AQL - How do I find Acceptable Quality Level?

qmslady - 2009 · May 11, 2009

I THINK I'M ON THE RIGHT PAGE.
I FORGOT, IS IT O.K. TO USE CAPS? I NEED SOME HELP WITH AN AQL QUESTION.

I am the Quality Management Systems Rep. and The Internal Auditor for for an aerospace machine shop, yes it's a cool job

.

I write all the policies and the procedures. This is a new company for me as I was laid off my last job, in Feb and got a job 2 days later. Thank God!! I was the AS9100 Auditor, a Lathe machinist and Tool Crib Manager, aerospace and oilfield.

So, this company I'm with now wants to get their certification and we are well on our way. But I have bumped into an area that I am not familiar with, and it has become my "pet peeve" :bonk:

also making me very crazy!!

I need to write a procedure on our AQL Statistical Acceptance Level. I have been researching this for about 3 days now and kept coming up on your website. So I thought I would ask someone for help.

I am finding that I don't really need to know our AQL, only how I arrived at our statistics. In aerospace parts we can have zero non-conformance, but we do run into problems and usually rework the parts or they are scrapped before anything leaves here. What I mean is we don't ship bad parts and our customers do not "accept" a certain level of defects. Our parts have to be precision machined 100% of the time.

We 100% inspect our parts and only want to have to "write down" a certain percentage, for our "in process inspection documentation" records and THAT the is the AQL number I am looking for and the methods I used to arrive at it.

Does anybody understand? Is there a chart online somewhere, that I can get this info...I would really appreciate some input.

Thanx...QMSlady :agree:

Gordon Clarke · May 11, 2009

qmslady said:
I THINK I'M ON THE RIGHT PAGE.
I FORGOT, IS IT O.K. TO USE CAPS? I NEED SOME HELP WITH AN AQL QUESTION.

Slightly (OK a lot) off subject, but reading that made me think of the old joke:
"Why do you always answer my questions with a question"?
"DO I"?

Years ago I could understand the value of AQLs but nowadays not so much. AQLs infer that are certain number of defects are allowable. A "Monday car" used to be a joke to all except the unfortunate buyer!
I'd hate to be passenger in a "Monday aeroplane" :mg:
With machined parts it is almost entirely about knowing what the machine is capable of manufacturing and that the operator pays attention to what is being produced.
It probably won't be "accepted" by the powers that be, but personally I'd never put in writing that I found it acceptable to manufacure even an infinitesmal quantity of faults.
Let the suppliers documented deliveries speak for themselves. Common sense would deem always to have at least two supliers for all (and definately for the critical) parts. Compare at regular intervals what the quality of the deliveries is and focus the suppliers attention to what is classified as critical. If it isn't critical and there are problems perhaps a new tolerance or acceptance criteria is the way forward.

I'll probably write more after I read the comments that come in

qmslady - 2009 · May 11, 2009

Hi gordon,

thanx for the jokes. Just so everybody knows...we are the sub-supplier, the last in the chain...like i said before we measaure all dims 100% inspection is criteria from some customers, we always check criticals, especially on parts with known problem specs. I need to know how to calculate an aql, show the criteria used to arrive at an aql of say 1.0? For inspecting all of our parts and get on with other things, i have a lot of procedures to write as well a quality manual and the training of the employees on the new standards they are being asked to perform...
Qmslady

howste · May 11, 2009

qmslady said:
I FORGOT, IS IT O.K. TO USE CAPS? I NEED SOME HELP WITH AN AQL QUESTION.

You can use all caps if you want us to think you're shouting at us...

qmslady said:
We 100% inspect our parts and only want to have to "write down" a certain percentage, for our "in process inspection documentation" records and THAT the is the AQL number I am looking for and the methods I used to arrive at it.

You're not really using an AQL, since you're already doing 100% inspection. So the question is, how many readings are you going to record?

The first question I have is: What do your customers want/need/require? If they want all values recorded, and are willing to pay for it, record them all.

If they don't specify or don't care, you get to decide. What is your batch size? If it's not too large, you might consider recording the range of readings for each batch.

alspread · May 11, 2009

An AQL is so much calculated as it is determined. You can pick any AQL level you want depending on what level of "quality" you want to live up to.

The AQL will determine you sample quantity to inspect from a lot. If you are performing 100% inspection, then you are not sampling and therefore don't need to have any AQL level. Just write your procedure that you do not use any sampling inspection (if that's the case).

If not (more likely) then you need to select the AQL that gives you the most assurance with the inspection resources you have available. Or (as is often the case in aerospace) your customer forces a certain minimum AQL level through their contract or specs.

You can (and will) use different AQL's for different characteristics depending on their individual signifigance, tolerance, customer designation, process capability, etc.

Good luck, and read your customer's requirements carefully on this subject and how to apply them in different manufacturing and lot situations.

