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CNC Machining Materials Guide: Choosing Between the Metals and Plastics We Cut

Material selection reference · Boyang Hardware, Zhongshan, Guangdong, China · Est. 2016

Direct answer Start with aluminium 6061-T6 — it is the cheapest to buy and the fastest to cut, and only move away when a requirement forces you to. The working set at Boyang Hardware is: aluminium (6061, 7075) for lightweight structures; stainless steel (304/316) for corrosion duty; brass (C36000) for fittings and threads; copper (C11000/C10100) for electrical and thermal parts; titanium (Gr 2/Gr 5) for strength-to-weight in harsh service; carbon steel for strength at lowest metal cost; and engineering plastics for insulation and low weight. Machine time is the real cost driver: cutting the same part in 316 stainless can take 3–5× the spindle time of 6061 aluminium.

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Material comparison for CNC machined parts

Table 1 — The material families machined at Boyang Hardware. Machinability is indicative only (see the FAQ on rating scales); "relative cost" combines stock price and machining time for a typical small part.
Material Key grades Machinability Strength Corrosion behaviour Relative cost Typical parts
Aluminium (general) 6061-T6, 6063 Excellent Moderate Good; anodising enhances it Lowest Brackets, housings, fixtures, enclosures
Aluminium (high strength) 7075-T6 Good High — near some steels Fair; needs anodise for wet service Low-mid Aerospace fittings, robotic arms, mould tools — 7075 details
Stainless steel 304, 316, 303 Moderate (303 easiest) High Excellent; 316 for chlorides Mid-high Food/medical hardware, marine fittings, pump parts
Brass C36000, C3604 Best of all metals (100% reference) Moderate Good; restricted where lead matters Low-mid Valves, fittings, threaded inserts — brass grades
Copper C11000, C10100 Low (gummy) Low Self-protecting patina Mid (metal) + slow cutting Busbars, heat sinks, electrodes — copper parts
Titanium Gr 2, Gr 5 (Ti-6Al-4V) Low Very high (Gr 5) Outstanding High Aerospace, medical, marine, sporting — titanium machining
Carbon steel 45#, A3/Q235 equivalents Moderate High (higher when hardened) Poor uncoated — needs plating or black oxide Low Machine bases, shafts, tooling, structural hardware
Engineering plastics POM/Delrin, Nylon, ABS, PTFE Excellent (low cutting forces) Low-mid Inherently inert or good Low-mid Insulators, guides, gears, seals-support parts

Five selection rules we apply at quotation

  1. Default to 6061-T6 unless a requirement — strength, corrosion media, conductivity, temperature — explicitly pushes another material.
  2. Let the environment pick the alloy. Chlorides and food contact → 316; seawater → 316 or titanium; indoor tooling → carbon steel with a finish.
  3. Price the machine time, not just the bar. A cheap billet that cuts three times slower often produces a dearer part — titanium and pure copper are the classic examples.
  4. Match the material to the process chain. Very high volumes with simple geometry may suit die casting or stamping instead of machining — compare routes in the die casting vs CNC break-even analysis; the material list shifts with that decision.
  5. Specify finishes with the material, not after it. Anodise (aluminium), plating (steel, copper), passivation (stainless) and other finishes — see the surface finish guide — change both cost and corrosion behaviour.

Engineering plastics — when to drop the metal entirely

POM (Delrin) machines to clean, dimensionally stable parts and is the default plastic for guides, gears and low-friction components. Nylon offers toughness and abrasion resistance with some moisture uptake to design around. ABS is economical for housings — though at higher housing volumes the route shifts to moulding, with tooling economics covered in the break-even comparison. PTFE and other fluoropolymers handle chemical and temperature extremes but are soft and demand gentle workholding. Plastics cut fast, hold ±0.02 mm realistically, and remove finishing steps entirely.

Honest limits: this guide is a starting point, not a materials engineering consultation — load cases, temperature extremes, media exposure and regulatory constraints (lead, food contact, medical) must be verified on the design side; we confirm manufacturability and cost, not fitness for purpose. Exotic grades outside the families listed are sourced case by case with variable lead times, and machinability ratings differ between scales and suppliers, so treat every table value as comparative rather than contractual. Where a requirement conflicts with cost, we say so plainly at quotation rather than quietly substituting a grade.

FAQs

Which aluminium should I use, 6061 or 7075?

6061 for general brackets, enclosures and anything welded or anodised on a budget; 7075 when strength-to-weight is the constraint — aerospace-style fittings, robot arms, high-load hinges. 7075 costs more and does not weld well. See 7075 machining capability and general aluminium machining services.

What tolerances does each material realistically hold?

Aluminium, brass and plastics hold ±0.01–0.02 mm routinely; stainless and carbon steel similar with attention to heat; titanium same process tolerances with adjusted parameters; 0.001 mm is reserved for critical features via grinding or wire EDM in any of them — see tolerance capability.

Can I get one drawing quoted in two materials to compare cost?

Yes — a common request. Both variants are quoted with the same inspection and delivery terms so the comparison is real, typically within the 1–2 working day quotation window.