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.

 

[tz101]

Critical dimensions are subjective, based upon the function of the particular component being inspected. You would have to know the function and fit requirements for the part in question to determine critical print dimensions. Non-critical dimensions are normally areas of the part that do not come into close proximity to mating assembly components, and that do not impact the design intent or the overall performance of said part.

Also, a dimension may be critical that sets up a secondary machining operation, but does not necessarily impact the final as-assembled envelope.

To make a short story long, you will most likely have to discuss with engineering and manufacturing personnel to get a good grasp on critical dimensions for your part in question.

 

[DanStro]

Where I work our QA department will ask for us (i.e. design/engineering) to mark up a print showing the critical dimensions. I cringe when I think of them determining that without our input. Or the customers if that is appropriate.

HTH,
Dan

Han primo incensus

 

[rnanj001]

Thank you. I will try to contact Engineering department to see if they can assist me in this matter.

 

[dgallup]

The automotive ISO quality standard TS16949 defines critical characteristics and their symbols. These are supposed to be decided on during your APQP multifunctional group meetings although usually design engineering decides on 90% of them. Generally it's going to be the dimensions that have the smallest tolerances, affect fit/function or have safety and regulatory affects.

Having these things specified on the part drawing is better than letting someone with no knowledge of how the part works arbitrarily decide what to measure.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[fsincox]

Critical dimensions are not well understood at my work. They wanted all indications of "critical dimensions" removed from drawings. They have always associated them with statistical analysis and told me when I started that they "stopped doing that here". Now we are part of a large company and everyone wants critical dimensions again.
Frank

 

[drawoh]

rnanj001,

I do not understand this concept of critical dimensions. On my drawings, all the dimensions are critical.

If the drawing is sloppily prepared, we can assume that many of the tolerances are complete nonsense. The critical dimensions are the ones where the tolerances are not complete nonsense.

There is the manufacturing issue that some tolerances are difficult to achieve. If some manufacturing process is running at or near its maximum capability, then that feature of the part must be inspected systematically.

JHG

 

[fsincox]

drawoh,
I used to be so idealistic, too. I still believe it should be that way if we studied our processes like the SPC theories say.

 

[KENAT]

Urgh, the idea of critical dimensions is a bit ridiculous.

What you're really worried about is meeting the specified tolerance on those dimensions.

Here there are at least 2 things to consider when you're looking to reduce from 100% inspection.

1. Relative ease of achieving stated tolerance with the manufacturing processes used.
2. Impact to end item function of not meeting that tolerance.

Now the thing is, most of the time if the drawing is correctly drawn & toleranced item 2 should be almost irrelevant. That is, the drawing allows maximum tolerance that will still allow proper function. Implying that if that tolerance is exceeded the part may not work and hence that is a critical tolerance.

In practice if you have a feature on the part that is in 'free space' in the assembled state then it may be tricky to define the ultimate limit based on function so you typically should put a correspondingly loose/wide tolerance such that any likely manufacturing process will easily achieve it. So you could argue this is no longer a 'critical dimension' hence implying there are others that are. However, if you look closely what we're really paying attention to is item 1 above, we've put the tolerance required well within the process capability.

So, I stand by my comment that from a dimensional tolerance point of view it's more about manufacturing process capability than end item function.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[fsincox]

The definition of critical dimensions is practically mandated by some companies/contracts aren't they?

 

[dingy2]

This is the wrong forum for asking questions on critical dimensions.

I remember years ago when I was the Quality Manager of an automotive supplier and I didn't know how the part fit or functioned and I had to come up with a Control Plan for our plant. I phoned the Customer drawing Designer and he stated "everything" on the drawing was critical. That didn't help so I phoned their Quality department and they told me how the part fit and how it functioned. The development of our Control Plan was based on that discussion. It costs $$$ to check or confirm every cycle a dimension that has no bearing on its function and relationship. That is a waste of $$ and drives the product costs up. All dimensions are confirmed yearly but on the line, only dimensions that pertain to its function, reliability and appearance are confirmed on a regular basis.

Critical is a dimension where if it is out of specification it could affect safety of the end product and has a rating of 9 or 10 in your process FMEA (failure mode and effects analysis). Other dimensions that may affect the function and relationship are considered "Major" and must be controlled on a regular cycle basis on the line. This statement is out of the AIAG process FMEA requirement and, I think, makes sense.

