Removing Inspection (Functional Tests) from a Process Step

  • Thread starter Quality Engineer66
  • Start date
Q

Quality Engineer66

#1
Good Afternoon,

I need your opinion on the following situation.

Assembly Process has three steps:

First step: functional test

Second step: weld the parts

Third step: functional test again.

Both the functional test are exactly the same. And I would like remove the first step and just do one test.

My question is there any quality tool or technique can I use to do this to feel confident that its ok to remove it. Or just do it the simple way: collect the data for few months and if there are no failures during first step then remove it.

Its an electrical test. so its a "pass" and "fail" test. If light is "on" is pass otherwise "fail".

I have the data for last one year as we record it every time. Whats the best way in this case?

I will highly appreciate if someone can guide me.

Thanks,
 
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Michael_M

Trusted Information Resource
#2
Re: Rmoving Inspection (Functional Tests) from a Process Steps

I am guessing as to what you are actually doing:

It looks like sometime in the past you had parts fail the second test and it was determined that the parts were 'bad' prior to welding. A test prior to welding was implemented to segregate the non-conforming parts prior to adding time (and material) by welding them. There is also the possibility of rework prior to weld that would not exist after welding.

If the above is the case, then the method I would use to eliminate the first step is 'cost' vs 'benefit'. What is the cost of performing the first test. How many 'non-conforming' parts does it detect. What would the cost be if these were discovered after welding.


This is just one example.
 
Q

Quality Engineer66

#3
Re: Rmoving Inspection (Functional Tests) from a Process Steps

Michael,

You are absolutely correct about history for doing this test.

Is there any standard template or tool can I use?

Thanks,
Ashok
 

Michael_M

Trusted Information Resource
#4
Re: Rmoving Inspection (Functional Tests) from a Process Steps

I am sure there is somewhere but I just put numbers on paper, verify the numbers are correct and not just saying what I want them to say, and make the deciding factor.

For example (I am going to make up numbers for this example):

Part costs $4.50 in material and processing to get to the first inspection stage. This point the part costs $4.50.

Inspection stage 1 costs $.50 to perform. This stage rejects 5% of the parts (price per part is $5.00--does not include rejections)

Welding costs $1.50 to perform (making the total price without rejection $6.50)

Inspection stage 2 costs $.50 to perform. This stage has 0% rejects (but is required per customer contract so we cannot eliminate it). Right now you have $7.00 into the part.

Parts rejected from the first stage cannot be repaired and are scrap. Assuming 100 parts:

Rejection after stage 1 will cost you $25.00 ($5.00 each *100 * .05 rejection rate)
Rejection after stage 2 will cost you $35.00 ($7.00 each *100 * .05 rejection rate).
Stage 1 inspection is costing you $50.00 to perform ($.50 *100) and the difference between stage 1 and stage 2 inspection is only $10.00. To me the cost vs. benefit is not worth keeping both inspections.

This is a pretty simplistic example, I did not take into account the cost if you can rework a part if discovered at stage 1 but not after stage 2, nor does it add in the cost of welding the 5% scrap ($7.50). I hope this may help a bit. One of the harder things to do is put real numbers down, not numbers that you want to see to get the conclusion you want.
 
Last edited:
M

MGMTREP

#5
Re: Removing Inspection (Functional Tests) from a Process Steps

Why not put effort into preventing the NC parts, so you know that all parts can be welded? Is there something about these parts that keep them from being 100% conforming?

Just a thought.
 
Q

Quality Engineer66

#6
Re: Rmoving Inspection (Functional Tests) from a Process Steps

Michael,

thanks a lot for such a detailed input. I will follow these guidelines.
 
Q

Quality Engineer66

#7
Re: Removing Inspection (Functional Tests) from a Process Steps

MGMTREP: There are not lot of failures. Its just that we have been doing this for years.
 
M

MGMTREP

#8
Ashok - You would know better than I about the number of failures. I wonder though: If there aren't enough failures to employ a prevention approach, why pre-test at all? If it's because, as Michael M posited, the cost of rework/scrap before/after welding is too high, then doesn't that still suggest a prevention approach may be the better long term solution?

You would know if there are other retraining forces e.g the assembly and/or process re-design that may be required isn't an option.
 

Helmut Jilling

Auditor / Consultant
#9
Re: Rmoving Inspection (Functional Tests) from a Process Steps

I am sure there is somewhere but I just put numbers on paper, verify the numbers are correct and not just saying what I want them to say, and make the deciding factor.

For example (I am going to make up numbers for this example):

Part costs $4.50 in material and processing to get to the first inspection stage. This point the part costs $4.50.

Inspection stage 1 costs $.50 to perform. This stage rejects 5% of the parts (price per part is $5.00--does not include rejections)

Welding costs $1.50 to perform (making the total price without rejection $6.50)

Inspection stage 2 costs $.50 to perform. This stage has 0% rejects (but is required per customer contract so we cannot eliminate it). Right now you have $7.00 into the part.

Parts rejected from the first stage cannot be repaired and are scrap. Assuming 100 parts:

Rejection after stage 1 will cost you $25.00 ($5.00 each *100 * .05 rejection rate)
Rejection after stage 2 will cost you $35.00 ($7.00 each *100 * .05 rejection rate).
Stage 1 inspection is costing you $50.00 to perform ($.50 *100) and the difference between stage 1 and stage 2 inspection is only $10.00. To me the cost vs. benefit is not worth keeping both inspections.

This is a pretty simplistic example, I did not take into account the cost if you can rework a part if discovered at stage 1 but not after stage 2, nor does it add in the cost of welding the 5% scrap ($7.50). I hope this may help a bit. One of the harder things to do is put real numbers down, not numbers that you want to see to get the conclusion you want.
If I read this right what you're really doing is combining inspection one into inspection two. If inspection two performs the same property tests as inspection one, that might be permissible. As you demonstrate it just raises the cost of each reject. If you have data to support that the customer is still protected, and if your data does demonstrate that the failure rate is low enough to justify this approach, the customer might prove it. Since you were dealing with welds, you probably need to include customer in the discussion.

In your hypothetical example, you indicated inspection one shows a 5% failure rate. A five % reject sounds very high. If it really is anywhere near that high, I doubt that this approach would be economically practical.
 
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