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The material you specify for a CNC machined part decides far more than its appearance. The alloy or plastic sets the cutting speed, tool life, achievable surface finish, and final cost before a single chip is removed. This guide compares the metals and engineering plastics machined every day — aluminum, stainless and alloy steel, titanium, brass, copper, and high-performance plastics such as PEEK and Ultem — and gives a practical selection framework so you can match material to load, environment, and budget. You will learn which grades shops reach for by default, where exotic materials are worth the premium, and how material choice interacts with tolerances and finishing.
Table of Contents
Machinability describes how easily a material cuts. Softer, free-cutting alloys remove material quickly with low tool wear; hard, abrasive, or heat-resistant alloys slow the spindle, dull tools, and stretch cycle time. Three cost levers trace directly back to material:
· Cycle time — aluminum cuts several times faster than titanium, so the same geometry costs far less in aluminum.
· Tool life — abrasive plastics and hard alloys consume end mills faster, adding tool-change and scrap cost.
· Material price — raw stock cost varies widely; over-specifying to an exotic grade inflates both stock and machining cost.
A useful rule: pick the cheapest grade that meets the service environment. Strength you do not need is money spent twice — once on stock, once on slower cutting. For the broader process picture, see the full CNC machining services guide.
Aluminum dominates rapid machining because it cuts cleanly, resists corrosion, and is light. KAIAO regularly runs 6061, 6061-T6, 7075, and 5052.
Grade | Key Properties | Typical Use | Machining Note |
6061 / 6061-T6 | Good strength, weldable, corrosion resistant | Brackets, enclosures, fixtures | Easiest and cheapest; anodizes well |
7075 | Very high strength, lower corrosion resistance | Aerospace frames, stressed parts | Cut slower than 6061; watch thin walls |
5052 | Formable, marine-grade corrosion resistance | Sheet parts, panels | Good for formed-then-machined work |
Aluminum is the default for prototypes and most production parts where weight and corrosion resistance matter more than extreme strength.
Steel covers the widest property range, from soft easy-cutting stock to hardened, high-strength alloys.
Grade | Key Properties | Typical Use | Machining Note |
1018 | Low carbon, easy to cut | Jigs, fixtures, general parts | Lowest steel cost |
4140 / 4340 | Hardenable, high strength, wear resistant | Gears, shafts, tooling | Rigid setups; may need stress relief |
304 | Corrosion resistant, formable | Commercial, food-grade parts | Work-hardens; keep tools sharp |
316L | Chloride resistant (molybdenum added) | Marine, medical, chemical | Harder than 304, slower feeds |
17-4PH | Precipitation hardened, strong | Aerospace, medical, valves | Age-hardened after machining |
420 | Hardenable stainless | Cutlery, wear faces | Hard; plan for finishing |
When corrosion resistance is decisive, comparing Stainless Steel 304 vs 316 for Machining prevents pitting in chloride or washdown environments.
Titanium offers an outstanding strength-to-weight ratio and is biocompatible, which is why it appears in implants, aerospace structures, and drone components. The trade-off is machinability: titanium has poor thermal conductivity, so heat builds at the cutting edge instead of dissipating into the chip. Shops counter this with rigid setups, slow feed rates, and high-pressure coolant — all of which lengthen cycle time and wear tools.
Use titanium when weight and strength or biocompatibility are non-negotiable. For non-critical structures, aluminum 7075 usually delivers most of the benefit at a fraction of the cost.
These conductive metals suit electrical and thermal parts. Brass machines beautifully — often the fastest of all — and is ideal for connectors, terminals, and EMI shields. Copper conducts even better but is gummy, so it needs sharp tools and the right geometry. Bronze wears well for bushings and bearings. Specify these when conductivity or low friction drives the design.
Inconel and similar nickel alloys resist heat and corrosion at extreme temperatures, serving turbine, exhaust, and energy parts — but they are among the hardest materials to cut, demanding low speeds and premium tooling. Magnesium is extremely light and easy to machine, yet its fines can ignite, so it requires strict chip and coolant control. Both are justified only when the operating environment leaves no alternative.
