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Significant digits when reporting measurements


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  Post Number #1  
Old 4th March 2004, 12:05 PM
vforge

 
 
Total Posts: 6
Please Help! Significant digits when reporting measurements

What is the accepted practice when reporting dimensional results with regard to significant digits and rounding?

For example, if a feature is toleranced as 5.00 +/- 0.25, should we report to 3 places (measurement is 5.252 = out of spec) or do we round down to 2 places and report 5.25 = in spec?

I'm new to quality, so trying to grasp the basics. Thanks.

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  Post Number #2  
Old 4th March 2004, 02:21 PM
Bill Ryan - 2007's Avatar
Bill Ryan - 2007

 
 
Total Posts: 978
Welcome to the Cove

Our normal "practice" is to report one digit further than the callout. We don't have that documented internally but I seem to remember seeing it as a "should" somewhere (I require it for any documentation I may be sending to our customer). You may be, momentarily, "fooling" the customer (and yourself) but if you take variability into account, you could end up shipping product which will cause a "no build" or loss of functionality.

Good luck!!

Bill
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  Post Number #3  
Old 4th March 2004, 02:58 PM
Wes Bucey's Avatar
Wes Bucey

 
 
Total Posts: 11,024
Quote:
In Reply to Parent Post by Bill Ryan

Welcome to the Cove

Our normal "practice" is to report one digit further than the callout. We don't have that documented internally but I seem to remember seeing it as a "should" somewhere (I require it for any documentation I may be sending to our customer). You may be, momentarily, "fooling" the customer (and yourself) but if you take variability into account, you could end up shipping product which will cause a "no build" or loss of functionality.

Good luck!!

Bill
When I was in the high tech aerospace/medical machining business, we made it a policy to cover this issue in Contract Review with the customer before we ever cut metal.

As an aside, we always discussed the form, fit, and function aspects of a component we made for customers (everything custom to customer's design.)

We found it helpful to request tolerances as either
(+0.000 / -0.xxx) or (+0.xxx / -0.000),
instead of (+/-0.yyy),
depending on the mating part. We had a nice little set of toys to illustrate our point (male mating parts with dimension at maximum material condition and female mating parts the opposite.)

Sometimes the customer's design engineers would have an epiphany and modify ALL their drawings to show tolerances in our suggested method.
  Post Number #4  
Old 4th March 2004, 03:09 PM
Sam's Avatar
Sam

 
 
Total Posts: 1,444
Normally you use an instrument that has a discrimination of ten times the tolerance or as Bill stated "one more place than the callout", which means you report the result to 3 places. Based on this I would say no you cannot round down.
  Post Number #5  
Old 22nd March 2004, 10:57 AM
Graeme's Avatar
Graeme

 
 
Total Posts: 425
I Say... The standard that applies is ...

Quote:
In Reply to Parent Post by vforge

What is the accepted practice when reporting dimensional results with regard to significant digits and rounding?
Practices vary, of course, and should conform to your customer's requirements. However, the applicable standard is:

ASTM E 29, Using Significant Digits in Test Data to Determine Conformance With Specifications.

Thist can be purchased (paper or electronic) from ASTM (www.astm.org).

According to the footnotes, this standard was first published in 1940 and reaffirmed in 1999.
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