Critical dimensions in the automotive field relate to dimensions where there is a statistical control required and it became a game in the past. We had charts all over the place with many having no meaning whatsoever. It is a bit better now but some Customers still plaster their drawings with symbols meaning "critical" so the game continues and the product costs increases.

At one time, GD&T applied to dimensions that affected the parts function and reliability but not now. Although this statement is in the newest ASME Y14.5-2009, most Designers apply GD&T on all dimensions except ones that are features of size which have a +/- tolerance. So, if you receive a drawing with the application of GD&T on pretty well everything, still find out which dimensions affect the part's function and reliability from a Customer's perspective. The term "Customer" is also the next operation on the line. Not all GD&T applications are "Critical" or even "Major" in importance.

If your company wants to place "Critical" dimensions on the drawing again, as Frank has stated, make sure that you can tell the supplier how it affects safety of the product or, at least, impair the function of the part.

Hope this helps.

Dave D.

http://www.qmsi.ca

 

[CheckerHater]

from a dimensional tolerance point of view it's more about manufacturing process capability than end item function

I agree.
Competent design engineer reflects fit, form, and function by applying tolerances to the drawing.
Dimension becomes critical when competent manufacturing engineer is looking at the blueprint and compares it to his shop’s capabilities.
Dimensions that can be achieved without effort are non-critical.
This is why I hate being asked to put “critical dimensions” on the drawing. To me all of my dimensions are critical. You tell me which ones are critical TO YOU.

 

[Belanger]

Good post Dave D! Critical dims can be good or bad, depending on what you mean. But not everything can be checked every darn time. Think even about flatness in the GD&T world: the definition of flatness is that every molecule of a surface be in the exact same plane. Can you really check that? No -- so we sample as many points as possible, be it with a CMM or even a feeler check while sitting on a surface plate.

So a similar philosophy can be extended to an overall part. The bad thing about critical dimensions (key characteristics, whatever) is that people often fail to state the statistical parameters that govern the idea, and another bad notion is that to some people they imply that other dimensions on the part can be totally ignored.

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

http://www.gdtseminars.com

 

[fsincox]

I agree, good post Dave, Thanks.

 

[TheTick]

Quite simply, critical dimensions should be determined and marked long before you see a print or part for inspection.

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

http://www.EsoxRepublic.com

-SolidWorks API VB programming help

 

[drawoh]

dingy2,

I am thinking this through as a designer, not as a plant manager or inspector. I am the guy who makes the drawings. I am not the guy who has to read them.

As the designer, I need to try very hard to ensure that critical features are well within the capability of the manufacturing process. As drafter, I look at every dimension and tolerance, and I make they are appropriate.

I worked with a guy who wrote notes on his drawings stating that special attention was to be paid to certain dimensions. To me, this implied that he did not care about the other dimensions.

If you are handed a package of drawings by a CAD operator who clicked on all the features and applied the default dimensions, then this discussion is relevant. You are going to have to track down someone who gives a s**t, to tell you which dimensions are important, and what the required tolerances are. The whole point of good drafting is that you see all the information you need on the drawing.

JHG

 

[dingy2]

Thanks drawoh:

I come from the Quality perspective and we always looks at the function and relationship of the part and its features. We rate them as critical, major, minor and incidental (who cares) with our Control Plan (Inspection Plan) reflecting this concept. Yes, we do end up tracking down someone who can tell us its function and relationship and, unfortunately, it usually is not the Designer. I really wish drawings would come out to the shop floor with a function and relationship of the features such as 4 bolts on centers go in these holes (positional), other holes are to lighten the part, etc. This information is critical for everyone to understand the part and to appreciate certain features within it.

We do care about all dimensions and if one is found out-of-specification, the run is red tagged and quarantined with corrections made to the process. If the dimension is deemed incidental and has no bearing on the part's function and relationship, then I am pretty sure the disposition would be "OK to Use". Sometimes we might contact the Customer's Quality Department for approval but that depends upon whether this is a Customer part or an in-house part.

In my opinion, good drafting does not give manufacturing all the information that is needed. Knowing the part's function and feature relationship within the part absolutely does give us the full information that is needed.

Again, I just see this differently than most people in this forum.

Dave D.

http://www.qmsi.ca

 

[fsincox]

The designer does not know the function of the part?

 

[drawoh]

fsincox,

The designer knows. The drafter, not necessarily.

JHG

 

[fsincox]

I would certainly hope so, the engineering representative (shop service/sustaining engineering) to the shop who is to handle shop questions (because the guy who designed it is too busy on new projects) that one I can believe too!