Rounding up Measurement results - Is it still within specification limits?

H

Helenliu

#1
I meet a problem during the audit, Pls kindly help me to answer.
********************************************
The spec is lower or equal to 1.5,
but the measurement value is 1.53, is it pass or fail?
For the last digit 0.03 is got by estimation, so the production department thinks it can be ignored or half adjust round-up. But from the points of quality, I think it should be fail. Am I correct? Or how can persuade the production enginner, if I am correct.
*******************************************:)
 
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Stijloor

Staff member
Super Moderator
#2
Re: Measurement results and spec

I meet a problem during the audit, Pls kindly help me to answer.
********************************************
The spec is lower or equal to 1.5,
but the measurement value is 1.53, is it pass or fail?
For the last digit 0.03 is got by estimation, so the production department thinks it can be ignored or half adjust round-up. But from the points of quality, I think it should be fail. Am I correct? Or how can persuade the production enginner, if I am correct.
*******************************************:)
Hello Helen,

Welcome to The Cove Forums! :bigwave: :bigwave:

How was the last digit "estimated?" I would be very hesitant to reject a part based on an estimation. To ensure the validity of the measurement: when you measure "tenths", your measuring device should be able to indicate "hundredths." Was this indeed the case?

Can you clarify?

Stijloor.
 
H

Helenliu

#3
Thanks for your quick reply:).
In fact, the digit read by the gauge is four like 1.5019, but the spec is always defined as <=1.50 . I don't know how many digital should be recorded for this case to decide the product pass or fail, 1.50 or 1.501, or all the digits can be read by the gauge.
As far as I know, the last digit read from the gauge like micrometer is estimated by operator, so can it be edvience that judge the reject or accept of the product?
 
Z

zancky

#4
Thanks for your quick reply:).
In fact, the digit read by the gauge is four like 1.5019, but the spec is always defined as <=1.50 . I don't know how many digital should be recorded for this case to decide the product pass or fail, 1.50 or 1.501, or all the digits can be read by the gauge.
As far as I know, the last digit read from the gauge like micrometer is estimated by operator, so can it be edvience that judge the reject or accept of the product?
in my modest opinion
first the part must be rejected as, supposing we are talking about an external diameter of a rod, that rod will not pass through a ring gage 1.50000 (Taylor principle is violated)
second the tolerance range must be reduced by the amount of the error of the measurement. i.e. if your instrument error (repeatibility / reproducibility etc) is 0.002 for instance Your tolerance should be <=1.4998 otherwise someone will use a device with accuracy 0.1 ...
 
A

andygr

#5
The standard practice that I have always followed is ASTM e29 for the precision that you have.
But when you move to the tight tollerance and fine precision work then measurement practices move to more of the absolute evaluation approach.

The criteria should be defined and stated in the design standard used but the engineering group responsable.

:2cents:
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#6
I believe that GD&T specifies that the spec is a dead end, that it should not be exceeded by any digit past the digit specified in the spec.:2cents:

More importantly, they should not be running any where near that spec - another tough lesson to teach manufacturing. :cool:
 
G

Geoff Withnell

#7
ANSI Y14.5 states (paraphrased) that all dimensions are considered to have an infinite string of trailing zeros. I have always believed it to be best practice to record the actual measurement, including all digits. Why throw away information? If a part is out of tolerance, it is out of tolerance, that is the inspection decision. The diposition decision may be to use as is, but information from inspection should ALWAYS be as complete and accurate as possible. That is inspection's job and anything less is p*ssing in the soup.

Geoff Withnell
 
A

andygr

#9
Granted working to the max or min tollerance is a problem and should never be done.

I have just have to go there and ask how the verification is to occure when taking a measurment if the requirement is an infinate number of digits?
The engineering call out is how the design function identifies to manafacturing and inspection what precision and tollerance is required to meet design function.
The real world is that that you can not seperate the measurement from the measurement function to determin conformance.:2cents:
 
G

Geoff Withnell

#10
Granted working to the max or min tollerance is a problem and should never be done.

I have just have to go there and ask how the verification is to occure when taking a measurment if the requirement is an infinate number of digits?
The engineering call out is how the design function identifies to manafacturing and inspection what precision and tollerance is required to meet design function.
The real world is that that you can not seperate the measurement from the measurement function to determin conformance.:2cents:
The requirement is not an infinite number of digits, and no one said it was. The idea is that the limit is the limit. ANY measurement beyond the limit, no matter how small the excursion beyond the limit, is a non-conformance. If the limit is 1.000 on the spec, then a measurement of 1.00001 is non-conforming. "But what if I were to measure that 1.00001 part with a handheld micrometer, and I get 1.0000? Fine. So far as you know you have a conforming part. "But if I measured it first on a super-mike and got 1.00001? Well then you know that the part is non-conforming. So the inspection decision is non-conforming. Given compliance to appropriate contractual issues, the disposition decision may be use-as-is. The two decisions are separate, one is information, the other is action. We do process and inspection planning to make sure we are using the appropriate tools for the process, which includes appropriate measuring instruments. Accurate enough to give us the required information, but not overly sensitive, thereby increasing costs.

Geoff Withnell
 
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