1. With Structural Member I created two vertical beams (89mm x 89mm).
2. Than I created a horizontal beam of 19mm x 38mm horizontaly to create the mortises in the vertical beams.
3. Deleted the body of the horizontal body
4. Created a new horizontal body (Weldment) of 19mm x 38mm.
5. Now I wanted to use Trim/Extend to trim the horizontal body and get tenons on both end of the size of the holes (mortises) in the vertical beam.

However if I use Tirm/Extend with bodies the horizontal body is trimmed at the vertical body and if I use Face/plane and select all the faces of the hole Solidworks keeps saying "Rebuild Errors, some of the selected bodies are missing".

What am I doing wrong? Is there a better way to create Mortise and Tenon?

Regards

Bart