SPC Coaching

If your process is in control & capable, why would you want to 100% inspect anyway?
100% inspection is only about 80% effective in the long term.
 

Read Jim’s response…Don’t confuse SPC and acceptance sampling.  SPC is based on the voice of the process, not the customer meaning that the control limits are set on historical process variation.  As long as you have a “capable” process with a Cpk>1.33 and your process remains in control, then there will be a very low probability of producing out of spec products. 

Learn more what is power and sensitivity of a control chart. Learn the basic first!

I agree that by changing the control limits from 3 to 2 sigma will cause more “out of control” parts to be captured and may require the operator to correct the process more, but this could cause over control resulting in more process variation that if the process was allowed to run in control (against control limits of 3 stdev).
My understanding of the original question was to do with capturing out of spec products which is more of a sampling plan question, not a SPC question.
If I am incorrect, please enlighten…

Down doggy,down!  Didn’t you read the question? He was asking about the control chart and how it relates to good and bad parts – which it doesn’t (in control).  You responded by saying “switch to 2-sigma limits”.  What does that have to do with good/bad parts doggy? He wants to show his operators how he can sample production and still keep bad parts from getting to the customer – that requires knowledge of the specification doggy.

You are totally clueless about how to re-set control limit, but still want to show off  like a guru.
A famous slogan for you, if you cannot attack a person’s idea, then the best thing you can do is personal attack the person.  

Have a good weekend…

Darth,
I think what they are getting at is that they are using go/ nogo gauges for the 20 for acceptance purposes but measure the 3 parts so that they can adjust the machine / tools – well that was my experience when I used to set up & run production machines.
Best regards,
Adam

This the original poster said their product has something to do with the medical field, maybe they should contact the HMO’s and ask them.  They seem to know all the answers anyway.  Of course medical devices could mean “Bedpans”.
Plastic

AA,
There is absolutely nothing in a control chart that will guarantee you ship good product. You can be “in control” and ship 100% bad product (your control limits can be located outside the spec limits). There is nothing that makes your control limits occur inside the specification. You can also have a distribution so tight inside the spec limits you could have a >6 sigma process.
One of your earlier post discussed using Capability Analysis to explain the probability of defects that is probably the easiest way to explain it. If you are using Minitab the numbers that occur outside the spect limits are at the bottom of the page. If your people believe that 100% inspection (which is actually sorting not inspection) will do better run a Attribute gage study with them.
Do yourself a favor – let go of the medical device stuff. The aircraft engine guys have a similar issue and so do the nuclear energy manufactureres (anyone for some defective fuel rods?) how about people who build bridges, cars, bomb fuses, etc. There are a lot of industries that have life threatening effects from defects.
Just my opinion.
Good luck

Hi:
I believe we need to know the process capability of the control chart establishment.
If the control chart is generated based on a capable process (e.g. Cpk>1.33), the in-control plot-point can ensure good shipment, because the possible escaped rate under this situation is less than the escaped rate of performing normal sample plan inspection(e.g. mill-std-105).
If the control chart is generated under a incapable process (e.g. CKP<1) , It is difficulty to explain the question, at that time, "in-control chart" only state the process is stable, but, can not ensure outgoing product quality.
Thanks!