Machining time is most of what a machined part costs. These rules cut that time without changing what the part does. We machine on 5-axis CNC, which reaches most faces in one set-up, but the same rules still save time on every machine.

Internal corners need a radius

A round cutter cannot make a sharp inside corner. Give every internal corner a radius of at least one third of the pocket depth, and a little larger than the cutter, so it can turn the corner without stopping. If a square part must sit in a pocket, add small relief cuts in the corners instead.

Keep pockets shallow

Deep, narrow pockets need long tools that cut slowly and chatter. Keep pocket depth to about four times the width of the pocket where you can.

Walls and thin features

Thin walls vibrate and bend while they are cut. Keep metal walls at least 0.8 mm (1/32 inch) thick, thicker for tall walls.

Holes and threads

Use standard drill sizes and standard threads, metric or imperial.
Thread depth beyond 1.5 times the diameter adds little strength.
Leave room at the bottom of blind tapped holes for the tap.

Tolerances

Unless your drawing says otherwise, we machine to ±0.005 inch (±0.13 mm). Put tighter tolerances only on the features that need them, such as bores, mating faces and locating holes, and use GD&T where position or flatness matters. Every tight tolerance adds machining and inspection time.

Surface finish

A standard machined finish is about 3.2 µm Ra (125 µin). Call out a finer finish only where a surface seals, slides or shows.

Allow for anodizing

Anodizing grows into and out of the aluminum surface. Type III hard anodizing adds about 0.001 inch (25 µm) to each surface, so tight bores and threads need to be masked or machined to allow for it. Tell us the finish and we will set the machining sizes. See Anodizing & Plating.

Prototype first

For a new design, we can 3D print it to check fit and form before metal is cut, as we did for the long-range scope rail. See Machining & Additive.

Designing machined parts