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.
July 27th, 2010 at 4:21 pm
Finally!!!!! someone who knows what he’s talking about when it comes to GD & T. I am so sick & tired of engineers using it incorrectly and then using their findings as the basis of rejecting parts.
January 3rd, 2012 at 8:42 am
I am relativly ‘new’ the the world of GD&T. I use a cnc controlled CMM and two different manual CMMs, with with cnc and manual CMMs having different software. I run into these type of things on print all the time and the people I report to can’t understand why my software will not report the things being specified on the prints. By the way, how do you do true position of a hole with no datum specified?
January 4th, 2012 at 12:10 am
The only way to use position tolerancing with no datum references is if there are two or more holes involved in the same callout. Then I suppose it’s possible: each hole simply becomes a datum for the other. The only example of this shown in the official GD&T standard is for coaxial (in-line) holes. In that case, they don’t even need to be perpendicular to anything; the only thing desired is to keep them in line with each other. On your CMM, it wouldn’t matter which hole you zero out on first.
February 16th, 2012 at 7:35 am
A pet peeve of mine is when the (I think) uninformed refer to the position symbol as “true position.” Am I correct in asserting that there is a concept of “true position” as mentioned in 14.5 and a “position symbol?”
March 13th, 2012 at 1:20 pm
Hi Dave … yes, you’ve nailed it. The standard calls the symbol itself “position.” (I believe the symbol was at one time called “true position,” but that’s been several decades!)
However, the term “true position” still has meaning. It is the theoretically perfect location that is desired. The “position” symbol then tells us how far from true position we may stray.
It’s not a huge deal, but yes, this is the idea that you were thinking of.
March 27th, 2012 at 12:11 pm
THANKS!
But I have to add that I dont care if you refrence 15 datums, or a pattern. Say what you mean and mean what you say. Design engineers should use the orientation callout that makes proper sense for manufacturablility. In this and many cases : . PLEASE USE IT! or LEAVE IT OFF THE PRINT!
Great article.