What defines a critical dimension

[zagwyn]

I am looking at my companies first article checklist and it ask for the critical dimensions to be checked. How is this defined? What is the criteria? Thanks

 

[ctopher]

thread1103-165581

Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 10-07-07)
ctopher's blog

 

[zagwyn]

ctopher,
I did go to that thread first but was looking for something more in layman terms. basically spell it out for me...

 

[TheTick]

Critical dimensions must be marked as such on an inspection print. It is not QC's job to decide, only to measure and document.

Engineers reviewing the print must mark dimensions "critical" as needed. If they didn't mark anything, well, then there are none.

Honesty may be the best policy, but insanity is a better defense.

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-SolidWorks API VB programming help

 

[fcsuper]

zaqwyn,

FAI can be done for all specifications on a drawing, or ones that are critical to form or processes. It isn't necessarily just dimensions, since specifications are also material, finish, notes, etc. You may be able to rely on a vendor provided Certificate on Compliance or Conformance (COC) from some non-dimension specifications if the vendor has been instructed to provide one on the PO.

FAI would cover anything that mates or touches to anything else for sure. It should also address the part form. It needs to be comprehensive.

FAI requirements shouldn't be mistaken for general incoming inspection requirements, which will cover only those inspection points that my be most vulnerable to variation and interface with other parts, or they may be just one or two points that can show the part is of the correct general shape (useful on less critical molded components).

Anyways, this is just some ideas. Good luck!

Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA
sw.fcsuper.com
Co-moderator of Solidworks Yahoo! Group

 

[powerhound]

Tick,
Did you actually mean that if no dimensions are marked critical, then they aren't critical? That gives a whole new meaning to the term critical from a manufacturing standpoint. Before I make any more comments, I'll wait for your response.

Powerhound, GDTP T-0419
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[ctopher]

In manufacturing, they are ALL critical, but the dim's that are most critical should be indicated on the dwg...thus, Critical Dimensions.

Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 10-07-07)
ctopher's blog

 

[TheTick]

powerhound:
Do you really believe that's what I meant? Sad.

 

[powerhound]

I'll take that as a no.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
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[zagwyn]

Okay here it is... If I were a mechanical engineer which obviously I am not what would be my criteria for a critical dimension?

 

[PSE]

From my point of view, while all dimensions/charateristics are important, a dimension or characteristic that directly affects safety or product compliance should be identified as a key or critical feature. Such analysis or programs should not be undertaken lightly.

A feature indicated as a critical characteristic should be 100% inspected with calibrated, traceable gauging that is known to be capable of the resolution required. Data is recorded and archived. This potentially gets quite expensive and again represents my opinion only.

Deciding what may be a critical characteristic or feature is up to the designer and indeed there may be none. In the worst case scenario world, look at a characteristic and think what could happen if it were wrong and what are the chances of it being wrong. Aside from potential hazards to the end user/customer, consider production processes and tooling that is involved. Could an out of tolerance electrical component/assembly cause a dangerous situation for an installer? Could a out of tolerance mechanical dimension cause an issue with fitting within a tool or machine with potentially dangerous results? Hardest to find might be a cascade type of failure mechanism where a series of otherwise benign conditional failures result in something serious.

Regards,

 

[fcsuper]

zagwyn,

Yeah, was PSE said (and me too, above ) I've seen a good practice is that all specifications on a drawing are inspected against on First Article Inspection (FAI). This includes all dimensions and non-dimensional specifications.

In some cases, this in not 100% practical, and only in those cases is there some scaling back of that requirement. Pretty much the rule is "verify all specifications that are verifiable."

Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA
sw.fcsuper.com
Co-moderator of Solidworks Yahoo! Group

 

[ctopher]

"Critical dimensions may be applied to undimensioned drawings where it is required to control the relationship of characteristics of parts in respect to each other closer than +/- .xx inch and also where tolerances less than +/- .xx inch are required. These dimensions are shown on the drawing in accordance with standard practice."

Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 10-07-07)
ctopher's blog

 

[KENAT]

To me the whole idea of 'critical dimensions' is a bit of a fallacy. If you've detailed & toleranced the component correctly then pretty much every dimension is critical. As such I have yet to be convinced that indicating it on the drawing is ever a good idea. Please note I'm talking about dimensions, not things like NDT inspection etc where this terminology is defined in an ASTM spec/old Mil Spec or something.

That said, the OP seems to be talking about a first article inspection not whether or not to indicate it on a drawing. Typically for a first article I've seen every dimension inspected, as FCsuper says in more detail. The point of a first article as I understand it is basically to prove the process, be it internal or a vendor.

If you're not going to fully inspect it I'd look at safety/compliance issues first. I'd then probably look at interfaces and then other features. However this will depend a lot on the end function of the part. For something like a lense the dimensions affecting optical performance may be arguably more critical than the interfaces etc.

What should be taken into acount too is process capability as related to the required tolerance. If you have a tolerance that you have reason to believe may be more difficult to meet with the likely process, then that should probably be inspected.

KENAT, probably the least qualified checker you'll ever meet...

 

[KENAT]

What I should have said is that once safety/compliance is covered then function is one of the deciding factors.

KENAT, probably the least qualified checker you'll ever meet...

 

[zagwyn]

Thanks all. Great input.

 

[desertfox]

Hi zagwyn

I agree with Kenat on this if the component is fully dimensioned with tolerances that should be sufficient to indicate which dimensions are critical or not.

regards

desertfox

 

[TheTick]

I've never actually worked anywhere where they used "critical" dimensions on drawings. I have worked at places that used "critical to function" and/or "critical to safety" dimensions.