Occasionally users of GD&T suggest that everything be simplified by just boiling all 14 symbols down to just two or three. (What, you didn’t know there were 14 symbols? Click here for a handy chart!)
There is some logic to what these people are saying — namely, that many GD&T symbols overlap others, and position and profile can be used in such a way as to cover the others. But as you might guess, there are pros and cons to this.
- First, realize that position always controls two qualities: location and orientation. Location is obvious, but don’t forget orientation — because position extends all the way through the depth of a feature, it will control any tilt or angling of that feature.
- Profile of a surface, if used with datum references and basic dimensions tying it back to those datums, can control all four required qualities: location, orientation, size, and form (shape). Since it covers ALL of these, it can be argued that the other GD&T symbols could be ignored and simply use this one symbol (well, two if you count profile of a line).
But there are two problems with this minimalist philosophy: For one thing, it may sometimes be necessary to really only control a particular aspect, such as parallelism. You wouldn’t want to use position, since we don’t care so much about location. And you wouldn’t use profile, because form control is not needed!
Second, though the minimalist philosophy seeks to simplify drawings, it can actually make it harder for people to decipher what you are trying to say. I mean, come on — if you want something to be perpendicular, what’s so hard about understanding the perpendicularity symbol? Profile might do the same thing, but recall that profile can be applied to any angle, so it doesn’t immediately mean 90º to the reader (although the drawing makes a corner look like 90º, what if it’s an 89º corner and you didn’t look too carefully for an angular dimension?).
So the bottom line is: While it is possible — and often desirable — to use position and profile to control multiple qualities of a feature, we shouldn’t ignore the other symbols, which have a definite role to play in the GD&T world.