A tolerance is the allowable range a dimension can vary and still be acceptable — for example, 25.0 mm ±0.05. It’s the language of “how precise does this need to be,” and it directly drives cost.
What’s achievable
Standard CNC machining commonly holds around ±0.005 in (±0.13 mm) without special effort, and tighter — into the thousandths or less — with more care, better machines, and inspection. Looser, general tolerances apply where precision doesn’t matter.
Why tighter costs more
Holding tight tolerances requires slower, lighter cuts, finishing passes, temperature control, better tooling, and more measurement — plus more scrap when parts drift out. The cost rises steeply as tolerances tighten, so over-specifying is a common and expensive mistake. Apply tight tolerances only to the features that need them (mating surfaces, bearing fits, sealing faces) and leave everything else at a general tolerance.
Frequently asked questions
What tolerance can CNC hold? Around ±0.005 in as standard; tighter with extra effort and cost.
Why not make everything tight? It dramatically raises cost and scrap for no benefit on non-critical features.
How should I specify tolerances? Tight only where needed; general everywhere else.
A practical habit: ask “what does this surface actually do?” before assigning a tolerance. Mating, sealing, and bearing surfaces earn tight numbers; cosmetic or clearance features rarely do. Communicating intent clearly on the drawing saves the shop from guessing and saves you from paying for precision you don’t need.