Machining Plastics, Wood and Composites
Acetal, nylon, acrylic, PEEK, hardwoods, MDF, carbon fiber and foam: melting, fraying, dust and tool choice.
How these materials differ from metal
- They are soft, so they cut at high spindle speed and fast feed. Cutting speeds of 200–400 m/min and more, with a chip load large enough for each tooth to take a real chip, not rub.
- Heat is the enemy in plastics. Thermoplastics melt at 100–250 °C. A tool that rubs, or a slow feed, melts the plastic, which re-welds behind the tool and gums the flutes.
- Fibers and grain decide the finish. Wood tears out across the grain; fiber composites delaminate and fray; both need the right tool geometry.
- Dust and fumes are the main safety concern, more than chips: see the dust section.
- Material moves. Plastics expand a lot with temperature; wood swells and shrinks with moisture; both can release stress and spring after cutting.
Plastics
| Plastic | Character | Machining notes |
|---|---|---|
| Acetal (POM, Delrin) | Stiff, low friction, dimensionally stable; the easiest engineering plastic | Cuts cleanly with sharp tools and air; produces good threads; keep chips from re-cutting |
| Nylon (PA6, PA66) | Tough, wear resistant; absorbs moisture and moves | Stress-relieve (anneal) before final cuts; sharp tools; leave allowance for moisture swelling |
| Acrylic (PMMA, cast and extruded) | Clear, brittle, melts and cracks | Use a single-flute O-flute bit at high feed; cast acrylic cuts better than extruded; flame or buff to polish |
| Polycarbonate | Tough, clear | Machines like acrylic but does not crack; keep it cool; it stress-cracks with some coolants |
| HDPE, UHMW | Soft, waxy, slippery | Stringy chips; sharp single-flute tools; clamp gently; it moves a lot |
| PVC | Rigid, flame-retardant | Machines well, but heating releases hydrogen chloride: keep it cool, extract fumes; never laser it |
| ABS | Tough, easy | Cuts cleanly; melts if rubbed |
| PEEK, PEI (Ultem), PPS | High-performance, strong, heat resistant | Machine like a metal: carbide, sharp tools, moderate speed, careful clamping |
| PTFE (Teflon) | Soft, slippery, creeps | Sharp tools, light clamping; it deforms under load and moves after cutting |
| Phenolic, G10/FR4, glass-filled | Hard, abrasive, dusty | Carbide or diamond tools; full dust extraction; see composites |
Plastics at a glance
Starting values for a 6 mm tool on a router at about 18 000 rpm. Use the lower values on a light machine.
| Material | Tool | Chip load fz | Coolant | Notes |
|---|---|---|---|---|
| Acetal, nylon | 1–2 flute, polished, O-flute or upcut | 0.05–0.15 mm | Air | Sharp tools; high feed; no coolant needed |
| Acrylic | 1-flute O-flute | 0.08–0.15 mm | Air; mist helps | High feed carries heat away in the chip; slow feed melts |
| HDPE, UHMW | 1-flute, sharp | 0.10–0.25 mm | Air | Very high feed; clamp lightly |
| Polycarbonate | 1–2 flute | 0.05–0.12 mm | Air | Cooler than acrylic; sharp tools |
| PEEK | 2 flute carbide | 0.03–0.08 mm | Air or mist | Moderate speed; stress-free stock |
| G10, FR4 | Diamond-coated or compression | 0.03–0.08 mm | Dry with extraction | Abrasive; very hazardous dust |
Worked example: a 6 mm tool at 18 000 rpm
- The cutting speed is π × 6 × 18 000 ÷ 1 000 = 339 m/min, far above any metal speed, and normal for plastic and wood.
- Acrylic, one-flute O-flute, 0.10 mm/tooth: Vf = 18 000 × 1 × 0.10 = 1 800 mm/min.
- MDF, two-flute upcut or compression, 0.10 mm/tooth: Vf = 18 000 × 2 × 0.10 = 3 600 mm/min.
- Slowing the feed to 600 mm/min in acrylic would reduce the chip load to 0.033 mm: the edge rubs, the plastic melts and re-welds, and the cut is spoiled.
- Many hobby routers cannot reach these feeds; then reduce the spindle speed (to 10 000–12 000 rpm) to keep the chip load, rather than slowing the feed.
