Process Capability Study which involves MMC (Maximum Material Condition)

D

D Moore

#1
Hello.

I have been asked to carry out a capability study which involves MMC.

We are manufacturing Flywheels for Ford motor company and a significant characteristic required by them is the position of a timing slot. The spec is 0.2 relative to a flat datum but also to to the flywheel bore and another datum hole.

Can a study be carried out as my UTL will be different for every component, dependent upon the measured size of the two datum holes?

If it is possible how do i go about it?

Thanks
 
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Stijloor

Staff member
Super Moderator
#2
Hello.

I have been asked to carry out a capability study which involves MMC.

We are manufacturing Flywheels for Ford motor company and a significant characteristic required by them is the position of a timing slot. The spec is 0.2 relative to a flat datum but also to to the flywheel bore and another datum hole.

Can a study be carried out as my UTL will be different for every component, dependent upon the measured size of the two datum holes?

If it is possible how do i go about it?

Thanks

Is it possible to attach a sketch? Does Ford refer to ASME or the ISO GD&T standard? For our Members: what does UTL mean?

Stijloor.
 
D

Duke Okes

#3
Why will USL be different? Isn't it relative to the center of the hole rather than to the edge?

If not, then I would guess you might need to record your values as +/- from nominal, or from the UTL.
 
R

Rand T

#4
I have struggled with this as well. The problem is that with bonus tolerance, the tolerance is increased depending upon the feature size. Therefore it may change with each and every part. It would appear that it may be necessary to perform the capability without consideration of the MMC bonus and hope it is capable and that the bonus will just act as a comfort factor.
 
W

world quality

#5
I agree with Randy, and sense there was not a drawing or sketch provide and to sense this is a flywheel, It has to be heat treated, what is the GDT, before HT and after HT. Each part will be different in dimensional size.
 

Stijloor

Staff member
Super Moderator
#6
Hello.

I have been asked to carry out a capability study which involves MMC.

We are manufacturing Flywheels for Ford motor company and a significant characteristic required by them is the position of a timing slot. The spec is 0.2 relative to a flat datum but also to to the flywheel bore and another datum hole.

Can a study be carried out as my UTL will be different for every component, dependent upon the measured size of the two datum holes?

If it is possible how do i go about it?

Thanks
A capability study that include the MMC principle is a quite a challenging issue.

This thread provides excellent insights.

As well as this thread.

BTW, we would like to see your drawing though...;)

Stijloor.
 

Paul F. Jackson

Quite Involved in Discussions
#7
D. Moore

If the MMC modifiers are designated only on the secondary and tertiary datum features…
then no “coordinate system mobility” shift allowance would be permitted to verify any one feature individually. The rules concerning simultaneous requirements provide that the “coordinate system mobility” shift allowance be applied to all identically datum referenced feature controls simultaneously.

Even though with some CMM software it is possible to apply this allowance independently when assessing individual feature displacements it is wrong. I used to counsel all of the CMM programmers to disable the datum feature bonus option. There are some gage simulation packages however that can perform the calculation correctly but care has to be taken that all affected feature tolerances are included in the simulation.

On the flex plate or flywheel the pilot diameter mating with the crankshaft generally is a very tight fit designed to prevent datum shift thereby preventing circumstantial imbalance. Even though one of the flywheel to crank fastener holes in the pattern is offset a few degrees from equal spacing to prevent the part from being assembled backwards… the bolt clearances in the pattern are generally identical and therefore would contribute to the functional variability of the timing mark to the crank rotation. That is a problem that would only be exacerbated by allowing increased rotational tolerance of the mark to the clearance hole pattern. Chances are the MMC modifiers are not designated with regard to function.

If the MMC modifiers are designated on the timing mark size itself
Typically the variable portion of location tolerance is ignored in capability prediction equations because variable limit geometric tolerances are not generally recognized or understood by statistical types.

To add to that...

MMC is often used on engineering drawings and applied to a "feature of size" simply because it is a "feature of size". That mentality has propogated itself throughout industry to serve those that want to have "GO attribute Gages" for "in-process" inspection.

Can the capability be predicted using the variability of the variable portion of tolerance? Yes... Read on in the threads that Stijloor referenced and find the thread that has my spreadsheets and powerpoint presentation in it.

Should the capability be predicted using the variability of the variable portion of tolerance? Sometimes Yes and Sometimes NO… In this case I would say NO!

You can easily determine if a material condition modifier (applied to the feature tolerance) is functionally appropriate by answering a simple question. "Does the feature's ideal function worsen as its location and/or orientation tolerance is permitted to increase with size" If the answer to that is No then the MMC capability is appropriate. If the answer to that is yes (as I suspect it is with the timing mark) then it is not!

The timing mark should be oriented to the datum features as good as possible all the time without respect to size.

In summary if the MMC modifiers are applied only to the datum features, they shouldn’t be functionally (my opinion) furthermore it is wrong to use the allowance in your timing mark location capability prediction independent of other identically referenced tolerances.

If the modifier is applied to the mark location then technically you have the right to use the variable portion of tolerance in the prediction even though it is non-functional application.

Tell your Ford STA person to look for Variable Limit Tolerance Capability Analysis on the Ford Powertrain Quality website then they might understand what they are asking for… I put it there before I left. When the modifiers are applied functionally then feature sizes can be targeted to optimize the capability of the variable limit tolerances and their associated sizes simultaneously i.e. Ppk of “zero at MMC” specifications.

Paul
 
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S

Sathyanarayanan

#8
I seem to be getting into this quite late and may be the interested parties are already clear on the issue. My recommendation would be to calculate the percentage of tolerance with bonus used on every reading and use the variance in percentages compared to 100% allowed as tolerance for calculating the Cpk. This method has some shortcomings but is the closest to what I would look for and the easiest to handle in the shop floor
 

Paul F. Jackson

Quite Involved in Discussions
#9
Sathyanarayanan,

There are several methods that people have used to predict conformance of "variable limit specifications" and you are correct that the "percent of tolerance" method (first used a long time ago in GM's fuel handling department and documented by Marty Ambrose) is probably the easiest to understand and apply... but I would say that it is the least accurate of the methods for making the conformance prediction.

It is that way because each instance of size and geometric deviation is converted to a ratio... so a large "bonus tolerance" and a large deviation can produce an identical ratio to a small "bonus tolerance" and a small deviation. The problem is that the underlying separate sources of variation are masked in the surrogate ratio.

This topic has been addressed in numerous other threads on this site for further reading.

Best regards,
Paul
 
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