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How to Improve Accuracy in CNC Machining Plastic Parts

2026-02-14
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How to Improve Accuracy in CNC Machining Plastic Parts? | Practical Factory Guide
Introduction – Why Plastic CNC Parts Often Miss Tolerance

If you have ever machined aluminum successfully and then switched to plastic CNC machining, you probably noticed something frustrating:

The same program suddenly produces:

  • oversize holes

  • warped surfaces

  • melted edges

  • ±0.2 mm deviation

We faced exactly this issue three years ago when producing POM (Delrin) precision gears for an automation customer.
The drawing tolerance was ±0.02 mm, but our first batch averaged ±0.08 mm — 4× out of spec.

After dozens of trials and real production adjustments, we reduced deviation to ±0.015 mm stable mass production.

In this guide, we’ll share the actual factory methods, tested parameters, and tooling solutions we use daily to improve:

  • plastic machining accuracy

  • dimensional stability

  • surface finish

  • repeatability

So you can avoid scrap and hit tolerance the first time.


H2 – Why Plastic Parts Are Harder to Machine Than Metal?

Before fixing accuracy, you must understand the physics.

Plastics behave completely differently from metals:

Property Metal Plastic
Thermal expansion Low 3–8× higher
Stiffness High Flexible
Heat resistance High Easy melting
Chip removal Stable Sticky/stringy
Stress recovery None Elastic rebound
Real impact in machining:
  • Heat → expansion → oversize parts

  • Clamping → deformation → shape spring-back

  • Tool friction → melting → poor tolerance