Using Inspection Instructions to Modify Drawing Requirements

M

Mgoul

#1
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.
 
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Jim Wynne

Staff member
Admin
#2
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.
Obviously, the orthodox way to proceed is to determine what works (assuming that the cylinder must fit around or inside of something else) and change the specifications accordingly. Your engineer seems to understand that the specification is "broken," but might be trying to following the path of least resistance in fixing it. The only acceptable alternative (in my mind) would be to add the inspection instructions (or explicit reference to them) to the drawing. This involves changing the drawing, of course, which might be what the engineer is trying to avoid.
 
J
#3
Having had some considerable experience with plastics I can understand the frustrations for both you and you supplier in meeting such a tight tolerance.

I guess the first question is who has control of the Drawing. Is it your company or customer drawing. I worked for a company that used almost ALL customer supplied drawings and these can be dvilishely hard to get changed sometimes.

If the Drawing is an in house drawing, then why is engineering so reluctant to change it??

Next question. How thick are the walls of the tube? How flexible? How flexible is it? How is it mounted on the product. How much "Out of Round" is acceptable.

If it is fairly flexible then an average might be an acceptable way to measure this. If it is not flexible then an average is no good since measurements could exceed a usable dim but still average out.

Bottom line on this is that The compnent Drawing IS part of your inspection instructions and it is the part of the instructions that needs changed if at all possible.

Peace
James
 
M

Mgoul

#4
You are exactly on point Jim. Thank you for your prompt reply.

Unfortunately, the engineer is unwilling to change the drawing. We have had this argument many times regarding this part as well as several others with similar situations. I won't bore you with all of his reasons but I will share my favorite one; the one that made my head turn purple (and I quote):

1) "I don't want to change the drawing because then the supplier will start making the parts even worse and then I won't be able to accept the N/C parts for use in production!"

I apologize to all the good, quality-oriented people here on the forums for putting that last statement in this thread. I'm sure some of your heads are purple right now as well.
 
M

MIREGMGR

#5
Is the component engineer arguing for only an averaging spec? So if they delivered a distorted part that measured 9.500", 9.750", 10.255", 10.505" at one end, that would be OK with the component engineer?

An averaging process would not be metrologically sound. It wouldn't even make common sense, as it would allow gross deviations as long as they average out.

If it were a conditional/secondary spec, applied only to parts that fall outside of the all-measurements-10.000"-to-10.005" primary spec but within the 9.998"-to-10.007" fallback spec, it in fact would give a different result in regards acceptable parts. A part that was 9.999", 10.000", 10.000", 10.000" would not meet the conditional spec as stated, even though per your description it sounds as if right now such a part would be passed by your material review process.

As suggested above, you need a primary (and only) spec that expresses what you require, and for which the supplier and your incoming inspection can have a common understanding of acceptable/non acceptable that is final.

Among other things, your current process must be having a negative impact on the quality culture at your supplier, since they're able to ship 30% of production to you that doesn't meet spec but does meet your requirements.
 

Jim Wynne

Staff member
Admin
#6
Among other things, your current process must be having a negative impact on the quality culture at your supplier, since they're able to ship 30% of production to you that doesn't meet spec but does meet your requirements.
I think this is a good point. You're inviting cognitive dissonance (the unsettling feeling one gets when trying to hold disparate points of view simultaneously) when you say, on the one hand, that "quality" requires conformance to specifications and on the other hand say, "well, sometimes it does" when the specifications are wonky.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#7
1) "I don't want to change the drawing because then the supplier will start making the parts even worse and then I won't be able to accept the N/C parts for use in production!"
Well, what is the supplier's response to the inability to meet the purchase order requirements? We have not heard that part of the story yet.

Beyond that point, using the average for a diameter is the weakest form of specification twisting. It has no meaning whatsoever. Min, max, true position, some functional approach has much more merit than a statistically insignificant piece of data as the average diameter.
 

Jim Wynne

Staff member
Admin
#8
You are exactly on point Jim. Thank you for your prompt reply.

Unfortunately, the engineer is unwilling to change the drawing. We have had this argument many times regarding this part as well as several others with similar situations. I won't bore you with all of his reasons but I will share my favorite one; the one that made my head turn purple (and I quote):

1) "I don't want to change the drawing because then the supplier will start making the parts even worse and then I won't be able to accept the N/C parts for use in production!"

I apologize to all the good, quality-oriented people here on the forums for putting that last statement in this thread. I'm sure some of your heads are purple right now as well.
What the engineer is likely saying is that he doesn't know exactly what will work and what won't, and can't be bothered to try and figure it out. In fairness to the engineer though, I've seen situations where "the system" is at least partially responsible for the phenomenon--the process for changing a drawing is too complex and slow, and the engineer is overloaded to begin with and has to resort to slapping band-aids on everything just to keep his head above water.

This sort of thing is one reason that it's good to have a formal engineering-change-request process. You should be able to submit a request in a situation like this, explaining the problem (and its potential for negative effects) and a proposed solution. The request should be initially reviewed at a level higher than the offending engineer (the engineering manager, e.g.) and that person should be ultimately responsible for deciding the course of action. Your situation is the type of thing that inevitably happens when engineers are left to their own devices (pun intended).
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#9
What the engineer is likely saying is that he doesn't know exactly what will work and what won't, and can't be bothered to try and figure it out. In fairness to the engineer though, I've seen situations where "the system" is at least partially responsible for the phenomenon--the process for changing a drawing is too complex and slow, and the engineer is overloaded to begin with and has to resort to slapping band-aids on everything just to keep his head above water.
In fairness to the engineer, I have seen the average scam used before - and the approached approved by customers. So, even the customer can be complicit in not knowing exactly what will work and what won't, and can't be bothered to try and figure it out.
 
M

MIREGMGR

#10
50% of the posts I make here are way off base in one direction, and the other 50% are way off base in the other direction...but on average I'm perfect. :)
 
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