The importance of holes on a cnc machined parts is no less than the threads. Parts are usually made to assemble with other ones. When it talks about assembly, the standards tolerances below takes important roles on it. In other words, the relationship between the two mating parts could be determined by "fitting". Generally speaking, there are three kinds of fitting below:

1.) transition fitting

A transition fitting is known as a slip or push fit for the mating parts. This kind of fitting is usually required a highly accurate alignment or tolerance between the mating parts. Under a transition fitting, the mating parts could perform a fitting without excess play or movement in the assembly.

2.) interference fitting

An interference fitting is so called press fit, tight fit or friction fit. It is usually used in fixing the bearing on to a shaft or a hole, a dowel pin into a hole. It may sometimes use to assembly spare parts into a single unit to reduce the raw material and production cost.

Besides, considering an optimize performance on fixing the bearing into the hole, the hole finished with burnishing process could provide a firm and consistence surface quality. You could refer to Sandy Munro's comments on the motor of Tesla Model S Plaid at 8'28''.

3.) slide fitting

The slide fitting is so called clearance fit or loose mating. The mating parts with a slide fitting could slide in and out without any restriction. A grinding process would be helpful to optimize the surface quality on the mating parts to slide freely between each other.