Sampling - Measuring and Inspecting a part with both Variable & Attribute Data

bobdoering

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#11
Having been in the industry making bone screws, I have found characteristics that proper control (including last off) will catch most rare conditions. It will not prevent them, but it will catch them. That will provide much more confidence for reducing inspection during final inspection - especially for template characteristics. As you know, the key difference between acceptance sampling and control is control reviews the part in time order and acceptance is random. Time order allows you to find a condition and remove any previous that show it back to its origin. So, if it is not statistically predicted, it will be found and contained.

The main exception is the chip impression. That may be able to be minimized with coolant flow and direction, but on many materials - and titanium has its issues - it is not possible to mistake proof. 100% visual is required - both at the machine and final.

But, there are dimensional characteristics that elude mistake proofing but can be better controlled - and small drilled holes are notorious example. You really do not know when a drill is going to fail - it may break or wear much differently than another drill. For that I inspected 100% with pin gages at 75% of the tolerance. They used to gage at 100% at full tolerance - and guess what happened? Yep, they would fail final. Goes back to what I always say - manufacturing does not own the full tolerance, if anyone does Final Inspection does, but manufacturing does not. It would not accommodate processes after the drilling - such as deburring - or even little variations like gage tolerance and fit. After changing the manufacturing floor gages to 75% - and keeping their mitts off of the gages in final inspection, final inspection was a breeze.

So, when final inspection is required the best approach is to make sure whatever you make will pass it. That cuts your overhead way down - you only inspect it once. It helps keep shipments on time. Mistake proofing is one good approach if you can do it. For swiss machines, most of the mistake proofing comes in clever programming and tool choice. I suppose dimension proofing, with durable controls on the shop floor, is another.:cool:
 
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bobdoering

Stop X-bar/R Madness!!
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#12
I'm just looking for easy-to-implement methods that will enable me to make my customer happy and reduce the inspection burden on my end.
There is one other thing you can do. Works great for your operation. Run all dimensions to 75% of the tolerance on the shop floor. There is a statistical reason behind it (http://elsmar.com/Forums/showthread.php?t=6658&page=3), but the short story is it is "poor man's SPC". I am betting you are already doing last off. Your final inspection will be a breeze. And that is the goal...:agree1:

One final question: Does the Z1.4 standard state anything about running at least 5 lots when checking attribute data?
I do not have the standard handy any more to answer that...sorry.:cool:
 

Bev D

Heretical Statistician
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Super Moderator
#13
Again, this all applies to long production runs at large organizations with big budgets and big quality departments full of people getting paid to study and prove theories related to SPC,
Sorry to upset you but these methods are not just for big companies or long runs. That's just a cop out response... (companies with a large work force don't have any more spare time than companies with a small work force) Nor do they take a long time or money compared to what you spend on inspection, scrap and rework. Bob has many good suggestions for improving your quality which is what enables you to reduce final inspection sample sizes. Additional mistake proofing is also not that expensive. I too have worked in high reliability low volume, short run precision machining environments with unreasonable customers that demand 100% inspection and have used many of the approaches that Bob describes as well as others - but you do have to research and learn about alternative methods, here or in other places - there are no quick fixes. There are some errors that create defects that come and go and unless you are inspecting 100% you will miss them with random or time sequence inspection/checking...but you may not have these events or they are rare enough that the defect rate they cause is acceptable to your end customer. Only you know that.

One of the great things about this forum: we can make that research faster and easier and more focused. We also are always trying to improve things not just provide point solution answers to narrow focused questions.

Bob has some good advice...good luck and I hope it works for you.
 
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