CNC accuracy: advantages of key industries

A drawing usually states a dimensional tolerance, a geometric tolerance and a surface finish. Each one is produced by a different part of the process. Dimensional tolerance comes from machine positioning and tool compensation. Geometric tolerance, such as flatness or coaxiality, depends more on how the part is held and how many setups it takes. Surface finish is set by tool geometry, feed rate and spindle speed.

Repeatability is the quieter requirement. A shop may hit ±0.005 mm on one part and drift across a 500-part run. That drift comes from thermal growth, tool wear and chip load changes. For assemblies, repeatability matters more than the best single-part result, because a pin that fits one housing must fit the next one too.

Surface finish also drives function. A seal running on a Ra 0.2–0.8 μm shaft behaves differently from the same shaft at Ra 3.2 μm. A smooth surface reduces friction and wear, but it can also hold oil poorly. The right finish is the one the mating part needs.

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  Dimensional tolerancePosition of a feature, held by machine accuracy and tool offset.
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  Geometric toleranceShape and orientation, driven by fixturing and setup count.
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  Surface finishCutting parameters; choose per function, not by habit.

Complex geometry is the clearest case. Deep cavities, thin ribs, undercuts and contoured surfaces are hard to reach with a three-axis setup because the tool has to approach from a fixed direction. A five-axis machine tilts the tool or the table, so the same feature can be cut in fewer setups. Fewer setups means fewer datum changes, and datum changes are where most tolerance stack-up comes from.

Thin-wall parts are a second case. As material is removed, the part relaxes and moves. A shop that understands this will plan the cut sequence, leave support ribs, and take light finishing passes. A shop that does not will deliver a part that measures well on the machine and fails on the CMM.

Third is part count. When the same drawing runs from one prototype to a few thousand pieces, the setup has to be repeatable, not just accurate. Fixtures, tool lists and inspection plans need to be written down so the tenth run matches the first.

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  Fewer setupsFive-axis work reaches angled faces without re-clamping.
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  Thin wallsCut sequence and support control distortion, not just feed rate.
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  Run repeatabilityDocumented fixtures and tool lists keep part 1,000 close to part 1.

Material changes how a cut behaves. Aluminium 6061 and 7075 cut fast and hold size well, but 7075 moves more after heavy material removal because of internal stress. Stainless 304 and 316L work-harden at the cut, so a light pass over a previously cut surface can push the tool instead of cutting. Titanium TC4 and Inconel conduct heat slowly, so the tool edge runs hot and wears quickly, which shows up as size drift late in the run.

The practical answer is to match the cutting strategy to the material. Hardened tool steel and Inconel usually need slower speeds, more coolant and more frequent tool changes. Plastics such as POM and PEEK need sharp tools and controlled clamping, because they deflect under light pressure and can be marked by a soft jaw that is too soft.

We keep material-specific programs for the grades we run most, which include 6061, 7075, 304, 316L, 17-4PH, 4140, TC4 and Inconel. If your part uses a grade outside that list, send the spec sheet with the drawing.

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  AluminiumFast to cut; 7075 needs stress relief planning on thin sections.
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  StainlessWork-hardening makes light finishing passes risky.
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  Titanium and InconelHeat stays at the edge; expect tool wear-driven drift.

A dimensional report is only useful if it states what was measured, with what instrument, and to what uncertainty. A caliper check on a 0.01 mm bore tolerance tells you almost nothing. For tight features, ask for CMM results with the datum scheme named, and for roundness or flatness, ask which instrument produced the number.

In-process monitoring matters more than final inspection on long runs. If the first part is checked and the last part is checked, the middle can drift unnoticed. We check raw material on receipt, monitor during machining and inspect 100% of parts before shipment, with reports on request.

For medical and automotive work, the documentation is part of the deliverable. Traceability of material, revision level and inspection records are usually required by the customer's own quality system, so it is worth agreeing the report format before the first cut, not after.

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  Ask for the datum schemeA number without a datum is not a result.
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  In-process beats end-of-lineDrift shows up mid-run, not on the last part.
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  Agree report format earlyEspecially for IATF 16949 and ISO 13485 programs.

Every tolerance has a price, and it is paid in cycle time, tool changes and inspection hours. If a feature only locates a cover panel, a ±0.1 mm tolerance and a Ra 3.2 μm finish will do the job. Tightening it to ±0.01 mm adds cost without adding function.

Sometimes the better answer is not a tighter tolerance but a different design. A slot that needs to align with a mating part can often be opened up and given a locating pin or a dowel instead. A sharp internal corner can be relieved so a standard end mill reaches it, which removes an EDM step. These changes usually come out of a DFM review before quoting.

The honest limit is that accuracy cannot fix a design that depends on it for the wrong reason. If two parts must align, alignment features are cheaper than extreme tolerances on every face. We would rather tell you that at the quote stage than ship parts that pass inspection and fail on the assembly line.

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  Cosmetic and clearance featuresLoose tolerance is usually enough.
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  Design change over toleranceLocating pins and corner reliefs cut cost.
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  Say it before the cutDFM comments are free at quote time.

Aerospace work leans on five-axis capability and material knowledge. Titanium and Inconel parts are often thin-walled and asymmetric, so the value of accuracy shows up as fewer setups and less distortion, not just a tighter number. Documentation for traceability is standard.

Automotive and EV programs lean on repeatability across volume. A housing that is accurate on part one but drifts by part five hundred will stop a line. Fixture design, tool life management and in-process checks do most of the work here. Our IATF 16949:2016 system is set up around that.

Medical devices lean on finish and cleanliness. Burrs on an instrument edge are a functional defect, not a cosmetic one, so deburring and surface finishing are planned into the route. Robotics and automation sit between these: bore alignment and gear housing geometry decide whether a joint runs quietly or vibrates.

Electronics and new energy parts often have large, thin features where flatness over a wide area is the hard part. A 4,000 mm maximum processing size helps here, but the real limit is usually how the part is supported during cutting.