Capabilities · Materials
Aluminum machining
BELL Machine Works machines aluminum 6061-T6, 7075 and 2024 to a standard tolerance of ±0.001", with surface finishes measured to 16 μin Ra, plus anodize and chem film to MIL specification, at our shop in Gilroy, California.
Choosing an alloy
6061-T6, the default, and correctly so. Good strength, excellent machinability, weldable, corrosion resistant, and it anodizes cleanly and consistently. Brackets, plates, housings, fixtures and thermal components. If you haven't specified and the application doesn't demand otherwise, this is usually the right answer.
7075, substantially higher strength, approaching some steels by weight, for structural components where strength-to-weight governs. The tradeoffs are real: lower corrosion resistance, not readily weldable, more expensive, and it anodizes to a less consistent color than 6061.
2024, high strength and excellent fatigue resistance, which is why it appears in aircraft structure and interior hardware. Poor corrosion resistance on its own, so it is usually clad, anodized or chem filmed. Specified when aviation requirements call for it rather than chosen for convenience.
Tell us the application if you're unsure. Alloy selection is part of the DFM review in every quote.
Parameters we hold in aluminum
| Alloys | 6061-T6 · 7075 · 2024 |
| Standard tolerance | ±0.001" |
| Surface finish | Measured to 16 μin Ra, held across multiple surfaces on optical-grade components |
| Thin sections | Sealing plates to 0.079", flat through machining and after release from the fixture |
| Finishing | Anodize Type I/II/III per MIL-A-8625 · chem film Type I/II per MIL-DTL-5541 · bead blast · precision masking |
Part classes we machine in aluminum
Optical-grade surfaces and multi-feature alignment. Backing blocks finished to 16 μin Ra across multiple faces, and transition adapters carrying ten or more tight-tolerance alignment locations where the relationship between features is the critical dimension, not the features themselves.
Vacuum and gas sealing hardware. O-ring sealed brackets and thin sealing plates down to 0.079", where flatness through machining and after release is the entire problem, and groove dimension and surface condition determine whether the seal holds.
Aircraft interior mounting hardware. Collar sets in 2024, machined as matched groups where all pieces have to close correctly on each other, with full material traceability and certification as part of the deliverable.
Mounting plates and tooling with threaded insert patterns for multi-point mounting, engraved with part identification for traceability through production.
Finished enclosures. Bead blast plus Type II anodize, including single part families run in multiple colours, a masking and scheduling problem as much as a chemistry one.
What actually goes wrong in aluminum
It's the forgiving material relative to what else we run, but three things still cost parts.
Thin sections move. Machining removes material asymmetrically and releases the residual stress in the plate. A thin part that measured flat in the fixture bows when the clamps come off. Roughing strategy, stock allowance and op sequence matter more than the finishing pass.
Built-up edge. Aluminum welds to the cutting edge, and once it does the tool stops cutting cleanly and the finish goes. Sharp tooling, correct chip load and proper evacuation prevent it.
Anodize grows into the hole. Type III hardcoat in particular adds meaningful thickness, on internal features as well as external ones. Tolerances on anodized features have to state whether they're before or after finish, and threads usually need to be masked or chased.
Specifications and traceability
Standard tolerance ±0.001", inspected on an in-house CMM. 5-axis work envelope 8" × 8" × 3", 45,000 RPM spindle. Anodize, chem film and blast through an ITAR-registered finishing house with certificates returning alongside the parts. Material certs and lot traceability ship with every order, mill certs go out with the parts. First article inspection on request.