Capability Study on a New Machining Process - Issues

Q

QE

Re: Capability Study - Issues

I am not able to open the data file.


All points are the same for each sheet. Example: upper left corner always location 1, upper right always location 3, etc...
I'm not sure what type of distribution I have. I looked at transforming the data using Box-Cox and got a lambda value of -5. I was not sure about this approach given the non-normal dist and being out of control. I have looked at the data and it does look like I have an issue with my sheet holding vacuum table. I noticed that location 1 is always low and location 4 is always high. I have attached the data.
 

bobdoering

Stop X-bar/R Madness!!
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Here is the file in XLS format to help folks out.
 

Attachments

  • Elsmar Excel 97.xls
    48.5 KB · Views: 217
B

brahmaiah

Not very - thickness is 0.046+/-0.004. We don't check parallelism.
Why do'nt you check parallelism? Do you have a customer waiver on that.? The general convension is parallelism should be within half the tolerance for the thickness or as specified by customer.
V.J.Brahmaiah
 
M

mayor

I'm not that familiar with Parrallelism. I assume this is a frontside to backside reference, but I don't know how it is determined. We take the sheets (which are urethane based) as supplied at 0.046" +/-0.004" and thin them to 0.0355" +/- 0.0005". The sheets are then used as a component in an assembled product.
 

bobdoering

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For simplicity's sake, the parallelism is the variation in thickness of one side to the other. A part with no variation - every location has the same thickness - would have a parallelism of 0.0000" As the variation increases, so does the parallelism. So, if you take the highest thickness from across the sheet, and subtract the lowest thickness from across the sheet, the difference is parallelism. I have prepared an X hi/lo-r chart on the last page of the attachment. It is designed to display and control that type of characteristic - particularly in precision machining. The upper char it the plot of the thickest and thinnest data on the part, the range is teh difference - or parallelism. You have precision tolerances - but I am not sure if your process falls into the definition of precision machining. In any event, it is telling.

First, you are not capable. Capability is the ratio of the process variation to the spec - and since you are both using a large amount of your spec, and have out of spec points, you are not capable. Whatever calculation you used to get 1.5 does not reflect reality.

Second, you data shows a downward trend overall. Point 4,5,6 show a shift. The other point do not show the shift as dramatically - so those locations may be influenced by what happened in locations 4,5 and 6, showing a gentler trend.


Not knowing your process (is it cutting or forming?) it is difficult to tell from a distance was the issue are. Not knowing what the process is exactly, I can not judge if you have - or should expect - a true trend. If you should expect a trend - such as in tool wear - you need to eliminate your strong special causes. As an outside dimension, tool wear should exhibit increase in size over time (if it is a cutting tool). If you have heat build-up issues, however, the tool may expand from part to part, causing a slow decrease in the dimension - similar to what you are seeing. Your fixturing may also be a problem - although it generally would affect all parts equally. Again, the variation of the raw material may also contribute to your overall variation - but without more understanding of the process, it is not clear how it might.

Hope this helps....
 

Attachments

  • Plastic Thinning.doc
    270.5 KB · Views: 128
M

mayor

Sounds like what we call TTV, total thickness variation, max-min. The charts are helpful, I like the hi/lo chart. We have ID'ed that the fixture (vacuum table) is a large source of the within variation. We are currently addressing that issue.
 

bobdoering

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I will add that I used the I-MR chart just as an analytical tool. It may not be the correct chart, nor may its calculations be correct - depending on the true underlying distribution.

However, I do recommend the X hi/lo-R chart for what you are doing. Forget averages - they are useless for tight tolerances. You need to see how much of the tolerance your process is eating up. At a minimum, your control limits should be 75% of the tolerance if you really wanted to feel as if you are in control for your process. Until then, you can rest assured you have some work to do.

Also, are you satisfied with your gage R&R? Gage error and measurement error can contribute to the total variation you see.
 

bobdoering

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Why don't you check parallelism? Do you have a customer waiver on that.? The general convention is parallelism should be within half the tolerance for the thickness or as specified by customer.
V.J.Brahmaiah

This was in reference to the raw material, so it is not a customer concern.

However, you are right in that any time you have a length or thickness, you should check parallelism (at least max-min over the area the dimension affects) or roundness for circular features. People measure one location out of an infinite number of locations - not good odds of getting a picture of what is happening...

As far as general convention, I would say if you are using more than 30% of your tolerance in parallelism (or roundness), you should look at another process. If you can not afford a better process, you better track that dimension like a hawk - usually using the X hi/lo-R charting methodology.
 

bobdoering

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In a control control chart a data should represent one characteristic fully and not 8 of them together represent one characteristic.At the end of measurement we have to answer how many parts are accepted and not how many spots are accepted.
V.J.Brahmaiah

The X hi/lo-R chart answers both what are all of the points doing, and is the part itself in spec (or control). It is nice to have the cake and eat it. Also, easier, less math, better visibility of the variation, and much, much more.


However to analyze the control within individual points to see if any one point is contributing more to the variation than another (which can readily be the case), individual charting is a very powerful tool.
 
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