generatrix of cylinder surface as datum?

[bxbzq]

This is a sheet metal bracket. I've attached a sketch. It's incomplete as usual and round corner at bending is not modeled yet, but it should not be a problem for discussion. There are two bumps stamped on the arms of the bracket. The far end surface of the bend and the generatrix of the bumps contact the mating part. So I want to use them all together as datum feature A. The questions are:
1. Is the datum feature A appropriate? I'm asking because for sheet metal parts, we typically use flat surface as primary datum, but in my case it's the far end of the bend and the generatrix of the bumps making contact. But being sloppy in form and interrelation location tolerance, I'm not confident to assign them as primary datum.
2. How do I assign the generatrix as part of datum features?

 

[drawoh]

bxbzq,

Your little bumps are two separate features. They cannot be directly used as one datum. You can combine two datums as a reference on your FCF. In ASME Y14.5M-1994, look at figure[ ]4.20 on page[ ]67. In ASME Y14.5-2009, this is figure[ ]4.23 on page[ ]67.

--
JHG

 

[TheTick]

While the datum callout might be correct in a strictly literal sense, you will never be able to get a good read on it when measuring actual parts.

 

[MechNorth]

Set the flat surface as datum target area A1, and make datum target line contact on the two bumps (A2 & A3). While you may not make perfect contact on all three targets, the part will be stable for inspection, though you may want to use magnetic holddowns on the datum target simulators to make sure it doesn't move.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[bxbzq]

Jim,
If use datum target lines, can the datum target simulator be flat surface making contact to the bumps on the furthest generatrix? I'm asking because I want the datum simulator be a flat surface just like the surface on the mating part. A gage pin does not make too much sense here. Also, if use datum target line, another question arises, what will be the tolerance for the basic dim 18.6? Standard says gaging tolerance applies but I think it would apply when using gage pin. If use a flat surface simulator, I would think something like a profile tolerance should come into play.

 

[MechNorth]

A flat plate is exactly how you would simulate the tangent line contact for the datum target line; two surfaces offset by the basic 5.1mm distance, distance and parallelism within gage maker tolerances. The features (humps) should be controlled relative to A,B; C doesn't add anything that I can think of, but does establish a consistent DRF with the other features.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com