonably accurate but inconsistent, or precise but offset by a constant amount.
Part tolerance
Part tolerance is the result the customer measures on the finished part. It includes:
- machine accuracy
- material movement
- knife deflection
- blade wear
- vacuum hold-down
- table surface condition
- cutting speed
- number of passes
- nesting and tool-path direction
- measurement method
A thin, stable gasket sheet may approach the published machine tolerance. A thick foam block or a soft felt panel can move more, so the practical part tolerance may be wider.
Why ±0.1 mm is not a promise for every material
The same machine can produce different results across materials. A dense, thin sheet may be held firmly and cut cleanly. A compressible foam can deflect under the blade and recover differently. A woven fabric can pull or fray.
The test condition matters. Ask:
- What material was cut?
- What thickness?
- Which knife and angle?
- What vacuum and table surface?
- What feed and oscillation?
- What was measured, and with what instrument?
- Was the measurement taken on the first part or across a production run?
A responsible supplier should provide this context, or at least confirm the test on your material.
How to improve the result
Match the knife and blade
The wrong blade can push the material instead of cutting it. A sharper or thinner blade may improve a soft material; a stronger blade may be needed for dense sheet.
Use the right hold-down
Zoned vacuum, a suitable sacrificial mat and correct loading help stabilise the material. Small parts may need a different zone strategy or a support material.
Control the speed
Higher speed is not always better. A slower feed can improve edge quality and dimensional stability in difficult material.
Use more than one pass
A controlled multi-pass cut can reduce deflection in thick or dense material. The right number of passes is determined by the test.
Maintain the table
A worn or contaminated mat changes hold-down and cut depth. Replace wear surfaces on a defined schedule.
Measure correctly
Use a calibrated instrument and a defined measurement method. Compare the same feature across parts. Record the result.
What to put in the RFQ
Instead of asking only for “±0.1 mm”, specify:
- critical dimensions and tolerance
- material and thickness
- part size
- cutting method allowed
- inspection method
- acceptable edge quality
- production rate
- whether the tolerance applies to the first article or every part
Then require a sample report that connects the test parameters to the measured result.
When to challenge a claim
Ask for evidence when a supplier:
- promises the same tolerance for every material
- cannot explain the test conditions
- measures a machine axis but not a finished part
- ignores vacuum, knife wear or material movement
- refuses a sample test
The right tolerance is the one the process can repeat on your material at the required output.
Request a cutting test and review the machine range before fixing the specification.