I’m in the middle of building my workbench. I screwed up a bit during both the chopping and paring of the mortise holes.
In order to correct this, the mortise holes will probably end up being around 23-25mm wide instead of the 18mm in the plans. My rails are about 42mm wide and 140mm tall.
Will this leave the shoulders too narrow, or will they be fine?
To join two boards of equal width with a M&T joint the one/third rule-of-thumb is usually applied. The tenon is about 1/3rd the thickness of the mortise piece so that there is 1/3rd each side.
If you reduce the cheek-side width of the mortise by increasing the tenon width, you must expect weakness in this area. If this is an acceptable strength compromise, it is for you to decide.
I decided the shoulders would become too narrow if I went with the wide tenons, but I really didn’t want to make four new legs.
Luckily I had some pine shelves lying around. I’ll rip those to match the height of the rails, and glue them on. That will make them almost as thick as the legs. I’ll then chamfer the legs to match the rails.
Hopefully it will look like it was meant to be that way…