Critical Dimension 9

[rnanj001]

Hi,
I am a QA Engineer and i have to highlight the critical dimension on the Mechanical drawing for QA Incoming Inspection, so that the QA Incoming inspector can inspect only those dimensions to accept or reject the part. My question is, what is considered as "Critical Dimension" and how should i select it. Any information reqarding this subject will be greatly appreciated.

 

[KENAT]

Dingy, I get the idea of safety related dimensions being considered 'CRITICAL' though if you're far enough off lots of dimensions could be argued to be critical.

However, did you not take into account process capability V required tolerance at all?

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[nithink]

Completely agree with Dave about critical dimensions,main conflicts emerge because of miscommunication between Customer and supplier.We use critical dimensions regulerly in our designs for dimensions which plays big role in function,safety and reliability.even in our process documents its clearly mentioned critical dimensions need to be given extra care

Nx 7.5.5.4

Teamcenter 8

 

[CheckerHater]

I am sorry, but let say I drill the hole.
The hole passes requirements specified on the print.
How can I possibly exercise "extra" care about the hole? Dance around it?

 

[Belanger]

All dims/tols on a print need to be met; that's not the issue. But when they say "critical," it means that they want to statistically monitor how well they actually hold the tolerance. Thus, they can be alerted about potential problems in the process before parts are made out of tolerance.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[tz101]

"Critical" means different things to different organizations. Like some have said here, Where SPC criteria needs to be ensured on a given feature, then that feature is deemed as critical. Other companies don't adhere to SPC practices, and still call critical features as those described in my original post on this thread. I have worked for a company that called all dimensions critical and precision toleranced every feature. Other companies don't specify any features as critical and seem to get along fine.

"Critical" is really a vague term that needs to be defined within the bounds of the intent and goals of the company you currently work for.

The question really boils down to the use of the term in your organization, and engineering and manufacturing both need to be in agreement that specified feature tolerances are reasonable given the available manufacturing processes that will be employed for the given part.

Specifying "critical" or overly restrictive tolerances on certain features means nothing if those tolerances cannot be reasonably held using the company's available manufacturing processes.

Again, go back to manufacturing and engineering and come to an agreement on which features are, in fact, critical on the part you are inspecting.

 

[dingy2]

Kenat:

The question was a definition of "critical" and I reflected the automotive (AIAG) definition. There are only a limited dimensions that fit this definition but we may have a few that fit under the "Major" banner where we may have an assembly problem rather than safety. Positional is a good example of a "Major" characteristic or feature as long positional is not default and it truly is a fit and function requirement.

All critical in the automotive world should be controlled through statistics with a short term requirement of 1.67 Ppk (+/- 5 estimated standard deviations) and some companies require a minimum value of 2.0. That means that the shop floor a working to 1/2 tolerances. Long term (on the shop floor) must be controlled using variable gauges with a Cpk of at least 1.33 or, in some cases, 1.67.

Most GD&T applications are not conducive to SPC applications. Imagine trying to figure out how one should measure flatness and get consistent readings between Operators since R & R studies are required on each measuring device used in SPC.

The term "critical" in the automotive sector has big implications and thus, costs.

Dave D.

http://www.qmsi.ca

 

[bxbzq]

I think critical dimension concept is common in automotive industry is because the product volume. In some industries, like heavy machine, the volume per year may be just tens. So the mfg line or assy line don't have to statistically predict the dimensions out of processes. They inspect everything on prints on every part. But in automotive insdustry, you cannot do that because the volume is huge. Instead, statistic control comes into to play.

 

[fsincox]

My impression is it leaches over into aerospace as "misson critical" so to speak. Here it may be applied at lower volumes of parts. This may be where the general confusion comes in.

 

[CheckerHater]

OK, before we go further into outer space, may I remind you what OP actually asked?

Quote: “I have to highlight the critical dimension…, so that the QA Incoming inspector can inspect only those dimensions to accept or reject the part”

Real life story: We are making parts and there are two little holes on it constantly getting out of whack. We spend more time checking parts and adjusting the machine to keep these holes in tolerance, than any other feature on the part.
When I have a chance to see completed assembly I find that holes are used to attach the little shield with company logo.

