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.