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# ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

## ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

(OP)
Good Afternoon,

I am looking at a drawing I clearly don't understand.

[/indent]I have a part that does mount on two surfaces although the surfaces are perpendicular to one another. Datum A on my drawing is parallel to the to the X-Axis. Datum D is a plane perpendicular to the X-Axis. I then have 3 cylindrical holes and one slotted hole in line on the Z Axis running through the Datum A plane. One cylindrical hole is Datum B and the slot below it is Datum C. When I come to the feature control frames for the holes / slots it goes like this. True Position | Ø.2 | A – D | B | C |

[/indent]Planes A-D would make an line axis in the z direction? With datum B hole tie down the x direction rotation? With Datum C being the “Z” direction zero? I know that’s not right because all the drawing dimensions are coming the Datum B.

I cannot wrap my head around the reason the engineer did not believe it would flush up against datum A and then rotate to datum D and align to datum B.

### RE: ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

Need to see drawing

### RE: ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

I guess I like to see the drawing too, or at least part of it......relevant to the OP question

### RE: ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

If I just can open the file attached.....File cannnot be open ....at least at my end.

### RE: ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

Must be using "&" in file name again

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

### RE: ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

I am not sure if using A-D as multiple datum feature primary--is a good idea. I do not think so.

Why? Since A and D are nominally perpendicular to each other, the question is which one is taking / removing the degrees of freedom and in what order or precedence? Sitting flush on A and also sitting flush on D in the same time? I am not sure how realistic din scenario is? Which is driving and which is driven?
What is the physical reality of this assembly? How this electronic case/ box is assembled?

I my opinion, but knowing nothing about this assembly, I would say that A primary, B secondary and C tertiary is more realistic approach.
Then D, as currently shown, should be profile to A, B, C (not to A-D, B, C)

Or even D primary, B secondary and C tertiary could be used as the global DRF for this assembly. Then maybe A should be profile to D, B and C.

### RE: ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

(OP)
Thanks, I was thinking that out the same way. I have never seen the primary plane consist of two perpendicular planes. I have seen two parallel planes as an example of a primary datum because it rests on both planes at the same time. Even if was referencing an axis then other two datums do not work.

### RE: ISO GD&T Question concerning using two adjacent planes to create a primary datum on a non-cylind

T. Schmidt: as you mentioned, multifeature datums are typically coplanar or coxial. I also noticed that there no geometric relationships between the datum features (like flatnees on A adn B perpediclaur to A , etc.). Not a recommended practice.

Certified Sr. GD&T Professional

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