CRITICAL on drawings 1

[powerhound]

Hello all,
I have a question about the addition of "CRITICAL" to a specific dimension on a drawing. Our sales guy wants to add it as a sort of flag to say "Hey guys, if you're going to miss a dimension, don't let it be this one." The tolerance on the dim is sufficient and it's really not any more critical than any of the other dimensions the only difference is if it's out of tolerance, it will show up later in a big way. I'm against putting this note on the drawing as I feel it may cause someone to think that dimensions not indicated as critical, are somehow unimportant. Is there standard out there that addresses the use of CRITICAL on a drawing? I hope I've been clear on the issue.
Thanks

Powerhound
Production Supervisor
Inventor 11
Mastercam X
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[thundair]

I did not see the mention of a 'material review board'(MRB) buying off a dimension that is out of spec.
A "critical" dimension would make them aware that it had to be in spec period, or that it may affect end item buy off.
Also could affect the interchangeability of components forward and backward
I will say that it is used less and less over the years since I started with the John Glen flight in 63.
Now that we are in MBD (model base definition) we rely on interchangeable requirements and assy level tolerance with no defined detail level tolerance


Cheers

I don't know anything but the people that do.

 

[swertel]

Kenat,

The trick with DOD-STD-2101 is to not let the tolerance of a dimension drive the decision as to a critical, major, or listed minor characteristic. Also, 2101 is not meant to be the driver for additional inspection (AQL) criteria. It is merely a flag to say that "This characteristic is important" and therefore whatever inspection level I define (either following ANSI AQLs or MIL-STD-1916) needs to be clearly documented because the customer is going to ask for inspection records of those characteristics.

Many people fall into the trap of wanting to inspect a dimension 100% and therefore call it "Critical" even when it is only critical in terms of scrap rate or assembly fits. Likewise, many designers/engineers/management insist that a tightly toleranced dimension must be a major or it would have a looser tolerance; but the tolerancing is determined based on fit, function, and part interchangeability and not on system failure.

It is true, though, that many dimensions that are critical, majors, or listed minors won't fit together if they are out of tolerance. If that's the case, then your inspection criteria can be "did it fit together." Talk about an easy and verifiable 100% inspection method.

--Scott

http://wertel.eng.pro

 

[ctopher]

http://www.facilityinformationcouncil.org/ncs/faq.php

See last paragraph at bottom.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)

 

[KENAT]

Like I said it was a while ago I used it so I don't remember it that well. For what I used it had absolutely nothing to do with dimensional issues.

It was to do with safety/system failure and that then led us to more tightly define NDT inspection requirements than normal.

As regards the use of critical as my companies procedure defines it or powerhounds sales guy wants to use it I'm still inclined to think that this is probably better put in a QA plan rather than the drawing.

 

[swertel]

Chris,

Based on that website, it appears that the standard uses a model-based design approach. In other words, the only dimensional data they put on the drawing are dimensions "critical" to the application. Any other manufacturing dimensions need to be interrogated from the model.

I just can't see that a corner fillet dimensioned to .25 MAX (to allow for any end mill to be used) is a critical dimension in a non-stress concentration application.

--Scott

http://wertel.eng.pro

 

[ewh]

That standard also refers only to building/civil areas. A machined part would be a different animal.

 

[swertel]

Thanks, ewh. I wasn't about to buy the standard just for this discussion.

--Scott

http://wertel.eng.pro

 

[ctopher]

Sorry, I didn't mean for the link to solve anything. It was just some FYI.

Chris
Systems Analyst, I.S.
SolidWorks 06 4.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 06-21-06)