Also called geometric tolerancing, ASME Y14.5, feature control frame
What is geometric dimensioning and tolerancing?
The symbol language on a drawing that says how much a feature may deviate, and from what.
What else it is called
- Other names
- geometric tolerancing, ASME Y14.5, feature control frame
- In French
- cotation et tolérancement géométriques
Related entries
Understanding geometric dimensioning and tolerancing
Rather than dimensions with plus-or-minus limits, it states the permitted variation in form, orientation, location and runout against named datums - the reference surfaces the part is measured from and, usually, the surfaces it actually sits on in the assembly. Measuring a part from the surfaces it actually sits on is the reason this system exists. It is also why a part can meet every plus-or-minus dimension on an older drawing and still not fit.
The practical complication for Canadian manufacturers is that two standards govern the symbol language and they differ in their default rules: the American Society of Mechanical Engineers standard, which United States customers draw to, and the International Organization for Standardization system, which European ones use. The symbols look largely the same. The assumptions behind them - how a tolerance applies at different feature sizes, what is implied when nothing is stated - are not.
The cost of getting this wrong is not an argument about interpretation, it is a rejected shipment with the part made exactly as somebody read the drawing.
In practice
When a drawing arrives, the first question is which standard it was drawn to. A part can satisfy one reading of a feature control frame and fail the other.
Frequently asked
Which standard should our drawings use?
Whichever your customers use, stated explicitly in the title block. If you sell into both the United States and Europe, say which standard governs on every drawing rather than leaving it to be inferred - that one line prevents a category of dispute.
Do we need a coordinate measuring machine to inspect it?
Not always, but true position and profile tolerances are difficult to verify reliably with hand tools, and functional gauges only cover the features they were built for. If you are quoting work with tight geometric tolerances, inspection capability is part of the quote.