What Types of Materials Can Be Processed Using CNC Machines?

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  Key takeaways
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  What materials processed using cnc machines actually includes
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  Metals: the default choice for materials processed using cnc machines
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  Plastics: fast to cut, easy to distort
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  Composites, ceramics, and where CNC stops being the right process
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  How to choose among materials processed using cnc machines
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  Machinability comparison for materials processed using cnc machines

The question sounds simple. The real answer depends on three things: how the material behaves under a cutting edge, how it holds its shape after the cut, and whether the finished surface meets the drawing. A material that cuts easily may still warp. A material that holds tolerance may destroy tooling in twenty minutes.

Subtractive machining removes material with a rotating cutter or a single-point tool. That means the workpiece must be stable enough to resist cutting force and stiff enough to avoid chatter. It also has to carry heat away from the cut zone. Materials that fail one of these tests can still be machined, but the process changes: slower speeds, lighter depths of cut, extra supports, or a different tool grade.

This page groups candidate materials by family, then gives the practical numbers we use on the floor: cutting speed ranges, achievable surface finish, and the point where the material stops being economical. It is written for design engineers and buyers who need to choose a stock material before a quote, not a materials science lecture.

One rule applies to every family. The condition of the stock matters as much as the alloy name. Annealed 6061 cuts differently from T6. A cast aluminum part with porosity will not hold a mirror finish no matter what the cutter does. Always specify temper, condition, and the form you are buying.

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  MachinabilityHow fast the tool can cut without failure.
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  Dimensional stabilityWhether the part moves after cooling or stress relief.
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  Surface responseWhether the finish meets Ra targets without extra steps.

Aluminum is the most common metal we machine. Grade 6061-T6 cuts at 300–600 m/min with carbide tooling and holds ±0.005 mm on a rigid setup. Grade 7075 machines well but is harder on tools and more prone to stress cracking after heavy stock removal. Cast alloy ADC12 cuts fast but often contains porosity, so cosmetic surfaces need care.

Stainless steel splits into two groups. Free-machining grades like 303 and 17-4PH (SUS630) form short chips and run at 120–200 m/min. Grades 304, 316, and 316L work-harden quickly, so the cutter must stay engaged. Never dwell in a 304 cut. If the tool rubs instead of cuts, the surface hardens and the next pass gets harder.

Carbon and alloy steels cover a wide range. Grades 1018 and 1045 are straightforward. Grades 4130, 4140, and 4340 machine well in the annealed state but need stress relief before finish cuts on thin sections. Tool steel in the hardened state usually goes to EDM or grinding rather than direct milling.

Titanium and nickel alloys sit at the difficult end. Ti-6Al-4V (TC4) conducts heat poorly, so the cutting edge absorbs almost all of it. Speeds drop to 30–60 m/min with high-pressure coolant. Inconel runs even slower. These materials remain machinable, but the cost per part reflects the cutter consumption and the cycle time.

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  Aluminum 6061-T6300–600 m/min; general parts, housings, fixtures.
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  Stainless 303120–200 m/min; shafts, fittings, food equipment.
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  Steel 4140100–180 m/min; gears, axles, tooling bodies.
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  Titanium TC430–60 m/min; aerospace brackets, medical implants.

Plastics machine at high spindle speeds but low cutting forces. The main problem is heat. A plastic chip that melts and re-welds to the cutter leaves a rough surface and a dimension that drifts. We run POM and ABS at 200–400 m/min with sharp, polished flutes and generous coolant or air blast. Dull tools are the usual cause of a bad plastic finish.

POM (Delrin) is the best general-purpose machining plastic. It holds ±0.01 mm on small parts and produces a clean chip. It is often used for gears, bearings, and sliding components. Nylon (PA) is tougher and more wear-resistant but absorbs moisture, so dimensions shift after machining. For tight-tolerance nylon parts, specify conditioning before the final cut.

PEEK and PC sit at the higher end. PEEK keeps its properties at high temperature and resists chemicals, which makes it common in medical and semiconductor equipment. It is expensive and abrasive, so tool life is short. PC is tough and transparent but prone to stress cracking around holes and threads. Use sharp tools and avoid excessive clamping pressure.

Acrylic (PMMA) is chosen for optical clarity. It cuts cleanly and polishes well, but it chips easily at the exit of a cut. Support the workpiece from both sides and use a slow feed on the final pass. PP and HDPE are soft and flexible. They are hard to hold to tight tolerances because the material deflects away from the cutter.

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  POMBest all-round machining plastic; holds tight tolerance.
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  PEEKHigh temperature and chemical resistance; abrasive.
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  PMMAOptical clarity; chips at cut exit without support.

Carbon fiber and glass fiber reinforced plastics are machined, but the cut is abrasive rather than ductile. The fibers wear carbide quickly, so diamond-coated tooling is standard. The bigger risk is delamination: if the cutter pushes fibers apart instead of shearing them, the edges fray and the part loses strength. Use down-cut or compression cutters on the top surface and keep the tool sharp.

Ceramics are usually machined in the green state, before firing. After firing, the material is too hard for standard milling. Fired alumina and zirconia go to diamond grinding or EDM. If your drawing calls for a fired ceramic with tight tolerance, plan the grinding step into the process from the start.

Magnesium alloys like AZ31B and AZ91D cut fast and produce an excellent finish. The catch is fire risk. Fine chips ignite easily, so the machine needs proper chip management and a no-water rule on the floor. We machine magnesium with dedicated tooling and strict housekeeping.

Some materials simply should not be milled. Very soft elastomers deflect under the cutter. Fully hardened tool steel above 55 HRC is better cut by EDM. And any material supplied as a rough casting with heavy porosity may need a different forming process before machining makes sense.

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  Carbon fiberDiamond tooling; watch delamination at edges.
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  Green ceramicMachinable before firing; grinding after.
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  MagnesiumFast cutting but fire risk; strict chip control.

Start with the function. If the part carries load, the material needs strength and stiffness. If it slides against another surface, wear resistance and friction matter more. If it conducts heat or current, that narrows the list quickly. Write down the two properties that matter most before looking at alloy tables.

Then check the geometry. A long, thin part in stainless will deflect during cutting. A deep pocket in aluminum is easy; the same pocket in titanium may need a different tool strategy and more time. The material and the shape interact. A material that works for a compact bracket may fail for a long shaft.

Finally, check the finish and tolerance callouts. A Ra 0.2–0.8 μm requirement on a large aluminum face is achievable with the right cutter and passes. The same callout on a cast part with porosity is not. Tight tolerance on a plastic part means controlling temperature, not just the machine.

When two materials both work, compare total cost. Titanium may save weight but cost three times as much to machine. Stainless may resist corrosion but double the cycle time versus aluminum. The cheapest material is rarely the cheapest part.

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  Function firstStrength, wear, or conductivity picks the family.
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  Geometry secondThin walls and deep pockets limit the alloy list.
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  Finish thirdTight Ra on porous stock is not realistic.