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

 

[KENAT]

If I remember correctly there is a mil spec or something that covers this. I looked at our company procedure thinking it would reference it but it doesn't, below is a definition though from from our prodecure.

Critical Characteristic - Product features, dimensions or notes that stand out as being primary to fit, function or application of a given part or assembly. Variation to these identified elements could directly affect the integrity of the design and its application.

Now I can't say my heart is really in what's there. To me in essence if the drawing is correctly toleranced then I would expect that any values outside of the tolerance of any dimension may cause problems while any value withing the range is OK. I would also expect all parts to meet said tolerances. In practice though maybe this is an over simplification.

Personally I think of it more as being an indication of a dimension that should be 100% inspected rather than just sampling/statistical but someone more familiar with the concept may be able to shed more light.

We use a diamond symbol which is explained by a note that references the company policy rather than writing critical next to the dim.

Hope this gives you a starter for 10!

Ken

 

[MintJulep]

If tolerances are designed properly, any dimension that is out of tolerance will cause the part to not work.

If you have dimensions that can be "out of tolerance", but the part is still acceptable, the tolerances are incorrect.

 

[KENAT]

http://sixsigmaspc.com/dictionary/criticalcharacteristic.html

if it helps

 

[powerhound]

MintJulep,
The tolerance is correct as the part will not work if the dimension in question is out of tolerance. The issue is whether or not it really makes sense to add the CRITICAL note to the dimension. To me, it makes no sense except to complicate the issue but I wanted to hear from people in the same discipline to get their opinions.

KENAT,
So you have a company policy governing the use of symbols and such on drawings? Can you elaborate a little? Maybe we need something similar.

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

 

[MintJulep]

Powerhound,
I agree, it just complicates things, and used as suggested by the marketing guy, is not consitant with the definition provided by Kenat.

 

[powerhound]

MintJulep and KENAT,
Thanks for the input. Based on what I've read, I'm going to leave the drawing as it is. I'm sure I'll hear about it but I think I'll survive.
Thanks again,
Powerhound

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

 

[MikeHalloran]

I had a go-round over that issue many years ago with the Drafting Supervisor, who wanted to establish a set of procedures and standards for identifying critical dimensions on drawings.

I told him that I had saved him the trouble, by removing the non- critical dimensions from my team's drawings entirely.

I suspect his true goal had more to do with adding paperwork and 'reports' to shuffle it, than it did with making the product better.




Mike Halloran
Pembroke Pines, FL, USA

 

[ctopher]

I agree with the others. There was another thread on this topic a few months ago, but can not find it.

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

 

[MechNorth]

"CRITICAL" is like saying "RUSH" in the shop...it's overused and usually ignored. If your shop is like so many others, they figure that their best efforts are good enough providing that they met the spec at some time in the past, so they don't inspect every piece anymore. The reality, of course, is that processes and equipment change over time and validation is needed frequently. If parts are qualified on statistical processing, then you're accepting that some bad parts may get through. If that's an acceptable risk, then live with it; if it's not, then you need to specify an inspection requirement. The symbol I've used in the past was a split oval shape where one half indicates 100% and the other half lists the dimension refeence letter for the inspection report. What I found that was with judicious use of this symbol, I got 100% inspection.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[swertel]

Classification of Characteristics is based on DOD-STD-2101. You'll want to follow that up with MIL-STD-1916 on how to inspect the characteristics.

Looking up the latest set of ASME Y14.X standards, you will also find standardized flags for Critical Safety Items as well as other "critical" callouts.

--Scott

http://wertel.eng.pro

 

[boottmills]

The company I work for used an * to denote "CRITICAL" dimensions and then later with an oval around the dimension. As soon as I began working here I started trying to eliminate this denotation and was met with a lot of complaints initially. Mr. Sykes is absolutely correct...CRITICAL=RUSH. We have had less issues with parts being out of spec in other areas because they are checking every dimension as opposed to just the "CRITICAL" ones.

Boottmills
SW2006 SP3.0

 

[dingy2]

Critical should not be placed on a drawing just to show that the feature (characteristic) is important. It is important that all dimensions meet the tolerances reflected. Important characteristics are usually toleranced a bit tighter.

