CNC materials used in machining and how to choose them
What CNC materials used means in practice
The phrase CNC materials used refers to the metals and plastics that can be milled, turned, drilled or routed while still meeting the required geometry, surface finish and in-service performance. There is no single best CNC material. The right choice depends on load, weight, corrosion exposure, temperature, electrical or thermal requirements, tolerance risk, finishing method and budget.
Aluminum may reduce machining time and part weight. Carbon steel can lower raw material cost. Stainless steel may improve corrosion resistance. Titanium can justify its price where strength-to-weight performance is critical, while plastics may solve insulation, friction or chemical-resistance problems. For more background on material selection, see the Materials section.
How machinists group CNC materials
Machinists do not look only at alloy names. They also group workpieces by cutting behavior because chip formation, heat flow, tool wear and coolant strategy all affect cost. ISO 513:2012 is a widely referenced standard for classifying and applying hard cutting materials for metal removal with defined cutting edges. Cutting-tool suppliers commonly connect it with workpiece groups such as P for steels, M for stainless steels, K for cast irons, N for non-ferrous materials, S for superalloys and titanium, and H for hardened materials. (iso.org)
This grouping is useful, but it does not replace the exact material grade. Aluminum 6061-T6 and 7075-T6 are both aluminum alloys, yet they differ in strength and cutting behavior. AISI 1018 and 4140 are both steels, but heat treatment and microstructure can change their machining response. Stainless 304, 316 and free-machining stainless grades can also behave differently. A reliable specification should combine the material family, exact grade, condition or temper, dimensional form, and any standard or certificate requirement.
Common CNC materials and where they fit
Aluminum alloys
Aluminum is often chosen when parts need low weight, good corrosion resistance, a clean surface finish and efficient cycle times. ASM Handbook material on machining aluminum alloys notes that aluminum alloys can generally be machined rapidly and economically, while alloy composition and temper affect chip control, built-up edge and finish. (materialsdata.nist.gov)
6061-T6 is widely used for brackets, housings, fixture plates, prototypes and general machine parts because it balances availability, cost, strength, corrosion behavior and finishing options such as anodizing. 7075-T6 is often selected when higher strength is needed at a similar density. A MatWeb data sheet for 7075-T6 lists density around 2.81 g/cc, typical ultimate tensile strength around 572 MPa and a machinability rating of 70 on a 0-100 aluminum-alloy scale. These values help explain why it is attractive for structural machined parts, although proper tool selection still matters. (asm.matweb.com)
Carbon and alloy steels
Carbon steels and alloy steels are selected for strength, wear resistance, toughness, weldability or low raw material cost. AISI 1018 is a common mild steel for general-purpose machined parts, shafts, spacers and brackets. MatWeb lists cold-drawn AISI 1018 with a machinability value of 70 percent based on AISI 1212 steel as the 100 percent reference. (asia.matweb.com)
Alloy steels such as 4140 are used when higher strength, fatigue resistance or through-hardening ability is required. ASM Handbook material on carbon and alloy steel machining emphasizes that chemical composition and microstructure influence the response of steels during turning, drilling, milling, thread cutting and grinding. (dl.asminternational.org) In practice, annealed, normalized, pre-hardened and quenched-and-tempered stock should not be treated as interchangeable for tooling or costing.
Stainless steels
Stainless steel is selected when corrosion resistance, cleanability, durability or appearance matters. 304 is common for general corrosion-resistant parts, while 316 is frequently considered for harsher service environments. The machining penalty is real: sources such as ASTM and the British Stainless Steel Association describe austenitic stainless steels as more difficult to machine than carbon steels because of high work-hardening rates, toughness, ductility and low thermal conductivity. (store.astm.org)
For CNC work, this usually means sharper tooling, rigid setups, correct feeds, controlled heat and avoiding rubbing cuts that harden the surface. Stainless may be the right material, but it should not be costed like mild steel or aluminum.
Titanium and heat-resistant alloys
Titanium alloys, especially Ti-6Al-4V, are used when high strength-to-weight ratio and corrosion resistance justify a higher machining cost. Published reviews of Ti-6Al-4V machining describe the material as challenging because of high strength, low thermal conductivity and chemical reactivity, all of which can concentrate heat at the cutting edge and accelerate tool wear. (pmc.ncbi.nlm.nih.gov)
Heat-resistant nickel alloys have a similar cost issue. They may be necessary for hot, corrosive or highly loaded applications, but they often require slower cutting, stronger fixturing, conservative tool engagement and tighter process control. They should be specified only when the application genuinely needs their performance.
Copper alloys and brass
Copper alloys are selected for electrical conductivity, thermal conductivity, corrosion behavior and appearance. Brass, especially free-cutting brass such as C36000, is valued for efficient machining. MatWeb describes free-cutting brass UNS C36000 as having excellent machinability and being fabricated by machining, roll threading and knurling. (matweb.com) ASM material on copper alloys also separates copper alloys into free-cutting, moderately machinable and difficult-to-machine groups depending on alloying additions and structure. (asminternational.org)
The main limitation is compliance. Leaded brass may not be suitable for every water-contact, food-contact, medical or regulated electrical application. When compliance matters, the grade and applicable regulation should be checked before stock is purchased. See also: Machines.
Engineering plastics
Plastics are common CNC materials for insulators, lightweight components, low-friction parts, chemical-resistant parts, prototypes and wear pads. Acetal, nylon, PTFE, UHMW, polycarbonate, ABS and PEEK each solve different problems. They also bring different machining risks, including thermal expansion, burrs, melting, moisture absorption, stress relief and part movement after unclamping.
