Three numbers do most of the work
GRACTS works to three figures: a standard tolerance of ±0.05 mm, a tight tolerance of ±0.01 mm for high-precision applications, and ISO 2768-m for dimensions you have not toleranced yourself. Knowing which of the three applies to each feature is most of what makes a drawing quotable.
ISO 2768-m is the default, not a gap
Every dimension on a drawing has a tolerance, whether you wrote one or not. ISO 2768-m is the general tolerance standard that covers the dimensions you left plain. It is a real specification, so a part made to it is not made carelessly — but it is looser than ±0.05 mm, and that matters on a fit.
Tolerance only the features that mate
A tight tolerance costs time on the machine. The features that earn it are the ones that touch another part: a bore that takes a bearing, a spigot that locates a shroud, a thread that has to index. An outside diameter nobody measures does not need ±0.01 mm, and asking for it on every dimension makes the quote worse without making the part better.
Say which dimensions are critical
If three dimensions on a twenty-dimension drawing decide whether the assembly works, mark them. A drawing that identifies its critical features is machined with the setups planned around those features, which is the practical difference between a part that fits and a part that measures.
Remember what the finish does
Anodizing and plating add thickness. If a bore is toleranced to ±0.01 mm and then anodized, it will not measure the same afterwards. Mark which features must be masked or machined with an allowance, and the sequence is planned before cutting starts.
