Significant Characteristics (SC) to Control Plan

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Johnnymo62

Haste Makes Waste
#32
Greetings. I work with industrial, automotive and military customers. We do capability studies of all dimensions at first run. If a dimension isn't capable we make it a Significant Characteristic and put appropriate controls in place. So, it is added to the FC/PFMEA/CP even if the drawing dosn't specify it to be a special characteristic.

I haven't found this requirement in anything but CSRs.
 

Jim Wynne

Staff member
Admin
#33
Greetings. I work with industrial, automotive and military customers. We do capability studies of all dimensions at first run. If a dimension isn't capable we make it a Significant Characteristic and put appropriate controls in place. So, it is added to the FC/PFMEA/CP even if the drawing dosn't specify it to be a special characteristic.

I haven't found this requirement in anything but CSRs.
If you do capability studies on all dimensions, you sure aren't working with something like castings, where there can be hundreds of dimensions. I think also that you might have the cart before the horse here. In general, dimensions don't become "significant" after discovering lack of capability in meeting the requirement. Special characteristics are almost always related to either safety or function in an assembly. In those cases, especially in automotive work, if you can't demonstrate capability you're looking at 100% inspection.
 

Miner

Forum Moderator
Staff member
Admin
#34
True there are many characteristics that are so unimportant that they can be 100% out of spec and it would make no difference in fit, function or reliability.
 

Jim Wynne

Staff member
Admin
#35
Another thing to consider is that when you add a feature/dimension to the control plan, it can trigger other requirements such as MSA.
 

toniriazor

Involved In Discussions
#37
Hello Elsmar Team

I am dealing with one business unit that want our plant to perform capability study on wiring harness branch dimensions - that means all dimenstion from retainer to connectors, retainers to take outs, connector to take outs shown on the drawing and etc., which on a handmade product build on assembly boards will bring up huge variation in the process. While the data is variable I really think this study will bring 0 value add and will not help to improve the process since measurements normally vary quite a lot, although are in their LSL / USL. What do you think, does it even make sense to do that and has anyone ever performed such study ? Also I came across an information that capability study normally should be done on independent process variables (i.e., unaffected by other characteristics or operations) and in such case isn't it that all dimensions at the end would be affected by previous operations such as wire cutting, tube cutting, taping over the branches and etc. What do you think or that just doesn't make sense :) ?

the particular BU wants to capture this requirement in SCCAF as well in the form of an SC and for a characteristic rated as SC we should do the capability according customer requirements.

I kind of need a good advice how to approach this situation, because never been asked before to do that for wire branch/bundle dimensions as described above.

thanks in advance for your thoughts
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#38
Aside from the fact that Cpk and Ppk indexes are useless a real capability study can be quite useful. While you might think that there is a "lot of variation" because it's a manual process but everything is always in spec, it would be beneficial to PROVE IT with the data - or discover that the manual process is not capable and you can improve it before Customers get failures. My crap detector goes up pretty fast when a supplier tries to tell me that a real capability study has no value. my advice is to do the study and then you will have the answer - debating science never works, but showing the data does.

I can't speak to the validity of these dimensions being critical characteristics...
 

Miner

Forum Moderator
Staff member
Admin
#39
Also I came across an information that capability study normally should be done on independent process variables (i.e., unaffected by other characteristics or operations) and in such case isn't it that all dimensions at the end would be affected by previous operations such as wire cutting, tube cutting, taping over the branches and etc. What do you think or that just doesn't make sense :) ?
I agree with Bev's comments that you really need to understand the variations in the output of your process, and a capability study is the way to do this. Your question (quoted above) really points to the best way to control the process, which is to go upstream to the process inputs (wire cutting, tube cutting, etc.) and control those. Controlling the inputs is the best way to control the output.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#40
I don't know where you read about not doing capability studies on 'dependent variables' or outputs. that simply isn't true. at all. In fact capability studies are specifically designed for output characteristics (dependent variables). There are two reasons for this:
(1) the goal of any process is that the output characteristics meet the specification requirements. If you don't have a well characterized process (inputs to outputs) then you can have all of the inputs demonstrating 'capability' yet still making out of spec parts.
(2) there are a whole bunch inputs that might matter. there is a whole of work to perform capability studies on the inputs in comparison to the relatively few outputs. and then of course often the critical input parameter may not even be known (see 1).

IF the output is not capable then you start diagnosing which input(s) form the causal mechanism.
 
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