General orientation for linear dimensions

[steveapathy]

I have designed a part containing both GD&T and ordinate dimensions. I have the edges of the part (an irregular shape, but mostly orthogonal) all defined by ordinate dimensions from a common origin, which is the primary locating hole in the part. Since a hole provides no rotational reference, the orientation of the part during measurement is not totally clear.

The part itself is assembled using this hole (near the bottom right corner of the part) as the primary locator, and a slot (near the top left corner of the part, in line with the hole) as the rotational control. For the purposes of the GD&T positional callouts, I have the hole as datum B, and the slot as datum C. Datum A is the large flat surface of the part.

When this part is inspected, they would most likely just use one of the long vertical edges to align the "y" axis, and measure all linear dimensions based on that orientation. Since the edge isn't what aligns my part in my assembly, that's not really capturing design intent. What I really want them to do, however, is align the part based on |A|B|C| prior to measuring linear dimensions from the alignment hole.

I know that a line profile all around my part would fulfill this need, but the vendor we receive the part from is insistent on having it defined by linear dimensions for inspection and qualification purposes.

Does anybody have any advice on how I can make it clear how to orient the part during measurement?

I included a made up part that illustrates what my part is like (made quickly, so ignore the fact that it's not fully defined... the real part has other features with a positional GD&T callout using |A|B|C|).

 

[Belanger]

Yes, plus/minus tolerances are ambiguous, unless they are merely defining size or a chamfer or a radius. And I can't envision a part that is defined by size only.

Certainly plus/minus can tackle form (which is what 2.7.1 refers to), but there are four essential things that we must know on every part: size, form location, and orientation. And for the latter two, plus/minus is usually open to multiple interpretations. (Yes, I'll hedge and add "usually.")

As an example of ambiguity, let's examine Figure 2-6. Sure, the diameter of the part is given. That's great for size and form on that feature. I'll even presume that they give us a length dimension, say 55 ± 0.2, which controls the lengthwise size and form. Are all features fully controlled for size, form, orientation, and location? No -- but of course it's OK to do that in the standard because the intent of that drawing is to show a principle, not define a real part.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[Belanger]

BTW, Steveapathy, I hope it doesn't sound like I'm picking on you! You've made clear that you are sold on the benefits of GD&T.
I'm just trying to impress that it's unfortunately used only when a designer thinks the "traditional" way won't cut it. That's a very low bar to set.

Understanding that issue will help you get across to your vendors why it's actually doing them a favor when drawings are air-tight and unambiguous.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems