Pre-Production Capability Assesment

#1
Hi all.

I need some help with presenting some data I've retrieved.

I was asked for an SPC analysis and X-bar chart for the capabilities of our 5 axis machines based on pattern block test pieces (see photo attached)

I have constructed a report for the test piece however i don't see anyway of achieving what has been asked with no control measures in place and nothing to measure it against.
After asking the QM i have been instructed just to set an upper and lower limit of 0.1 and construct a line graph with the results however this gives me many graphs as opposed to the one graph they are after.

Could anyone offer any advice on how i can achieve the results they're after as i have hit a brick wall.
 

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Proud Liberal

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#2
You can't do capability on one piece. They need to produce a sufficient quantity to have your sigma value be meaningful. A 30 piece sample would be the minimum I would consider.

A single line graph per feature is of value to evaluate stability but a histogram would point to a better understanding of the process (bi-modal, skewness, kurtosis, etc.).

If you have Minitab, a 6-pack capability analysis is the automotive industry standard for capability. If you can share the data, we can point you in the right direction as far as presentation goes.
 
#3
You can't do capability on one piece. They need to produce a sufficient quantity to have your sigma value be meaningful. A 30 piece sample would be the minimum I would consider.

A single line graph per feature is of value to evaluate stability but a histogram would point to a better understanding of the process (bi-modal, skewness, kurtosis, etc.).

If you have Minitab, a 6-pack capability analysis is the automotive industry standard for capability. If you can share the data, we can point you in the right direction as far as presentation goes.

Thankyou for replying so soon. The report may be a tad tricky to decipher, i'll work on presenting it a bit better shortly.
the end result i am trying to achieve is the average deviation about the 6 degrees of freedom I.e. average x axis, y axis, z axis then rotational about, each axis, to determine whether the machine is capable of producing the parts required and whether there is a trend in declining capability.

These blocks are being produced at a rate of 1 per week. and i have been requested to split the part into 8 quadrants for reporting.

In my mind this isn't sigma its a simple chart showing a risk analysis however after being told i need to produce SPC 3 sigma graphs ive become confused with whats required.

The other problem i've come across is that there could be an error in a positive x direction one side then an error in a negative direction the other side however the average then shows 0 error within that axis as i'm not allowed to present deviation as just a positive value from nominal.

Thankyou for your help
 

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Proud Liberal

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#4
It sounds like the confusion rests with those making the request. At the rate of one per week, this is going to be a lengthy process (more than 6 months). I doubt that's what the QM wants. But based on what you've said, I don't think they really know what they want (distilling this information into a single chart is not a reasonable request). Before you go too far down any path, you need clarification on what the goal is.

The data shows that you are clearly NOT capable to the specifications listed when numerous features are measuring out of tolerance. If on the other hand, the goal is to determine what tolerances can be held, a short term study could be used for that purpose.

Also note, that breaking up the analysis of the sphere geometry into quadrants will impact the analysis. Reductions in the percent of feature measured will shift the means and increase the standard deviations of size and location measurements. You can easily demonstrate this by writing a program to repeatedly measure the CMM's reference sphere (calibration ball) using decreasing % of feature.

Another thing to consider is how the data is being collected. Are you taking discreet points or scanning?
 
#5
It sounds like the confusion rests with those making the request. At the rate of one per week, this is going to be a lengthy process (more than 6 months). I doubt that's what the QM wants. But based on what you've said, I don't think they really know what they want (distilling this information into a single chart is not a reasonable request). Before you go too far down any path, you need clarification on what the goal is.

The data shows that you are clearly NOT capable to the specifications listed when numerous features are measuring out of tolerance. If on the other hand, the goal is to determine what tolerances can be held, a short term study could be used for that purpose.

Also note, that breaking up the analysis of the sphere geometry into quadrants will impact the analysis. Reductions in the percent of feature measured will shift the means and increase the standard deviations of size and location measurements. You can easily demonstrate this by writing a program to repeatedly measure the CMM's reference sphere (calibration ball) using decreasing % of feature.

Another thing to consider is how the data is being collected. Are you taking discreet points or scanning?

After speaking with the QM i have been told just to put it into line graphs with +/- 0.1 limits and go from there. I would agree that they are unsure as to what asked because as soon as i mention standard deviation or Cp/Cpk i get a blank response.

The part is measured using discreet points on the CMM, no scanning.

Could you advise on another short term study i could use that might be of some use for determining machine tolerances? my main concern with the data i've collected is that with no control log to restrict tool length, size etc, the data means nothing.

Thankyou again
 

toniriazor

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#6
Where did these limits came from ? Reading above it seems you into a big mess. +/- 0.1 limits is the control limit or product LSL/USL ? Before you move forward the goal and direction must be clear.
 
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