<< Back

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

1. Tighter tolerances mean increased costs

It is important to remember that tighter tolerances cost more due to increased scrap, extra fixtures, special measurement tools and/or longer cycle times, as the machine may need to be slowed down to maintain tighter tolerances. Depending on the tolerance callout and the geometry associated with it, the cost can be more than twice as much as maintaining standard tolerances.

Global 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 increased inspection time.

The best way to apply tolerances is to only apply tight or geometric tolerances to critical areas when it is necessary to meet design criteria to minimize cost.

2. Tighter tolerances could mean changes to the manufacturing process

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

3. Tighter tolerances can change inspection requirements

Remember that when adding tolerances to a part, you should consider how features will be checked. If a feature is difficult to machine, it is likely to be difficult to measure as well. Certain functions require specialized inspection equipment, which can increase part costs.

4. Tolerance depends on material

The difficulty of manufacturing a part to a specific tolerance can be very material dependent. Generally, the softer the material, the harder it is to maintain the specified tolerances as the material will bend when cut. Plastics such as nylon, HDPE, and PEEK may not have the tight tolerances that steel or aluminum do without special tooling considerations.