2009 standard - figure 7-45 ???

[AndrewTT]

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.

 

[drawoh]

AndrewTT,

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

 

[3DDave]

C is nominally perpendicular to the mutual axis of the holes and sets orientation. B only sets a distance into/out-of the view and does not limit any constraints that C has.

Why do you think it does not fully constrain the holes?

 

[Belanger]

Andrew, notice that datum feature B is a diameter, while datum feature C is a slot/channel. So looking at this part from the bottom you would see something that looks like a slotted screw.

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

 

[AndrewTT]

I totally missed the diameter symbol. Ignore this post. Mods can delete it.

I feel much better now....

Thank you.

 

[mkcski]

AndreTT

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

 

[mkcski]

AndrewTT
S
Sorry for the typos but... Datum axis B should be 2 linear motions not 3

Certified Sr. GD&T Professional