Figure 4-8 of ASME Y14.5-2009

[jasoncwells]

Figure 4-8 in the GD&T spec shows a datum reference frame. I am aware of the two mutually perpendicular planes that exist on the axis of the part per section 4.10.3. The part that puzzles me is, how are those two planes oriented about the part axis? That is, why did the author of figure 4-8 align those planes with the holes in the part?

Perhaps the more important question, does the orientation of the u and v planes about the Z axis even matter? (On a side note, a purely cylindrical part, there would be no feature at all to control orientation of the two planes associated with datum K.)

My understanding persuades me that one of those holes has to be a datum for the orientation of the two planes of datum K to be restrained in rotation about the part axis. That's not how figure 4-8 is shown, so my understanding must not be complete.

And if you are so kind as to help me, would you please point me to the part of the spec that is pertinent? I've spent quite a bit of time studying this, and I still am not satisfied with my understanding and my command of the spec.

Thanks,
Jason C. Wells

 

[Belanger]

You nailed it in your second paragraph. Since it's purely cylindrical part with no feature to distinguish the clocking of the part, then there is no tertiary datum feature.

It's still a complete datum reference frame, but if an inspector really needs to stabilize those intersecting plane (such as on a CMM), then simply pick any one of the four holes to zero out on. As long as the holes are all 90 degrees from each other (within the position tolerance), then you could really spin the part all day long and it doesn't matter.

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

 

[mkcski]

joasoncwells:

I agree with J-P. There is typically confusion with hole locations when coordinate dimension from the X-Y planes is applied. This dimensioning schema is typically used because many "think" of the X and Y movements of a machine tool or CMM. In reality polar coordinates could be used. In this example, because there in no tertiary datum to control the rotation of the planes, you could dimension the (4) holes using a 48 diameter BSC "bolt circle" and pictorially rotate them 45-degrees off of the X-Y datum lanes (no 45-degree BSC dim from the planes) and accomplish the same location for the pattern.

Certified Sr. GD&T Professional

 

[jasoncwells]

Thank you, folks. This gives me quite a bit more confidence in my application of the spec. Mkcski is correct in that I was thinking of X and Y movements of a mill.