Rectangular Coordinate Dimensioning

[mgri]

Hi everyone and thanks for taking the time to help out. I think my question is fairly simple. Here at work we are dimensioning a moderately complex part and want to apply some tolerances to the position of a few mounting holes. You will see that we've defined a datum "A-B" (Aside: I need to update the drawing to say "2 Surfaces" for A and B), and also used two holes for the secondary and tertiary datums. You'll also see that there are some coordinate dimensions on this drawing in the left most view.

My concern is that these coordinates are aligned with the part, but are not aligned in a perpendicular and parallel direction to the datum frame set up by "A-B,C,D". I always thought that the ordinate system needed to be strictly perpendicular to the reference frame? The guys here say that there is a implied "horizontal" datum set up by the basic dimensions that define the positions of datum D, so it's ok to reference the hole position to "A-B,C,D". Is this valid? It seems to contradict this from the standard:

1.9.1 Rectangular Coordinate Dimensioning. Where rectangular coordinate dimensioning is used to locate features, linear dimensions specify distances in coordinate directions from two or three mutually perpendicular planes. See Fig. 1-49. Coordinate dimensioning
must clearly indicate which features of the part establish these planes.​

 

[powerhound]

What you have on this drawing is okay. You have a primary datum plane of A-B, a secondary datum axis of C, and a tertiary datum axis of D. These do sufficiently constrain all degress of freedom, so your datum reference frame is okay. You also have the holes located from the datum features using basic dimensions and with respect to the datum reference frame via the feature control frames so that part is correct.

I think your drawing is set up correctly. There is no tolerance on two of the four corner holes because you only have 2X above the feature control frame.

The guys you work with are only incorrect in that they state that the horizontal and vertical relationship is implied. The relationship is not implied, it is specifically stated im the feature control frame and no implication is required.

Okay...I just re-read your post and I think I know what's going on. I don't want to delete what I've already written since it is still valid.

NO. There is no implied horizontal relationship of the holes to the outside edge of the part. I think that's what you're trying to say, right? The outside edge of the part still needs to be controlled to the |A-B|C(M)|D(M)| datum reference frame. As the drawing stands, all the holes are in the right place relative to each other but they can be anywhere on the part. Since two of the holes comprise the datum reference frame, you need to control the part around the holes, not the holes within the part.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[mgri]

Thanks John. I appreciate your thoughts and yes, the parts still needs to be related to position of the holes.

Really, to clarify, my only concern with the drawing is that the co-ordinate dimensions do not run parallel and perpendicular to the reference frame established by the datums A-B,C and D. I always understood needed to happen but now I'm unsure.

Again from the standard: "Where rectangular coordinate dimensioning is used to locate features, linear dimensions specify distances in coordinate directions from two or three mutually perpendicular planes."

To me the co-ordinates on the drawing I attached don't satisfy that. What do you think?