Why the blank decides the part before the first cut
Before a spindle removes any stock, the raw material has already set most of the part's fate. Weight, stiffness, and corrosion are fixed by the alloy or plastic you clamp. Pick wrong stock and the whole job must be re-cut.
CNC is forgiving because it is subtractive: almost any solid stock that clamps and cuts works, yet that breadth means a long menu of aluminum, steels, brasses, titanium, and engineering plastics, each with its own cost and machinability.
Aluminum 6061: the default workhorse
If you make one choice, make it 6061-T6 aluminum. It is light, naturally corrosion resistant, and easy to machine. Its moderate strength and good weldability make it the go-to for enclosures, brackets, fixtures, and most functional prototypes.
Shops love 6061 because it cuts cleanly, makes short chips, and rarely work-hardens, so tool wear stays low and cycle times stay short. From one to a few thousand parts it gives the best balance of cost, speed, and performance, which is why it is the default quote for most CNC prototypes.
Aluminum 7075 and 2024: when strength rules
When 6061 is not strong enough, 7075-T6 steps up with tensile strength near 570 MPa and excellent fatigue resistance. The trade-off is lower corrosion resistance and slightly worse machinability, so parts are often anodized afterward for protection.
2024 brings high strength and good fatigue life but is less corrosion resistant and harder to weld, so it appears mainly in aircraft structures. Both grades machine well with sharp tooling; plan for their lower ductility and avoid aggressive depths of cut that could crack thin features.
Stainless steel 304 and 316: corrosion resistance
For parts that see water, chemicals, food, or the body, stainless is the answer. 304 is the general-purpose grade with good corrosion resistance and formability; 316 adds molybdenum for superior resistance to chlorides and marine use, the standard for medical and outdoor hardware.
The cost is machinability: stainless work-hardens fast and wears tools quicker than aluminum, so cycle times and cost rise. The payoff is a part that survives where carbon steel would rust, finished by passivation or bead blast.
Carbon and alloy steel: hardness and wear
When a part must resist wear, hold an edge, or carry heavy load, carbon and alloy steels such as 1018, 4140, or tool steels like A2 and D2 earn their place. They deliver high strength and hardness and can be heat-treated after machining for dies and moving components.
Machining these is demanding: hardness drives slow feeds, rigid setups, and real tool wear. For most prototypes the extra cost is unnecessary, but for end-use tooling and load-bearing hardware, steel is often the only material that lasts.
Brass and copper: conductivity and aesthetics
Brass, typically free-machining C360, is a joy to cut: it chips beautifully, holds tight tolerances, and leaves a golden, corrosion-resistant finish. That makes it ideal for connectors, terminals, decorative hardware, and low-friction bushings where its self-lubricating nature helps.
Copper and alloys such as beryllium copper carry excellent electrical and thermal conductivity for heat sinks and contacts. Copper is gummy and tricky to machine, so shops often substitute brass when conductivity allows. Both are right whenever the part's job is to move electrons or heat, not bear load.
Titanium: strength-to-weight and biocompatibility
Titanium grades like Ti-6Al-4V combine high strength with very low density and outstanding corrosion resistance, plus full biocompatibility, which explains their role in medical implants, aerospace, and high-end cycling. They resist attack from body fluids and seawater alike.
The downside is machinability: titanium has low thermal conductivity, so heat concentrates at the edge and tools wear fast. It pays off only when weight, strength, or biocompatibility are non-negotiable; else aluminum or steel is far cheaper.
Engineering plastics: POM, nylon, PEEK, ABS, PC
Plastics fill niches metals cannot: they insulate electrically and thermally, damp vibration, and weigh a fraction as much. POM offers low friction and tight stability for gears and bushings; nylon absorbs impact and resists wear; ABS and PC suit housings and tinted or clear covers.
PEEK sits at the top, with continuous service near 250 C, excellent chemical resistance, and medical and semiconductor approval. Choose plastic when weight, insulation, or inertness beat raw strength, while accepting its lower absolute strength.
Machinability drives cost and lead time
Machinability is the silent driver of price. Free-machining metals like 6061 and C360 cut fast with low tool wear, while titanium, stainless, and hardened steel slow everything and burn tooling.
Design choices compound it. Thin walls, deep pockets, and tight tolerances on hard materials multiply cycle time and scrap risk. When budget is tight, specify the easiest grade that meets the need rather than the strongest; that often cuts both cost and lead time without hurting the prototype's purpose.
Surface finishing and material compatibility
The material also decides which finishes are possible. Aluminum anodizes to a durable, colored, corrosion-resistant coat; steel and stainless take plating, black oxide, or passivation; brass polishes to a mirror; titanium can be bead-blasted or anodized in subtle hues.
Plastics limit the menu: most take bead blast, paint, or laser marking, but few tolerate high-temperature plating. Knowing the end finish early avoids surprises, because a clear anodize on a steel part is impossible.
A practical selection checklist and FabProto's help
A simple filter settles most choices: does the part see load, heat, or corrosion? Must it conduct? Is weight critical? Aluminum wins by default; steel for wear and load; stainless for corrosion; brass or copper for conduction; titanium for weight plus strength; plastics for insulation and light weight.
FabProto stocks the common grades and runs them daily, so we recommend the cheapest stock that meets your spec and quote finishes in the same pass. Send one CAD model and tell us the environment; we return a material recommendation, tolerance capability, and cost, so you never over- or under-build the part.