Choosing a material for a machined part is really three decisions at once: what the part must do, what the material costs to buy, and what it costs to cut. The third is the one most often left out, and on a part with a lot of material removal it can dominate the other two combined.
The metals, in rough order of cost to cut
Free machining brass and aluminium are the fastest and cheapest. Mild steel is straightforward. Free machining stainless is manageable; the standard austenitic stainless grades work harden and cost appreciably more. Tool steels, titanium and nickel superalloys are slower again and consume tooling. Cast iron machines well and is dirty. The gap between the extremes is a multiple, not a margin.
The plastics, and why they are not soft metal
Acetal machines best and holds tolerance. Nylon absorbs moisture and moves. Polycarbonate is tough and stress cracks. PTFE creeps under the clamp. PEEK machines well and is expensive. All of them conduct heat poorly, so the practical constraint is getting heat out of the cut rather than cutting force, which is the opposite of metal.
Stock availability changes lead time
Shops hold common grades in common sizes. An unusual alloy, an unusual temper or an unusual bar size means the clock starts when the material arrives, which can add a week or more. Where the specification allows a range, saying so lets the shop use what is on the rack and can save days at no cost to function.
Matching the material to the post processing
Anodising needs aluminium and the alloy affects the colour. Black oxide needs ferrous material. Passivation applies to stainless. Hardening needs a hardenable grade and changes dimensions, which is why parts are often machined soft, hardened and then ground. Choosing the material without checking the finishing chain is how a part becomes unfinishable.
Questions people ask about cnc machining materials
Which material is cheapest to machine?
Free machining brass and aluminium by a wide margin. On a heavily cut part the difference against stainless or titanium is a multiple of the cost, not a small increment.
Does the shop choose the material?
Only if the drawing does not specify. Where the alloy affects strength, corrosion or finish appearance, specify it; otherwise letting the shop use stock material saves time.
Why are plastics harder to machine than they look?
Because they conduct heat poorly, move with temperature and humidity, and carry stock stress that releases as material comes off.
How does material affect lead time?
Common grades in common sizes are on the shelf. An unusual alloy or size means waiting for the mill or the stockist, which can add a week or more to the quoted date.