Almost every machine shop advertises precision, which makes the word useless as a filter. What actually separates a precision shop is a controlled environment, a metrology capability that can prove what it made, fixturing designed to hold a part without distorting it, and a process that is stable rather than heroic. All four cost money and all four are visible if you ask.
Temperature is the first constraint
Metal expands, and at tight tolerances the expansion over a few degrees is a meaningful fraction of the tolerance band. A precision shop controls the temperature of the room, lets material stabilise before machining, and measures in the same conditions it cut in. A shop that machines in an unconditioned bay and measures in an office is measuring the walk between them.
Fixturing that does not distort the part
Clamping force deforms a part, and it springs back after release, so a feature machined under clamp is not where it was cut. Precision work uses low stress workholding, vacuum chucks, custom soft jaws, and sometimes machining in stages with re fixturing between them. It is invisible in a quote and it is most of what you are paying for.
Metrology, and being able to prove it
A shop cannot hold what it cannot measure. Ask what inspection equipment is on the floor, whether it is calibrated and to what, and whether they will provide a dimensional report. A coordinate measuring machine with a documented calibration is a different proposition from a set of micrometers, and on a geometric callout it is the only thing that answers the drawing.
What to put on the drawing and what to leave off
Tolerance every feature at the loosest value its function permits, and reserve tight values for the few that do work. Say which datums matter and in what order. State the temperature at which the tolerances apply on anything large or in a material with high expansion. And if a feature is critical, say so in words as well as in numbers, because it changes how the part is fixtured.
Questions people ask about precision cnc machining
What counts as precision machining?
There is no threshold. In practice it means a controlled process with the environment, fixturing and metrology to hold and prove tolerances tighter than general commercial practice.
Does precision cost more?
Yes, in setup, in slower finishing passes, in inspection and often in scrap. That is why tolerances should be tight only where the function needs them.
How do I know a shop can hold my tolerance?
Ask what they will measure it with and whether they will supply a report. A shop that answers with specific equipment and a calibration record is a shop that has thought about it.
Does material choice affect achievable precision?
Considerably. Stable materials in stable sections hold tolerance; thin walls, high expansion alloys and stressed stock all fight you, and plastics fight hardest.