Significant Characteristics (SC) to Control Plan

Jim Wynne

Staff member
Admin
#41
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.
What is the reason for the request?
 
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Johnnymo62

Haste Makes Waste
#42
My take on the wire harness issue is that by using the assembly tables you are basically 100% verifying every assembly made is correct.

This is how we treated hose/tube assemblies, at a previous company. We were Tier 1 to Ford.

Just measure all dimensions on the original PPAP quantity and this verifies the process and tables are correct. Then you just have to control the tables like any other fixtures.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#43
A fixture should substantially reduce the valuation if designed appropriately. The OP hasn’t yet said they use a ‘fixture’ to create the wire harnesses - the OP has said that the manual process results in ‘large variation’.
 

toniriazor

Involved In Discussions
#44
What is the reason for the request?
Hi Jim, reason for request is that this is captured in SCCAF as an SC and as part of FORD requirements the plant should have a capability done on that. The whole story is a complex one, because that means on each dimension shown on drawing x 125. On some products there are 40 + dimensions to be verified through measurement. I can't even imagine the workload of that and the rational sampling plan. In addition it will take quite a lot of time, but that's another story.

long story short about SCCAF: this SCCAF plant does not approve, because it is a carry-over from another plant because the project was transferred to us. First the product characteristic is not correct, it is not specific to certain measurement and is too vague which is wrong and should never be the case in those documents. We asked previous plant to provide us capability results, they refused. Those are issues within the org. We have escalated this to top management, however BU wants that study.
 

Sebastian

Trusted Information Resource
#45
I have a very little experience with Ford project, but first of all I would challenge SCCAF content.
I guess, selected items may not be properly justified in existing DFMEA/PFMEA.
Simply someone arbitrary decided "this and that is SC and that's all."
These items were inspected by previous plant, but there was no capability analysis, because someone found it stupid, or real results were not satisfactory.
 

toniriazor

Involved In Discussions
#46
I have a very little experience with Ford project, but first of all I would challenge SCCAF content.
I guess, selected items may not be properly justified in existing DFMEA/PFMEA.
Simply someone arbitrary decided "this and that is SC and that's all."
These items were inspected by previous plant, but there was no capability analysis, because someone found it stupid, or real results were not satisfactory.
Hi Sebastian, yes you are right the DFMEA input was not refined properly as a result the output was really poor SCCAF and characteristics. Its really a long story, however it is what it is and we have to cope with it.

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.
Hi Bev, this didn't come from myself, it is a part of training material I found from FORD. It is exactly written like that:
Selection Criteria
characteristics, or variables, must be:
- Independent process variables (i.e., unaffected
by other characteristics or operations)
- Feasible to measure on the plant floor (i.e.,
measured the way they are machined)


so I just wanted to see what are the thoughts on that here in the forum. To be honest myself I am a little bit confused for both and struggled to find a clear examples and luckily came across a good information adapted for this discussion that I describe below. However I'd appreciate if you could list a few that you think fit in those categories as well.
So I think both can be described like that:
independent process variable: bolt tightening process ( you get the bolt or nut, place it where its required and use torque analyzer to get required N value to achieve strong connection) this operation is not affected by other previous operations, it's a simple process if it could be said like that and the operator has a direct control over it - starting material example - correct bolt/ nut and etc. or correct tool or die geometry with other words control is direct and immediate
for the other extremity what I found on internet is that:
dependent variables are: those that are determined by the independent variable selection (example: material properties of the bolt/ nut or force or nuton requirements needed to get the bolt/nut well tightened) - control is directly indirect. Also I found that: If a dependent variable needs to be controlled, the designer must select the proper independent variable that changes the dependent variable. Got confused already :)

I think topic is quite huge, but eventually we will get somewhere with that.

P.S : found something like a good example in attached.

Cheers.
 

Attachments

Last edited:

Johnnymo62

Haste Makes Waste
#47
A fixture should substantially reduce the valuation if designed appropriately. The OP hasn’t yet said they use a ‘fixture’ to create the wire harnesses - the OP has said that the manual process results in ‘large variation’.
A wire harness assembly table is a like a fixture and unique to the one part number, in my experience. It is not a table like a work bench where anything can be made.
 

Miner

Forum Moderator
Staff member
Admin
#48
Hi Bev, this didn't come from myself, it is a part of training material I found from FORD. It is exactly written like that:
Selection Criteria
characteristics, or variables, must be:
- Independent process variables (i.e., unaffected
by other characteristics or operations)
- Feasible to measure on the plant floor (i.e.,
measured the way they are machined)


so I just wanted to see what are the thoughts on that here in the forum. To be honest myself I am a little bit confused for both and struggled to find a clear examples and luckily came across a good information adapted for this discussion that I describe below. However I'd appreciate if you could list a few that you think fit in those categories as well.
So I think both can be described like that:
independent process variable: bolt tightening process ( you get the bolt or nut, place it where its required and use torque analyzer to get required N value to achieve strong connection) this operation is not affected by other previous operations, it's a simple process if it could be said like that and the operator has a direct control over it - starting material example - correct bolt/ nut and etc. or correct tool or die geometry with other words control is direct and immediate
for the other extremity what I found on internet is that:
dependent variables are: those that are determined by the independent variable selection (example: material properties of the bolt/ nut or force or nuton requirements needed to get the bolt/nut well tightened) - control is directly indirect. Also I found that: If a dependent variable needs to be controlled, the designer must select the proper independent variable that changes the dependent variable. Got confused already :)

I think topic is quite huge, but eventually we will get somewhere with that.

P.S : found something like a good example in attached.

Cheers.
Based on the attached example, I think you may be misinterpreting what is meant by independent. Think of it in terms of a regression (i.e., the independent variable is an input, and the dependent variable is the output). I think the intended meaning is that the variable is an input process variable.
 
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