zero tolerance with MMC

[jay R]

Its been a while since I reared my ugly head in here, so what the heck, this one is giving me fits.

I have a FCF that specifies, 0 tolerance with MMC, relative to A datum.

The hole spec is as follows, 21.15+/-.65 mm

Its location measures 0,0 as it is my origin, the hole diameter measures 21.547 mm

When I try to apply the TP via pcdmis, it fails.

My understanding from my textbooks and the ASME standard is that the hard gage would be a pin of size 20.5 mm, at location 0,0. This being the case, this should pass.

Why does it fail, what am I missing?

Thanks to everyone willing to help!!

[bw_bob]

In your TP dimension box do you have the m checked for bonus ??
If its your datum it should be 0,0

[Rich P]

We have several parts that have a zero tolerance MMC modifier callout--and 20.5 would be your min ID size and it would have to be at 0,0 perfect location at this size and my understanding

Are you measuring a cylinder or a circle?
Sometimes deviation from form will give you funky read on position if cylinder is not made corrrectly
also try switching off or on use datums checkbox pc-dmis trys to best fit into datum structure off alignment which may move location
also try auto checkbox on position box and see where error is coming from
this should give you an idea as to where to look for error issues

[CGoode]

I see this type of GDT misuse alot. I am taking it for granted that datum -A- is the base plane and is perpendicular to the axis of the feature being true positioned. This in fact turns the dimension into perpendicularity with the cylindar bonus as the tolerance.

[jay R]

Thanks for the input guys.

First question, yes I do have mmc selected for the feature.

Second and third question simultaneously;

I am measuring a circle, and according to my textbook, and the asme standard, the zero tolerance mmc callout is used to reduce manufacturing costs without resulting in scrap parts. By default the virtual condition of the hole, ie the hard gage to inspect this part manually, would be the 20.5 mm pin in a plate at location 0,0. This being the case, my part easily passes, yet pcdmis applies a -.65 tolerance. Which is more or less correct, in that the virtual condition for the hole is the 20.5 mm pin. However, it shows up as failed and out of tolerance.

Is it as simple as just adding the positive half of the hole tolerance, the +/-.65 into the plus field for the hole?

See attached, I have tried it both ways with deviation to perp on and off, same result

[than_357]

Jay,

Your tolerances in the attached pic aren't bilateral as was the first post's tolearance. (ie- you have 21.150 -.65 instead of +/- .65)

I have a gut feeling that no matter where you go with this, the zeroing on the feature is not a valid PC-DMIS concept. The only deviation which would show would be axial and you just removed it.

Tom H

[jay R]

Hi Tom, that is where things got confused I think. On the feature itself, I had applied a +/-.65 tolerance when defining the hole. For whatever reason, pcdmis only applied the minus side of the tolerance when I did the TP callout for the hole. This makes sense when determining the VC of the hole, but pcdmis, in my mind, should still have applied the +/- tolerance in the TP call out.

After I posted yesterday, I went back into the program and edited the TP callout to include the + side of tolerance in the description of the feature and now it passes.

I still don't understand why pcdmis didn't do it, but this seem to have corrected the error.

Thank you for the help, at least now I know what to watch for, as this isn't the first time this has happened to me.