Tolerance is the acceptable range for a dimension which is determined by the designer based on the form, fit and function of a part. Unless specifically called out by the designer, the standard tolerance used by Xometry for CNC machining is +/- .005” for metal parts and +/- .010” for plastic parts.
If tighter tolerances (less than the standard, e.g. +/- .002”) are required, the information must be communicated as to which dimensions require a more narrow range. This can be specified at the part modification level in our instant quoting platform. As a point of reference, a piece of paper is about 0.003” thick.
It is important to keep in mind that a tighter tolerance can result in additional cost because of increased scrap, additional fixturing, special measurement tools and/or longer cycle times because the machine may need to slow down in order to hold the tighter tolerance. Depending on the tolerance call out and the geometry associated with it, costs can be more than double of what it would be to hold the standard tolerance. Tighter tolerances should only be used when it is necessary to meet the design criteria for the part.
Overall geometric tolerances can also be applied to the drawing for the part. Based on the geometric tolerance and type of tolerance applied, additional cost may be the result because of increased inspection times.
The best way to apply tolerances is to only apply tight and/or geometric tolerances to critical areas which will help in minimizing costs.
You can find additional resources on CNC tolerances by reading our blog post, "5 Things to Know About Part Tolerances for CNC Machining" or download our ebook, Mastering Tolerances for CNC Machined Parts.