Wood and wood products
| Material | Character | Machining notes |
|---|---|---|
| Hardwoods (oak, maple, walnut) | Dense, strong grain | Sharp carbide bits; moderate feed; burns if slow; cut across the grain with light passes |
| Softwoods (pine, cedar) | Soft, resinous, fuzzy | Fast feed; resin builds up on the bit: clean it |
| Plywood | Layers of veneer with glue | Compression bit for clean edges; abrasive glue lines; chip-out on the face |
| MDF | Uniform, no grain, abrasive, very dusty | Carbide; high feed; dust extraction is essential |
| HDF, hardboard, particle board | Hard, abrasive | Carbide; fast feed; edges chip |
| Bamboo, laminated panels | Hard, fibrous | Sharp carbide; compression bit; strong holding |
- Grain direction. Cutting with or across the grain tears the fibers on the exit side. Take a light finishing pass; use a downcut bit on the top face and a compression bit on veneered sheet.
- Burning. Dark marks mean the bit is rubbing: too slow a feed, too high a speed, a dull bit, or a bit loaded with resin. Speed up the feed, lower the speed, and clean or change the bit.
- Moisture. Wood expands and shrinks across the grain by 2–8 % with humidity. Keep stock indoors for a few days and cut parts to fit with a margin.
- Fuzzy grain and tear-out on end grain: use a sharp bit and climb cut, and back the cut with a sacrificial board or tape.
- Dust extraction is required. Wood dust, especially hardwood and MDF dust, is a respiratory hazard and a fire and explosion risk.
Composites and sheet laminates
| Material | Hazard or problem | Approach |
|---|---|---|
| Carbon fiber (CFRP) | Delamination, fiber pull-out; abrasive; the fine dust is conductive and hazardous | Diamond-coated or compression routers, sharp tools, backing material; vacuum extraction; keep dust out of electronics |
| Fiberglass (GFRP), G10/FR4 | Very abrasive; glass dust irritates skin, eyes and lungs | Carbide or diamond tools; wet cutting or full extraction; respirator, gloves and long sleeves |
| Aluminum composite panel (Dibond) | Aluminum skin over a plastic core | Single flute O-flute; do not cut through the back skin; tape to a flat bed |
| Honeycomb and sandwich panels | Crushes; frays | Very sharp, fast tools; support the core |
| Corian and solid surface | Dense, abrasive | Carbide, 2-flute, fast feed with extraction |
| Acrylic mirror, laminates | Coating chips | Compression bit, backing, low load |
- Tools wear fast in glass and carbon fiber: plan on diamond-coated or polycrystalline tools for volume.
- Delamination (peeling of the outer ply) is worst at the exit side: back the part with a sacrificial board, use a compression bit, and keep tools sharp.
- Dust control. The dust is a serious health hazard. Cut wet or with a vacuum at the tool, wear a P3/N100 respirator, and do not blow it with air.
- Seal the edges of cut composites that will be exposed to moisture.
Foam and soft materials
- Rigid foam (XPS, EPS, polyurethane tooling board): cut with a sharp single- or two-flute bit at high feed and moderate spindle speed. The dust is fine and static; use a vacuum.
- Cork, rubber, felt and leather: cut with a very sharp tool, or a knife or laser rather than a bit.
- Soft materials push away from the tool; hold them flat, and use a downcut tool to keep them in place.
Router bits and tool geometry
| Bit | Best for | Watch out for |
|---|---|---|
| Upcut spiral | Clearing chips from deep slots and pockets; clean bottom edge; metals and plastics | Lifts the top face and may fray it; can lift a loose sheet |
| Downcut spiral | Clean top edge; holds sheet down | Packs chips into the slot, which heats; use shallow passes |
| Compression | Laminates, plywood, melamine: both faces clean | Needs the cut deeper than the up-cut section, to work as a compression cut |
| Straight flute | Plastics and thin sheet; low-lift | Higher cutting noise; less chip clearance |
| O-flute (single flute, polished) | Acrylic, plastics, aluminum composite | Needs high chip load; one flute only |
| Surfacing (spoilboard) bit | Flattening the spoilboard and large faces | A large cutter; light cuts |
| V-bit, engraving bit | Signs and engraving | Cutting speed depends on the contact diameter |
Heat, melting and movement
Plastics expand 5–10 times as much as metal. A part cut warm, or in a warm shop, is smaller when it cools. For 100 mm of material warmed by 20 °C:
| Material | Expansion coefficient | Growth over 100 mm |
|---|---|---|
| Acetal (POM) | 110 × 10−6/°C | 0.220 mm |
| Nylon 6/6 | 80 × 10−6/°C | 0.160 mm |
| Acrylic (PMMA) | 70 × 10−6/°C | 0.140 mm |
| HDPE | 200 × 10−6/°C | 0.400 mm |
| Aluminum | 24 × 10−6/°C | 0.047 mm |
| Steel | 12 × 10−6/°C | 0.023 mm |
- Let the part cool before the final measurement, and measure at room temperature.
