How to choose the right metal for your CNC part.

The 7 dimensions that drive material choice.

Material selection for CNC parts is a constrained optimization — you're minimizing cost while meeting seven performance requirements. For most parts, only 2–3 of these actually constrain the choice. Identify yours first, then pick the cheapest material that satisfies them.

Starting defaults by use case.

When to pick aluminum — 60% of parts.

Aluminum is the default CNC material because it combines low density, good machinability, corrosion resistance, and reasonable cost. Most brackets, housings, enclosures, mounts, and structural frames should start as aluminum and only move up if there's a specific reason.

• 6061-T6 — the baseline. Good strength (276 MPa yield), excellent weldability, anodizes cleanly. Use for 80% of aluminum parts. See full spec →
• 7075-T6 — when 6061 is strength-limited. Yield 503 MPa, but 2.5× the material cost, slightly lower corrosion resistance. See full spec →
• 5052-H32 — sheet-forming grade. Excellent corrosion resistance (marine-rated), lower strength than 6061. Mostly for bent/formed enclosures.
• 2024-T3 — aircraft-skin grade. Similar strength to 7075 but better fatigue resistance. Poor weldability, poor corrosion (usually clad).
• 6063 — extruded heat-sink grade. Better surface finish in anodizing, lower strength than 6061.

When to pick carbon or alloy steel.

Steel comes in when you need strength at 3–4× the density of aluminum and low cost. Heat-treatable alloy steels are the default for shafts, gears, and load-bearing machine parts. Plain carbon steels are for jigs, fixtures, and non-corrosive structural use.

• 1018 mild steel — lowest cost, easy to machine, easy to weld. Use for non-heat-treated brackets, fixtures, and jigs. No corrosion resistance — paint or plate.
• 1045 medium carbon — can be flame-hardened on surfaces. Used for shafts, keys, and gears in non-corrosive service.
• 4140 (42CrMo4) — the default alloy steel. Heat-treatable to ~45 HRC. Used for shafts, connecting rods, tooling bodies, and anything that sees cyclic load.
• 4340 — 4140 with more nickel. Better impact toughness; use when 4140 is underperforming in fatigue. More expensive.
• A2 / D2 tool steel — air-hardening, very wear resistant. For dies, punches, stamping tools, wear plates.

When to pick stainless steel.

Stainless is the answer whenever moisture, washdown, food, blood, or mild chemicals are involved. It's 2–3× harder to machine than aluminum, so expect 30–80% higher part cost for the same geometry.

• 303 — free-machining austenitic. Has sulfur added for cleaner chips; use for turned parts in mild environments. Avoid if corrosion is critical or if welding is needed.
• 304 / 304L — general-purpose stainless. Good corrosion in food, beverage, pharmaceutical, and most outdoor environments. See full spec →
• 316 / 316L — added molybdenum gives chloride resistance. Use for marine, coastal outdoor, and all sour-service environments. See full spec →
• 17-4 PH — precipitation-hardening. Can be aged to ~1200 MPa yield. Used where high strength + corrosion are both required (aerospace fasteners, pump shafts).
• 420 — martensitic. Hardens to 50+ HRC. Used for cutlery, surgical instruments, corrosion-resistant dies.

When to pick titanium.

Titanium's sweet spot is when you need stainless-equivalent corrosion resistance but at half the density. It's 4–6× more expensive than stainless and harder to machine, so it shouldn't be a default — but it's excellent for weight-critical, biocompatible, or chloride-aggressive applications.

• Grade 2 (commercially pure) — low-strength titanium. Best formability and corrosion resistance. Used for chemical process parts, marine heat exchangers, and biocompatible non-structural devices.
• Grade 5 (Ti-6Al-4V) — the titanium workhorse. Yield 830 MPa, biocompatible, weldable. Used for aerospace structural parts, orthopedic implants, race bike components. See full spec →
• Grade 23 (Ti-6Al-4V ELI) — extra-low interstitial. The medical-implant version of Grade 5. Slightly lower strength but better toughness and fracture resistance.