Machined plastics give stronger, more dimensionally stable parts than molded ones during validation, and some outperform metal in weight or chemical resistance.
Material | Key Properties | Typical Use | Machining Note |
ABS / PC | Easy to cut, good strength | Enclosures, electronic prototypes | Watch heat; PC can stress-crack |
PMMA (acrylic) | Optically clear | Lenses, covers, light pipes | Polishes to glass-like clarity |
POM (Delrin) | Low friction, stable dimensions | Gears, bushings, slides | Excellent for tight-tolerance motion |
Nylon (PA) | Tough, wear resistant | Brackets, spacers | Absorbs moisture; verify post-conditioning size |
PEEK | High temp, chemical resistant, implantable-grade | Medical, aerospace, semiconductor | Cut slower; needs sharp tools |
PEI (Ultem) | High temp, flame retardant | Aerospace, electronics | Strong but abrasive on tools |
PPSU / PPS | Steam-sterilizable, chemical resistant | Medical, fluid parts | Stable; verify food/ISO 10993 grade |
Engineers often use CNC plastic prototyping to prove mechanical function before committing to steel injection tooling.
Apply these rules in order, stopping at the first grade that fits:
13. Corrosion or washdown? Choose 316L stainless or anodized aluminum; avoid plain steel.
14. Strength-to-weight critical? Choose titanium or aluminum 7075.
15. High temperature or aggressive chemical? Choose PEEK, PEI, or Inconel.
16. Biocompatible or sterilizable? Choose titanium, 316L, or medical-grade PEEK/PPSU.
17. Electrical conductivity needed? Choose brass or copper.
18. None of the above, cost-first? Choose aluminum 6061 or ABS — the default workhorses.
Over-specifying to aerospace-grade titanium when 6061 aluminum suffices is the most common avoidable cost mistake.
· Medical — titanium, 316L, and PEEK under ISO 13485 documentation; see CNC machining for medical devices.
· Aerospace — 7075 aluminum, titanium, and Inconel where heat rules; detail in the aerospace CNC machining guide.
· Automotive and EV — aluminum and steel for housings and structural parts, with some engineering plastics; explore CNC machining for automotive.
· Electronics — aluminum for enclosures and heat sinks, PC or PEEK for insulators.
Thin-wall or hard materials often benefit from 5-axis CNC machining, which holds accuracy on tricky geometry in fewer setups.
CNC machining materials set the floor for cost, strength, and lead time before cutting begins. Match the grade to the real service environment, avoid paying for properties you do not need, and let tolerance and finishing follow from the material rather than the reverse. A short design-for-manufacturability review with your shop usually catches the expensive over-specifications early. Ready to specify your next part? Share your drawing and requirements with how to choose a CNC manufacturer in China to find a partner who reviews material choice, not just price.
Can CNC machines cut hardened steel?
Yes. Shops machine 4140, 4340, and hardened stainless such as 420, though harder grades need rigid setups, slower feeds, and often a stress-relief step before finish cutting.
Which plastic is best for a sterilizable medical part?
PEEK and PPSU tolerate steam sterilization and are available in implantable or ISO 10993 grades; confirm the exact grade and certs with your supplier before design release.
Does aluminum or steel weigh less for the same part?
Aluminum is about one-third the density of steel, so an aluminum part is far lighter at equal size — a major reason it leads aerospace and automotive work.
How does material affect achievable tolerance?
Plastics expand more with heat than metals, so stable plastics like POM and PEEK hold tight tolerances better than nylon, which shifts as it absorbs moisture. The CNC machining tolerances guide covers this in detail.
Will my shop provide material certificates?
A capable partner supplies material certs (COA/COC) on request, which is mandatory for medical and aerospace traceability.
Is recycled or specific-origin stock available?
Stock origin can often be specified, but certification matters more than "recycled" labeling for critical parts — ask for the mill cert that proves the grade.
