General Tolerances

General tolerances are the tolerances that apply to every dimension on a drawing that has no tolerance written beside it, and they are the quiet default that governs most of a machined part. A drawing cannot carry an explicit tolerance on every one of its dimensions — the page would be unreadable — so the drawing’s title block, or a general note on the sheet, declares a general tolerance: a stated allowance that covers all the untoleranced linear dimensions, angular dimensions and, in many drawings, the chamfers and radii. The best known of the systems that set these is the standard ISO 2768, which defines the general tolerances by class; this wiki’s entry on reading engineering drawings names the general tolerance as one of the title block’s two governing notes, and this entry sets out what it is, how it is read, and why the machinist must know it exactly — because a dimension without a written tolerance is not a dimension without a tolerance.

The tolerance that needs no number

The general tolerance exists because dimensions need tolerances, but not every dimension needs to say so. When a designer draws a feature that is not critical, writing ±0.2 beside every such dimension would clutter the drawing and say nothing new; instead the title block carries a note that the general tolerance is, say, the medium class of ISO 2768, and every untoleranced dimension on the sheet is then governed by that class. The machinist reading the drawing therefore reads every bare number twice: once as the nominal size, and once through the general tolerance note that gives it its allowable band. The same number on two drawings with different general tolerances is two different requirements — a 50 millimetre dimension allowed a few tenths under a fine class, or allowed a visible fraction under a coarse class — which is why the first questions about any bare dimension are which standard and which class the title block names, and what band that class gives for the size in question.

The classes of ISO 2768

ISO 2768 gives the general tolerances in a small ladder of classes, and the drawing chooses which rung applies. For linear and angular dimensions the standard defines classes running from a fine grade, through medium and coarse, to a very coarse grade; each class fixes the allowance for a given size of feature — the larger the dimension, the larger the allowance, and the finer the class, the smaller it is — and the angle tolerances are graded by the length of the shorter leg of the angle, so that a short angled surface is held to the same practical variation as a long one. The designer selects the class that the part’s ordinary, non-critical features can tolerate, balancing cost against function: a fine class holds everything closely and costs more to make and check, while a coarse class lets the shop cut freely and is chosen where nothing on the part needs more. The machinist does not memorise the tables; the machinist reads the class on the drawing, knows what it permits for the sizes in hand, and recognises that a drawing calling the fine class is quietly asking for more careful work on every bare dimension than one calling coarse.

What the general tolerance covers

The general tolerance is broader than size, and reading its note is reading the drawing’s whole default for ordinary features. ISO 2768’s first part covers linear and angular dimensions — the lengths, diameters, distances and angles drawn without a written tolerance. Its second part covers the general geometrical tolerances: the default form — straightness, flatness, and the rest of the shape controls treated in this group — that applies to features where no geometric tolerance is called out, along with general rules for features such as drilled holes, so that a feature which is not otherwise controlled still has a defined limit of form. Many drawings add a general note for the edge conditions too — the chamfers and corner radii, and the treatment of edges that must be broken or kept sharp — which the dimensioning conventions of the sheet name once for every edge that does not carry its own call-out. The general tolerance is thus the drawing’s blanket: whatever a dimension or a feature does not say explicitly, the general note says for it.

The machinist’s reading

For the machinist the general tolerance is a decision-maker on the floor, because it separates the features that need care from the features that do not. A dimension governed by a coarse general tolerance is cut and forgotten — the ordinary pass holds it without thought, and it needs no special measurement. A dimension under a fine general tolerance may still be a normal cut, but it is one the machinist cuts deliberately and may check, because the allowed band is no longer generous. And the same logic applies to the whole job: quoting, planning and inspection all depend on which class is in force, since a drawing under a fine class quietly demands better work, tighter setups and more checking across every untoleranced feature than the same drawing under a coarse class. The general tolerance also marks the boundary of what the drawing has thought about: when a feature’s variation genuinely matters, the designer does not leave it to the general note but writes a specific tolerance, calls a fit or uses GD&T — the explicit controls this group treats — and the general tolerance retreats to the background it is meant to occupy.

The default that still governs

The general tolerance is easy to overlook precisely because it is everywhere, and the discipline of reading it is the discipline this wiki sets at the centre of reading drawings: read the title block before the geometry. The note names the standard, the class and the edge conditions, and from that note every bare dimension on the sheet receives its band. The machinist who reads it knows that the part’s ordinary features have an agreed looseness — that a chamfer may be broken within reason, that an untoleranced length may sit anywhere in its class’s band, that a face not otherwise controlled still has a limit of flatness — and knows equally when a number matters enough that the drawing has taken it out of the general tolerance and given it its own. No number on a drawing is without a tolerance; the general tolerance is the one that carries the numbers no one wrote a tolerance for, and the part is made right only when the shop knows which numbers those are and what the default allows them.