Critical, Major and Minor Dimensions 2

[Euan Foster]

Hey all,

I have a few questions about critical, major and minor dimensions and if anyone uses them in relation to mechanical parts? My work are currently reviewing their process and are seeking to implement something better so thought I would ask the experts here.

Now there is a lot of good chat on the subject - Link & Link & Link

From the above links and relevant standards, I have been able to decipher how companies usually classify critical dimensions, how they denote them on a drawing (usually the race track symbol) and how often they are inspected (usually ISO2859 or ANSI Z1.4-2003 with a relevant AQL).

However, I am yet to find anything on minor and major dimensions (i.e. how they are commonly denoted on drawings, how often they are inspected in relation to critical dimensions and if any other companies really use this). The youtube video hints at a chinese definition on major and minor dimensions but I think the approach mentioned doesn't really add any value.

I am therefore wondering the following:
- Does anyone work at a company that makes the distinction between critical, major and minor dimensions?
- If yes, how do they make this distinction on the drawing and how are critical, major and minor dimensions inspected in relation to one and other (i.e. frequency of inspection etc)?
- If yes to the first point, do they think the distinction adds value? Please bare in mind that I work in a low volume, high value manufacturing sector.

Looking forward to hearing back from you all

Euan

 

[CheckerHater]

Generally, there is no standard or generally accepted practice to identify "critical" dimensions.

(Some) reasons are outlined in the enclosed document.

There were several discussions on this forum on this topic. (Example:

http://www.eng-tips.com/viewthread.cfm?qid=322065)

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[AndrewTT]

We indicate some dimensions on drawings as critical. There is no other designation. So a dimension is either critical or non-critical for us.

However, critical dimensions are broken down into categories (variable or attribute). The variable designation requires capability to be shown. The attribute designation does not require capability to be shown (i.e. a go/no-go gage could be used). This is a new system for use and there is ongoing discussion about somehow requiring some capability demonstration for attribute dimensions during PPAP to avoid excessive measuring of attribute dimensions after PPAP.

 

[dgallup]

I've never seen any standard for major and minor dimensions, I think those are company specific anachronisms. Most companies these days are getting away from such non-standardized notations. However, critical dimensions are a major part of the TS16949 quality standard used world wide in the automotive industry. It does not use the race track symbol. Instead, the symbols depend on the customer (which I hate, if you are creating a standard, standardize it). Within the critical characteristic symbols, most likely you will find one for fit & function, another for safety & regulatory and a third for statistical process control. Again, the actual symbols used depend on your customer. These same symbols must be used in your design and process FMEA and control plans. The control plan will specify the frequency of inspection.

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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.

 

[swertel]

The specification you want is DOD-STD-2101.

Critical Characteristic: Any attribute that if fails, could cause serious injury, damage, or harm.
Major Characteristic: Any attribute that if fails, could cause mission failure (or failure of the product).
Minor Characteristic: Any attribute that requires special inspection techniques.

Classification of Characteristics is called out on the drawing by numbering each characteristic within parenthesis.
(C1), (C2), etc.
(M101), (M102), etc.
(m201) or (201), (m202) or (202), etc.

The inspection level is often pre-defined, so a critical is 100% inspection, Majors have a higher percentage, and so on. But, you shall not apply a classification of characteristic onto a dimension to force inspection levels. There are other ways to force a dimension to be 100% inspected without listing it as a critical.

--Scott
www.aerornd.com

 

[3DDave]

Link to DOD-STD-2101

http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=37253