Understanding CNC Machining Tolerances

The concepts of component interchangeability and dimensional tolerancing have since become an accepted part of manufacturing. Unfortunately, the misapplication of the latter can cause a host of problems. For instance, an overly stringent tolerance might require that parts go to a secondary grinding or EDM operation for completion, unnecessarily increasing costs and lead-time. Tolerances that are “too loose” or that aren’t in line with those of mating parts can make assembly impossible, leading to required rework, or in the worst case, making the finished product unusable.

To help avoid these unpleasant situations, this design tip includes some guidelines on how to properly apply part tolerances at Protolabs, along with a few definitions of the more commonly used callouts. We’ll also explore the industry standard for part tolerancing known as Geometric Dimensioning and Tolerancing (GD&T), a system defined under the American Society of Mechanical Engineers (ASME) Y14.5 specification.

Standardized Tolerances for CNC Machining

The standard prototype and production machining tolerance at Protolabs is +/- 0.005 in. (0.13mm). This means any part feature’s location, width, length, thickness, or diameter will not deviate by more than this amount from nominal. For example, the 1 in. (25.4mm)-wide bracket you're planning to order will measure between 0.995 and 1.005 in. (25.273 and 25.527mm) across, while the 0.25 in. (6.35mm) hole on one leg of that bracket will come in at 0.245 to 0.255 in. (6.223 to 6.477mm) diameter.

That’s pretty close, but if you need greater accuracy, there's our standard precision or production machining tolerance of +/-0.002 in. (0.051 mm). We're also able to hold +/- 0.0005 in. (0.0127mm) on reamed holes, and  +/- 0.002 (0.051mm) on feature locations, provided those features are machined on the same side of the part. Depending on the part geometry and material, however, we can often achieve even greater accuracy, provided you make us aware of your requirements. For these and other exceptions, please be sure to note them on your part design when you upload the file(s) for quoting.