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#### Coleman Donnelly

I have about a dozen or so features that are being inspected in process and I have boiled down my control variables to Spindle Speed, feed rate and fixturing pressure (vacuum pressure).

So I am trying to set-up my study so I can establish the number of samples that will be needed.

The effect that I am looking for is to reduce common cause variation from the process. By reducing the overall variation of the process (or sigma value) I can improve the CPK, reduce scrap rates and potentially reduce my inspection frequency accordingly since the process inspection results will be using control charts to control the process.

Back to my problem:

I have 3 variables that I can play with which gives me 8 corner points. I believe that I want to use center points as well in order to improve the vissibility of curvature of the settings.

I also heard something about star points, I don't know what they are or when they are required/applicable...

So this means that one run of my DOE=9 samples

Now I need to figure out how many replicates and repeats I am going to want to do.

If I do a power and sample size study I need to plug in 3 factors and it will give me the 4th...

Number of replicates

Effect

Power

Center points

So -

Center Points -1

Power - ?

Effect - ?

Number of replicates - ?

I am assuming that Power should be between 80 and 90%, but I am unsure of this. My perfectionist brain is telling me that I should try to get close to 100%, but I have been cautioned that if Power is to high than I will get a false signal in my design.

so I run power at both 80 and 90

Effect. I am trying to improve my process Cp/Pp. If my current process state yields a Cp of 1.15 and my target is 1.66 does this mean that my effect response I am looking for should be set to 0.51? It seems that any effect in the right direction is a good thing...

So I am not sure what to put here.

I have:

center points 1

Power 80, 90%

Effect 0.10

Replicates?

So here is the problem... Typically I would run a 30pc short run Cpk study to evaluate the Cp of a given characteristic of my part... This takes my original DOE sample size from 9 to 270 with 0 repeats and 0 replicates... this seems to me like a bit much...

So now I back up a step:

If I can calculate my sigma from an initial capability study for each of my characteristics than cant I determine a size smaller than 30 to establish my Cp/Cpk within a reasonable 95% confidence interval. Can I than reduce my 30 pc requirement to the largest requirement to have 95% confidence that my returned sigma value from the DOE is stable?

This is my idea, but I don't exactly know how to do this...

I should mention that I have never actually done a DOE study - this is my first one so if someone can proof my strategy I would feel a lot better.