GD&T Centering 1

[cdeggz]

I have two slots on a plate, and want the centerlines of the two slots to be within a specified tolerance (they are nominall aligned). How would I put this on the print?

The sizes of the slots are not controlled and not important, as long as the centers are aligned.

 

[KENAT]

Could you give us a picture and a description of the application + what standard you work to.

It's not often that someone really cares about the center (though folks often care they do), usually it's where the edges are that matter and so that is the more common case an application of GD&T.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[cdeggz]

Sorry it confidential so I can't share it. This is a strange case where I don't care where the edges are, only the center.

I'm trying to follow ASME Y12.5M '94

The only idea i have is to define a datum as the center of the first slot, define the second slot centerline, and then put a positional feature contorl frame on the second one relative to the first datum.

 

[Belanger]

Yes, your idea mentioned in the last post would be the way to do it, with the position tolerance tagged with the slot's width, not really the center line itself.
Although keep in mind that the datum and the position tolerance are derived from the "actual mating envelope" of the two slots (i.e., derived from just the high points).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[cdeggz]

Thanks Belanger. How would I define the datum? Just put a line through the slot with a note saying it is datum?

 

[Belanger]

In the GD&T system, the datum symbol would be tagged not to the centerline but to the size dimension. See the attached graphic -- it's a slot centered on a block, but it's close enough to your situation.

Notice the datum symbol is in line with the dimension arrow for the width. This means that the true datum is the center of the block, not the one wall on the right.

In GD&T a datum is often a center line/plane, but the symbol itself must be identified with the physical feature that the center is derived from. I know this might not be intuitive, but them's the rules!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

http://www.gdtseminars.com

 

[CheckerHater]

The ways to align your slots may actually differ depending on overall design of your part (some other features, the way you want them to be controlled; what is called “design intent”)

 

[pmarc]

Of course keep in mind that depending on scenario chosen (from those presented by CH) maximum possible misalignment of two center planes will be different.

 

[cdeggz]

Got it. Thanks dudes!