Polycarbonate Machining Tolerances: What OEM Buyers Should Specify in Drawings
Machining Tolerances Are a Negotiation, Not a Default
Every polycarbonate part you order cut, drilled, routed or CNC-machined is produced to a tolerance — and if you don’t specify it, the factory picks one. OEM buyers who skip tolerance callouts end up with parts that don’t fit, rework costs and production delays. This guide gives the realistic tolerance bands for polycarbonate machining so you can put the right numbers on your drawings.
Standard Tolerance Table for PC Machining
| Operation | Standard tolerance | Precision tolerance | Notes |
|---|---|---|---|
| Cut-to-size (sheet saw) | ±1.0 mm | ±0.5 mm | Edge finish varies by blade |
| CNC routing (profile) | ±0.3 mm | ±0.1 mm | Precision needs rigid workholding |
| CNC drilling (hole) | ±0.2 mm | ±0.05 mm | Drill type and speed critical |
| Laser cutting | ±0.2 mm | ±0.1 mm | Edge discolouration on cut face |
| Thermoforming (shrink) | ±1.5% | ±0.8% | Material and temperature dependent |
| Bending (radius) | ±1.0 mm | ±0.5 mm | Spring-back varies with thickness |
Polycarbonate is not metal: thermal expansion (~0.065 mm/m/°C) and sheet thickness tolerance mean sub-±0.05 mm tolerances are only achievable on small features with controlled environment.
What Affects Machining Tolerance
- Sheet thickness tolerance — extruded PC is typically ±5–10% of nominal thickness (e.g. 10 mm sheet = 9.5–10.5 mm)
- Thermal drift — cutting heat expands the material; measure parts at room temperature
- Workholding — vacuum tables and clamps prevent vibration, which is the #1 cause of tolerance loss
- Tool sharpness — dull tools melt rather than cut, leaving oversized holes and ragged edges
- Batch consistency — resin lots vary slightly; keep +±0.3 mm for multi-batch parts
How to Write Tolerances on Your Drawing
- Call out critical features only — hole centres, mating edges; leave non-critical features loose
- Use GD&T where parts mate — position tolerances beat loose dimension stacks
- Specify edge finish — polished, matte, or “as machined” (affects cost and appearance)
- State the measurement condition — “at 23°C” removes thermal ambiguity
- Ask for a first-article report — before mass production, verify critical dimensions on a sample
FAQ: Polycarbonate Machining Tolerances
What is the standard tolerance for CNC-cut polycarbonate?
±0.3 mm is standard for CNC routing; ±0.1 mm is achievable on critical features with rigid setups. Hole centres hold ±0.2 mm typically.
Can you hold ±0.05 mm on polycarbonate?
Only on small features in a temperature-controlled shop. The sheet’s own thickness tolerance and thermal expansion make sub-0.1 mm across large parts impractical — design around ±0.1–0.3 mm for plastic parts.
Why are my laser-cut holes undersized?
Laser cutting melts a kerf wider than the beam; the hole closes as the material cools. Specify laser kerf compensation (+0.1–0.2 mm) or use CNC drilling for precise holes.
How accurate is thermoformed polycarbonate?
Thermoforming shrinks and thins the sheet unevenly; expect ±1.5% on overall dimensions and more on deep draws. Confirm critical post-form dimensions with a sample.
Common Machining Defects and Their Causes
| Defect | Cause | Fix |
|---|---|---|
| Melting / burnt edges | Dull tool, high feed, no coolant | Sharp carbide tools, lower RPM, compressed-air cooling |
| Oversized holes | Heat expansion during drilling | Pre-drill undersize, finish with reamer at low speed |
| Cracking at corners | Sharp internal corners concentrate stress | Specify ≥ 3 mm corner radii |
| Stress whitening | Excessive clamping pressure | Softer workholding, lower clamp force |
| Warping | Uneven material removal releases internal stress | Machine both sides alternately; anneal for tight parts |
| Fuzzy edges on laser cut | Nitrogen pressure too low | Increase assist gas pressure |
Measurement Best Practices
- Measure at 23°C — a 1 m part measured at 35°C is ~0.8 mm larger; thermal error swamps tolerance
- Use calibrated tools — callipers to ±0.02 mm, CMM for critical features
- Measure in the same direction — extrusion direction vs cross-direction can differ in thickness
- First-article + in-process — verify the first part fully, then check critical dimensions every batch
- Record and share — a first-article report (FAR) with measured values protects both buyer and supplier
When to Choose Each Machining Method
- Laser — fastest for 2D profiles under 10 mm, but leaves a melted edge; good for prototypes and non-critical parts
- CNC routing — best balance of precision, edge quality and cost for production parts; 3-axis covers most needs
- Waterjet — thick sections (15 mm+) without heat-affected edges; slower, higher cost
- Drilling — use step drilling or backing plates to prevent exit-side chipping
- Thermoforming + trimming — for formed parts, trim after forming to control final dimensions
Standards and References
- ISO 2768-1 — general tolerances for linear and angular dimensions (m/c/f/v classes)
- ISO 8015 — GD&T fundamentals (tolerance interpretation)
- ASME Y14.5 — GD&T standard for drawings (North America)
- ISO 1101 — geometric tolerancing
- ASTM D3841 — polycarbonate sheet specification reference for material variability
Get Machined Parts Quoted to Your Tolerance
Send your drawing with tolerance callouts (or ask us to suggest them). Bakway CNC-routes, drills, lasers and forms polycarbonate parts with first-article reports on request — IATF 16949 process control.