The most common over-specification we see is a plastic part quoted to metal tolerances. It is not that the machine cannot hold them — it is that the material will not stay there afterwards.
Plastics move with temperature roughly ten times as much as steel. A 200 mm PEEK part will change dimension by around 0.3 mm over a 40 C swing, so a plus or minus 0.02 mm callout across that distance is meaningful only at a stated temperature. Our default is to machine and measure at 20 C and to state that on the inspection report.
As a working guide for machined features: plus or minus 0.05 mm is routine on parts under 100 mm, plus or minus 0.1 mm is comfortable to 300 mm, and beyond that you should expect to specify plus or minus 0.3 mm or accept the part is a fitting rather than a precision component. For hygroscopic materials — nylon and acetal — add a moisture allowance, because a dry-machined nylon part will grow one to two percent as it takes up ambient moisture. If a tolerance is critical, tell us why and we will tell you whether the material can hold it.
Need a material recommendation?
Send us the application and we will reply with two or three candidate grades and a price.
