ASME Y14.5-2009 - Development of a Datum Reference Frame with Translation Modifier

[swoop360]

Decided to get my nose dirty and go page for page through the GD&T standard and either I am not understanding the concept of a translation modifier or they have made an error on Fig. 4-19. My understanding from the text and from 4-19(e) is that the datum simulator can be adjusted along the [57.4] basic dimension, however in figure 4-19(a) they have not indicated that datum C is permitted to move. I suspect that they meant to write [POS|Ø0.1(M)|A|B{trans-mod}].

Also, is there anything that is restricting the datum simulator from translating along the [12] dimension (within the bounds of the positional tolerance) or have they simply omitted the extra DOF for clarity?

 

[Belanger]

in figure 4-19(a) they have not indicated that datum C is permitted to move.

Datum feature C is indeed allowed to move; it has a position tolerance of 0.1 at MMC. Therefore, the real hole might not be exactly at 57.4.

is there anything that is restricting the datum simulator from translating along the [12] dimension?

No there isn't, because the only position tolerance being held on that datum feature C (in the x-y direction) is a straight-down distance of 57.4. If that hole is off to the left or right we won't know it because all subsequent tolerances on the print come from that new datum reference frame.

In other words, the basic dim of 12 is really telling us what the other 4 holes are doing, with respect to datum C once it's established.

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