True Position and Datum requirements 2

[joejack7]

I need some justification on this one...

Our design group has released a drawing with a hole pattern on a sheet of honeycomb core. The requirement of the holes is TP of .250 LMC relative to Datum 'A' only. Datum 'A' is the planar surface of the honeycomb. My question is...

In controlling the position of these holes is it possible to control to only 1 datum feature? Since this is concerning POSITION with basic dimensions, doesn't there need to be more controlling features, ie. datum 'B' and/or 'C'?

Joe Jackson
Goodrich Corporation

 

[ringman]

Joejack,

Im not sure whether you got the justification you were looking for or not. After reading through the responses and trying to digest, I think not so.

I am of the opinion that for you to apply LMC to ANY hole, a datum, other than a plane would be required. (Or in addition to). This will effectively control the minimum edge distance. Do you agree and has that portion of your situation been resolved?

 

[Tunalover]

Ringman and others-
I should have chosen my wording more carefully. I said:

"Normally, the dimensioning method will infer the coordinate system (often with one datum running lengthwise of the board and the other at a right angle to it)."

What I should have said:

"Normally, the dimensions will infer the coordinate system (often with one of the datum reference planes running lengthwise of the board and the other at a right angle to it)."

The datum assigned to the hole axis has two mutually perpendicular datum reference planes passing through the axis. The best choice of the angular orientation of these two planes is inferred from the dimensions between the two holes.

Tunalover

 

[drawoh]

tunalover,

You are describing something I have done several times, mostly on sheet metal. The extreme case is that I bent all four edges to work as gussets. It is difficult to locate holes from an edge, and easy to locate the holes from each other, since they are located on the same flat surface.

Datum_A is the bottom surface. Datum_B is a hole, and Datum_C is a second hole. The flanged edges are located by sloppy tolerances or a sloppy GD&T profile.

I can now register the part precisely, and inspect it.

The GD&T standard shows simple parts with circular shapes and hole patterns inside, where there are only two datums, the base and the OD. This makes sense to me, as long as there is only one hole pattern, and there are no rotation issues.

Once the geometry requires you to control rotation, you need three datums.

JHG

 

[Tunalover]

drawoh-
This practice is good for when one or more holes are created BEFORE the part outline. In PCB manufacturing, the router often uses one or two holes as a reference when forming the outline. This is amenable to "step and repeat" processes where many circuit boards are created from a large sheet of material.

You said "This makes sense to me, as long as there is only one hole pattern, and there are no rotation issues."

I've had many hole patterns on the same part dimensioned this way using 'A' as the primary and 'B' as the secondary. There is no limit to how many hole patterns can be dimensioned this way in one part.








Tunalover