# Ford Method - Position Capability with MMC Modifier

#### Ron Rompen

Trusted Information Resource
Is anyone familiar with the FMC methodology of calculating positional capability using residual tolerances/residual tolerance mirror?

This is a completely new one on me, but one of our customers has 'suggested' that we use it rather than my preferred method (total tolerance used as a percentage of total tolerance available).

I am puzzling through the worksheet that was sent to me, however I am not at all confident that I understand it completely, and if I can't understand it, then I can't explain why I do (or do not) use it.

Anyone able to assist?

#### Miner

##### Forum Moderator
I have seen several methods on this, but not the Ford method specifically. If you can share, I will certainly try to assist.

#### Paul F. Jackson

##### Quite Involved in Discussions
I don't believe there is an official Ford method other than the advice published in the AIAG but while I was in Ford Powertrain I published method attached. There are other threads containing spreadsheets that accomplish this method and enable suggested process optimization adjustments.

Paul Jackson
.

#### Attachments

• ExpandingSPC.pdf
612.6 KB · Views: 486

#### Miner

##### Forum Moderator
A link to the discussions to which Paul referred.

Thanks Paul.

#### Paul F. Jackson

##### Quite Involved in Discussions
In my first attempt at addressing the "Variable Tolerance Limit Problem" I offered the Residual Tolerance Capability Method but I recognized later that by using the individual size and position measurements to create a surrogate statistic that could be compared to a constant limit I was actually altering the underlying variation of the independent measures of size and position. Therefore I wrote in another thread, "I would not recommend using: the residual tolerance method described in "Simple Process Capability" Quality Magazine by me, the percent of tolerance method described in "Calculating MMC Cpk" by Marty Ambrose, or the adjusted true position method "Calculation of Cpk under conditions of Variable tolerances" Quality Engineering by Glen Gruner because each one of those methods uses an individual pair of variables for size and geometric deviation to produce a surrogate variable the can be compared to a constant limit. In so doing the underlying variation from the independent sources can either be amplified or moderated in the surrogate."
Paul

#### Miner

##### Forum Moderator
Paul,

What do you now recommend using?

#### Paul F. Jackson

##### Quite Involved in Discussions
The method described in the attachment "Expanding SPC" for which I've provided the spreadsheets is the best that I know of. It is reflective of the independent variation of both size and position and it attempts to predict the area of the intersecting distributions (non conformance). There is error in the prediction relative to an attribute check (hard gauge) but it is resoundingly more predictive of % defective than ignoring the variable tolerance as the predominant practice has been. Furthermore it permits process owners to examine whether a process can be improved in terms of collaborative conformance between location and size simply by making size or axis translation adjustments to a feature or pattern-of-features.
Paul

#### Ron Rompen

Trusted Information Resource
Thanks for the detailed response, Paul.

The method I am using was created by Doug Hutchins on April last year, if that provides any additional information. I will try and scan the document and upload it (later) for others to review.

I am going through the PDF file you attached, and was looking for the spreadsheets which you mentioned in your last message - I am unable to locate them (perhaps I misunderstood something)?

If these are available, could you please upload them, so that I can try them out, and review with the rest of my team here?

Thanks again.

#### Paul F. Jackson

##### Quite Involved in Discussions
How to calculate a True Position toleranced at MMC - Capability of True Position
Paul