Every machined part has a tolerance that comes free with a normal finishing cut, and a tolerance beyond that which has to be bought with a slower pass, a controlled environment, a better fixture or an extra operation. Knowing roughly where that boundary sits is what lets a designer tolerance a drawing without wasting money on features that do no work.
What comes free
A competent shop holds general commercial tolerances on a normal finishing cut with no special measures, and shops publish a general tolerance block for exactly that reason. Everything toleranced at or looser than that block costs nothing extra. The first job in tolerancing a drawing is therefore deciding which features can simply be left to it, and it is usually most of them.
What tight tolerances actually buy you
Slower finishing passes with lighter cuts. Temperature stabilisation of the material and the machine. Better workholding that does not distort the part. Measurement on equipment good enough to prove the result, which itself needs calibration and a controlled environment. And a higher scrap rate, because the process window is narrower. All of that is on the invoice.
Where tolerances stack
Features machined in one setup are related to each other very precisely. Features on opposite faces inherit the accuracy of the re fixturing. So the cheapest way to hold two features closely to each other is to make them reachable in the same clamping, and the most expensive drawing is one that demands tight relationships between features on faces that cannot be reached together.
Tolerancing a drawing cheaply
Use the general tolerance block for everything that does no work. Reserve tight values for fits, sealing faces and interfaces. Say which datums matter and in what order. Group tightly related features onto one face. And state the temperature for anything large or in a high expansion material, because otherwise the tolerance means different things in different rooms.
Questions people ask about cnc machining tolerances
What tolerance comes as standard?
Whatever the shop's general tolerance block says, which is what a normal finishing cut holds without special measures. Everything at or looser than that is free.
Why do tight tolerances cost so much?
Slower passes, temperature control, better fixturing, calibrated metrology and a higher scrap rate. The tolerance narrows the process window and every part pays for it.
How do I keep two features accurately related?
Design them so both can be machined in the same setup. Features on faces that cannot be reached together inherit re fixturing error.
Should I put a tolerance on every dimension?
No. Use the general tolerance block for anything that does no work and reserve explicit tolerances for fits, seals and interfaces.