Dimensions and tolerances apply only at the drawing level where they a

[drdherl]

Dimensions and tolerances apply only at the drawing level where they are specified."

Can someone clarify what this statement means? I realize we should be inspecting a part at the level of the part in which the features are added....

Does statement mean that requirements don't flow up?
If I have a box with 4 mounting holes .250 +/-.001 in diameter. Assume that hole dia was verified at box level. Box is then stuffed with components per an electronics assembly dwg and then that assy was subjected to lots of elec testing. After testing we find that holes have been damaged and no longer meet +/-.001 tolerance. Is the box assy requirements compliant or would I need to re-specify requirements at the highest level to ensure that part is still reqts compliant?

(obviously, in reality, I'd need to fix the root problem)

 

[looslib]

You would need to respecify the requirements at the assembly level.
The part did meet the spec at the free-state, so changes brought about by the assembly have to accounted for at the assembly level drawing.


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[ctopher]

I agree with Ben.
The ICD could also be used here.
thread1103-235979

Chris
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[powerhound]

I don't think I agree. As drdherl stated, the root cause (such as poor handling practices) would need to be addressed. I completely understand about the need to ensure that dings or dents don't affect final assembly but I don't think simply restating tolerances is the answer. It will open up the door to more deviation from what the standard says. Believe me, it's not a can of worms you want to open if compliance with the ASME standard is your intent.
It's not likely that any amount of mishandling will move two holes closer together or further apart so a dimensional check of the spacing probably isn't necessary. My opinion is that there should be a note stating to check the condition of the holes before attempting assembly. I'm under the impression that your example of the box and holes is just an example so please take my reference to it as an example of one course of action.

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[ctopher]

The condition of the part is up to QA and inspection. There should not be a note on the dwg. If there is a note added, then dim/tol should be also added, which do not belong on the assy dwg unless the holes are drilled at the assy level.

Chris
SolidWorks 08, CATIA V5
ctopher's home (updated Aug 5, 2008)
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[JamesBarlow]

I would assume that it more likely applies to assemblies were there is a possibility of things changing durring assembly.

If a bearing is pressed into a housing some dimensions on the housing may change and no longer be in part tolerance but the part is still acceptable.

This is common for us when using a bronze bushing pressed into something then machined thru after. Rather then calculating how much the housing will expand after everything is done we just dimension the housing without the bearing and don't worry about it after the bearing is inserted. If the housing dimension is important we will then require the effected surfaces to be machine after assembly.

 

[drawoh]

drdherl,

This sounds like an extension to your earlier post. Let us walk through a practical scenario.

I work in Massive Engineering Inc's department[ ]A. I need a Nutating Widget, and it is to be designed by someone in department[ ]B for some reason. I create an interface drawing showing the maximum outline and the mounting holes. I apply a composite positional tolerance to the mounting holes...

TruePos Ø1 A B C
TruePos Ø0.2@MMC A

You are handed the design and you start work. You work out the design and prepare fabrication drawings. Probably, you will apply the above FCF to your fabrication drawing. You will send the drawing out to get the part fabricated, and your QC person will inspect the incoming parts. You assemble the test everything and ship the final widget to me.

My QC person uses my interface drawing to do his inspection. We are not aware of your fabrication drawings. We can assume you did them, but in a strict technical sense, your process is a mystery to us.

Each of us generates documentation that specifies something in a manner that QC can use to accept or reject the item. I do not see an alternative to dimensions and tolerances.

JHG

 

[drdherl]

DrawOH
Not sure I follow you....

Here's a quick summary of my concern.

My dftg dept wants to change ICD dims from hard dims and hard tols to Ref Dims and Ref tols since they are specified on various lower level fab dwgs.

(belief is that hard dims on ICD would replicate inspection that hopefully would occur on lower level and thus dims/tols need only be reference)

My question is that if we do put dims and tols on lowest level dwgs, are those dims and tols still binding at the highest level dwg if the statement "dims and tols apply only at the dwg level specified" is the rule. In other words, If I convert ICD to ref dim philosophy, can I legitatemly assume that the requirement at the highest assembly is the same as the requirement at the lower level?
If mfg doesn't inspect lower level part and moves discrepant parts up the dwg chain, can part be rejected as not meeting a requirement post mortem?
I'm afraid by converting to ref dims and ref tols on ICD, I am losing some amount of requirements control (not that I am in control in my massive company)
Thx

 

[drawoh]

drdherl,

Please note that I am not following a standard here. I am trying to solve a problem.

You apply a reference dimension to a drawing to provide information that is controlled elsewhere. You can apply interface dimensions to your assembly drawing because you a nice person and you don't want to force people to search down through the documents to the fabrication drawings. Your features are controlled by the fabrication drawings.

Who generates the ICD? If the ICD is generated by the designer/drafter who generated the assembly, arrangement and fabrication drawings, then the dimensions are reference.

If you generate the ICD and send it to an outside department for design and documentation, then the dimensions are an instruction and a requirement, to be met by the other department. The ICD is an inspection document, and the dimensions are not for reference. You may need new terminology for such a document.

JHG

 

[aardvarkdw]

The way I have always thought of this is that if a hole/feature is placed in a part at the part level, it should be dimensioned on that drawing level. If it is critical to form/fit/function at the assembly level if can be detailed as a reference dimension but not specifically toleranced with respect to the original part features(If it was in tolerance before it should still be since nothing has changed in the part definition). However a dimension to other parts, from that feature, is acceptable. Also if the original part will be modified at the assembly level, features can be dimensioned to show relationship to MODIFIED features but not to static features dimensioned in the original part definition.

David