CNC Machining After Extrusion: ±0.01 mm Where It Counts
Extrusion gets the shape close. Machining decides whether the lid closes, the connector seals and the hinge holds. We machine our own profiles, so the feature tolerance and the wall it sits in are agreed together.
- Machined features
- ±0.01 mm
- Bore and OD
- ±0.04 mm
- Minimum wall
- 0.3 mm
- Alloys
- 6063 / 6061
Why machine at all, if the profile is already the right shape?
An extruded profile cannot hold a bore position to ±0.01 mm, cannot produce a flat sealing face, and cannot cut a thread. Those features have to be machined — but only those features. Everything the die already formed stays as it is.
That is the difference between machining a blank and machining a block. On an extruded blank, cycle time goes into the bore and the sealing face instead of into clearing a pocket that the die could have formed for free.
The trade-off is fixturing. A 0.3 mm wall will deflect under clamping force if you hold it the way you would hold a solid block. Our answer is a fixture cut from the same profile, light clamping, staged cuts and stress relief between roughing and finishing.
Machining facts
- ±0.01 mm on machined features
- ±0.04 mm bore and OD
- ±0.02 mm capability on wall thickness, minimum 0.3 mm
- Fixture built from the production profile — one datum
- Dimensional report before finishing
Five stages from blank to finished feature
Written the way it runs on the floor, not the way it reads in a brochure.
-
Fixture from the profile
The fixture is built from the same extruded section the production run uses, so the machined bore and the outer wall share one datum.
-
Rough then finish
Material is removed in stages with stress relief in between. Cutting a 0.3 mm wall in one pass is how parts come back out of tolerance.
-
Critical features to ±0.01 mm
Bores, sealing faces, hinge pockets, connector cut-outs and threads. Bore and OD are held to ±0.04 mm.
-
Deburr and clean
Edges are broken by hand where a tool cannot reach. Burrs left in a connector pocket become a field failure after anodizing.
-
Measure before finishing
Dimensional report on the machined part, before the anodic layer changes the surface. Colour and texture are checked after finishing.
What we hold, and what you should ask for
Over-tolerancing is the most common cost driver we see on incoming drawings. A ±0.01 mm callout on a non-functional surface adds cost without adding function.
| Feature | Value | Typical use |
|---|---|---|
| Machined pocket, slot, step | ±0.01 mm | Lid fits, PCB seating, connector cut-outs |
| Bore and OD | ±0.04 mm | Hinge barrels, tube joints, bearing seats |
| Wall thickness capability | ±0.02 mm | Thin-wall housings, down to 0.3 mm |
| Profile cross-section | ±0.05 mm | Outer profile as extruded, before machining |
| Sealing face flatness | Agreed per part | Gasketed enclosures — specifies the gasket groove, not just the face |
Flatness and coaxiality are set per part because they depend on the section. Send the assembly constraint and we will define the callout with you.
Where machining gets expensive
- Deep narrow pockets that need long, slender tools
- Walls under 0.3 mm — possible in theory, unreliable in a production lot
- Tight true position across two faces that cannot be reached in one setup
- Cosmetic faces that need to stay free of clamp marks through every operation
We flag these at DFM review with a specific change — usually moving a feature to the die, or opening a radius — rather than quoting a price that assumes everything goes right.
Where machining decided the outcome
Machining questions from design engineers
Three questions that shape how we quote a machined part.
Do you machine parts you did not extrude?
Only when the blank makes sense for the job — for example when a customer already holds profile stock. Our tolerance advantage comes from controlling the die and the fixture together, so we normally quote the pair.
How do you stop a 0.3 mm wall from moving during machining?
Three things: a fixture cut from the same profile, light and evenly distributed clamping, and staged cuts with stress relief between roughing and finishing. The wall is measured after the part has relaxed, not immediately after the cut.
What inspection data comes with the parts?
A dimensional report covering the drawing's critical features, produced before finishing so the anodic layer does not mask a size issue. Appearance and colour are signed off against the approved sample and colour chip. See Quality & Inspection.
Send the CAD File. We Will Mark Up What Costs Money.
A STEP file plus the critical dimensions is enough for a DFM note: what we can hold, what we would change, and what the change saves.
What to send
- 2D drawing or 3D model (STEP, IGES, DWG, DXF, PDF)
- Alloy and temper
- Surface finish and color
- Estimated annual quantity