Aluminum is the metal most machined parts are made from, and for good reason: it cuts several times faster than steel, holds tolerance well, takes a good finish straight off the tool and accepts anodising afterwards. That combination makes it the default for prototypes and for a great many production parts where the strength of steel is not required.
Which alloy to ask for
The general purpose choice is a free machining grade that cuts cleanly and holds tolerance, and it is what most shops keep in stock. Structural and marine grades trade machinability for strength or corrosion resistance. Where the part is anodised for appearance, alloy choice changes the color, so a cosmetic part should specify the alloy rather than leave it to the shop's bar rack.
Why CNC machining aluminum is cheap
High cutting speeds, low cutting forces and good chip evacuation mean material comes off quickly and the tool lasts. The result is that a part machined from aluminum can cost a fraction of the same part in stainless, and the difference grows with the amount of material removed. On a heavily pocketed part the material choice can dominate the whole quote.
What still costs money in custom aluminum CNC work
Setups, fixturing and tolerances, exactly as with any material. Thin walls, which chatter and deflect. Very deep pockets with small radii. Cosmetic surfaces, because aluminum marks easily and handling damage is a real scrap cause. And anodising, which is a separate operation with its own lead time, masking and color matching problems.
Finishing choices after aluminum machining
As machined is fine for internal parts. Bead blasting evens out the tool marks and is the usual precursor to anodising. Clear anodising adds hardness and corrosion resistance with little color change; color anodising is cosmetic and varies with alloy and batch. Hard anodising is a functional wear coating and grows the surface, so allow for the thickness on fitted features.
Questions people ask about aluminum machining
Which aluminium alloy machines best?
The free machining general purpose grades that most shops stock. If the drawing does not specify, the shop will use what is on the rack, so specify when the alloy affects strength, corrosion or anodised color.
Is aluminium cheaper to machine than steel?
Usually much cheaper, because cutting speeds are far higher and tools last longer. The material itself may cost more per pound, but on a machined part the labor saving dominates.
Does anodising change dimensions?
Yes. The coating grows from the surface as well as into it, and hard anodising grows appreciably. Allow for it on any fitted or threaded feature, or have those features masked.
Can CNC machined aluminum hold tight tolerances?
Yes, with the usual caveats about thin sections and temperature. It moves more with temperature than steel, so a very tight tolerance on a large part needs the measurement conditions stated.