Aluminum CNC machining positioner: design, setup and tolerance control

Most positioner bodies we machine are 6061-T6. It machines cleanly, holds a thread well, and stays stable after stress relief. A rotary table housing or an index plate that sees repeated clamp cycles usually starts here. Wall thickness of 3 mm or more is comfortable; below 2 mm the part starts to move during clamping and after machining.

When the arm carries a load at the end of a long reach, 7075-T6 earns its higher cost. Yield strength runs roughly 1.6 times that of 6061-T6, so a thinner section can carry the same moment. The trade-off is machinability: 7075 chips harder, wants sharper tooling and more coolant, and thin webs are more likely to spring.

6082-T6 sits close to 6061 and is common on European drawings. Its strength is slightly higher and its corrosion behavior is similar. If your print already calls out 6082, we machine 6082; there is no reason to switch just for our convenience.

Casting alloys such as ADC12 appear in die-cast positioner housings, not in machined ones. A machined body from ADC12 billet is unusual and usually a sign the drawing was copied from a casting print.

Repeatability comes from the locating features, not from the outside shape. A bore that holds a bearing or a spigot needs to be machined in the same setup as the face it sits against. Split those two features across two setups and the perpendicularity error shows up as tilt in the finished assembly.

H7 bores from Ø10 mm to Ø80 mm are routine on our 5-axis centers. We bore rather than ream when the diameter is above roughly Ø20 mm, because a boring head lets us dial in the size and correct for tool wear without scrapping the part. Below that, reaming is faster and just as accurate.

Pin holes for dowels deserve their own note. A dowel hole at Ø6 H7 with a 20 mm depth wants a drill, a boring pass or a ream, and a chamfer. Skip the chamfer and the lead-in burr pushes the pin off position by a few micrometres, which is enough to matter on an index plate.

Slot and keyway features are where 3-axis work is often enough. If the slot is open on one end and the tolerance is no tighter than ±0.02 mm, a 3-axis mill with a good vise does the job at lower cost than a 5-axis cycle.

Aluminum moves under clamp pressure. A positioner plate that is 8 mm thick and 300 mm long will bow if you clamp it hard in the middle. We hold thin plates on a fixture plate with low-profile clamps at the edges, or on vacuum for parts under 5 mm.

For a housing with five machined faces, we run a 5-axis cycle with the part on a Ø400 mm rotary table. One setup, one datum, all faces related to each other. The alternative is four or five 3-axis setups with re-fixturing between each, which adds stack-up error and handling time.

Long arms over 1,000 mm go on the large-travel machines at 4,000 × 400 × 150 mm. Support the overhang with a tailstock or a steady, or the arm will chatter and the surface finish will show it.

We machine soft jaws to the part profile for runs above a handful of pieces. It costs a little setup time and removes the guesswork from the second operation.

Our working tolerance is ±0.005 mm on critical features, which is ±0.0002 in. That number applies to a bore, a face, or a pin hole on a part that is stable and reachable. It does not mean every dimension on a 400 mm plate comes in at ±0.005 mm; thermal drift and material movement make that unrealistic.

Inspection is 100% before shipment. Raw material gets checked on arrival, in-process dimensions are monitored at each operation, and the finished part gets a final inspection. Reports are available on request, including dimensional reports and material certificates.

Where a positioner interacts with a purchased bearing or a mating casting, we prefer to see the mating part drawing. Matching the bore to the actual bearing rather than to a nominal size removes one source of assembly trouble.

Hardcoat anodizing adds roughly half the coating thickness per surface, so a 25 μm coating grows the part about 25 μm per side. If a bore is anodized, either mask it or open the pre-plate dimension to compensate. We do this on a case-by-case basis with the drawing in hand.

Clear or colored anodizing is fine for most positioner housings and keeps a clean look. Conductive anodizing is the choice when the housing is part of a grounding path.

Bead blasting before anodizing gives a matte, even appearance and hides light tool marks. Brushed or polished finishes are available when the part is visible on a machine. Laser marking works for part numbers and alignment marks; minimum character height is 1.5 mm.