What is the optimum number of check items should an inspector look into a product?

reynald

Quite Involved in Discussions
#1
Suppose i have many quality characteristics that i need to inspect in the product and I can choose to design a series of inspection processes and allocate how many of these check items each station whould check.

How would you answer this question: What is the optimum number of check items should an inspector look into a product?

Example: 40 check items.
Should i do one inspection process where an inspector would look at the product if it deviates any of the 40 check items

or

should i do a series of 4 inspection stations each station owning 10 of the check items?

What should be done to answer that question?

Thanks
Reynald
 
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I

isoalchemist

#2
Re: What is the optimum number of check items should an inspector look into a product

It really depends on the process and the product. Are the checks similar in nature, is special equipment ($$$) needed, do you have the physical space, manpower etc.

Also what are the 40 checks? Is visual appearance one check or are you filling out a form for scratches,dents, surface reflection, etc. A little more information would be helpful because IMHO both are viable options depending on the inspection/product.
 
J

Jon Nonns

#3
Re: What is the optimum number of check items should an inspector look into a product

Suppose i have many quality characteristics that i need to inspect in the product and I can choose to design a series of inspection processes and allocate how many of these check items each station whould check.

How would you answer this question: What is the optimum number of check items should an inspector look into a product?

Example: 40 check items.
Should i do one inspection process where an inspector would look at the product if it deviates any of the 40 check items

or

should i do a series of 4 inspection stations each station owning 10 of the check items?

What should be done to answer that question?

Thanks
Reynald
Another thing to really consider is the qualification of the inspector(s). If specialty equipment is involved are all inspector(s) qualified to use it. Additionally look at it in a process flow and try to balance the inspection line. Even if there are 40 inspection items some may take much longer than others, some might be able to be done in parallel by the same inspector. Don't be afraid to think outside the proverbial box and try to make the inspection process as lean as possible. You have a unique opportunity to design a lean process from the start. Good luck and remember that this is just my :2cents:.

Jon
 

reynald

Quite Involved in Discussions
#4
Re: What is the optimum number of check items should an inspector look into a product

Thank you for the responses.
All check items are visual in nature, under about 100x magnification (using microscope). The check items are similar to scratches, dents, foreign materials, chips, and others.

I might have space and some extra/idle equipments but im not really sure how to measure/quantify the improvement in terms if effectiveness.
 
S

slack157

#5
Re: What is the optimum number of check items should an inspector look into a product

Inspection is necessary evil, it must be as lean as possible.
It's necessary to spend time on process control instead of purely relying on inspection.

In case of special process, a process validation is necessary with all the concerned inspection, but not mass production.
 
H

hinzchaa

#6
Re: What is the optimum number of check items should an inspector look into a product

Depends on the AQL:

The acceptable quality level or AQL of a sampling plan is a measure of the level of quality routinely accepted by that sampling plan. It is defined as the percent defective (defectives per hundred units X 100%) that the sampling plan will accept 95% of the time. This means lots at or better than the AQL are accepted at least 95% of the time and rejected at most 5% of the time.


The AQL can be determined using the operating characteristic (OC) curve by finding that quality level on the bottom axis that corresponds to a probability of acceptance of 0.95 (95%) on the left axis.
Associated with the AQL is [FONT=Arial, helvetica, sans-serif]a confidence statement one can make. If the lot passes the sampling plan, one can state with 95% confidence that the quality level of the lot is equal to or better than the AQL (i.e., the defective rate of the lot < AQL). On the other hand, if the lot fails the sampling plan, one can state with 95% confidence that the quality level of the lot is worse than the AQL.
[/FONT]

"When I was in QC for a recieving inspection dock, we would start with a given number base on the sampling plan. If any characteritics were non-conforming the inspection level would then be determined as to the criticality of the diemension out of specification. If further inspection was necessary, the lot would go into a hold area for further inspection."


Lot tolerance % acceptance:

Acceptance sampling is an important aspect of statistical quality control. It originated back in World War II when the military had to determine which batches of ammunition to accept and which ones to reject. They knew that they couldn't test every bullet to determine if it will do its job in the field. On the other hand, they had to be confident that the bullets they're getting will not fail when their lives are already on the line. Acceptance sampling was the answer - testing a few representative bullets from the lot so they'll know how the rest of the bullets will perform.
 
H

hinzchaa

#7
Re: What is the optimum number of check items should an inspector look into a product

Sources:
siliconfareast.com/ltpd aql.htm and siliconfareast.com/ltpd aql2.htm
 
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