another gd&t question

[george frick]

do you apply the full mmc allowable on datums, or 1/2, or a percentage?we have this "discussion" all the time.

[Paul Sarrach]

I have actully never added bonus tolerance from my datums. I have always made them fix it if it was that close to being out. I figured that the little extra was my insurance that I would not ever submit a a patyt that was not functional for build or PPAP.

[bw_bob]

MMC is based off the size of your datums. If its a hole say 5.0 +/-.10 and it measures 5.10 at then you get .20
Your modifiers are there for a reason.........use them

[rebeldude]

You use the full amount. It figuires it out for you

[tking]

My understanding of the usage of MMC on datum features is that it allows you to use the math to calculate the virtual size of datum pins on hard gages to provide for "datum shift", the ability to shift the parts around on the datum pins until all the feature check pins "pin" (good part) or not (bad part). You can also do this by translating and rotating your alignment on the CMM till all features show good (or not).

The problem (and probably your "discussion") is people want to independantly apply the "bonus" to individual features and call them good. It's difficult to get them to understand that it must be done as a feature "set". Mostly because they want the one or two bad features on a report to show good (not because they don't understand). I've had the most luck by drawing a plate with 4 holes in it that has each hole at it's max deviation away from the center of the plate. I show them that I can shift the part around the datums and make each hole "pin" one at a time but can't find a shifted position that will allow all 4 to "pin" at the same time. At that point if they want to call the bad feature good by including all the datum bonus in the FCF I leave it to them to explain why a "good" part won't assemble.

Wild guess, this thread will provide for much debate

TK

[kbotta]

George,
great topic!

Here is my stand on it.

Datum 'shift' (or datum displacement) is not to be confused with additional ("bonus") tolerance. I do use the additional tolerance from the feature that is being inspected.

However, I do not use the datum MMC option in Pcdmis.

In order to fully understand this (datum MMC), one must also understand simultaneous requirements. There is a section in Y14.5 4.5.12 (pg. 68) that goes into detail (and pg 92 5.3.6). Without going to far into it, it says this: Where 2 or more features or patterns are located by basic dims relative to common datum features, referenced in the same order of precedence, and at the same material conditions (MMC.rfs etc) they are considered a composite pattern and the Geometric tolerances are applied simultaneously. See FIG 4-26 for a good graphic. So, if you shift one way (or direction) ALL of your inspected features using the sim. REQ rule need to go the same way. One feature cannot go +x, and another -x.

Pcdmis cannot 'break free' from an alignment. You are always aligned to something, even if it is the machine coordinate system.
If it is a bolt hole pattern, and it is absolutely necessary, I will use the 'paper gaging' method and plot the points to see if it will pass or fail. Cumbersome.

The nice thing is with functional gages, the 'shift' is built right in. So, if it is close on the cmm (but slightly out) NOT using the shift, I verify the shift on the gage.
Make sense??

Good topic,
Kevin

[Rich P]

My understanding on MMC is that if you were to build a gage this is what the tool would be made and set to target the dimensions ie a functional fit- form -function of part to feature control frame callout.
in essence the bonus tolerance is built into the check to give additional tolerance--and report this the same way_Position is toleranced to Print unless controls are set by engineer.
On a PPAP I highlight so manager and engineer to to acknowledge/sign off so that without MMC we would fail this position callout is understood by them.
For a production run QA manager would be alerted that release was Ok with a bonus tolerance applied.

[kbotta]

Per your question
"do you apply the full mmc allowable on datums, or 1/2, or a percentage?"

You are allowed an amount equal to 1/2 the DIFFERENCE between its actual mating size and VC size. The actual mating size ( or actual mating envelope AME) can be a different value than if you simply put a pin though the hole.
That one threw me off. You have to really understand the deffinitions (Para. 1.3) to get a true handle on this. Very complex.

KB

[\v/inston]

KB = GD&TJM

[tking]

KB = GD&TJM

OK, I'm guess'in GD&T Jedi Master?

TK

[kbotta]

Ha! not hardly...More like padawan learner...

[\v/inston]

OK, I'm guess'in GD&T Jedi Master?

TK

...

[STANLEY]

All production wants from you, is that the part is ok. Be carefull on bonus tol. Think like a gage. If your using all the bonus tol to make a part good, chances are, your running parts out of spec.

[Paul Sarrach]

All production wants from you, is that the part is ok. Be carefull on bonus tol. Think like a gage. If your using all the bonus tol to make a part good, chances are, your running parts out of spec.

When you get down needing datum to make a good part then all you are doing is splitting hairs.

I check all my parts on gages. So I assume its already added. And yes in theory you can. But in theory I can come to everyone who reads this house and have a beer. Theories are like *********, everyone has one.

[STANLEY]

I make production ajust the process if i'm using bonus to approve a part, Here you can't shut the job down if the part is fuctional, i have in the past though. Like Paul said everyone has there own theories on this subject.