How many engineers on this forum will consider those holes “critical”?

 

[btrueblood]

Depends - does my boss want that badge on the parts, and does the misalignment reach a point where the badge won't go on, or might fall off in the field?

 

[CheckerHater]

Well, the badge with the logo is the most important part of any machine, if you ask marketing; but it’s not my point.
It may be difficult to predict on the design stage which feature will give manufacturer or customer more trouble.
This is why I believe the feature becomes critical when customer says: “it’s important” and shop says: “it’s hard to do”.

 

[btrueblood]

Ah, ok. With the added explanation I now concur - and I think your definition bears repeating and remembering:

The feature becomes critical when customer says: “it’s important” and shop says: “it’s hard to do”

Lps from me.

 

[drawoh]

...
When I have a chance to see completed assembly I find that holes are used to attach the little shield with company logo.

How many engineers on this forum will consider those holes “critical”?

This is closer to my point. If the logo badge must go on, the holes are critical. This is a design problem somebody can solve. Did the tolerances actually reflect the need to clear screws? Could the clearance holes be made bigger, thus opening up tolerances?

What has happened here is that one set of dimensions and tolerances are marginally within your manufacturing capabilities. Consequently, your process fails frequently. Perhaps there are more critical features on your part, but since these are easy to do, you don't have problems, and you don't have to monitor the process as closely.

JHG

 

[ewh]

Why should the drafter care about critical dimensions? His job is to completely define the part, including the proper use of any GD&T and general dimensions - not to specify how or which machine to use to achieve that definition, or which dimensions are more likely to fail inspection. That responsibility falls on the manufacturing and/or quality departments, using the part definition provided by the drawing. Granted, engineering should be aware of any process limitations, but other methods should be utilized to address these and not the part definition itself.
Of course, this is an idealized position, but I am among those who feel that part definition should not include process definition unless absolutely necessary. As long as the part meets the definition on the drawing, it shouldn't matter if it were machined or created with a hammer and chisel. Separate documentation should be created to address such other issues.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[fsincox]

ewh,
I am believe I have the same understanding as you, in theory, that is the way the book seems to direct it. It never ceases to amaze me how far we are from the intent of the standard, at least as I would have interpreted it.

 

[tz101]

Why should the drafter care about critical dimensions? His job is to completely define the part, including the proper use of any GD&T and general dimensions - not to specify how or which machine to use to achieve that definition, or which dimensions are more likely to fail inspection. That responsibility falls on the manufacturing and/or quality departments, using the part definition provided by the drawing.

You have evidently worked in a vacuum. Companies these days often shove other departments' responsibilities off on drafters. Happens all the time. I have worked at more than one place where we were expected to specify critical dimension designators on each part print that went out.

But, to the original post, if in doubt, place the ball back in manufacturing and engineering's court.

 

[CheckerHater]

Tz101,
No, we don’t work in vacuum.
I put “critical” dimensions on my drawings every day, that’s why I say I hate doing it.
But if my boss ever visits this forum I want him to know that this is WRONG.
PS: I do marketing materials too. :-(

 

[ewh]

I also never said or meant to imply that it isn't done, just that it defeats the original purpose of the definition drawing, bordering on specifying method of manufacture (which IS usually against the standard).

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[drawoh]

tz101,

We are discussing a quality issue here. That fact the people in manufacturing are responsible for quality does not does not relieve design and drafting of responsibility. It sounds to me and a couple others here that manufacturing and QA are working around inferior drawings. If the drawings are crap, you need to talk to someone and find out which dimensions are critical, and what the tolerances are.

All of our fabrication is subcontracted. I regard the final functionality of our parts at least somewhat proprietary. If my drawings are prepared properly, the fabricator and inspector can simply believe my dimensions and tolerances.

JHG

 

[KENAT]

No tz101, not a vacuum. However, I managed to pretty much persuade folk that the way they were wanting to use the term critical dimensions and indicate them on the drawing was inappropriate and didn't really comply withe the invoked drawing standard.

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