Every ±0.01
costs money.
Specify by function.

ISO 2768 defines general tolerances for linear and angular dimensions when specific tolerances are not given on the drawing. Four classes: fine (f) for precision work, medium (m) most common, coarse (c) for general fabrication, very coarse (v) for rough work. Specify one class as a drawing note to avoid tolerancing every dimension. Most CNC drawings use ISO 2768-m as default.

Work backwards from function. What must fit, mate, or move relative to what? That relationship dictates tolerance. For example: a bolt through a clearance hole needs the hole tolerance ±(bolt tolerance + clearance desired). A shaft in a bearing needs IT6 or better. A bracket bolted to a flat plate needs just enough tolerance to avoid bind (±0.1 mm usually works). Don't default to ±0.01 mm everywhere — that drives cost up 5–10× for most parts.

Standard: ±0.1 mm (ISO 2768-m default). Precision: ±0.025 mm. Tight with 5-axis and CMM verification: ±0.01 mm on critical features. For features below ±0.005 mm, grinding or EDM is required. Hole positional tolerance 0.05 mm typical, 0.02 mm with boring bar. Concentricity 0.01 mm achievable on turned parts in a single setup.

Standard injection: ±0.1 mm for features under 25 mm, ±0.2 mm for larger features. Tight injection: ±0.05 mm achievable on critical dimensions in single-cavity molds with temperature control. Multi-cavity molds (4+ cavities) introduce cavity-to-cavity variation typically ±0.025 mm additional. Shrinkage varies by material (PA66 shrinks 1.5%, POM 2%, glass-filled less).

Often yes. GD&T lets you express real functional requirements (a bore must be concentric to a reference, a mating face must be flat) instead of falling back on over-tight +/- tolerances. A hole with 0.05 mm concentricity to datum A is easier and cheaper to produce than a hole with ±0.01 mm on every dimension. GD&T rewards good design and penalizes lazy tolerancing.

On specific features, yes — precision grinding can reach ±0.002 mm on cylindrical diameters. Lapping reaches ±0.001 mm on small flat surfaces. These processes are slower and more expensive than standard machining, so reserve for features where sub-micron actually matters. Most parts have 2–5 critical features at ±0.01 mm and everything else at ±0.1 mm.