Accurate CNC Machining for Accurate Results
This page explains what actually drives dimensional accuracy in CNC machining, and where the cost sits. It is written for design engineers and sourcing engineers who need to judge whether a drawing can be held on the shop floor. After reading it you can tell which tolerances are routine, which ones need a specific machine or setup, and which ones should be relaxed.
Accuracy is a process result, not a machine spec
A machine that quotes ±0.005 mm does not automatically produce parts at ±0.005 mm.
What accurate CNC machining actually means on a drawing
A tolerance block on a drawing states an allowed range. Whether that range is easy or hard to hold depends on the feature, the material, and how many times the part is handled between operations. A single bore in aluminium held to ±0.005 mm is normal work. The same tolerance on a thin wall 4,000 mm long, in titanium, after heat treat, is a different project.
When we quote accurate CNC machining, we are not quoting a machine. We are quoting a sequence: which machine, which fixture, how many setups, how the part is measured, and what happens if the first article drifts. That sequence is what the tolerance actually costs.
The 99.99% qualification rate in our shop is a result of that sequence, not a claim about any single spindle. Parts are measured against the drawing at the machine, again after finishing, and a third time before shipment. Reports go out on request.
- 1Routine±0.05 mm on milled pockets and drilled holes in aluminium or brass
- 2Tight but standard±0.01 mm on bores, journals and mating faces with a clean setup
- 3Needs planning±0.005 mm across multiple features or after heat treat
- 4Ask firstSub-micron flatness, thin walls under 0.8 mm, long unsupported bores
Which machine holds which tolerance
Three-axis machines handle flat plates, pockets, and drilled patterns where all features are reachable from one direction. They are fast and cheap to set up. The limit is not the control resolution, it is the number of setups, because every re-clamp adds a position error.
Four-axis mills add a rotary table, typically Ø400 mm, so features on four sides can be cut without re-fixturing. That removes a whole class of stack-up error. It also lets the tool stay normal to the surface on cylindrical parts, which improves both accuracy and finish.
Simultaneous five-axis centers cut the part from any angle in one setup. For accurate CNC machining of angled bosses, sloping faces, and contoured pockets, this is the difference between holding ±0.01 mm and chasing it across three operations. We run 16 of these centers, plus 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers.
Size matters as much as axis count. Our largest travel is 4,000 × 400 × 150 mm. Medium frames run 750 × 1,150 × 550 mm and 600 × 600 × 600 mm. Compact work sits on 500 × 500 × 450 mm or 500 × 310 × 200 mm machines.
Matching the machine to the feature
Use this as a first filter before you send a drawing.
| Feature type | Suggested machine | Typical tolerance | Watch out for |
|---|---|---|---|
| Flat plate, one-side pockets | 3-axis | ±0.05 mm | Re-clamping for the back side |
| Four-sided housing | 4-axis | ±0.01 mm | Rotary table runout |
| Angled bosses, contoured pockets | 5-axis simultaneous | ±0.005–0.01 mm | Fixture stiffness |
| Shaft with cross holes | Mill-turn | ±0.01 mm | Bar pull-out on long parts |
| Long frame, 4,000 mm | Large-travel 3-axis | ±0.05 mm | Thermal growth over the run |
| Thin wall under 1 mm | 5-axis, light passes | ±0.02 mm | Chatter and spring-back |
Where accuracy is actually lost
Thermal drift is the quiet one. A spindle warms up over the first hour and the part grows with it. On a 500 mm aluminium part, a 5 °C swing moves the material about 0.06 mm. That is larger than the tolerance we are trying to hold. We rough, let the part cool, then finish.
Fixture deflection shows up as a taper. A part clamped on one end and cut on the other will bend under tool pressure. The fix is support, not a slower feed. Adding a jack or a soft jaw under the work often recovers more accuracy than any change to the cutting parameters.
Tool wear drifts gradually, so the first hundred parts are good and the next hundred creep out of tolerance. In-process probing or a scheduled offset check catches it. We monitor during the run and inspect 100% before shipment, with raw material checks at the front and final inspection at the back.
Material behavior matters too. Heat-treated 17-4PH and 4140 move after machining. Titanium springs back. PEEK and other plastics grow with humidity. The drawing may allow ±0.005 mm, but the material may not settle there for days.
When tighter tolerance is not worth the cost
Tolerances should follow function. A bore that locates a bearing needs a tight fit. A clearance hole for an M6 bolt does not. Applying a tight block tolerance across an entire drawing raises cost on every feature without improving the assembly.
Tolerance stack-up is the usual culprit. Five features each held to ±0.01 mm can put the final assembly 0.05 mm off. Fixing the stack by tightening one feature further rarely works. Datum choice and a smaller number of controlled features work better.
Surface finish and tolerance are separate calls. Ra 0.2–0.8 μm is a fine finish, Ra 0.8–1.6 μm is a high finish, and Ra 1.6–3.2 μm is as-machined. A sealing face may need both a tight tolerance and a fine finish. A bracket usually needs neither.
If a feature is genuinely hard to hold, say so in the RFQ. We can often propose a small geometry change, an added relief, or a different datum that keeps the function and drops the cost. Our DFM analysis comes back with the quote, within 12 hours.
Questions engineers ask before they send a drawing
Can you hold ±0.005 mm on every feature of a part?
On a rigid part with accessible features and a stable material, yes. On thin walls, long unsupported bores, or parts that get heat treated after machining, the achievable number depends on the geometry.
Send the drawing and we will tell you which features we can hold at that level and which ones we would hold at ±0.01 mm instead.
Do you charge more for tight tolerances?
Yes, because tight tolerances need more setups, slower finishing passes, and more measurement time. The increase depends on how many features carry the tight call.
Relaxing a non-functional tolerance is the fastest way to cut cost.
What inspection data comes with the parts?
Every order is inspected 100% before shipment. That includes a raw material check, in-process monitoring, and final inspection.
Dimensional reports, material certificates, and first article inspection reports are available on request.
Which materials are the hardest to hold accurately?
Titanium grades like TC4 (Ti-6Al-4V) and Inconel move and work-harden during cutting. Plastics such as PEEK and PA move with temperature and moisture.
Aluminium 6061 and brass C36000 are the most predictable for tight work.
How small a batch can you run without losing accuracy?
There is no minimum order quantity. We run from a single prototype to 10,000+ part runs.
On small batches the setup dominates the accuracy, so the same fixture and inspection plan applies as on a production run.
How do you protect the drawing we send?
Uploads are secure and confidential. We do not share files outside the project team.
An NDA is available on request if your program requires one before the files move.
Send the drawing and get a tolerance answer, not a guess
Quotation and free DFM analysis within 12 hours. Production can start within 24 hours of approval, and parts ship in 3–5 days.
12-hour quote100% inspectionNo minimum order quantityNDA on request
