Mating part

I have sketched and extruded a boss on a part. It has the "T’ cross-section. I now want to create a pocket for this boss to fit into on another part. It is a “T” section so that it can only be assembled one way. The pocket needs to be slightly larger than the boss, so they fit.

I am a newby just learning to work with surfaces.

Hi @davidullman Welcome to the Forum
There are so many ways to achieve your objective within S3D, the following may help
Bear in mind these are a guide only, choose the actual shapes required:

Sketch your Design and Copy:

Alter the view and Select the Shape(s) to be Pulled or Pushed
In both cases both Shapes were Selected the left by 30mm and the right by 20mm:

Next the 'T’on the left was Pushed down and the ‘T’ on the right Pulled up:

To get the ‘fit’ needed Pull or Push individual Faces in the appropriate direction
While the Measurement/Dimension Figures are visible click on them and alter to requirement in the Pop Up as shown above.

Just shout if you need more help, the Beginners Tutorials accessible from the ? top left menu is great place to start.

Happy S3Ding

Thanks. Most helpful.

You are most welcome.
Feedback regardless of being positive or negative, is helpful those who try to help.