Measurement System Capability with a Continuous Measurement System

E

Elwood

#1
I am trying to set up a SPC scheme on several production lines that extrude electrical conductors and the like. We use a laser measurement system that measures the outer diameter in the x and y direction and then takes that average of these two values for the purpose of process control. How do I conduct a measurement system capability study on this device? The ideal way, I think, would be to run a calibrated cable of sorts through our process, collect data, and then analyze that data. Unfortunately, we do not possess such a cable nor do I know where to find one. Is there any other way I can do this? I do not feel comfortable implementing SPC on these processes without knowing to what degree I can trust my measurement system.
Thanks for the help.
 
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Miner

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#3
Is this an automated test? That is, does the wire run through the measurement device continuously during the measurement, or does an operator collect a sample and present it to the device?
 
E

Elwood

#4
Yes, it is an automated test. The product being extruded passes through the device and is measured on two planes and then averaged.
 
P

prototyper

#5
How is the system calibrated?
This may give an indication as to how you could conduct an MSA study!
 

Miner

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#6
Because this is an extrusion, the process is probably strongly autocorrelated for some time period. This means that sections in close physical proximity are virtually identical except for measurement error. Take repeated measures over a short period of time for one "part". Wait beond the period of autocorrelation (you will have to determine this statistically), and take more repeated measurements for the next "part". Repeat. There will be no reproducibility.

I worked with a rubber extrusion process and the period of autocorrelation was 20 minutes. You will need to determine this period, because this will force you into an I-MR type of control chart with a sampling frequency longer than the period of autocorrelation to obtain control limits based on more than just measurement variation.
 
E

Elwood

#7
Thank you. How do I go about statistically determining the autocorrelation period? I imagine that I would have to determine that for each product type and size, correct? We do have the ability to re-run an extruded product through the measurement equipment. Would that provide the reproducibility component?
 

Miner

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#8
If you have statistical software such as Minitab, the autocorrelation analysis is straightforward. Just take measurements at small, equal intervals for a period of time that you know will have actual part variation such as a material change. Perform the analysis. Minitab shows a graph by time interval with the autocorrelation coefficient versus limits. Once the coefficient falls inside the limits, you are seeing actual product variation.

It would, but if that is not a normal practice, it is not part of your normal measurement error, so I would not do it.
 
Last edited:
E

Elwood

#9
I will have to think about how to collect the data properly on our process. This should get me going in the right direction I think. Thanks again for the help.
 
E

Elwood

#10
Miner,
What data source do I use to perform the autocorrelation analysis? Do I analyze the raw data from one regular production run and repeat this process for all of the different product types and sizes we run?

Also, just to make sure that I understand how to interpret the graph from Minitab, I first look for the first blue lag line that is below the dashed red line. If that line corresponds to a lag of 10, for example, and my data points are 5 seconds apart then I need to sample at a frequency greater than 50 seconds in order to separate actual process variation from variation due to measurement error. Am I correct in my understanding so far? Say for example I choose 55 seconds as a sampling frequency, do I still collect data every 5 seconds and subgroup the 11 data points into 1?

Once I have done all of that, what is the next step? Do I perform a capability study on the process for different products? What actually provides the repeatability component?

Thanks again.
 
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