Read the CNC Guide Main Tips
This guide is for engineers and buyers who run or source CNC work. It covers the setup, tool holding, and inspection steps that decide whether a part lands at ±0.005 mm or drifts out of tolerance. Read it before you release a drawing or a program.
Key takeaways
Read the CNC guide main tips from the drawing first
Most scrap starts on paper. Open the drawing and find the functional datum before you open CAM. The datum is the surface that mates with the next part in the assembly, not the surface that is easiest to clamp. If the drawing calls A-B-C, check whether the fixture can actually reach all three faces in one setup.
Then read the tolerance stack. A hole at Ø10 H7 with a true position of 0.05 mm is a different job from a clearance hole at Ø10.5. On a 4,000 mm part, thermal drift and machine geometry both matter; on a 20 mm bracket, tool runout matters more. The print tells you which.
Surface finish notes set the finishing pass. Ra 0.8–1.6 μm is a normal machined finish. Ra 0.2–0.8 μm needs a sharp tool, a light radial stepover, and a stable setup. If the note says Ra 1.6–3.2 μm, you can take a heavier cut and save cycle time.
- 1Check the datumConfirm which faces locate the part in the assembly.
- 2Check the stackAdd up the tolerances that stack from the datum.
- 3Check the finishMatch the finishing pass to the Ra callout.
Workholding: where accuracy is won or lost
A 5-axis machine can position a tool to ±0.005 mm, but a loose vise will not hold that. Clamp on a machined face, not on raw stock. Raw saw-cut surfaces vary by 0.2 mm or more, so the part shifts when the cut starts. Face the clamping face first if you have to.
Watch for lift. A vise with 40 kN of clamping force can lift the movable jaw by 0.02–0.05 mm on tall parts. Support the part with a screw jack or a matched parallel. For thin walls, use a soft jaw machined to the part profile so the pressure spreads over more area.
For long parts, use a tombstone or a fixture plate with more than one clamp point. A 4,000 mm travel machine will cut the full length; a part clamped at two points will vibrate in the middle. Add a mid-support and re-check the top face after each roughing pass.
- 1Clamp on machined facesRaw stock moves under load; a faced surface does not.
- 2Support tall partsJacks and parallels stop jaw lift.
- 3Add mid-supportsLong parts need more than two clamp points.
Tool holding and runout
Tool runout is the error you can fix in five minutes. A holder with 0.02 mm runout will cut an oversized slot and wear one flute harder than the others. Check with a dial indicator on the flute, not on the shank. If the runout is over 0.01 mm, clean the taper, re-seat the holder, and check again.
Use the shortest tool that reaches. A Ø6 mm end mill sticking 60 mm out of the holder will deflect under a normal cut. Keep the gauge length under 4× diameter for finishing passes. For deep pockets, use a necked tool or a smaller stepdown instead of a long reach.
Match the coating to the material. Aluminum likes polished flutes and a ZrN or uncoated surface; titanium and Inconel need AlTiN or AlCrN. The wrong coating will built-up-edge on aluminum and pull the edge off on titanium. Check the chip color: aluminum chips should be bright, not blue.
- 1Measure runout on the fluteKeep it under 0.01 mm for finishing.
- 2Keep the tool shortGauge length under 4× diameter for finishing.
- 3Use the right coatingPolished for aluminum, AlTiN for titanium.
Thermal control and in-process checks
Aluminum expands about 23 μm per meter per °C. A 300 mm part that warms by 1 °C grows about 7 μm. That is more than the ±0.005 mm band on a tight feature. Let the part and the machine reach the same temperature before the finishing pass. In a shop with no climate control, this can take 30–60 minutes.
Use coolant consistently. Interrupted coolant changes the temperature of the part and the tool, which changes the cut. Flood coolant on steel and stainless; air blast or minimum quantity lubrication on aluminum and plastics. Do not switch mid-run.
Check the part before the last pass. Measure a critical feature with a micrometer or a bore gauge. If it is 0.01 mm over, adjust the wear offset before finishing. This is cheaper than scrapping the part and re-cutting it from stock.
- 1Let the part stabilize30–60 minutes in a non-climate-controlled shop.