Automotive OEM place a symbol for critical to reflect SPC applications and most of those are not really critical or are on inappropriate features such as positional tolerances. Some companies were overusing this and it just increased the costs with little benefit.

It is up to your Quality department and the Supplier's Quality department to control important characteristics. The "Control Plan" or in non-automotive companies, the "Inspection Plan" would show the controls - methods of measuring, frequencies, sample size, etc. for confirming the important features are within specifications. That is the way to go.

 

[KENAT]

The particular procedure I reference litterally just covers the use of the symbol for critical characteristics.

As well as the above the main parts of relevance are:

Minor Characteristic - Product features, dimensions, or notes that establish a secondary requirement within the part or assembly. Reasonably anticipated variation of these characteristics should not affect principal fit, function, durability or customer satisfaction of the component or its assembly.

A Design team consisting of a Designer, Product Engineer of Record, Design Checker, and a Manufacturing Engineer to examine the characteristic on each part and assembly drawing and identify the Critical Characteristics. By implication, all other characteristics are Minor. Determination of Critical Characteristics should be based on the product application, manufacturing and assembly knowledge, engineering judgment and the results of a DFMA analysis. Other considerations should be given to knowledge of particular customer expectations and requirements.

Designate characteristics classified as Critical on product drawings by the use of the black diamond symbol. (?) The symbol will be placed after the numeric value of the dimension or a leader from the symbol leading to the critical item should be used for clarity. The following note will be added to the face of the drawing “ X. SYMBOL ? DENOTES CRITICAL DIMENSION / FEATURE IN ACCORDANCE WITH **** (ref company procedure)”. No designation is required for Minor Characteristics.

Like I said in my first post my heart isn't in it and actually I think the definition of minor characteristic could lead to problems.

If the 'reasonable variation' is within tolerance then why bring it up. If it is outside of tolerance but doesn't cause a problem then the drawing tolerance should be relaxed but maybe that's just me.

 

[ctopher]

Some major aerospace companies and the military require there to be "critical" dimensions shown on the drawings using a symbol.

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

 

[ewh]

I'm in agreement with the majority here. I understand it's statistical use, but it really has no place on the drawing, the purpose of which is to define the part. Other documentation would be a much better place to include such dimensions.

 

[swertel]

Actually, DOD-STD-2101 specifies that you show critical, major, and minor characteristics on the drawing. I do believe, though, that the definition of these classifications as defined in 2101 differ from what the majority of this thread is referring to.


--Scott

http://wertel.eng.pro

 

[KENAT]

If I remember correctly I've used DOD-STD-2101 or something similar at a previous job. If it's the one I'm thinking of you actually define critical characteristics based more on safety/effect on end performance than dimensional criticality. This then leads you to determine what level of inspection and what NDA or other techniques are required.

For example at my last place I designed a folding lug for a US program. This lug was the main suspension for the store. Accordingly the area around the pivot was critical, I think class B, so had to have a heavy inspection criteria. The rest of the lug had much higher safety factors and different failure modes so was only class D or something like that requiring much lower sampling.

Like I said this is all from memory and I may be confusing with another spec but I think this is it.

 

[swertel]

Yes, that about sums it up. As the versions change, so do the criteria, but the basic premis remains.

A critical characteristic is something that would cause personal harm or death (safety issue) at the system level.
A major characteristic is one that would cause of failure at the system level (resulting in a mission failure) but not so extreme as to injure anyone.
A minor characteristic is everything else, but a listed minor is a characteristic that has a significant quality impact or requires special inspection techniques.

So basically, what this thread has described as a "Critical" is really just a listed minor if following the spec.

--Scott

http://wertel.eng.pro

 

[KENAT]

I'm thinking that while there are occasional dimensions that are 'critical', eg if a firing pin was too long so caused the explosive chain to fire during assembly, generally they are few and far between.

The way the sales guy is proposing to use the term 'critical' or the way our company spec defines it is more along the lines of at most a major characteristic. Furthermore I'd expect this failure to normally be apparant during assembly, ie parts don't fit or it doesn't pass system test, than in service.

However, from what I remember the spec is more about invoking NDT etc rather than about dimensional inspection although sometimes dimensional variation could directly lead to failure.