Position with Only One Datum?
This time around, I’d like to present another “pet peeve” of mine, at least in the world of GD&T. It involves using the position symbol when the only quality being controlled is perpendicularity.
This is very common — it stems from some subconscious notion that if GD&T is going to be used on a hole, it’s got to be the “true position” symbol. NO!
Consider the following example. There is a position tolerance applied to the large hole on the left, and the datum being referenced is A.
But let’s go to the standard and examine how the geometric control called “position” should be used: ASME Y14.5-2009 (and prior editions) state that position’s main job is to control location — meaning that it involves a distance — and perpendicularity often comes along as part of that position control.
Since the large hole given above is already distanced from the edges by plus/minus dimensions, the geometric tolerancing has nothing to do with location. The only relationship that the large hole has with datum A is one of orientation. Therefore, an orientation symbol must be used:
Notice the perpendicularity symbol. This is the correct way to identify this hole, since the hole itself now becomes the datum feature for other features to locate back to.
That said, there are no GD&T police that will haul you to jail if you insist on using the position symbol. But recall that the purpose of geometric tolerancing is to use a standardized language to express the design requirements. So it’s best to stick with the proper terms and symbols if you want to minimize confusion when expressing your requirements!
One final footnote — there are times when a position tolerance may reference only one datum: If a pattern of features (two or more) are being positioned with one feature control frame, then a single datum plane is allowed (because there is the location between the two features that position controls). And another example is that of coaxial features; we may have one diameter positioned to another diameter, and this “coaxiality” is indeed a location control.