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