- 2Keep coolant steadyDo not switch coolant type mid-run.
- 3Measure before finishingAdjust the wear offset, then take the last pass.
A 7-step setup routine for a first article
Run this sequence on the first part of every job. It takes about 40 minutes and catches most of the errors that cause scrap.
- 1Face the clamping faceTake a 0.3–0.5 mm cut on the face that will sit in the vise. This gives you a true surface and removes saw-cut variation.
- 2Set the work offset on the datumTouch off the datum faces from the drawing. Use a probe if the machine has one; a 0.02 mm error here shows up on every feature.
- 3Check tool runoutIndicate every finishing tool on the flute. Re-seat or replace any holder over 0.01 mm.
- 4Cut a test featureMachine one pocket or one hole and measure it. Compare to the target before running the rest of the part.
- 5Adjust the wear offsetIf the test feature is 0.01–0.02 mm off, correct it in the offset, not in the program. Re-cut and re-measure.
- 6Run the finishing passUse the finishing parameters: light radial stepover, steady coolant, shortest tool that reaches.
- 7Inspect the first articleMeasure all critical features and record the numbers. Release the run only after the first article passes.
When to machine in-house and when to outsource
Use this table to decide where a job belongs. The right answer depends on tolerance, volume, and how soon you need the part.
| Situation | In-house CNC | Outsource to a partner |
|---|---|---|
| Tolerance tighter than ±0.01 mm | Only if the machine is calibrated and the shop is stable | Use a partner with 5-axis and in-process probing |
| One prototype, 3–5 day need | Possible if the machine is free | Faster if the partner has capacity and free DFM |
| 10,000+ parts per year | Needs dedicated capacity and tooling | Better if the partner runs mill-turn and automated cells |
| Material is Inconel or titanium | Needs the right tools and coolant | Use a partner that cuts these weekly |
| Part is 4,000 mm long | Limited by machine travel | Use a partner with 4,000 mm travel machines |
| Certification is required | ISO 9001 alone may not cover it | IATF 16949, ISO 13485, or ISO 27001 partners |
The short version
Read the drawing, clamp on a machined face, measure the tool, and check the part before the last pass. That sequence holds ±0.005 mm on a good machine and catches most errors before they become scrap.
Questions engineers ask
What tolerance can a normal CNC shop hold?
±0.005 mm is achievable on a rigid machine with a stable setup and a controlled room. On a 4,000 mm part, that tolerance is harder because thermal drift and machine geometry both add error.
For most brackets and housings, ±0.02 mm is enough. Ask for the tight tolerance only on the features that need it.
How do I know if a part should be 5-axis or 3-axis?
If the part has features on four or more faces, or if a 3-axis setup would need three or four refixturings, use 5-axis. One setup removes the stack-up error from re-clamping.
If the part is a flat plate with holes on one face, 3-axis is faster and cheaper. Do not pay for 5-axis on a simple part.
What surface finish should I call out?
Ra 1.6–3.2 μm is a normal machined finish and is fine for most functional surfaces. Ra 0.8–1.6 μm needs a finishing pass with a sharp tool and a light stepover.
Ra 0.2–0.8 μm is a fine finish. It adds cycle time and needs a stable setup. Call it out only where the drawing needs it, such as a seal face or a sliding surface.
Why does my part measure differently in the shop and in the inspection room?
Temperature is the usual cause. A part measured at 25 °C in the shop and at 20 °C in the inspection room will read differently. On aluminum, the difference is about 23 μm per meter per °C.
Let the part soak at the inspection temperature before you measure it. For tight features, record the temperature with the measurement.
What files do I need to send for a quote?
Send a 3D model (STEP or IGES) and a 2D drawing with tolerances, datums, and surface finish notes. The model shows the shape; the drawing shows what matters.
If you only have a drawing, send it. A partner can model the part, but the quote will take longer and may carry more assumptions.
How do I keep a long run consistent?
Check the first article, then check at fixed intervals. Measure a critical feature every 20–50 parts, depending on tool wear and material.
Replace tools on a schedule, not on failure. A worn tool changes the cut size before it breaks.
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