All three early plastics machine well with sharp high-speed-steel tools at a low cutting speed, about half what you would use for brass. Celluloid takes no water coolant because water makes it swell and it must never be cut near sparks or flame, and Bakelite makes abrasive dust that needs a mask, because phenolic dust is harmful to breathe.
Part of Plastics and Polymers
Plastic machining is the cutting, drilling, turning and filing of finished plastic stock to final shape.
Rules From The Carrier
Plastics from Scratch gives three:
- Use sharp high-speed-steel (HSS) tools.
- Run at low cutting speed, half what you would use for brass.
- No coolant for celluloid: water causes swelling. Bakelite produces abrasive dust, so wear a mask.
Why Low Speed And Sharp Tools
Plastic conducts heat poorly, so the heat of cutting stays at the edge. At high speed it softens or melts a thermoplastic into a gummy smear, burns a casein piece and, for celluloid, brings the heat of a cut close to a fire hazard. A sharp tool at low speed cuts a clean chip. A dull tool rubs, heats and tears.
Material By Material
Casein. Machines like horn: drill, turn and cut. Keep it cool and dry, because heat scorches it and water softens it. A pole lathe suits buttons and beads: see Pole Lathe Build and Treadle Lathe. See Casein: Applications and Limits for what the material can do.
Celluloid. Cut by hand or at slow speed, with light cuts, and no sparks, no flame, no power-tool friction heat. Collect shavings and dust and take them out of the workshop, because they are fuel. Do not machine a piece showing decay. See Celluloid Safety.
Bakelite. Hard, brittle and abrasive, so tools dull faster than on wood; sharpen often. Support the work to avoid chipping, and drill with a backing block to prevent breakout. Sanding or machining cured phenolic makes fine dust to avoid breathing, as Phenolic Resins warns. Use a mask, preferably work outdoors or with extraction, and clean up dust wet or by vacuum rather than blowing it about.
Thermoplastics (salvaged PE, PP, PVC). They melt and gum under a hot tool. Cut slowly and cool the tool in air. PVC and unknown plastics are not machined hot: see Fume Safety.
What You Need
- Sharp HSS tools, and a way to sharpen them often
- A slow, controllable drive: pole lathe, treadle lathe or hand tools
- A mask, and ideally a dust-collection route
- A clamp or fixture that grips without cracking brittle stock
Worked Example: Setting The Speed
The carrier gives the rule as half the speed you use for brass, but it gives no brass speed, so measure your own. If your lathe turns a brass rod cleanly at N rpm for a given diameter, run the plastic at N / 2 for the same diameter. A pole lathe stroke rate is set by the operator, so the equivalent is a slower, lighter treadle with a lighter cut. Halving again is safe if the plastic starts to gum.
Common Mistakes
- Cutting dry celluloid fast — friction heat near a flammable plastic.
- Water on celluloid — it swells.
- Dull tools — tearing, heating and chipping.
- No mask for Bakelite — dust inhalation.
Dust, heat and fire
Bakelite dust is abrasive and harmful to the lungs; wear a mask and keep dust out of the air. Celluloid shavings burn violently, so no sparks or flame and no friction heat. Heated thermoplastic gives off fume. Wear eye protection against chips, and keep long hair and loose clothing away from the lathe.
What’s Next
- Learn next: Buttons, Handles and Hardware — what machining is for
- Combine with: Compression Mold — the parts you will finish