Using Inspection Instructions to Modify Drawing Requirements

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J
#12
Not saying this is the proper solution but have you considered some sort of Poke-a-yoke system?

Instead of measuring the parts in four places and then needing to do some gymnastics to get it accepted, why not create a Go-NoGo type of Gage. Such a gage, or gages, properly designed could take into account out of round condition, min-max acceptable etc.

Which makes me think of another question. Is there an out of round allowable on the part?

Peace
James
 

Jim Wynne

Staff member
Admin
#13
Not saying this is the proper solution but have you considered some sort of Poke-a-yoke system?

Instead of measuring the parts in four places and then needing to do some gymnastics to get it accepted, why not create a Go-NoGo type of Gage. Such a gage, or gages, properly designed could take into account out of round condition, min-max acceptable etc.
A hard gage of some sort might be a good idea to facilitate inspection, but it doesn't address the issue of specifications. If you have a cylinder that fits the gage but not the specification, you're in the same place where you started.

Which makes me think of another question. Is there an out of round allowable on the part?
It's at least implicitly allowed, given the proposed inspection method of averaging four measurements.
 

somashekar

Staff member
Super Moderator
#14
We buy plastic cylinders with an outer diameter of 10.000" - 10.005". Our supplier fails to meet this requirement about 30% of the time. However, the parts rarely measure outside the range of 9.998" - 10.007". As long as the material does fall within the 9.998" - 10.007" range we are always able to accept it as nonconforming.

Rather then modify the drawings, the component engineer for these parts keeps attempting to modify our inspection instructions to allow for the discrepancies.

We measure the OD in 4 places on each end using calipers. If any one of these 4 measurements is out of spec then the parts go into our nonconforming material review process. The component engineer wants to modify our inspection instruction to state that if the average of the 4 measurements meets the 10.000" - 10.005" then the parts are acceptable without issue and without documenting the discrepancy.

I need help making my case against this practice. I have argued it every way that I can think of and I'm not getting through. So please, lend your expertise to this issue and help me build a case or if you disagree, please tell me your side so that I can reconsider my position.
If you have sufficient data to prove that 9.998 to 10.007 works for you and that repeated deviations for acceptance is recorded and no rejections subsequently have come up due to this tolerancing and no such customer complaints have surfaced due to this tolerance, then you may make an engineering change request (ECR) supported by your purchasing who can compare the price for the 9.998 to 10.007 (present price) and a revised price for the same part within 10.000 to 10.005 tolerance(1.n times the present price). The component engineer will then has to justify why the change cannot be made OR make the change. Sometimes MONEY speaks more than words ... Your ECR will have a great chance for an ECN on the drawing.
 
P

Phil Fields

#15
Maybe I missed this detail in previous posts, but what does the OD mate to? Does it mate to AIR, or to another part, does it have an interference fit or a loose fit? Is the OD dimension/tolerance realistic for its use?

Phil
 
J
#16
A hard gage of some sort might be a good idea to facilitate inspection, but it doesn't address the issue of specifications. If you have a cylinder that fits the gage but not the specification, you're in the same place where you started.
Agreed. Just thought I'd toss it out there.

It's at least implicitly allowed, given the proposed inspection method of averaging four measurements.
Yes, but the out of tolerance is implicitly allowed also given the number of times the parts have been OK'd out of tolerance. The point I was trying to make was that a Roundness tolerance might provide the room needed to avoid rejecting the parts in the first place. Of course if no roundess tolerance is on the drawing then we are back to the beginning once again. :frust:

Actually to answer the engineer's concern about opening the tolerance resulting in even worse parts, I might offer to send a letter along with the revised drawing telling the vendor why the change was made and that these new dims are the absolute outside acceptable dims and any product recieved out of print will be returned. Perhaps you could even promise to send along with an invoice to the supplier for the cost of sorting should you be forced to sort bad from good.

Peace
James
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#17
Actually to answer the engineer's concern about opening the tolerance resulting in even worse parts, I might offer to send a letter along with the revised drawing telling the vendor why the change was made and that these new dims are the absolute outside acceptable dims and any product received out of print will be returned. Perhaps you could even promise to send along with an invoice to the supplier for the cost of sorting should you be forced to sort bad from good.

I would still be interested in knowing why the vendor can not meet the print tolerances as they currently stand. Without that information, the engineer may be correct in his fears...and you can play open the tolerance game for a long time.

The OP needs some real process capability information before changing the print. He needs to know what they can sustain - and that is around the whole part, not average, not just the good points. Then, ponder the print. This whole specification mess is spiraling out of control. Nobody in the OP's company or the vendor have any facts...at least that we have seen.
 

Jim Wynne

Staff member
Admin
#18
I would still be interested in knowing why the vendor can not meet the print tolerances as they currently stand. Without that information, the engineer may be correct in his fears...and you can play open the tolerance game for a long time.

The OP needs some real process capability information before changing the print. He needs to know what they can sustain - and that is around the whole part, not average, not just the good points. Then, ponder the print. This whole specification mess is spiraling out of control. Nobody in the OP's company or the vendor have any facts...at least that we have seen.
You're right--one would think that with a ten-inch plastic cylinder, a total, unilateral tolerance of .005" would be indicative of something. All we know though, is that the supplier misses it 30% of the time, which isn't enough information to be writing prescriptions.
 
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