When to pick nickel alloys.

Nickel superalloys earn their cost when you need strength at temperature above 500°C, or aggressive corrosion resistance in oil/gas service. Expensive to buy and machine — budget 8–12× the aluminum equivalent.

• Inconel 625 — solid-solution-strengthened. Best weldability, service to 980°C. Used for chemical process vessels, exhaust components, subsea manifolds.
• Inconel 718 — precipitation-hardened. 1240 MPa yield in aged condition, service to 650°C. The dominant alloy for aerospace hot-section parts and O&G downhole. See full spec →
• Hastelloy C-276 — extreme chloride and acid resistance. Used in chemical plants, chlor-alkali, wet FGD scrubbers.
• Monel 400 / K-500 — nickel-copper. Best choice for seawater service at moderate temps.

When to pick copper or brass.

Copper alloys win when electrical or thermal conductivity is the primary requirement, or when you need non-sparking / non-magnetic behavior.

• C110 (OFHC copper) — pure copper, best conductivity. Used for bus bars, electrodes, heat sinks, RF components.
• C360 free-machining brass — the fastest-machining metal. Used for fittings, fasteners, connectors, small electrical components. See full spec →
• C260 cartridge brass — drawn/formed. Used for ammunition cases, deep-drawn parts.
• Beryllium copper (C17200) — age-hardenable copper. High strength + high conductivity + non-sparking. Used for springs, electrical contacts, and oil/gas non-sparking tools.
• Bronze (C932 / C954) — excellent bearing properties. Used for bushings, thrust washers, worm gears.

When to pick machined plastics.

Plastics machine cleanly on CNC and make sense for electrical insulators, low-load structural parts, and chemically aggressive environments where metal won't survive.

• Delrin (POM acetal) — best general-purpose machining plastic. Low friction, dimensionally stable, easy to cut. Used for gears, bushings, small structural parts. See full spec →
• PEEK — high-performance. 250°C continuous service, chemically resistant, biocompatible. Expensive. Used for medical implants, aerospace brackets, semiconductor wafer carriers. See full spec →
• PTFE (Teflon) — lowest friction, chemically inert. Soft, hard to machine to tight tolerance. Used for seals, bearing surfaces, chemical process parts.
• Ultem (PEI) — high-temp, high-strength. Flame-retardant, dimensionally stable. Used for aircraft interiors, electrical housings.
• Nylon (PA6, PA66) — tough, wear-resistant. Absorbs moisture (swells slightly). Used for gears, wheels, bushings.
• UHMW / HDPE — low-cost. Low friction, impact resistant. Used for wear strips, guides, non-structural parts.

Material-overspec traps we see most often.

Quick reference chart.

For a printable version, see our materials library index which includes all of the above with linked detailed spec pages.

Relative cost is per finished part with identical geometry (machining + material), benchmarked against 6061 = 1.0×, production quantity 100 parts.

Decision flow — how to actually pick a material

The mistake most engineers make: they start from "I need a material" and scan spec sheets. Better approach is to start from requirements and narrow down. A systematic flow:

Material substitution patterns

When an exotic material is specified, ask if a more common substitute works:

• 7075 → 6061: if strength margin allows. Saves 33% on material cost.
• 316 → 304: if chloride exposure is not an issue. Saves 30%.
• Titanium Gr5 → 17-4 PH stainless: in many structural applications, 17-4 offers similar strength at 40% the cost. Titanium wins only when weight or biocompatibility are paramount.
• PEEK → Delrin: if service temp < 100°C and no chemical exposure. Saves 10-15× on material.
• Inconel 718 → 17-4 PH: for non-high-temperature applications. Inconel wins above 500°C; 17-4 is adequate below that and costs 60% less.
• Aerospace-grade aluminum (AMS spec) → commercial: if certification is not required. Same alloy, same properties, sometimes different source. AMS adds 20-40% for traceability paperwork alone.

