2009 standard - figure 7-45 ???
2009 standard - figure 7-45 ???
(OP)
1. Can someone enlighten me as to why the datum scheme for this figure does not fully constrain the part against all DOF?
2. I know the figure was intentionally left unfinished so that the concept of a three level composite FCF could be demonstrated, but going with what is on the figure I have another question. Couldn't the reference to C at MMB be left off of the top level of the FCF? Doesn't B at MMB give you what you everything you need and C at MMB is redundant? I can't find it in the standard right now but I remember reading that a datum feature cannot constrain a DOF that has already been constrained by another datum feature. An example would be a cylindrical surface (primary datum) that is coaxial with a conical surface (secondary datum). In this instance the conical surface would only constrain 1 DOF because the cylindrical surface has already constrained 4 DOF. If this same conical surface had been the primary datum it would constrain 5 DOF.
3. What information would need to be added to this drawing to make this FCF scheme choice more obvious and/or necessary?
Thank you.
2. I know the figure was intentionally left unfinished so that the concept of a three level composite FCF could be demonstrated, but going with what is on the figure I have another question. Couldn't the reference to C at MMB be left off of the top level of the FCF? Doesn't B at MMB give you what you everything you need and C at MMB is redundant? I can't find it in the standard right now but I remember reading that a datum feature cannot constrain a DOF that has already been constrained by another datum feature. An example would be a cylindrical surface (primary datum) that is coaxial with a conical surface (secondary datum). In this instance the conical surface would only constrain 1 DOF because the cylindrical surface has already constrained 4 DOF. If this same conical surface had been the primary datum it would constrain 5 DOF.
3. What information would need to be added to this drawing to make this FCF scheme choice more obvious and/or necessary?
Thank you.





RE: 2009 standard - figure 7-45 ???
This could be a mistake. I don't see this figure in my copy of ASME Y14.5M-1994, so it must be new. It looks like the datum C feature ought to be rotated 90°.
--
JHG
RE: 2009 standard - figure 7-45 ???
Why do you think it does not fully constrain the holes?
RE: 2009 standard - figure 7-45 ???
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: 2009 standard - figure 7-45 ???
I feel much better now....
Thank you.
RE: 2009 standard - figure 7-45 ???
I does control all 6 DOF.
Datum plane A - 1 linear and 2 rotation
Datum axis B - 3 linear
Datum center-plane C - 1 rotation (around Datum axis B)
The axis of the tolerance zone for the pattern of 2 coaxial holes will be located:
157.7 BSC up for datum plane A
be alinged with (pass through) Datum axis B
be perpendicular to Datum center-plane C
Certified Sr. GD&T Professional
RE: 2009 standard - figure 7-45 ???
S
Sorry for the typos but... Datum axis B should be 2 linear motions not 3
Certified Sr. GD&T Professional