Materials: grades we machine, how they behave, what to specify
The alloy on a drawing decides the price as much as the geometry does. This page covers the grades that actually run through the plant, how each behaves at the cutter, and what a complete material callout looks like.
Aluminium: the default unless there is a reason
Aluminium is the bulk of what leaves the plant. It machines fast, accepts anodizing, and is cheap per kilo. The choice between grades matters more than most drawings admit.
| Grade | Tensile strength | Machinability | Typical use |
|---|---|---|---|
| 6061-T6 | 310 MPa | Excellent | Housings, brackets, fixtures, heat sinks |
| 6063-T5 | 190 MPa | Excellent | Extruded profiles, trim, lighting parts |
| 7075-T6 | 572 MPa | Good | High-stress brackets, hubs, structural parts |
| 5052 | 275 MPa | Fair | Sheet metal, marine and outdoor parts |
| 2017 | 430 MPa | Good | Screw-machine parts, legacy drawings |
| ADC12 | ~230 MPa | Cast, then machined | Die cast housings, covers |
Stainless and steel
Stainless is usually chosen for corrosion, not strength. Where both are needed, 17-4 PH does the job in one grade and removes the plating step.
| Grade | Condition | Why specify it | Watch out for |
|---|---|---|---|
| 303 | Annealed | Free-cutting, fast on a lathe | Lower corrosion resistance than 304 |
| 304 | Annealed | General corrosion, food contact | Galls when threaded against itself |
| 316 | Annealed | Chloride and marine environments | Costs more, machines slower |
| 420 | Hardenable | Cutlery, instruments, wear parts | Distortion after hardening |
| 4140 | Pre-hard 28–32 HRC | Gearbox bodies, shafts, flanges | Hard turning needed above 45 HRC |
| 42CrMo | Quenched and tempered | Gears to 48–55 HRC | Plan grinding allowance before hardening |
| 12L14 / 1213 | Free-cutting | High-volume shafts and fittings | Poor corrosion resistance, needs finishing |
Titanium, copper alloys and plastics
High strength to weight with poor heat conduction. Heat stays at the cutting edge, tools wear quickly and feeds drop. Expect three to five times the machining time of 6061 for the same geometry, and design generous radii because thin walls spring back.
Softer and more formable, with better corrosion resistance in chemical service. Cheaper than TC4 and adequate wherever the part is not strength critical.
The easiest metal on this page to machine. Good for electrical contacts, fittings and small turned parts. Dezincification is the risk in hot water, so plumbing parts move to bronze.
Bearing bushes, wear plates and worm wheels. Softer than steel, kinder to a shaft, and available in bar for turned parts.
High conductivity, gummy at the cutter, built-up edge on the tool. Needs sharp positive geometry and frequent tool changes; surface finish is harder to control than the tolerance.
Dimensionally stable, chemically resistant, expensive. Machines like a hard wood with sharp tools; internal stress in the stock means thin sections can warp, so stress relief between rough and finish is normal.
Fast to machine, low friction, low cost. POM holds tolerance better than nylon because it absorbs less moisture; nylon is tougher but moves with humidity and needs a tolerance allowance.
PC and PMMA for windows and light covers, with stress cracking as the main risk after machining. PTFE for seals and insulation, where creep limits the achievable tolerance.
Stock forms and lead time
| Material | Bar | Plate | Notes |
|---|---|---|---|
| 6061 | Ø5–Ø250 mm | 6–200 mm thick | Stocked; plate cut to size |
| 7075 | Ø10–Ø200 mm | 10–150 mm thick | Stocked in common thicknesses |
| 303 / 304 / 316 | Ø5–Ø120 mm | 3–100 mm thick | Rounds stocked, plate on order |
| 17-4 PH | Ø10–Ø150 mm | On order | Condition confirmed before machining |
| TA2 / TC4 | Ø10–Ø120 mm | On order | Adds 1–2 weeks, mill cert supplied |
| PEEK / POM | Ø10–Ø150 mm | 6–80 mm | Natural and black stocked |
How to write the material callout
A line that says only "aluminium" or "stainless" will get a question back, and a question costs a day.
"6061-T651, ASTM B209" is a complete callout. "Aluminium" is not. The temper changes machinability and the standard tells the mill what to certify.
H900 and H1025 machine differently and reach different strengths. If the drawing does not say, the shop picks, and it may not be the condition your stress calculation assumed.
Certificates are standard on request but add handling. If the programme needs heat-lot traceability, put it on the drawing so it is quoted, not discovered at shipping.
Hardening and anodizing change dimensions. State the final condition and let the process route carry the allowance, instead of tolerancing the pre-plate size yourself.
Tell us the material and the environment it lives in.
If the grade on the drawing is over-specified for the job, the quote will say so, with an alternative and the price difference.
From the shop notes
Longer answers to the questions these pages raise.