CNC part tolerances every designer needs to know

Tolerance is the acceptable range of dimensions determined by the designer according to the shape, fit and function of the part. Understanding how CNC machining tolerances affect costs, manufacturing process selection, inspection options, and materials can help you better determine product design.



1. Tighter tolerances mean increased costs

It is important to remember that due to increased scrap, additional fixtures, special measurement tools, and/or longer cycle times, the tighter the tolerance, the higher the cost, as the machine may need to slow down to maintain tighter tolerances. Depending on the tolerance markings and related geometric figures, the cost may be more than twice the cost of maintaining standard tolerances.

Overall geometric tolerances can also be applied to drawings of parts. Depending on the geometric tolerance and the type of tolerance applied, additional costs may be incurred due to the increase in inspection time.

The best way to apply tolerances is to apply tight tolerances or geometric tolerances only to critical areas when design standards need to be met to minimize costs.



2. Tighter tolerances may mean changes in manufacturing processes

Specifying tolerances that are tighter than standard tolerances can actually change the optimal manufacturing process for parts. For example, a hole that can be machined on an end mill within a tolerance range may need to be drilled on a lathe within a tighter tolerance range, or even needs to be ground, thereby increasing installation costs and lead times.



3. Tighter tolerances can change inspection requirements

Remember, when adding tolerances to parts, you should consider how to check features. If features are difficult to machine, they are probably also difficult to measure. Certain functions require special inspection equipment, which may increase the cost of CNC milling parts.



4. Tolerance depends on the material

The difficulty of manufacturing parts according to certain tolerances can be very dependent on the material. Generally, the softer the material, the more difficult it is to maintain the specified tolerances because the material will bend when cut. Without special tool considerations, plastics such as nylon, HDPE, and PEEK may not have the strict tolerances of steel or aluminum.