Control Chart CpK dilemma - Defect Per Unit data on a single production line

[sbickley]

All,

Please provide some input to the question I pose below. Please keep in mind that I fully support SPC but am not a statitician and don't want to be. I use automated SPC packages, understand the basic concepts of SPC, but need some technical guidance - in a form that I can use to communicate to others in my organization.

The Issue: I want to generate a control chart for Defect Per Unit data on a single production line, by week. What type of control chart should I use, keeping in mind that the LSL is always 0?

2nd - How can I calculate CpK? (histogram?)

3rd - I don't have a set USL to work with from management, should I arbitrarily pick one or do I need one to control chart the process?

Thanks in advance for your help!
Scott

[cncmarine]

Let me try to understand this:

1.You want chart nonconformities on the production line.

2.Do you need to have it in CPK????

3. Do you have access to the production numbers. Units produced?

[The Taz!]

Originally Posted by sbickley

All,

The Issue: I want to generate a control chart for Defect Per Unit data on a single production line, by week. What type of control chart should I use, keeping in mind that the LSL is always 0?

I am somewhat confused. . . it sounds like you want to capture attribute data. If you can determine a "sample space". . . you can use a U-chart. . .

If you have a fixed number of potential defects, you can chart the number of defects found on a part on a C-chart if you use a constant sample size. . .

I am not sure why you would want to chart defects per unit. . . a more meaningful way might be to determine then chart the individual defects on a p-chart as a % of pieces produced between sampling. . .

Originally Posted by sbickley

2nd - How can I calculate CpK? (histogram?)

A histogram is typically for variable data. A frequency diagram may be more meaningful. If you are in fact looking at attribute data, I'd suggest calculating PPM and using that as a measure. PPM = (Defects/Total parts) x 1,000,000

Originally Posted by sbickley

3rd - I don't have a set USL to work with from management, should I arbitrarily pick one or do I need one to control chart the process?

Depends on what sort of analysis/monitoring tool you choose. Control Limits for u, C and p charts are calculable.

Originally Posted by sbickley

Thanks in advance for your help! Scott

Not sure if I deserve thanks yet. . .

[cncmarine]

I have to agree with TAZ on this.

Go with the p chart.


If you are looking for CPK then you might look into precontrol on the production line it self. That way you can be charting nonconfromities and getting life input from the operators.

[Rob Nix]

A "U" chart (as Taz mentioned) is designed for defects per unit (Juran's Handbook pg. 45.15). Establish a baseline, calculate control limits (simple equation - if you can't find it, post request), and get it in control.

You can possibly use a concentration diagram, a picture of the product with a dot or other character representing each defect and where it happens on the product (you will see clusters in certain areas).

Also, categorize the types of defects and do a pareto analysis.

As far as Cpks and USLs, don't bother worrying about that. Remember Deming's warning about numerical targets. Just analyze the process (as above) and make improvements where you can.

Hope this helps.

[sbickley]

OK - let me get a bit more specific - my company is not even in the infancy of implementing an SPC program. We are building Slot Machines; the DPU measure I have is the total defects found by QA/Total machines produced.

No analysis has been conducted on how many opportunities for defects there are - it is literally thousands - and changes with each machine configuration, as each machine is custom built.

A p chart is not feasible in light of this, from my limited experience. I want to generate a chart to show if the process is/is not in control, i.e. is the DPU number stable over time or all over the map. Any ideas on that one?

Ultimately, I'd like to establish an USL on the allowable DPU (target) and measure CpK to that - does that make sense?

[sbickley]

Thank Rob - planned on doing the pareto by defect type. I will also try the u chart - once I plug the data into my software package, I'll be asking some more questions!!!

Thanks!

[The Taz!]

Originally Posted by sbickley

Thank Rob - planned on doing the pareto by defect type. I will also try the u chart - once I plug the data into my software package, I'll be asking some more questions!!! Thanks!

Be careful with the U-chart. . . FOLLOW THE RULES.

You may be better off listing all defects and doing some basic problem solving to minimize them instead of charting them. . . Trend the data as each machine is produced. Statistical studies and their calculations are typically not setup well for one-of-a-kind hand built machines. People are the process there and usually don't lend well to statistical control. You may need to re-evaluate the tools you are using.

By the way, can I have the serial number and location of the slot machine with the most defects or lowest quality level??