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Critical Dimension
9

Critical Dimension

Critical Dimension

(OP)
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.

RE: Critical Dimension

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.

RE: Critical Dimension

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

RE: Critical Dimension

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

RE: Critical Dimension

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.

RE: Critical Dimension

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

RE: Critical Dimension

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

RE: Critical Dimension

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.

RE: Critical Dimension

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Critical Dimension

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

RE: Critical Dimension

3
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.
www.qmsi.ca

RE: Critical Dimension

Quote (KENAT)

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.

RE: Critical Dimension

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

RE: Critical Dimension

I agree, good post Dave, Thanks.

RE: Critical Dimension

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

batHonesty may be the best policy, but insanity is a better defense.bat
http://www.EsoxRepublic.com-SolidWorks API VB programming help

RE: Critical Dimension

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

RE: Critical Dimension

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.
www.qmsi.ca

RE: Critical Dimension

The designer does not know the function of the part?

RE: Critical Dimension

fsincox,

The designer knows. The drafter, not necessarily. :)

JHG

RE: Critical Dimension

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!

RE: Critical Dimension

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?

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Critical Dimension

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

RE: Critical Dimension

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?

RE: Critical Dimension

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

RE: Critical Dimension

"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.

RE: Critical Dimension

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.
www.qmsi.ca

RE: Critical Dimension

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.

RE: Critical Dimension

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.

RE: Critical Dimension

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”?

RE: Critical Dimension

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?

RE: Critical Dimension

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”.

RE: Critical Dimension

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.

RE: Critical Dimension

Quote (CheckerHater)

...
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

RE: Critical Dimension

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

RE: Critical Dimension

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.

RE: Critical Dimension

Quote:

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.

RE: Critical Dimension

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. sad

RE: Critical Dimension

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

RE: Critical Dimension

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

RE: Critical Dimension

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.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Critical Dimension

My comment about working in a vacuum was only directed at a single person, based on the quote I included with my post.

RE: Critical Dimension

tz101,
Well, since you have seen fit to point your finger at me, just let me say that some people can be of the opinion that over 30 years experience, starting on the board, moving on to CAD not too long after it first appeared, supervising drafting departments, and spending a dozen years contracting at various companies across the south US could be considered "working in a vacuum", but IMO that it is not an accurate assessment.
Please don't take your frustrations out on those of us about which you know little or nothing. Not all industries or companies follow the same practices in this regard, and I am offended at your knee-jerk reaction at being exposed to experiences other than your own with which you may disagree (you know, those occurring in that vacuum). Just because other methods have been common in your experience does not mean that they are the only allowable (or preferable, depending) methods used.
There are good and valid reasons for using differing methods, but product definition does not have to be one of them. Product definition is just that, not process definition which statistical methods tend to direct.
I still stand by my statement with which you seen to have taken offense.

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

RE: Critical Dimension

The term "critical" is misapplied when being applied to dimensions. Dimensions are a definition of the product. Asking which ones are critical versus other ones is a bit like asking which words in this sentence are critical to this sentence's meaning.

Criticality is determined by experience with the product. What are the specifications that the vendor finds challenging? Where have failures occurred?

If you want a starting point, you can use crutches, like defaulting to dimensions that define features that mate with other parts.

On molded products, you can determine "process control" dimensions; these are dimensions that are nominally no more important than any other dimension, but can serve as an indication as to the overall compliance of the product.

Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion

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