Most shops that can hold ±0.02 mm on 7075 aluminum will struggle to hold ±0.1 mm on a PC bracket — not because plastics are hard to cut, but because the rules flip. Metals are stiff and tolerate aggressive clamping; plastics deflect, grow with heat, and dent under jaws that would be gentle on steel. If your prototype is a machined plastic part, these are the rules that actually change.
Deflection Sets Your Tolerance, Not the Machine
Aluminum's elastic modulus is about 69 GPa. Polycarbonate is roughly 2.4 GPa — under identical cutting forces the part deflects nearly 30 times more. That is why thin ribs and walls in plastics should be supported or left thick and finish-machined last. A 1 mm wall in ABS that you machined "successfully" will relax and shift for hours after unclamping as stored stress equalizes.
Practical consequences we design around:
- Hold tight tolerances (±0.05 mm) only on features within about 2–3 material thicknesses of a solid datum; expect ±0.1–0.2 mm on long unsupported spans.
- Let the part rest after roughing — 30 to 60 minutes out of the fixture before finishing often saves a scrapped dimension.
- Thermal expansion is the second thief: PC grows around 65–70 µm/m·°C versus 23 for aluminum. A 200 mm PC part measured at coolant temperature will be a different size on the inspection bench. Measure everything at the same temperature, and tolerance anything long accordingly.
Clamping: Pressure Is the Enemy
Jaws that grip 7075 without a trace will leave visible dents in PC and POM, and worse, squeeze thin sections into a bow that springs back after release. On plastics we routinely use soft jaws bored oversize, vise stops with minimum torque, talon or serrated pads for the roughing cut only, and vacuum or adhesive fixturing for thin plates. A good test: if you can see a jaw mark on the part when it comes off, your finishing operation inherits that distortion.
Tooling: Sharp Beats Coated
Plastics melt locally instead of shear cleanly once the edge dulls — the result is fuzzy burrs, smeared surfaces and dimensional drift as heat soaks in. The tooling rules:
- Sharp, polished, high-clearance edges. Single-flute (O-flute) or two-flute geometry for soft plastics like PE, PP and ABS; chip loads err on the generous side because rubbing generates heat faster than cutting does.
- Uncoated carbide or DLC for most unfilled grades; built-up edge on PE and PP is solved with polish and lubrication, not coatings.
- Glass-filled grades are a different animal. PA6-GF30 or glass-filled PC behave abrasively — expect tool life a fraction of unfilled material, and reach for diamond or DLC coatings. Peeling, delaminated edges at hole exits usually mean the edge is already gone.
- PEEK: machine it sharp and preferably annealed first. PEEK's glass transition sits near 143°C, so localized heat softens the cut zone within seconds; moderate depths and air blast or light mist beat flood coolant for chips that clear instead of packing.
Heat, Chips and Coolant
Plastic chips are stringy, light and static-prone. They pack into pockets, re-cut, and weld to surfaces. Compressed air blast with chip evacuation beats flood coolant in most cases — and some coolants cause environmental stress cracking in PC and ABS over time, a delayed failure that shows up days later as cracks near machined edges. If a machined plastic part must be cleaned, mild aqueous detergent and a blow-off beats solvent wiping unless the solvent compatibility is confirmed.
Surface Finish and Deburring Reality
Fine finishes are achievable — polished tools can push PC and POM to Ra 0.4–0.8 µm, optics-grade even finer. But plastic burrs behave differently from metal burrs: they are elastic, tear rather than shear, and hand-deburring leaves witness marks easily. Sharp tools and climb milling prevent most of the problem; a deburring blade with light passes handles the rest. Do not specify Ra 0.4 on a cosmetic plastic face without confirming the supplier can inspect it repeatably — surface texture on plastics reads differently under profilometry than on metal.
Material Selection Shortcuts
- ABS: cheap, easy to cut, moderate stiffness; the default for cosmetic housings. Watch shrinkage and heat deflection near 90–100°C.
- PC: tough and dimensionally stable, notch-sensitive; stress-relieve after heavy machining.
- POM (Delrin): the machinist's favorite — stiff for a plastic, excellent stability, low friction. Best choice for gears, latches and precision clips.
- PA (Nylon): absorbs moisture and moves with it — 2–3% is possible in humid conditions, so tolerance-critical PA parts should be machined after conditioning or specified in a drier grade. This is the same moisture physics that matters in molding.
- PEEK: when temperature and chemical resistance demand it; budget accordingly and anneal before final passes.
When to Machine Plastic Instead of Printing or Molding
Machined plastic wins when you need material properties identical to the production resin, tolerances tighter than print processes deliver, or 5–50 parts within days. If you are still comparing routes, our 3D printing service covers the quick-iteration end of the spectrum, while plastic injection molding takes over when volumes justify tooling. Machining sits between them: no tooling cost, production-grade material, and per-part cost that stays honest up to a few hundred pieces.
Send us the CAD model and the function the part serves — our CNC machining team will flag the deflection, clamping and moisture issues specific to your material before the first chip is cut, which is exactly when they are cheap to fix.
