5 Things to Know About Part Tolerances for CNC Machining

Learn about tolerances for CNC machining, including the implications of tightening the range and how your available tolerance range is dependent on certain factors.

By Serena Ngoh · April 12, 2019

1. Tolerances are the acceptable range for a dimension, which is determined by engineers based on the form, fit, and function of the part

Unless specified, Xometry's standard tolerances are +/-.005” for metal parts and +/-.010” for plastic parts. As a point of reference, a piece of paper is about 0.003” thick. If you need tighter tolerances, just specify your tolerances and we'll get your parts made to a T!

2. Tighter tolerances result in additional cost due to increased set-up requirements, longer cycle times, and more in-process metrology

Increased cost and lead times are a result of CNC machines potentially slowing down to hold the tighter tolerance. Additionally, the set-up will require time-intensive labor. Tight tolerances should only be applied to critical areas to help minimize cost.

3. Specifying tighter than standard tolerances can actually change the optimal manufacturing process for your parts

For example, a hole that can be machined on a vertical mill at one tolerance range may need to be bored on a lathe in a tighter tolerance range, adding set-up costs and lead time. Even tighter tolerance requirements may require grinding or lapping. 

4. Keep in mind how features will be inspected when adding tolerances to your parts

 If a feature is difficult to machine, it is likely also difficult to measure. Certain features require specialized inspection equipment, and this may increase part costs.

5. The difficulty of manufacturing a part to a specific tolerance can be very material dependent

Generally, the softer a material is, the harder it is to hold a specified tolerance due to the material flexing while cutting. Plastics such as Nylon, HDPE, and PEEK may not hold as tight tolerances as steel or aluminum without special tooling considerations.

Congratulations! You're now on your way to building high precision parts. Read more design tips from our CNC Design Guide or quote your CNC parts now.