What Materials Can a CNC Machine Cut?

• 1
  CNC machine cut materials: the short answer
• 2
  Aluminum, stainless steel, and titanium
• 3
  Plastics and polymers on a CNC machine
• 4
  Carbon fiber, Inconel, and other specialty stock
• 5
  How material choice changes the part, the cost, and the finish
• 6
  CNC machine cut materials compared

A CNC machine cuts by moving a hardened tool against stock and shearing away chips. That single action works on anything softer than the tool and hard enough to hold its shape while being cut. In practice, the list of CNC machine cut materials covers metals, plastics, composites, and a few ceramics and wood products. What changes is not the principle but the parameters.

Machinability is a shop-floor term, not a lab one. It combines cutting speed, tool wear, chip formation, and how much the part moves under clamping and heat. Two alloys with the same hardness can behave very differently because one galls on the tool and the other breaks chips cleanly. That is why we quote by grade, not by material family.

The practical ceiling is set by three things: tool material, machine rigidity, and heat removal. Carbide tooling handles hardened steel up to roughly 45 HRC in milling. Beyond that, the tool edge breaks down faster than the part can be finished. Heat is the other wall. Titanium and Inconel carry heat into the cutting edge instead of into the chip, so feeds slow down and coolant becomes mandatory.

Below we group the materials we run most often and explain what each one does well, what it does badly, and when a different process makes more sense.

Aluminum is the default for machined parts. Grades 6061 and 6061-T6 cut fast, hold ±0.005 mm without drama, and take anodizing well. 7075 adds strength for aerospace brackets but chips harder on deep pockets. 2024 machines well but corrodes without coating. Copper grades like C101 and C110 cut like soft aluminum but gum up tools, so we use sharp geometry, high rake angles, and air blast instead of flood coolant.

Stainless steel splits into two camps. Free-machining 303 turns beautifully and is the right pick for shafts and fittings. The 304 and 316 grades resist corrosion and weld well, but they work-harden the moment the tool rubs instead of cuts. Keep the feed per tooth high and never dwell. 17-4PH (SUS630) machines in the annealed state and gains strength after heat treatment, which suits medical and aerospace parts.

Titanium TC4 (Ti-6Al-4V) is the grade most engineers ask about. It cuts at roughly one-third the surface speed of 6061, and the chips do not carry heat away. Flood coolant and rigid setups are not optional. Thin walls below 1 mm will move. We plan extra passes and leave stock for a finishing cut.

Tool steel and 4140/4340 alloys are common for molds and structural parts. Pre-hardened 4140 at 28–32 HRC machines with carbide at moderate speeds. Anything above 45 HRC usually goes to grinding or EDM instead.

Plastics machine fast but they are unforgiving in different ways. POM (Delrin) is the best-behaved engineering plastic. It holds tight tolerance, chips cleanly, and resists moisture. ABS and PC are softer and cheaper but tend to smear if the tool gets hot. Sharp two-flute cutters with high helix and generous chip clearance fix most of that.

PEEK and PA (nylon) are used for high-temperature and wear parts. PEEK machines at high spindle speed but is abrasive and expensive, so we treat the material cost as part of the quote. Nylon absorbs moisture from the air and can grow after machining, which matters for close-tolerance fits. Dry it before cutting and finish the part after a stabilization period.

PMMA (acrylic) is clear and brittle. It cuts well but cracks easily at drilled holes and thin edges. We use slow feed, high rake, and support the exit side with backing. PP and HDPE are chemically resistant and cheap, but they are gummy and deflect under clamping pressure, so light passes and soft jaws are standard practice.

One rule covers most plastics: keep the tool cool and the chip clear. Recutting a chip is what causes melted edges, not the cutting itself.

Carbon fiber reinforced polymer cuts as an abrasive stack of fiber and resin. The resin smears if the tool rubs, and the fibers fray if the tool is dull. Diamond-coated or polycrystalline diamond (PCD) tooling is standard for production. We cut dry or with light mist, use down-cut geometry on the top face, and support the underside to prevent delamination.

Inconel and other nickel alloys are the hardest common materials on a milling machine. They work-harden aggressively and hold heat at the edge, so surface speeds drop to 20–30 m/min in many cases. Each pass must cut below the hardened layer from the previous one. This material eats tooling, and the quote reflects that.

Magnesium AZ31B and AZ91D cut easily and fast, but the chips are flammable. We never use water-based coolant, and we keep the work area clean of fine swarf. The trade-off is worth it when weight matters, as in drone frames and handheld tool housings.

Beryllium copper sits in its own category. It machines well and conducts heat and electricity, but the dust is toxic. Enclosed machines, HEPA extraction, and wet cutting keep the operation safe.

Material choice drives three things at once: how the part performs, how much it costs to cut, and what finishes are available. Harder and tougher materials cut slower, wear tooling faster, and need more rigid fixturing. That is the entire cost story in one sentence.

Finish availability depends on the substrate. Aluminum takes hardcoat and clear anodizing, and laser marking holds at 1.5 mm character height. Stainless takes passivation, bead blasting, and electroless nickel. Plastics take tumbling, brushing, and polishing, but not anodizing or plating. Carbon fiber usually stays as-machined or gets a matte clear coat.

Tolerance is not a single number across all materials. We hold ±0.005 mm on metals under stable conditions. Plastics and composites are temperature and moisture sensitive, so we quote practical tolerances based on the grade and the wall thickness.

If the goal is a prototype in days, aluminum and POM get you there fastest. If the goal is a production part in a high-temperature or corrosive environment, the material picks itself, and the machining plan follows.