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  Miscellaneous Quality Topics
  "Significant" Dimensions

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Author Topic:   "Significant" Dimensions
Johnny Jewel
Lurker (<10 Posts)

Posts: 2
From:Mid-MO
Registered: Apr 2001

posted 11 April 2001 09:54 PM     Click Here to See the Profile for Johnny Jewel     Edit/Delete Message   Reply w/Quote
Here is a question that, although more design-related, definitely impacts
the quality field in the area of Quality Plans, measurement, and
effectiveness of design documentation. First, let me describe one way the
organization I work for portrays the importance of drawing dimensions.

Critical Dimensions -- Dimensions determined to be critical to the
part's fit and function. Critical dimensions will be indicated on the print
by brackets or other indicators. These dimensions must meet a Cp of 2.0 and
Cpk of 1.5.
Major Dimensions -- Dimensions that are toleranced beyond the standard
block tolerance. This would include all GD&T dimensions.!!!!!!!!!
(!!!!!!!!!!! mine)
Minor Dimensions -- Dimensions whose tolerances are not explicitly
written on the drawing (non-functional dimensions) and whose limits are
defined by general tolerances or as indicated in the title block area of
the print.!!!!!!!!!!!!!!(Mine, again).

The last 2 must pass a First Article Inspection with Majors having 5
parts measured, and 4 of the 5 must be within 63% of the tolerance
range.!!!!!!!!!!(Oops, those darn!!!!!!!!!'s again!). Minors have 1 part
measured, and must be in tolerance.
(Regarding "standard block tolerance" and "title block area", our
drawings have default tolerances something like 3-place decimals tolerance
is ±.005, 2-place decimals ±.010, and so on.)

1.) Has anyone seen anything similar?
2.) Does anyone know the source of this?
3.) Can you tell I am not neutral on this method?

I believe that the designer, thru design reviews, drawing reviews,
tolerance analysis, FMEA's, and other means should decide which dimensions
are significant and at what level. These dimensions must all meet the same
capability minimums the company has decreed as the goal, and the tolerances
can take up the slack of how important each is. By the way, the "63% IT",
as it has been nicknamed, has been put forth as being a Cpk of 1.33. It is,
if the process is perfectly centered, and you agree that capability can be
estimated with 5 parts.
Aaand, there is no hierarchical nature to characteristics described
with Geometric Dimensioning and Tolerancing -- it is merely the way the
designer has chosen to describe the particular feature, usually because it
is the best way; most descriptive and most related to the function of the
part.
Finally, without telling you the name of my company, I will tell you
we are a leader in our field -- and yet our design department can't
tolerance analysis its way out of a paper bag. They work strictly with
nominals in the development phases and throw in the standard ±'s with
little thought. It is rare that prototype parts are even measured to see
"what worked". Oh, and they also try to designate as few Critical's as
possible so that meeting those pesky Process Capability targets won't hold
up release of the product to the producing plant.

4.) Are these attributes common?

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CarolX
Forum Contributor

Posts: 108
From:Illinois, USA
Registered: Jun 2000

posted 12 April 2001 10:20 AM     Click Here to See the Profile for CarolX   Click Here to Email CarolX     Edit/Delete Message   Reply w/Quote
Johnny,

I'll answer question 4 first -
I Won't say common - but standard. I have yet to see an engineer who will classify characteristics as critical, major and minor.

As for your other question, I can tell you my experience years ago working for defense contractors.

DoD drawings use to have a reference sheet which listed critical, majors and minors. Quality requirements were as follows
Critical - 100% inspection
Major - Sampling inspection performed using MIL-STD-105, Genreal Inpection Level II, .65 AQL
Minor - Sampling inspection performed using MIL-STD-105, General Inspection Level II, 2.5 AQL
Usually anything not specified as critical or major was considered minor.

I don't know if this information will help you. But without a doubt, good luck trying to get engineering to define these for you.

CarolX

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