Guide
Plastic machining tolerances
Why the tolerance a machined plastic part can hold depends on the plastic, and what moves its size after it leaves the shop.
Critical dimensions
±0.002"–±0.005"
Depending on the plastic.
By plastic
How common plastics hold their size
Three things decide how closely a plastic holds size: how much it grows with heat, how much water it takes up, and how firmly it stands up to the cutter. Listed from the most stable to the least.
| Plastic | Expansion* | Water, 24 h | Under the cutter | How it holds size | |
|---|---|---|---|---|---|
| 01 | PEEK | 2.6 | 0.1–0.5% | Stiff | Among the most stable, including in heat and steam. |
| 02 | PET | 3.9 | 0.1% | Stiff | Low expansion and low water uptake, so it holds size in wet service. |
| 03 | Acetal | 6.1–6.8 | 0.20–0.25% | Stiff | Cuts cleanly and takes up little water. The common choice for close-tolerance parts. |
| 04 | Nylon | 4.5 | 1.2% (cast 0.6%) | Stiff when dry | Stable when dry, but grows as it takes up moisture. |
| 05 | UHMW | 11.1 | Slight | Soft | Expands the most with heat, and gives under clamping. |
| 06 | PTFE | 8.9 | <0.01% | Soft | Compresses under the tool and creeps under a sustained load. |
*Coefficient of linear thermal expansion, in/in/°F × 10⁻⁵. Steel is about 0.65 and aluminum about 1.3. Typical published values. Grades vary.
Filled grades Glass- and carbon-filled grades of these plastics expand less and are stiffer, so they generally hold size better than the unfilled plastic.
Temperature and length
How much a plastic part moves with temperature
Drawing dimensions apply at room temperature, 68°F, unless the drawing says otherwise. A part that runs hotter or colder than that is a slightly different size in service, and plastic moves several times more than metal.
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Short features barely move
A 30°F change moves a 2" acetal dimension by about 0.004".
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Long lengths move a lot
The same change moves a 48" UHMW length by about 0.16".
That’s why a bore, a slot or a hole size can hold a close tolerance, while the overall length of a long plastic part usually carries a wider one.
Replacing a metal part with plasticSize change calculator
How much will it move?
Change in size 0.0037"
Beyond ±0.002", within ±0.005"
- The same dimension in aluminum
- 0.0008"
- The same dimension in steel
- 0.0004"
Change = length × expansion × temperature change. Scale is logarithmic. Typical published expansion values; grades vary.
The calculator needs JavaScript. The figures shown are for a 2" acetal dimension and a 30°F change.
Beyond temperature
Moisture, stress and softness
Three other things move a plastic part’s size, during machining or after it ships.
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Moisture
Nylon takes up water from humid air and washdown, and grows slightly as it does. Cast nylon takes up less than extruded nylon 6/6. Acetal, PET, PEEK, UHMW and PTFE barely change.
Acetal vs nylonMatters most for Nylon parts in wet or humid service
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Internal stress
Rod and plate carry some stress from how the stock was made. When heavy machining takes a lot of material from one side, that stress can bow a large flat part. Stress-relieved stock keeps it in check.
Matters most for Large plate parts and deep pockets
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Softness
UHMW and PTFE give under the cutter and under clamping, then spring back when released. PTFE also creeps under a sustained load. That’s why soft plastics sit toward the ±0.005" end of our range, and stiff plastics such as acetal, PET and PEEK toward the ±0.002" end.
Matters most for UHMW and PTFE
In our Aurora shop
Holding the tolerance
- Roughing, then finishing passes, so heat and stress come out before the final cut.
- Workholding that supports the part without squeezing it out of shape.
- Light passes on thin walls and floors.
- Parts measured to the drawing in our Aurora shop.
Questions
Tolerance FAQs
Something else? Call (630) 904-2525.
Yes. Parts are machined and inspected in our Aurora shop, and measured to the dimensions and tolerances on your drawing.
Quality and documentationYes. We measure the sample and machine the new part to its sizes, in the plastic you choose or one we suggest.
Reverse engineeringHave a drawing with tight tolerances?
Send it as it is. We machine to the tolerances on your drawing, and if one is tighter than we can hold in that plastic, we’ll tell you when we quote.