Plastic CNC Processing Guide

• 1
  Why plastic cuts differently from aluminum
• 2
  Material behavior that decides the cut
• 3
  Feeds, speeds and fixturing in practice
• 4
  What tolerance is realistic on plastic
• 5
  CNC versus injection molding versus 3D printing

A plastic CNC processing guide has to start with heat, not with cutting tools. Aluminum carries heat away in the chip and through the workpiece. Plastics do not. Thermal conductivity of POM or ABS is roughly one-thousandth that of aluminum. Heat generated at the cutting edge has nowhere to go. It stays in the chip, in the workpiece, and on the tool.

That single fact drives almost every parameter decision. Cutting speed goes up, tool life drops and the part starts to smear. Feed per tooth goes too high and the edge chips the workpiece instead of shearing it. The window between a clean cut and a melted one is narrow, and it moves with every polymer grade.

Plastics also spring back. Elastic recovery of POM, PA and PP means the finished surface sits slightly behind where the tool passed. A sharp, polished, positive-rake cutter reduces that rebound. A dull tool rubs instead of cuts, which raises heat and makes the rebound worse.

Chip evacuation matters more than most people expect. Long stringy chips from PP or HDPE wrap around the tool and recut, which doubles the heat in the cut. Air blast or high-pressure coolant clears them, and it also cools the part without thermal shock.

• 1
  Low thermal conductivityHeat stays in the cut zone instead of leaving with the chip.
• 2
  Elastic recoveryPOM, PA and PP rebound after the edge passes, so the finished surface differs from the programmed path.
• 3
  Stringy chipsPP and HDPE need air blast or high-pressure coolant to stop recutting.

ABS is the forgiving one. It machines with general-purpose carbide, holds a decent edge, and accepts most surface finishes. Use it for housings, covers and prototypes where the part needs to look right and fit right, not survive chemical exposure.

PC and PMMA are the optical and impact grades. Both are notch-sensitive, so sharp internal corners become crack starters. Corner radii of at least 0.5 mm, and drilling with a pilot then a reamer rather than a single plunge, keeps them intact. PMMA polishes to an optical finish with the right sequence but will stress-craze if the coolant is wrong.

POM is the bearing and gear material. It machines beautifully, holds ±0.005 mm on a stable setup, and has low friction. The trade-off is that it does not bond well and it releases formaldehyde if overheated, so shop ventilation matters during heavy cuts.

PEEK, PA and carbon-fibre grades sit at the other end. PEEK needs sharp tooling and generous coolant because it is abrasive and expensive to scrap. Carbon-fibre reinforced stock wears carbide fast, so diamond-coated tools are the practical choice. PA absorbs moisture and will move after machining unless it is dried and kept dry.

• 1
  ABSGeneral-purpose carbide, forgiving, good for housings and covers.
• 2
  PC and PMMANotch-sensitive; add radii and drill in stages.
• 3
  POMHolds tight tolerance, low friction, ventilate during heavy cuts.
• 4
  PEEK and carbon fibreAbrasive; diamond-coated tooling and coolant control.

As a starting point, run plastics faster in surface speed and lighter in chip load than aluminum. Spindle speeds in the 8,000 to 24,000 rpm range suit small-diameter cutters, with feed per tooth kept low so the chip carries heat away without overloading the edge. Every grade moves that window, so treat these as a first trial, not a fixed recipe.

Coolant choice is not cosmetic. Compressed air handles most ABS and POM work. Soluble oil or a mist suits PC and PMMA because it cools without flooding the part. Water-based flood coolant can stain or craze some grades, so confirm compatibility before the first cut.

Workholding is where plastic jobs fail quietly. Clamping force distorts thin walls and leaves a spring-back error once the vise opens. Use soft jaws machined to the part profile, vacuum chucks for flat panels, and light passes to keep the load low. For long parts, support the middle with a steady or a sacrificial block.

Program the toolpath to enter the material gradually. Ramp or helical entry beats a straight plunge, which pushes the tool straight down into a low-conductivity material. Finish passes should be light and climb-milled to reduce the rebound mark on the final surface.

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  Surface speedHigher than aluminum; small cutters at 8,000–24,000 rpm.
• 2
  CoolantAir for ABS and POM; mist or soluble oil for PC and PMMA.
• 3
  WorkholdingSoft jaws and vacuum chucks to avoid distortion.
• 4
  EntryRamp or helical, never a straight plunge.

Plastic moves after the machine stops. Moisture uptake, thermal contraction and internal stress release all change the dimension between the last cut and the inspection bench. That is why a plastic part rarely holds the same tolerance band as an aluminum part of the same geometry.

General machined features on stable grades like POM and ABS can meet ±0.05 mm without special effort. Tight features, thin walls and long unsupported spans need to be reviewed case by case. On our five-axis centers we work to ±0.005 mm on metals, and we apply the same metrology discipline to plastics, but the achievable band depends on the grade and the wall.

The most common tolerance problem is not the machining, it is the drawing. A ±0.02 mm callout on a 2 mm PC wall is a design problem, not a machining problem. Relax it, thicken the wall, or add a rib. Those changes cost less than chasing a number the material will not hold.

• 1
  Stable gradesPOM and ABS hold ±0.05 mm on general features.
• 2
  Thin wallsDistortion dominates; tolerance must be reviewed per part.
• 3
  Drawing reviewRelax, thicken or rib before trying to machine to an impossible callout.