Machining Plastics & Composites

Machining plastics and composites is the craft of cutting the lightest end of the material range, and it is a craft with its own rules, because the plastics do not machine like metal at all. Where a metal chip is strong, hot and predictable, a plastic chip is soft, heat-sensitive and springy: plastics melt and smear under a dull tool, deflect and burr under the lightest pressure, and recover their shape after the cut, so a part machined like metal comes off the machine with melted edges, torn fibres and dimensions that are not what the tool cut. The shop that machines plastics and composites well treats them as what they are — soft, heat-sensitive, abrasive or brittle by turns — and cuts them with sharp, keen tools, controlled heat, gentle clamping and a respect for their weaknesses. This entry sets out how the thermoplastics and the fibre composites are machined.

How plastics differ from metals

The differences that govern plastic machining are three, and the first is heat. Plastics are poor conductors with low melting or softening points, so the heat of cutting stays where the tool rubs, softening and melting the material into smears and burrs; the metal’s habit of carrying the heat away in a strong hot chip does not happen here. The second is stiffness: plastics are soft and elastic, so they deflect under cutting pressure, spring back behind the tool and tear into burrs instead of shearing cleanly, and they distort under a clamp as easily as under a cutter. The third is their variety: a thermoplastic like acetal machines like a soft brass, while a thermosetting composite of carbon fibre and resin is abrasive, brittle and entirely unlike it. The cutting geometry that suits them all, though, is the same first rule: sharp, high-rake tools that shear the soft material cleanly, cutting with low force and little heat, run at high speed and kept always cutting — the heat of the cut is the enemy, and the sharp edge is its cure.

Machining the thermoplastics

The common engineering thermoplastics each have their own character, and the shop reads it before cutting. Acetal (POM), the “brass of plastics”, is stiff, stable and low in moisture, and it machines cleanly with sharp tools into accurate, low-friction parts — gears, bearings, bushings and insulators. Nylon is tough and wear-resistant but absorbs moisture and swells, so it is machined and then sized with that in mind. Acrylic (PMMA) is glass-clear and brittle: it cuts well with sharp, polished tools but crazes, chips and cracks at an edge, especially where a drill breaks through, so exits are supported and the material is kept cool. Polycarbonate is tough and prone to stress-cracking, and the high-temperature engineering plastics — PEEK, PEI and the like — machine like a stiff, slightly abrasive metal and hold their accuracy at temperatures that melt the ordinary plastics. Across all of them the practice is the same: sharp high-rake tools, climb milling for a clean edge instead of a torn one, coolant or an air blast to carry the heat away, and thin, well-supported work — plastic parts distort under the cutting force of a heavy cut and spring back out of tolerance when the tool passes.

Machining the fibre composites

The fibre composites are a different world, and the shop that machines carbon-fibre (CFRP) and glass-fibre (GFRP) laminates meets a material that is at once strong, abrasive and delicate. The carbon fibres that give the laminate its strength are harder than the tool that cuts them: they wear a carbide edge rapidly, so production trimming of carbon fibre is done with polycrystalline diamond (PCD) tooling, and even carbide is reserved for short runs. The composite’s weakness is its layer structure: where a drill or a cutter breaks through the far face it does not cut the fibres but pushes them, delaminating the laminate and leaving a torn, furry exit, so holes in composites are drilled with specialist tooling and the exit face is supported, as the drilling practice of this wiki describes. The cut itself produces a fine, conductive dust that is a health hazard and must be extracted, the part is usually machined dry (cutting fluid wicks into the laminate and contaminates it), and the laminates are trimmed and profiled at high speed on the router or the machining centre with the sharp, diamond-edged tools that can part the fibres cleanly.

Workholding and the gentle clamp

Plastics and composites are held as the delicate work of the shop is held, because the materials that deflect under a cutter deflect under a clamp too. A plastic block clamped in hard jaws is squeezed and springs when released; a laminate clamped hard at its edges bows in the middle; so the work is held gently and evenly — soft jaws machined to the part, a vacuum table for sheet and laminate, support beneath the thin sections — with no more force than keeps the work still. The clamping is the quiet half of plastic machining: the sharp tool cuts the material cleanly, but the workholding decides whether the part keeps the shape it was cut in. And because plastics relax and release their moulded-in stress when material is cut away, a part is often roughed, allowed to settle, and finished, so that the distortion happens before the final cut rather than after it.

Plastics and composites in the shop

Plastics and composites earn their place by the properties no metal offers: the clarity of acrylic, the low friction of acetal, the electrical insulation of PEEK, the stiffness and lightness of carbon fibre, and the corrosion-free service of them all. They are machined by the practice this entry has set out — sharp high-rake tools that shear without heat, controlled heat and speed, climb milling for clean edges, supported exits where drills break through, PCD where carbon fibre wears the edge, and gentle clamping that holds the work without crushing it. They are not difficult materials in the sense of the hard alloys — the tools cut them easily — but they are unforgiving of the habits learned on metal, and the shop that cuts them as if they were aluminium gets melted edges, delaminated holes and parts that are not to size. Cut as their nature demands, they give the cleanest, lightest, most precise work in the shop, and they complete the materials picture: at the far end from the nickel superalloys that fight the cutter with strength, the plastics yield to it — and must be coaxed all the same.