# A lesson in Datums and Geometric Tolerancing Please

#### tham

##### New member
I understand how to select datum planes and axis to make them datums to use with my Geometric Tolerancing.

Can you select surfaces of a feature? I have a plate that is round, I need the outside round surface to be a datum for geometric tolerancing. How can I select this round surface? What am I supposed to use in place of the surface?

Help!!

Thanks

#### tham

##### New member
No, that's not what I'm after. its not the thickness, its the outside diameter.

#### especial

##### New member
hammer without seeing your round part and assuming you have datum A, B, C, D, ... . Datum A being the flat side , Datum B and C Being hole on the flat surface and Datum D being the round. Dia .XXX with a Plus minus and gd&t perpendicular within dia .005 max material condition to Datum A. This is just my guess and hope it helps you.

Thanks

AS

#### Rajesh

##### New member
If I understand correctly,

Create a datum parallel to a datum(that passes through the axis) and tangrnt to the outside diameter.

Rajesh

#### especial

##### New member
Attatch Datum to diameter (dimension) and move Datum only (not dimension) to the extension line . I left this little tip bit out .

Thanks

AS

#### ProFishent

##### New member
Like especial says you can set the datum in a dimension. In the SETUP/GEOM TOL/SET DATUM menu under Placement, change to In Dim. Then pick the diameter dimension of the surface you want to reference. Select a datum tangent to the surface. In the drawing the datum will show with the dimension.

#### dougr

##### New member
It's been a while since I did some serious detailing but can't you rename the internal axis from your feature and set that ??

After all it's really the feature axis that's the datum..

#### ProFishent

##### New member
Yes, you can select a tangent plane or an axis.

#### tham

##### New member
Thanks everyone. I did as especial and Profishent said and created a datum plain tangent to the circle, then attached the datum to the circle Diameter Dimension.

This looks a little strange in the model, but I think that is how PTC intends us to do it, so what can you do but follow it.

Thanks again.

#### mekaneck

##### New member
wow, I'm responding to a really old post, but I'll put this here anyway:

If you want the round surface as the datum, you create an axis (by referencing that round surface), then go to properties on that new axis and make it a datum. You can also make the internal axis (from the original feature) as the datum (as dougr suggested) but I wouldn't recommend that.

In GD&T (ASME Y14.5), if you are calling out a round feature as a datum, the datum is the feature's axis. Therefore, in Pro/E, you want to create an axis as your datum, not a plane.
Edited by: mekaneck