Accurate CNC Machining Services: What Holds a Tolerance

Accuracy breaks into four numbers a machinist has to hit at the same time. Dimensional accuracy is how close a feature lands to its nominal size: a 20 mm bore that measures 20.004 mm is out by 4 μm. Geometric accuracy covers form and position — flatness, perpendicularity, true position, runout. A bore can be dead on size and still be oval, which fails a roundness callout even though the caliper says it is fine.

Surface finish is the third. Ra 0.8–1.6 μm is a normal machined finish on aluminum and steel; sealing faces and bearing journals often need Ra 0.2–0.8 μm, which means a finishing pass at low feed or a subsequent grinding operation. The fourth is repeatability: the same feature on part 1 and part 500 must land in the same place. A shop that hits tolerance once on a prototype and drifts across a production run has not solved the problem.

These four pull against each other. Tightening a positional tolerance usually costs setups and cycle time. Polishing to Ra 0.2 μm removes material, so the finishing pass has to be planned before the roughing cut, not after. When a drawing asks for ±0.005 mm and Ra 0.2 μm on the same face, the process plan is doing most of the work.

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  DimensionalFeature size against nominal, measured in μm.
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  GeometricForm and location: flatness, position, runout.
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  SurfaceRa value; drives sealing, wear and fit.
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  RepeatabilityHolds across the run, not just on part one.

The machine sets the floor on what is possible. A 3-axis mill is the right answer for a flat plate with holes and pockets on one face. As soon as a part needs features on four or five sides, each extra setup adds re-fixturing error. That error is often larger than the machine's own positioning accuracy. A simultaneous 5-axis center cuts those faces in one setup, so the datum never moves.

We run 127 high-precision CNC machines: 16 simultaneous 5-axis centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers. That mix matters because accuracy is cheaper to buy on the right platform. A small prismatic part with a single critical bore does not need 5-axis; putting it on one wastes spindle time and does not improve the result. A housing with angled ports and a true-position callout does need it.

Size decides the rest. Travels run from compact 500 × 500 × 450 mm cells up to 4,000 × 400 × 150 mm for long parts, with a Ø400 mm rotary table for round work. Large parts add a thermal problem: a 4,000 mm steel part grows roughly 0.05 mm over a 10 °C shop swing. On that class of work we cut after the part has soaked, not straight off the truck.

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  One setup5-axis removes re-fixturing error on multi-face parts.
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  Right platform3-axis is often more accurate per dollar on flat work.
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  Size and soakLong parts need thermal stabilization before the finish pass.

Most tolerance failures we see are setup failures, not machine failures. A vise holding a part on two parallels lets it bow under clamping force; release the vise and the part springs back out of tolerance. For anything thin or long, we prefer a fixture that supports the full underside, or we clamp on a sacrificial tab and cut it off in a later operation.

Heat is the second lever. Roughing a 7075 aluminum block removes a lot of material fast, and the part can move 0.02–0.05 mm as it cools. The fix is boring but effective: take a roughing pass, let the part rest, then finish. On tight work we split the finishing pass into a semi-finish and a light final cut so the last 0.2 mm comes off on a settled part with a sharp tool.

Tool choice follows the same logic. Carbide with a balanced flute count and a coating suited to the material keeps cutting forces predictable. A long reach tool deflects, so we keep the flute length as short as the geometry allows. When a feature needs a sharp internal corner that no end mill can reach, wire EDM or precision grinding finishes the job — those are the operations that hold ±0.005 mm on hardened or awkward geometry.

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  Support the partFull-underside fixtures beat two parallels on thin work.
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  Cut, rest, finishLet the part cool before the final pass.
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  Short toolsLess flute length means less deflection.
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  EDM or grindFor sharp internal corners and hardened material.

Aluminum is the easy case. 6061-T6 and 7075 machine cleanly and hold ±0.005 mm on most features. Thin sections still move, and 7075 stresses relieve more than 6061 after heavy roughing, so the rough-rest-finish sequence matters more there.

Stainless 303 and 304 work harden, which pushes cutting forces up and dulls tools faster; a dull tool rubs instead of cutting and pulls the part. 17-4PH in the H900 condition is often finished by grinding rather than milling. Titanium TC4 (Ti-6Al-4V) is worse on heat: it conducts poorly, so the cutting edge runs hot and the tool wears quickly. We slow the surface speed and accept longer cycle times to keep the tolerance.

Plastics are the counterintuitive ones. POM and PEEK cut easily but expand with heat and spring back after the cutter passes. A tolerance of ±0.05 mm is realistic on a PEEK part; demanding ±0.005 mm on a long thin plastic section is usually a mistake. If a plastic part truly needs that band, the design should add ribs or the feature should move to a metal insert.

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  Aluminum 6061 / 7075Holds ±0.005 mm; 7075 needs stress relief.
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  Stainless 303 / 304Work hardening; keep tools sharp.
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  17-4PH, Ti-6Al-4VGrind or slow down; heat is the limit.
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  POM, PEEKThermal growth and spring; ±0.05 mm is realistic.

A tolerance you cannot measure is not a tolerance you can hold. We inspect 100% of parts before shipment, and the loop starts earlier: incoming raw material is checked against certs, in-process dimensions are measured as features are cut, and a final inspection confirms the drawing before packing. Reports are available on request.

Which instrument depends on the callout. Calipers and micrometers cover most dimensional checks. A CMM handles true position, profile and parts with many related features, because it works from a coordinate frame rather than a single reading. Height gauges and pin gauges cover bores and depths quickly on the shop floor. For surface finish we use a profilometer when the drawing names an Ra value.

Measurement itself carries uncertainty. A caliper read by hand is not the tool for a ±0.005 mm callout; the gauge resolution and the operator's touch both add error. On tight work the part is measured on the machine or on the CMM, at the same temperature the cut was made. If a drawing has a tolerance band tighter than what the gauge can resolve, the inspection plan needs to change before the first chip is cut.

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  IncomingRaw material checked against mill certs.
• 2
  In-processDimensions measured while features are cut.
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  Final100% inspection before shipment.
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  ReportsInspection data supplied on request.