True Position Interpretation: Length of Boss

[[Be...]]

Hi All, first off wanted to appreciate anyone who discusses on this form. I started a new Quality Engineer role a few months back and have been cutting my teeth learning Calypso on a Zeiss Duramax. The discussion posts here are a great resource and I appreciate anyone who shares their knowledge. 

As for business, wanted to get others interpretation of some GD&T that came through our shop earlier. Part simplified for customer protection but drawing below follows how they used three true position callouts. My best understanding is there are better ways of controlling this part, and might be some sloppy tolerancing, but wanted to confirm my interpretation.

 

     TP 1: TP of a boss tied to -A-, the normal plane.

The normal plane shouldn't control any location, so am treating this as a "perpendicularity" for the cylinder of datum -B-. Assume a perpendicularity constraint would make more sense?

     TP 2: TP of opposing boss tied to -B-

To me, this reads as concentricity. Having -B- for a datum to tie the second boss too means this TP makes the most sense to me out of the three, but again would thing a concentricity control frame would make the most sense. (Fixturing it on the CMM to inspect in one setup is a different story tho 🙂)

     TP 3: TP of the OAL between the bosses? 

This feature makes the lease sense to me out of the three. Don't believe I've ever seen a True Position of the length of a shaft

The ABC datums used make sense in theory. -A- is my plane, -B- gives location, and -C- clocks my rotation. Perfectly constrained.

But in how its applied to the .600 dim, I get confused. Thinking about the form and location, the form should be controlled by the dimensional sizes (Boss Ø .200, OAL .600) and the location should be already established by the first and second TP's, giving the locations of the boss's centerline.

I believe this means that the third TP is redundant, as the part is already entirely constrained for form and location?

 

Wanted to see if anyone else had a better understanding of what the engineer was trying to accomplish in dimensioning this way and if there was anything I missed. Should i chalk it up to a new engineer over eager to use GD&T, or someone who knows what they're doing and there's some reasoning I'm missing

 

Thanks again, 

Ben V

Edited Friday at 05:52 PM
better clarified

[[Pa...]]

Let me make some small adjustments; hopefully this helps:

First, datum A is the centerplane of the (0.300) feature, not its surface.  Datum B is the centerline of the boss, not its surfaces.

 TP 1: TP of a boss tied to -A-, the normal plane.

The normal plane shouldn't control any location, so am treating this as a "perpendicularity" for the cylinder of datum -B-. Assume a perpendicularity constraint would make more sense?

The primary datum sets the orientation of the coordinate system.  In this case, you're very close.  Datum A constrains the orientation of the tolerance zone to be perpendicular to A.  The entire axis of the considered feature needs to fall within that zone to pass.  In this case I'd expect (as you do) to get similar results from a perpendicularity evaluation.

   TP 2: TP of opposing boss tied to -B-

To me, this reads as concentricity. Having -B- for a datum to tie the second boss too means this TP makes the most sense to me out of the three, but again would thing a concentricity control frame would make the most sense. (Fixturing it on the CMM to inspect in one setup is a different story tho 🙂)

Here I disagree.  The DRF sets the orientation of the tolerance zone to be (first) perpendicular to A and (second) centered about B.  This is not the same as concentricity for a number of reasons.  Some of these reasons were the justification for doing away with concentricity years ago. 

TP 3: TP of the OAL between the bosses? 

This feature makes the lease sense to me out of the three. Don't believe I've ever seen a True Position of the length of a shaft

The ABC datums used make sense in theory. -A- is my plane, -B- gives location, and -C- clocks my rotation. Perfectly constrained.

But in how its applied to the .600 dim, I get confused. Thinking about the form and location, the form should be controlled by the dimensional sizes (Boss Ø .200, OAL .600) and the location should be already established by the first and second TP's, giving the locations of the boss's centerline.

I believe this means that the third TP is redundant, as the part is already entirely constrained for form and location?

Yep.  This one is weird. What is says is that the calculated centerplane between the two planes (0.600) apart must lie between two parallel planes (distance variable thanks to MMC) which are (first) parallel to A (not the surface), (second) centered about B (this has no effect on the evaluation; test it and you'll see you can ignore this), and clocked to C (again, this has no effect).  If the distance between the two measured planes is at MMC, you need that centerplane perfectly located and oriented, and of perfect form, or it fails.

I have no idea what the intent of this drawing is.  That GD&T looks on its surface like an assembly tolerance, but I can't think of a real-world scenario that fits this.  I'm guessing new guy at the drafting table.