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

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#11
Granted working to the max or min tolerance is a problem and should never be done.
True. :agree1: To further the point:

One of the toughest things to instill into the production mind is that they do not "own" the full print tolerance. The only people who "own" the full print tolerance is final inspection, as that represents the customer's dock OR (if one ignores some measurement error) 100% inspection. Manufacturing is restricted by the virtue of their sampling error, measurement error (no matter how small) at a minimum. The first thing I hear is "You are cutting my tolerance!" Guess what, you never had it - you just didn't realize it. That is one reason why they shy away from SPC, it appears to "cut their tolerance." Again, you never had it. If you want it back, all you need to do is 100% inspection! Sorry for your luck.

But the good news is, once they figure it out, 0.000001 is a non-issue! :cool:
 
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howste

Thaumaturge
Super Moderator
#12
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.
*******************************************:)
Yes, it's out of tolerance. What they are doing is called "Flinching." The QCI Certified Quality Technician Primer gives this definition: "Flinching: Giving a reading “the benefit of doubt” by rounding to the next decimal inside the specification." It's listed under the category of "Known Errors."

If you plot a histogram of measurements collected this way it will show the tails of the distribution truncated, meaning that some out-of-tolerance parts are accepted.
 

Randy

Super Moderator
#13
What's with all the jabbering over jibberish? Here's the question...

The spec is lower or equal to 1.5,
but the measurement value is 1.53, is it pass or fail?
IT'S A FAIL!

The max allowable was 1.5 and the dude says it's been determined to be 1.53, so what's the big deal? The whatever is toast, it's too big, it's broke, no good, trash, out of limits, IT AIN"T RIGHT! Who cares what it's called or the science behind it.

This is like saying a roadkill is 106% dead. Now, the question we have presented is "Is it too dead?" ..............When it became 100% dead that did it!

When that whatever started measuring past 1.5 it was over, finished, done, terminal, quitting time.

If someone comes up and asks..."Is a 1.53" peg to big to go into a 1" hole?", what do you think the answer should be?

Goodness guys, is it so hard to just give a direct answer and say "Yep, too big" ( I see one or two folks could without all the bru-hah. Good for them)
 
S

sustain

#17
Helen, can you measure more than one sample? If so, you can quantify the expected variation. The variation estimate and the average compared to the specification will provide more information regarding the number you can expect to pass/fail the specification.
 
F

fortyfour73

#18
I guess my only question is, is this your tolerance or the customers? (if there is one).

1.5 and 1.53 are two very different things, but if the allowable tolerance from the customer is 1.8 and you are just trying to hold tighter ones for the gold star and cookie, then so be it.

Also, I did not read all the postings, but what are you using to check the dimensions with? If you are using calipers or something related that only is accurate to the third place ( i.e. .001"), then you may be alright.

:rar:
 

Randy

Super Moderator
#19
I guess my only question is, is this your tolerance or the customers? (if there is one).

1.5 and 1.53 are two very different things, but if the allowable tolerance from the customer is 1.8 and you are just trying to hold tighter ones for the gold star and cookie, then so be it.

Also, I did not read all the postings, but what are you using to check the dimensions with? If you are using calipers or something related that only is accurate to the third place ( i.e. .001"), then you may be alright.

:rar:
The OP clearly stated 1.5 was the max allowable, period, so your allowable tolerance isn't a valid argument here.

Additionally
you may be alright
is playing horseshoes and this isn't horseshoes.
 
M

MIREGMGR

#20
This kind of problem can be prevented either by correcting the training of the responsible specification creator(s) so that they clarify this particular question on the drawing or in a workplace standard, or by assuring that production uses measurement techniques with precision capabilities that match the precision expectation established by the drawing. Production shouldn't involve judgment calls as to what measurement tool to use, how to do that measurement, or where the boundaries of "good" are.

When an argument arises over measurement tolerance and precision, it has to be due to a preventable system problem.
 
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