- Stress-relieve nylon, acetal and polycarbonate before finish machining: cut roughly, rest or anneal, then finish. Stock from the supplier may already be stress-relieved.
- Moisture. Nylon absorbs up to 2–3 % moisture and grows by 0.2–0.5 %; keep it dry and finish to size in the environment it will live in.
- Keep the cut cool with high chip load, air or mist; do not use water-based coolant on stress-sensitive plastics such as polycarbonate.
- Plastics do not weld to the tool in the same way as metal, but molten plastic re-welds behind the tool: clear the chips and keep the tool sharp.
Workholding
- Vacuum tables and double-sided tape are the usual hold for sheet; see Workholding and Fixturing.
- Clamps must not squeeze. Plastics and thin wood distort, and a part clamped hard springs back when released. Use many light clamps or tape.
- Tabs hold parts in a sheet; use small tabs, or an onion-skin final layer.
- Spoilboard. Cut slightly into it so the part separates cleanly; skim it flat regularly.
- Support thin parts from below to avoid vibration and chatter.
Dust, fumes and safety
| Material | Hazard | Precaution |
|---|---|---|
| Wood dust (hardwoods, MDF) | Respiratory sensitizer; hardwood dust and MDF dust are carcinogenic; fire and explosion hazard | Dust collector with a good filter and a cyclone, a respirator (P2/N95 or better), and regular cleaning; never blow it with air |
| Plastic dust and fumes | Fumes from hot plastic; PVC makes hydrogen chloride | Keep the cut cool; extract; never cut PVC with a laser |
| Glass fiber, carbon fiber | Irritant fibers; carbon dust is conductive | Vacuum at the tool, wet cutting, respirator (P3/N100), gloves and sleeves |
| Phenolic and epoxy laminates | Allergenic dust and fumes | Extraction; avoid skin contact |
| Magnesium-based and metallic composites | Fire | See Machining Metals |
| Noise | High-speed routers are loud (90–100 dB) | Hearing protection |
- Extraction is not optional for MDF, composites and fine dust. A shop vacuum with a cyclone separator and a HEPA filter is the minimum.
- Dust fire. Fine dust in a collector can ignite from a spark; use a metal duct, ground the system, and empty the collector often.
- Eye and ear protection and a mask for every cut. See Workshop Safety and Troubleshooting, Accuracy and Safety.
Related
- CNC Machining: the overview of this guide.
- Machining Metals: machining metals, coolant and hazards.
- Workholding and Fixturing: vacuum, tape and tabs.
- Laser Cutting: cutting sheet with light, and the materials that are not safe.
- Workshop Safety.
FAQ
Why does my acrylic melt and stick to the bit?
The feed is too slow (so the bit rubs), the speed is too high, or the bit is dull. Use a single-flute O-flute bit, a high feed (0.08–0.15 mm per tooth), and air to cool.
How do I get a clean edge in plywood?
Use a compression bit (cuts down on the top and up on the bottom), a sharp tool, and enough feed to avoid burning. Support the underside.
Can I cut carbon fiber on a hobby router?
Yes, with sharp diamond-coated or compression bits and strong dust control, but the dust is a hazard and conductive. It is easier to cut it with a waterjet or send it out.
Why is my nylon part the wrong size?
Nylon absorbs moisture and expands with temperature, and it releases stress after cutting. Stress-relieve before finishing, keep it dry, and measure at room temperature.
Do I need a dust collector for MDF?
Yes. MDF dust is very fine, irritates lungs and is classed as a hazard; extraction at the tool and a good filter are needed.
References
- Plastics machining guides from stock-shape suppliers: Ensinger, Quadrant (Mitsubishi Chemical), Curbell Plastics, Professional Plastics.
- Router-bit makers’ technical notes: Amana Tool, Whiteside, Onsrud (chip load charts and bit selection).
- Composites machining guidance: Toray, Hexcel and ASM Handbook Vol. 21 (Composites).
- HSE (UK) and OSHA guidance on wood dust, glass and carbon fiber, and silica.
- Oberg, E. et al., Machinery’s Handbook, 31st ed., Industrial Press.
Last reviewed: October 2026. Cutting data are starting points: machines, tools and materials differ, so test on scrap and follow the tool maker’s data.