SPC for Diameters - Parts not Perfectly round so charts are useless

[bobdoering]

The distribution is a rectangle, centered over the nominal (do not like to use the term mean, since it plays no role in this control). Should be shown on slide 9 or so of the presentation. So, as long as the LCL and UCL are 75% of the specification, the capability is the tolerance divided by the process spread - or (USL-LSL)/(UCL-LCL), an at 75% the capability is 1.33.

 

[True Position]

If the concern is the washer fitting over the mating part, why not simply use go/no go pin gages? A pin controls for your egg shape condition and custom pins aren't too expensive. For the OD issue, you could use ring gages.

 

[bobdoering]

Fit really is not the issue - control is the issue. To use fit, you would have to inspect 100%, but with control, you might reduce that down to between 1 every 5 pcs, or 1 every 30 pcs (or more), depending on the tool wear rate. Cuts down inspection, provides variable data for making adjustments, more information about the process, etc. Win-win.

 

[RMedrano]

Fit really is not the issue - control is the issue. To use fit, you would have to inspect 100%, but with control, you might reduce that down to between 1 every 5 pcs, or 1 every 30 pcs (or more), depending on the tool wear rate. Cuts down inspection, provides variable data for making adjustments, more information about the process, etc. Win-win.

Yeah, basically we are doing a 300 percent inspection right now with 2 sets of Jo Blocks and a 3 point diameter gage.

I am going to have the operators get me some sequential sample data off 30 pcs. and plug them into the spreadsheet you gave me to see what things look like. I know for a fact that we are not capable, otherwise we wouldnt be 300% inspecting.

Another thing is... these charts, do they assume that all special causes of variation have been identified leaving only tool wear as a concern? The Tooling engineers are telling me that this isn't a tool wear issue, they just cant hold the dimension wanted by the customer.

This die that runs these parts is really a pain in the butt! It crashes constantly. I cant wait until its in our facility full time

 

[bobdoering]

Although the uniform distribution assumes the primary variable is tool wear, the charting will still prevent reject parts from being made if the total variation is consistant and predictable. Not sure if it is at this point - but this chart is a clearer picture of the process than measuring one diameter per part, or averaging the measurement mess! If it is not predictable, then you really do not have a controllable process, and you are left to sort for life. I hope that is not the case!!

And, if it is not a tool wear issue, then what are they offering up as the root cause? Material variation? Machine capability? Poor tool design? There are only so many things...

 

[RMedrano]

Well, if it is not a tool wear issue, then what are they offering up as the root cause? Material variation? Machine capability? Poor tool design? There are only so many things...

I would say a combo of Tool Design and Machine Capability. I know they are in the process of putting together a huge Capital Appropriation request.

 

[bobdoering]

Well, don't jump to conclusions until the data shows it. And, if you do go for a new process, use this charting system to verify it is an improvement!

 

[RMedrano]

Well, don't jump to conclusions until the data shows it. And, if you do go for a new process, use this charting system to verify it is an improvement!

Oh totally agree there.

 

[RMedrano]

Well, don't jump to conclusions until the data shows it. And, if you do go for a new process, use this charting system to verify it is an improvement!

heres some data, as you can see... one Diameter is better than the other, but still out of control.

View attachment B800 CAP.pdf

 

[bobdoering]

Yes, the OD tolerance is clearly outside of the capability of the process. The out-of-roundness is eating up over 50% of your tolerance!! The ID would have been OK, except for that blip at the end. Was it real? Anyway, now you can say without exception that you are stuck with sorting the OD - unless someone that knows your process can make some improvements. If that ID had not had the one bad data point, you could have SPC'ed the ID one very 10 parts or so and been done with it. Any theories why the ID is so much better than the OD? Any lessons learned there?

Do you feel as if you really know more about your process with this charting technique than the ones you have tried before?