PEEK is a high-performance thermoplastic used where temperature, chemical resistance and mechanical performance are important. Mitsubishi Chemical Group data for Ketron 1000 PEEK lists density of 1.31 g/cm³, tensile strength of 115 MPa and melting temperature of 340°C under cited test methods. (mcam.com) Those values help explain why PEEK is specified for demanding plastic components, but they do not remove the need to confirm the exact grade, filler content and operating environment.
A practical selection matrix
| Part requirement | Materials often considered | Why they fit | Main caution |
|---|---|---|---|
| Fast machining and low weight | 6061-T6 aluminum, 6082 aluminum, 7075-T6 aluminum | Good strength-to-weight ratio and efficient metal removal | Strength, anodizing response and corrosion behavior vary by alloy and temper |
| Low-cost strength | 1018 steel, 1045 steel, 4140 steel | Good availability and broad mechanical-property range | Heat treatment and hardness strongly affect machining time |
| Corrosion-resistant metal parts | 304 stainless, 316 stainless, selected free-machining stainless grades | Useful where rust resistance, cleaning or appearance matters | Work hardening and heat concentration increase tooling risk |
| High strength with weight reduction | 7075-T6 aluminum, Ti-6Al-4V titanium | Higher performance per unit mass than many general materials | Titanium is much slower and more tool-sensitive than aluminum |
| Conductive or decorative parts | Copper, brass, bronze | Electrical, thermal and appearance advantages | Some copper alloys are gummy; some brasses raise lead-compliance questions |
| Insulation, low friction or chemical resistance | Acetal, nylon, PTFE, UHMW, PEEK | Nonmetallic performance that metals cannot provide | Movement, creep, moisture and temperature may affect tolerance |
Why machinability changes total cost
Raw material price is only one part of CNC cost. Machinability affects spindle time, tool life, burr removal, scrap risk, fixture rigidity, coolant needs, inspection time and whether a part can be finished consistently. A cheap material that work-hardens, distorts or destroys tools may cost more than a higher-priced material that machines predictably.
Machinability numbers are useful only when the reference scale is understood. Some steel ratings are based on AISI 1212 as a 100 percent reference, while copper-alloy systems commonly reference free-cutting brass. Aluminum alloys may use a separate aluminum-alloy scale. Comparing ratings across different scales without context can lead to poor decisions. A better approach is to compare materials within the same family, then validate the choice against tooling recommendations, stock condition and expected tolerance.
Specification details that prevent avoidable problems
- State the exact grade and condition. Write 6061-T6, 7075-T651, 4140 pre-hard, 304 annealed or PEEK unfilled when that is what the design requires. Generic labels such as “aluminum” or “stainless” are not enough.
- Separate prototypes from production material. A prototype may be cut from 6061 for speed, while production may require stainless, titanium or PEEK for service conditions. If the prototype material is only a stand-in, document that clearly.
- Match material to finishing. Anodizing, passivation, black oxide, nickel plating, bead blasting and polishing all depend on alloy and surface condition. Finishing requirements should be considered before toolpath planning.
- Check dimensional stability. Thin-wall aluminum, stress-relieved plastics, hardened steels and high aspect-ratio parts may move during or after machining. Sometimes a stress-relief step, rough-and-finish sequence or altered stock size reduces risk.
- Use data sheets for screening, not final design authority. Public material databases are useful for comparison, but final drawings should cite applicable ASTM, AMS, EN, ISO, customer or supplier specifications when performance is critical.
- Confirm compliance early. Leaded brass, flame-rated plastics, food-contact polymers, medical materials and aerospace alloys may require documentation beyond ordinary purchase descriptions.
Common selection mistakes
One common mistake is choosing the strongest material when the part mainly needs stiffness, geometry and corrosion resistance. Another is choosing stainless steel for every corrosion concern without considering aluminum with coating, plastic, brass or a different stainless grade. A third is treating plastics like soft metals. Plastics may cut easily, but they can deflect, melt, absorb moisture or creep under load.
The most expensive mistake is selecting the material after the drawing is complete. Material, tolerance, wall thickness, thread depth, finish and inspection method interact. A small change from a sharp internal corner to a realistic radius, or from a difficult alloy to a more machinable grade, can reduce cost without reducing function.
Frequently asked questions
What is the most common CNC material?
There is no single material used for all CNC work. Aluminum 6061-T6 is common for general machined components and prototypes, but steel, stainless steel, brass, copper, titanium and engineering plastics are also common when their properties match the application.
Which CNC material is easiest to machine?
Free-cutting brass and many aluminum alloys are often among the easier materials to machine, but “easy” depends on the exact grade, temper, cutter, setup and tolerance. Some plastics also machine quickly, although dimensional movement and burr control can make them less simple than they appear.
Is stainless steel harder to CNC machine than aluminum?
Usually, yes. Austenitic stainless steels tend to work harden and conduct heat poorly compared with aluminum, so they generally require more careful feeds, rigid fixturing and tool control. Aluminum usually allows higher metal-removal rates, but it can still create built-up edge or chip-control issues in the wrong condition.
Can CNC machines cut plastic materials?
Yes. CNC milling, turning and routing are widely used for plastics such as acetal, nylon, PTFE, UHMW, ABS, polycarbonate and PEEK. The main concerns are heat, clamping pressure, chip evacuation, sharp tooling and dimensional change after machining.
How should I choose between aluminum, steel and plastic?
Start with the service requirement. Choose aluminum when weight, speed and corrosion resistance matter; steel when strength and cost matter; stainless when corrosion resistance and durability matter; and plastic when insulation, low friction, chemical resistance or weight reduction is more important than metal strength.