Gordon Clarke · May 11, 2009

qmslady said:
Hi gordon,

thanx for the jokes. Just so everybody knows...we are the sub-supplier, the last in the chain...like i said before we measaure all dims 100% inspection is criteria from some customers, we always check criticals, especially on parts with known problem specs. I need to know how to calculate an aql, show the criteria used to arrive at an aql of say 1.0? For inspecting all of our parts and get on with other things, i have a lot of procedures to write as well a quality manual and the training of the employees on the new standards they are being asked to perform...
Qmslady

Hi M'lady

I don't get it. Usually (always?) it's the customer that specifies the AQL level - not the supplier.
If you really inspect 100% then how often do you delivery faulty parts?
I think you are paying the price due to the fact that your customer has had a bad experience at some time. Either that or the ones interested in AQLs must have been around when man (and woman) started to walk upright on two legs.
To me, if the dimensions can be measured, supply the customer with documentation that you're within tolerance. SPC. If your spread is 70% or less of +/-3 sigma then you have the process well under control.
An AQL of 1.o???? One percent is acceptable????? You've never delivered to Europe or Japan

In MIL STD 105D or whatever you use, the sampling size depends on quantity.

Another (poor) joke:
If you did 100% control on a box of matches you could always tell the customer they were all OK and every single one ignited. :bonk:

Jim Wynne · May 11, 2009

qmslady said:
I THINK I'M ON THE RIGHT PAGE.
I FORGOT, IS IT O.K. TO USE CAPS? I NEED SOME HELP WITH AN AQL QUESTION.

I am the Quality Management Systems Rep. and The Internal Auditor for for an aerospace machine shop, yes it's a cool job.

I write all the policies and the procedures. This is a new company for me as I was laid off my last job, in Feb and got a job 2 days later. Thank God!! I was the AS9100 Auditor, a Lathe machinist and Tool Crib Manager, aerospace and oilfield.

So, this company I'm with now wants to get their certification and we are well on our way. But I have bumped into an area that I am not familiar with, and it has become my "pet peeve"also making me very crazy!!

I need to write a procedure on our AQL Statistical Acceptance Level. I have been researching this for about 3 days now and kept coming up on your website. So I thought I would ask someone for help.

I am finding that I don't really need to know our AQL, only how I arrived at our statistics. In aerospace parts we can have zero non-conformance, but we do run into problems and usually rework the parts or they are scrapped before anything leaves here. What I mean is we don't ship bad parts and our customers do not "accept" a certain level of defects. Our parts have to be precision machined 100% of the time.

We 100% inspect our parts and only want to have to "write down" a certain percentage, for our "in process inspection documentation" records and THAT the is the AQL number I am looking for and the methods I used to arrive at it.

Does anybody understand? Is there a chart online somewhere, that I can get this info...I would really appreciate some input.

Thanx...QMSlady

Are you trying to establish a sampling plan?

Gordon Clarke · May 12, 2009

M'lady,
All of the replies/suggestions you have been getting are serious and express the same doubt as myself, although I do tend to be less serious than most. If I were you, I'd pick one of the "contributors" (cheapest within the USA) and give the person a phonecall. That's probably the quickest way to solve your apparent dilemma. You can then write and let the rest of us know how things progressed.

qmslady - 2009 · May 12, 2009

alspread said:
An AQL is so much calculated as it is determined. You ould pick any AQL level you wanted depending on what level of quality you want to live up to.

The AQL will determine you sample quantity to inspect from a lot. If you are performing 100% inspection, then you are not sampling and therefore don't need to have any AQL level. Just write your procedure that you do not use any sampling inspection (if that's the case).

If not (more likely) then you need to select the AQL that gives you the most assurance with the inspection resources you have available. Or (as is often the case in aerospace) your customer forces a certain minimum AQL level through their contract or specs.

You can (and will) use different AQL's for different characteristics depending on their individual signifigance, tolerance, customer designation, process capability, etc.

Good luck, and read your customer's requirements carefully on this subject and how to apply them in different manufacturing and lot situations.

o.k.
I think I get what your saying, say what we do, and do what we say? We inspect 100% and write down 10%. Our customers have NO "set requirements" as yet, but we just picked up a new job. Our batches run from 1 piece to 100 pieces. Mostly around 25 to 50 pieces, so 100% is not too time consuming, and is checked while the next part is running.

So you are right in saying that we do not do "sampling inspection". But how can I justify the percentage that we write down? To say that we are writing down 10% of 100% inspected parts over the quanity of, say, 20 pieces and every part of 15 pieces or less? What's the AQL for that measurement?

Most of our customers do not specify their inspection criteria in "terms", only the tolerances given in the specs, which we abide by, usually running nominal. There are some parts that have pretty tight tolerances, critical dims and we have been writing down every other part to show consistancy in conformity, unless the lot is small, we write 1st piece, (FAI), every piece "in process" and Final.

If it's just a matter of writing down "how" we do it, that I can handle, but if we have to "justify" how we came to this conclusion, of writing down only 10%, I need an AQL number and how we arrived at that number, being careful not to "pin" us down to any one way.
Appreciate this, you have been very helpful...qmslady

Jen Kirley · May 12, 2009

Unless you can justify which parts you choose to write down, you can't justify writing down only 10% or 15% when inspecting 100%.

I have found a tutorial on acceptance sampling. I am not affiliated with Servicco. I hope this helps.

AQL - How do I find Acceptable Quality Level?

qmslady - 2009

Gordon Clarke

qmslady - 2009

howste

Thaumaturge

alspread

Gordon Clarke

Jim Wynne

Gordon Clarke

qmslady - 2009

Jen Kirley

Quality and Auditing Expert

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