Material DFM considerations by family

Each material family has specific design quirks that increase cost if ignored:

Aluminum alloys: thin walls under 0.8mm warp during machining. Sharp internal corners (smaller than R0.5mm) require slow-feed finishing passes. 7075 doesn't weld reliably — design in separable pieces if welding is needed.

Stainless steels: work-hardens during machining — sharp tools and adequate coolant mandatory. Post-weld passivation almost always required. For tight tolerances, account for higher thermal expansion than carbon steel.

Titanium alloys: fire hazard during machining (fine chips ignite) — requires flood coolant. Galling with steel tools requires coated tools. Stress-relief anneal often required after heavy machining. Cost 10-20× aluminum in materials alone.

Superalloys (Inconel, Hastelloy): extreme tool wear — expect 5-10× higher tooling cost. Slow cutting speeds (30-50 SFM vs 300+ for aluminum). Design for minimum material removal — every cubic millimeter matters.

Engineering plastics: dimensional instability with temperature and humidity. Plan for thermal-stress relief (anneal before or after machining) on PEEK and nylon. UHMW and other rubbery plastics can't hold tight tolerances; design for ±0.15mm.

Certification requirements that drive material choice

Beyond mechanical properties, certifications often dictate material:

• ISO 10993 biocompatibility: required for medical devices contacting the body. Limits choices to 316L stainless, Ti-6Al-4V ELI, PEEK, and specific plastic grades. Non-certified grades of the same alloys don't qualify.
• USP Class VI: short-term body contact plastics. PEEK, POM (Delrin 150 grade), and specific polycarbonate grades qualify. Not all grades of each plastic qualify — must be certified lot.
• FDA food contact: indirect and direct food contact have different requirements. Stainless 304/316, PE, PP, PET, and specific Delrin grades (Delrin 150 NC010) qualify.
• AS9100D / AMS specifications: aerospace materials require traceability from mill through fabrication. AMS-spec alloys cost 20-40% more than commercial equivalents — pay for the paperwork.
• NORSOK M-650 / MDS: oil and gas subsea applications. Specific stainless (UNS S31803 duplex, Inconel 625, Monel 400) required. Testing and qualification requirements are extensive.
• UL / IEC flame ratings: V-0, V-1, V-2 plastics for electrical enclosures. Specific grades and colors qualify; not interchangeable across grades.

Confirm certification requirements early. Discovering at production that your chosen material doesn't qualify for your regulated application is catastrophic — you've potentially committed to a material that must be switched, triggering requalification, tool changes, and schedule slip.

Thermal considerations beyond service temperature

Service temperature is often framed as a single number, but real thermal analysis considers:

• Coefficient of thermal expansion (CTE): differences between mated materials cause stress during temperature cycles. Aluminum 23 ×10⁻⁶/°C, steel 12 ×10⁻⁶/°C, Invar 1.5 ×10⁻⁶/°C. Mating aluminum to steel in precision fixtures can create interference or gaps across even modest temperature ranges.
• Thermal conductivity: matters for heat sinks, cooling paths, and maintaining dimensional stability during machining. Copper 400 W/m·K (best conductor), aluminum 237, carbon steel 52, stainless 16, titanium 22, PEEK 0.25.
• Glass transition temperature (Tg): for plastics, properties change dramatically near Tg. Delrin's Tg is around -60°C — fine for normal service. PEEK's Tg is 143°C — parts exposed above this temperature soften even though the part doesn't melt.
• Creep: plastics deform slowly under sustained load, even below service temperature. UHMW creeps substantially. PEEK creeps minimally. Design fasteners and load-bearing plastic parts with creep in mind.

Designing against thermal effects requires integrating all four factors. A metal bracket clamping a plastic part at elevated temperature will see: different CTE pulling the assembly apart as it heats, creep in the plastic relieving clamping force, potential glass transition softening at the upper service limit. Any of these can cause the assembly to fail even though no material was individually overstressed.

Material-selection FAQ