CNC completion accuracy is important because the last cut sets the fit
Roughing removes the bulk. The finishing pass decides the final geometry, surface texture and residual stress your part carries into assembly. This page explains what the finishing stage actually controls, which tolerances are realistic, and when chasing another micron costs more than it returns.
What the finishing pass actually decides
Roughing is a material-removal operation. It runs high chip loads, leaves stock on the walls, and produces a near-net shape with tool marks, a work-hardened skin and residual stress. Nobody measures a rough part against a drawing because the geometry is not there yet.
The finishing pass is different in kind, not just in scale. It removes the last few tenths of a millimeter with a finer tool, smaller stepover, higher spindle speed and a tool path that follows the final surface. What it leaves behind is the geometry the drawing describes and the surface the next process inherits.
That is why the accuracy of this final pass matters beyond cosmetics. Tolerances stack from the last cut outward, so a wall that comes out 20 μm thin does not get corrected later.
Thermal drift works the same way. A spindle that grows 15 μm over a long finishing cycle will cut the first pocket and the last pocket at different sizes, even on the same program.
Which tolerances are realistic, and where they stop
A general machining tolerance of ±0.1 mm is comfortable on most 3-axis work. Move to ±0.05 mm and you need sharp tools, stable fixturing and a machine that holds its geometry through the cycle. GreatLight runs a standard tolerance of ±0.005 mm (±0.0002 in) on qualified features.
Feature type matters more than the number on the title block. A bored hole can hold ±0.005 mm because a boring head cuts a circle with a single point. A pocket corner radius is harder because the tool corner radius and deflection both enter the result.
Surface finish and dimensional accuracy are separate requirements that interact. Finishing to Ra 0.2–0.8 μm usually needs a light finishing pass, higher surface speed and a sharp edge. Push feed too hard and the finish degrades before the size drifts.
Deep cavities and thin walls bend under cutting force. A 1 mm wall on a 60 mm tall pocket will deflect during the finishing pass and spring back after the tool leaves. No amount of machine accuracy fixes that. Support the wall, take lighter passes, or accept a wider tolerance.
Fixturing, datum strategy and error stacking
Every setup adds error. A part machined in five setups accumulates five datum transfers, and each transfer carries its own positional uncertainty. A 5-axis machine that reaches five faces in one setup removes four of those transfers.
Datum choice drives the result more than most people expect. If the drawing dimensions from a face that the fixture covers, the operator has to pick an indirect reference. That reference may be 30 μm off before the first cut.
Workholding force is a real variable. Vise pressure on a thin aluminum bracket can distort it by tens of microns. The part measures correctly in the vise and springs out of tolerance when released.
Probing helps, but only if the probe is calibrated and the stylus is rigid enough for the feature. A long stylus on a small bore deflects and reports a size that is not there.
How the finishing result gets verified
Inspection should mirror the drawing, not the machine. A CMM report that measures the same three points the operator used tells you very little. Map each critical dimension to a measurement method before the first chip.
First article inspection catches setup errors before the run continues. It confirms the datum, the tool offsets and the program in one pass. If the first article is out, the fix is usually an offset, not a new program.
In-process checks matter on long runs. Tool wear on a 10,000 part order will drift the size gradually, so an operator who checks every 50 parts catches the trend before the last parts go out of tolerance.
Final inspection covers what the machine cannot measure, including burrs, thread depth and surface defects. GreatLight inspects 100% of parts before shipment and provides reports on request.
Which completion approach fits which part
| Part situation | Recommended approach | Why |
|---|---|---|
| Prototype, simple geometry | 3-axis, ±0.05 mm | Fast setup, low cost, easy to rework |
| Multi-face housing | 5-axis, one setup | Fewer datum transfers, tighter position |
| Thin wall, tall pocket | Light finishing passes, support | Reduces deflection and spring-back |
| Sealing face or bearing bore | Fine finish Ra 0.2–0.8 μm | Controls leak path and fit clearance |
| Loose cosmetic bracket | As-machined Ra 1.6–3.2 μm | Finish cost buys nothing here |
| Hardened tool steel insert | Grind after heat treat | Machining cannot hold size after hardening |
Pick the tolerance the function needs
If the feature controls fit, sealing or safety, pay for ±0.005 mm and inspect it. If it only holds a cover or carries no load, a ±0.1 mm as-machined surface does the job at lower cost.
Common questions on finishing accuracy
What does CNC completion accuracy mean exactly?
It describes what the final machining pass produces: the dimensional size, the geometric form and the surface texture the part carries when it leaves the machine.
Roughing only removes bulk. The finishing operation sets the geometry that assembly and inspection actually see.
Can a 3-axis machine hold ±0.005 mm?
Yes, on features that are reachable in one setup with a rigid setup and sharp tooling. Bores, flat faces and simple profiles are realistic.
Once the part needs several faces and datum transfers, the accumulated position error usually exceeds that budget. A 5-axis machine that finishes more faces in one setup is the more reliable route.
Why does my part measure correct in the vise and wrong after?
Clamping force distorts the part during cutting. The tool cuts a shape that matches the clamped state, and the part springs back when released.
Reduce vise pressure, add support under thin sections, or machine the critical faces with lighter passes and lower radial engagement.
Does a finer surface finish always mean a tighter tolerance?
No. Finish and size are separate. A part can have Ra 0.4 μm and still be 30 μm oversize.
They interact during the pass, because feed, speed and tool condition affect both. Check both on the drawing rather than assuming one follows the other.
When should we grind instead of machine?
After hardening, or when the tolerance is tighter than the milling setup can hold. Hardened tool steel at 55 HRC will wear a carbide cutter and drift the size.
Grinding removes very little material with a rigid wheel and holds size well on hardened surfaces. It also adds a process step and cost, so use it only where function demands it.
How does inspection fit into the completion pass?
Plan the measurement before the first cut. Map each critical dimension to a method and a datum that match the drawing.
First article inspection confirms the setup. In-process checks track tool wear on longer runs. Final inspection covers burrs, threads and surfaces the machine cannot verify.
Send the drawing, get a finishing plan
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More machining notes
We publish setup notes, tooling trials and inspection data from the factory floor.
