Why every dimension needs a tolerance
A drawing that says a shaft is 20 mm is not manufacturable as written. No machine hits 20.000 mm exactly, and no measuring tool would agree it did. What the shop needs to know is the range that still works: 20 mm with, say, ±0.1 mm means anything from 19.9 to 20.1 mm ships. That range is the tolerance.
Tolerances carry cost. Halving a tolerance can double machining time, because it demands slower feeds, better tooling, and more inspection. The engineer's job is to make each tolerance as loose as the function allows — tight only where two parts actually have to mate.
General tolerances: ISO 2768
Putting a tolerance on every single dimension would bury the drawing. Instead, most of them are covered once by a general tolerance note, and only the critical few get their own value. The common standard is ISO 2768, which has tolerance classes by size range:
| Class | Meaning | e.g. 6–30 mm |
|---|---|---|
| f (fine) | fine | ±0.1 mm |
| m (medium) | medium | ±0.2 mm |
| c (coarse) | coarse | ±0.5 mm |
| v (very coarse) | very coarse | ±1.0 mm |
The full note is usually written as two letters, one for linear sizes and one for angles and form — for example ISO 2768-mK means the medium class for lengths and the K class for geometry. It sits in the title block and applies to everything without an explicit tolerance.
Fits: when two parts share a dimension
Where a shaft goes into a bore, the two tolerances together decide whether it slides, locates, or presses in. The ISO system names these with a letter and a number — for example H7/g6, a classic "close running" clearance fit. You don't have to memorise them; you pick the behaviour you want (clearance, transition, or interference) and the standard gives the pair of tolerances.
When plus/minus is not enough: GD&T
A ±0.1 on a hole's position controls its coordinates, but not squarely what you care about — is the hole round, is it perpendicular to the face, is the pattern true to the datum? That is what Geometric Dimensioning & Tolerancing (ISO 1101) adds: symbols for flatness, perpendicularity, position, concentricity and the rest, each tied to a datum — the reference the feature is measured from. Reach for GD&T when the function is about a relationship (alignment, seating, rotation), not just a length.
How to choose, in one line
- Start with a general tolerance for the whole part, so 90% of dimensions are covered at once.
- Tighten only the dimensions that mate with another part.
- Use a fit (H7/g6 and friends) where a shaft meets a bore.
- Use GD&T when a relationship matters more than a size, and always name the datum.
How Vernia handles it
Vernia fills in the parts a senior would add from memory. A new drawing ships with a general-tolerance note of ISO 2768-mK already in the title block, so most dimensions are covered before you place the first one. Any single dimension can carry its own plus/minus when it matters.
For geometry, Vernia has the full GD&T toolkit to ISO 1101: feature control frames with the standard symbols, plus datum labels, placed straight on the drawing. Surface roughness follows ISO 1302, and hole callouts carry their own note. The drawing goes out as PDF with the font embedded, so the tolerances read the same on every machine that opens it.