Accurate CNC Machining: What It Takes to Hold a Tolerance

The finished part has to match the drawing within the stated tolerance, and it has to do so on every part in the run, not only on the first article. The number on the drawing is the target. What matters is how far the process drifts across 50 or 500 pieces.

Three things decide the result: machine geometry and thermal stability, the fixture that holds the part, and the tool path. A five-axis center can produce a bad part if the fixture lets the blank move 0.02 mm under cut. A three-axis mill can hold ±0.005 mm on a simple plate if the setup is rigid and the stock is stable.

So when a supplier tells you they offer accurate CNC machining, ask what tolerance they hold on which feature, on which material, over how many parts. That answer tells you more than a machine list.

Our general working tolerance is ±0.005 mm (±0.0002 in) on critical features, with surface finish from Ra 0.2–0.8 μm on fine-finished surfaces up to Ra 1.6–3.2 μm as machined. Those numbers are not universal. They apply to specific features, materials and geometry, and they get harder as part size grows.

Thermal growth is the first limit. Aluminum 6061 expands about 23 μm per meter per °C. A 300 mm part that warms 5 °C during roughing moves roughly 0.035 mm before finishing starts. We rough, let the part settle, then finish. On long parts, we sometimes leave 0.3–0.5 mm of stock and take the final pass after cooling.

Tool deflection is the second limit. A Ø6 mm end mill with 60 mm of stickout bends under load. Long reach tools force lighter depths of cut and slower feed, which raises cost per part. If a feature sits 80 mm deep in a pocket, expect the tolerance to loosen or the price to rise.

Material also sets the ceiling. Stainless 316 and titanium TC4 (Ti-6Al-4V) work-harden and push back on the tool. Copper and plastics cut freely but move after machining because of internal stress. We adjust speeds, stepovers and clamping per material.

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  Small features, short toolsBest accuracy comes from features within 5× the tool diameter.
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  Long partsA 4,000 mm part cannot be held to ±0.005 mm on every feature; call out the critical ones.
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  Thin wallsWalls under 1 mm deflect during clamping, not during cutting.

A number on a CMM report is only useful if the measurement plan matches the drawing. We build the inspection plan from the GD&T callouts, not from a generic scan. Datum features get measured first, then the toleranced features relative to those datums.

The inspection chain runs three steps: raw material check on incoming stock, in-process monitoring during the run, and final inspection before shipment. We inspect 100% of parts before they ship. Reports are available on request, and for medical and automotive work we keep records tied to the lot.

Temperature matters in measurement too. A part measured at 28 °C reads differently than the same part at 20 °C. For tight work we let parts stabilize before final inspection. Otherwise you are measuring the shop, not the part.

Our qualification rate is 99.99% across shipped parts. That figure comes from the inspection record, not from a marketing claim. When a feature is out, we know before the box is closed.

Sometimes the honest answer is to change the process. If your part has a ±0.005 mm bore but the rest of the geometry is ±0.2 mm, you are paying for accuracy in places it does not matter. Split the tolerances. Tight only where the function needs it.

Casting and die casting make sense when wall thickness is generous and the tolerance is ±0.1 mm or looser. Sheet metal holds ±0.1 mm on formed features at a fraction of the machining cost. Add machining only on the critical faces.

If the part is a one-off bracket with no mating features, a ±0.1 mm process will do. If it is a mating component in a medical device or a fuel system, the tolerance is not negotiable and the cost is justified.

We will tell you when a cheaper route fits. Free DFM analysis at quote stage often removes a tight tolerance that was never needed.

We run 127 high-precision CNC machines across 3 wholly-owned plants covering 7,600 m², with 150 technicians. That includes 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers.

Travel sizes cover most work: 4,000 × 400 × 150 mm for long parts, 750 × 1,150 × 550 mm and 600 × 600 × 600 mm for medium work, and 500 × 500 × 450 mm or 500 × 310 × 200 mm for compact parts. A Ø400 mm rotary table handles round features in one setup.

Materials we machine daily include 6061-T6, 7075 and 2024 aluminum, 303 and 316L stainless, 17-4PH, 4140 and 4340 steel, C36000 brass, TC4 titanium and PEEK. Each has its own cutting strategy and its own drift behavior.

Finishes run from anodizing and electroless nickel to bead blasting and laser marking. Finishing can move a dimension, so we plan stock for it at the CAM stage.