Is the top hole oval? 1

[ImnotfromMars]

I am having difficulty understanding a section of GD&T.

I have a flanged pipe with a boss on the side. The hole in the boss is located with a basic dimension and a true position in which Datum C is a hole in the flange at an angle relative to the top hole.

Because the size of the C datum is at an angle relative to the hole location, does this result in an oval tolerance zone for the hole? In other words would the C hole contribute to the stack up distance between the hole and datum A?

see the attached rough drawing.

http://www.flickr.com/photos/33544050@N00/416926366/

Robert

 

[ImnotfromMars]

On the drawing the .500 +/-.010 True positioned hole should have a dia symbol in front of it.

Robert

 

[drawoh]

ImnotfromMars,

Datums primarily are a fixturing specification. In your case, the fixture could be a flat plate with two pins, one .900"DIA and one diamond shaped one at .247"DIA at the nominal position.

Datum_A, the flat surface, locates Z and rotation about the XX and YY axes. Datum_B, the .900"DIA pin locates in X and Y when the hole is at MMC. Datum_C, the .247"DIA pin, controls rotation about ZZ, again when the hole is at MMC.

Your part is orthogonally located, and your tolerance zone is round, as specified.

JHG

 

[CheckerRon]

Your hole location dimensions should be basic, and the anglular tolerance to datum C should also be basic. Then the Datum c is only for clocking, and there is no ovality to the tolerance zone.

 

[ImnotfromMars]

Thanks for the postings and the help.

If I understand correctly the C at MMC in the TP for the Dia .500 hole on the top, has no effect if I stack up the axial distance between this hole and datum A. The datum C would be controlled with a pin at dia .247. This would stop rotation about the ZZ.

If the C hole measured dia .253 this extra distance would only be reflected angularly against the TP and therefore the acceptable area where the TP could fall in for the hole would be oval in shape due to the contribution of the tertiary datum.

Robert

 

[drawoh]

ImnotfromMars,

As specified on your drawing, datum_C controls rotation about ZZ. If there is clearance in datum_C your part is allowed to rotate a bit. If your tool is positioned accurately to datums_A and_B, you will have a linear error and a rotation error. That is a bit more complicated than an oval.

For that matter, datum_B applies at MMC, so clearance here allows the hole to move sideways. Given the non-orthogonal location of datum_C, even this error is complex.

JHG

 

[ImnotfromMars]

Thank you for the post and the help with understanding.

So the contribution of datum C at MMC and datum B at MMC would not have any effect on the stack up distance between the dia .500 hole and datum A.

Robert

 

[ringman]

Notfrommars,

It would be interesting and beneficial to those contributing to have some insight as to the functioning of this part you have described, and how it interfaces with the next assy. Is that a possibility?

Thanks

 

[ImnotfromMars]

ringman,

Thank you for the interest. I have created a simple model zeroing in on the area I am having difficulty understanding.

If I elaborated enough to give a complete understanding of the components involved I would be treading in the area of disclosing company proprietary data and I am sure this is an area which all of us would want to avoid.

Robert