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In general production, cnc plastic machined parts typically hold a dimensional accuracy of ±0.05 mm to ±0.1 mm. On critical features machined from dimensionally stable grades such as POM (Delrin) and PEEK, ±0.01 mm to ±0.025 mm is achievable when cutting heat, fixturing, and inspection conditions are properly controlled. The exact figure is never set by the machine alone. It depends on the plastic grade, part geometry, wall thickness, and even the temperature at which the part is measured. This guide explains what those numbers mean in practice and how to specify them on your drawings.
Dimensional accuracy describes how closely a finished part matches the dimensions on the drawing. It is expressed through tolerance: the permitted deviation above and below a nominal size. A bore drawn as 10.00 mm ±0.05 mm is acceptable anywhere between 9.95 mm and 10.05 mm. Accuracy claims without a stated tolerance are meaningless, which is why professional machining quotes always tie dimensions to a drawing standard or a feature-specific limit.
For plastics, many drawings reference ISO 2768-m (medium) as the general tolerance for non-critical dimensions, then add tighter feature-level limits only where function demands them: bores, pin locations, sealing faces, and alignment features.
Three working ranges cover most real projects:
For comparison, metal parts can be held to a few microns because metals are dimensionally stable. Plastics behave differently: they expand with heat, absorb moisture, and relax internal stress after cutting, so tolerance targets must be material-specific rather than copied from a metal drawing.
1. Material behavior. Engineering plastics have a coefficient of thermal expansion of roughly 50–150 µm/m·°C, about 4–10 times higher than aluminum. A nylon part measured at 30°C can differ by nearly 0.1 mm per 100 mm of length from the same part measured at 20°C. Hygroscopic grades add another variable: nylon can absorb 2–9% moisture by weight, changing dimensions after machining as it equilibrates to ambient humidity.
2. Cutting heat. Plastics conduct heat poorly (typically 0.1–0.4 W/m·K), so heat concentrates at the tool edge. Above the softening point the material smears instead of cutting cleanly, destroying both finish and size. Sharp tools, high spindle speeds, light finishing passes, and air-blast cooling are the standard countermeasures.
3. Part geometry. A stable block with short holes holds tolerance far more easily than a thin cover with long slots, snap features, or deep bores. Thin walls deflect under clamping and cutting forces, so the measured size can change the moment the part is unclamped.
4. Residual stress. Extruded or pressed plastic stock carries internal stress. Asymmetric material removal releases it and the part warps. Stress-relieved (annealed) stock, balanced machining passes, and rest time between roughing and finishing keep dimensions stable.
5. Measurement conditions. Precision plastic parts should be inspected at the 20°C reference temperature, after the part has been conditioned to the state in which it will be used. A dimension checked immediately after machining on a warm part is not the dimension the customer will receive.
Material choice is the single biggest lever on dimensional accuracy. The table below gives realistic working values for common grades:
| Material | Standard Tolerance | Precision Achievable | Main Limiting Factor |
|---|---|---|---|
| POM (Delrin / Acetal) | ±0.05 mm | ±0.01–0.02 mm | Thermal expansion, fixturing |
| PEEK | ±0.05 mm | ±0.01–0.02 mm | Stress relief, cost control |
| Polycarbonate (PC) | ±0.05 mm | ±0.02–0.03 mm | Heat sensitivity |
| Nylon (PA6 / PA66) | ±0.1 mm | ±0.025–0.05 mm | Moisture absorption |
| ABS | ±0.1 mm | ±0.05 mm | Material compliance |
| PTFE | ±0.1 mm | Case-dependent; needs special control | Creep, softness |
At ANOK Precision Manufacturing, accuracy on plastics starts with material knowledge. Our plastic cnc machining work covers ABS, Nylon (PA6, PA66, and glass-filled grades), POM, PEEK, PET, PC, PTFE, PMMA, Delrin, and ULTEM, from one-off prototypes to high-volume production. Each grade gets its own cutting strategy: sharp polished tooling, high spindle speeds, light finishing passes, and air cooling to keep heat out of the workpiece.
Process discipline does the rest. Roughing and finishing are separated so internal stress can relax before final passes; 4-axis and 5-axis machining centers keep related features on the same datum in a single setup; and every job runs under an ISO 9001:2015 quality system with dimensional inspection on calibrated equipment. As a precision cnc machining factory serving medical, aerospace, automation, and food equipment customers, we report dimensions against the drawing standard and inspection conditions agreed at quotation, so the tolerance you approve is the tolerance you receive.
The dimensional accuracy of CNC plastic machined parts is not a single number. Expect ±0.05–0.1 mm as a dependable general range, and ±0.01–0.025 mm on critical features when the material is stable and the process is controlled. The shortest path to the right accuracy is a drawing that tolerances by function and a machining partner who understands how each plastic behaves. Send your drawings to ANOK for a free DFM review and quotation, and we will recommend the material and tolerance scheme that fits your application.