Sampling Plan for Inspection of Metal Parts after CNC Machining

Mike S.

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#21
Oh. Well, Figure B1 kinda forces you into some rather high inspection levels. It is not a mistake in the customer requirements. Probability of conformance is another way of saying a protection level to the consumer. ERP = 1- probability of conformance.

So for example if you are inspecting a machined part with a thickness tolerance of +/- .004" you need to use an ARP9013 sampling plan with IRR of >/= 97% or ERP </= 3%.
 
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bobdoering

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#22
Here is my take: sampling plans are great for incoming inspection, but of no value on the machining shop floor. The sampling frequency for a CNC should be 5 samples (not parts, samples) between offset adjustments. You can use 7 if you need more resolution. Generally, X hi/lo-R charting will provide much more information to control the process than sample and data collection. Much more....
 

Mike S.

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#23
Sure, SPC is valuable. But if your customer demands a certain sampling plan be used on each lot prior to shipment, you must comply. If you can use your SPC data as your sample quantity in the sampling plan, all the better.
 

bobdoering

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#24
Sure, meeting the customer requirements is nice, but of no informational value about the process or its output. So, in that case just find any old C=0 plan to meet the requirement. You can tell they are not too deep into sampling or they would have identified the plan more specifically. Its just report card data - has little to no value beyond that. It is the customer's attempt for cheap insurance.
 

Mike S.

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#25
No, with all due respect, meeting the customer's requirements is a NECESSITY.

It sounds like a Boeing requirement -- either direct or flowed-down.
 

bobdoering

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#26
No, with all due respect, meeting the customer's requirements is a NECESSITY.

It sounds like a Boeing requirement -- either direct or flowed-down.
I agree is must be done - but that does not mean it is of any value besides window dressing.
 
R

Rio BP Simbolon

#28
Sampling in production process should consider when probably instability factors arise. Example : Stamping process, sample should take when: Resetting, lot-material change, changing of shift.. etc. And if there's no instability factors arise, we take sample in every first part and every 2 hours...
 

bobdoering

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#29
Sampling in production process should consider when probably instability factors arise. Example : Stamping process, sample should take when: Resetting, lot-material change, changing of shift.. etc. And if there's no instability factors arise, we take sample in every first part and every 2 hours...
For CNC machining, it is primarily a function of tool wear and associated offset adjustment. The sampling should be a frequency of 5 to 7 samples between each offset. That could be 2 hours...it could be 15 minutes...it depends on the tool wear, tolerances and amount of roundness or parallelism used up within the tolerance. You can't just pick a time period out of the air.
 
G

GRISHAG

#30
For CNC machining, taking 5-7- samples and measuring of critical dimensions is not a problem. The problems are a) measuring of all dimensions 2) some of dimensional requirements (roundness, flatness, etc) can be measured using CMM only.
 
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