datum consumption
datum consumption
(OP)
Not sure if I need to post a picture so I will try to describe this as best as I can.
I have a hole pattern in a stamping located with a positional callout (MMC) to three datums:
A = plane, surface of part
B = central pilot diameter (MMC)
C = hole, not contained in hole pattern (MMC)
When the stamped part is used to produce the final part, a rivet is orbitally formed into datum C. After all other parts are added to finalize the assembly, the rivet cannot be reached to use as a datum feature.
So in the assembly I have specified the position relative to two datums (A and B as defined above).
The other engineer in the group thinks that I have to have a third datum to "clock" the hole pattern. In general I think she is correct, otherwise the pattern can technically rotate around datum B. However the higher level drawing does prevent this to a certain extent. The problem is that I do not really have a valid feature that I can use for a tertiary datum that makes functional sense (I don't want to pick a bogus datum just for the sake of doing so).
Any ideas (do I really need a tertiary datum)? Thanks!
I have a hole pattern in a stamping located with a positional callout (MMC) to three datums:
A = plane, surface of part
B = central pilot diameter (MMC)
C = hole, not contained in hole pattern (MMC)
When the stamped part is used to produce the final part, a rivet is orbitally formed into datum C. After all other parts are added to finalize the assembly, the rivet cannot be reached to use as a datum feature.
So in the assembly I have specified the position relative to two datums (A and B as defined above).
The other engineer in the group thinks that I have to have a third datum to "clock" the hole pattern. In general I think she is correct, otherwise the pattern can technically rotate around datum B. However the higher level drawing does prevent this to a certain extent. The problem is that I do not really have a valid feature that I can use for a tertiary datum that makes functional sense (I don't want to pick a bogus datum just for the sake of doing so).
Any ideas (do I really need a tertiary datum)? Thanks!





RE: datum consumption
I thought dimensioning only applies at the drawing level it is specified on. ASME Y14.5 1.4(n)
So inspecting it after it is assembled for compliance with dimensions given on a lower level drawing is bogus isn't it?
Id think its, ‘clocking’ should be inspected before assembly.
Did I understand correctly, does this help?
KENAT, probably the least qualified checker you'll ever meet...
RE: datum consumption
I understand where she is coming from but I really don't want to add a tertiary datum that I know to be bogus and will probably create problems.
RE: datum consumption
However this should be detailed on the part detail drawing & inspected on the part before it is assembled, so the fact the rivet fills the hole/hole is otherwise in acessible is irelevant.
Or do you have a detailed assembly or something? Even so if the dimensioning is given on a view of the unassembled plate I'd think this was how it should be inspected, but now I'm pushing the limits of my knowledge.
Or is the datum so that parts are assembled to the whole pattern in the correct orientation, I'm starting to get confused.
As to the company policy, while any check is probably better than none there's a massive difference between regular peer checking and getting detailed checking from knowledgable senior designer or dedicated checkers. See thread1103-193286: Drawing Checker/Approver Qualifications for a related discussion.
KENAT, probably the least qualified checker you'll ever meet...
RE: datum consumption
Now the assembly drawing.
Why would you want to place positional tolerances on the assembly drawing when the pattern was produced at the stamping operation? I guess that there must be a reason and if there is a reason, you do need a tertiary datum for orientation but now you have a problem if you do not use the same tertiary as the stamping.
Dave D.
www.qmsi.ca
RE: datum consumption
So the positional tolerance is typically included on the final assembly drawing because our customers use this for fit-up.
I have been thinking about it, I don't think the tertiary datum on the drawing brings value to the assembly drawing since it is controlled earlier. I think it will only confuse matters since the mating part almost never includes a tertiary datum (or positional tolerance at all in most cases).
Thanks for you help and opinions, it is good to get other points of view.
RE: datum consumption
Here is another thought.
The stamping drawing absolutely needs the 3 datums. This will include the feature to feature position and also the pattern position relative to datum hole B and tertiary hole C.
On the final assembly drawing, show the positional tolerances at MMC with only a reference datum A. In other words, we have a hole to hole positional tolerance and perpendicular to datum A. This is called the FRTZF per ASME page 95. This would reflect the features within a pattern and is conducive to assembly of the mating parts.
Dave D.
www.qmsi.ca
RE: datum consumption
RE: datum consumption
I only mentioned referencing the positional tolerances at MMC to datum A. This excludes orientation of the pattern which is on the stamping drawing.
This application is conducive to a situation where we have 4 pins on a fixed location that have to assemble in the 4 holes of your part.
Usually, when one has a positional tolerance at FRTZF referencing the primary datum, the value is a bit less than the positional tolerances to datums A, B & C.
Dave D.
www.qmsi.ca