Ok, here’s how I make mortise and tenon joints with a dado blade and mortising machine. My zeal for hand tools aside, I can’t get away from this method. It’s just so darn efficient and consistent in results.
Most of us have a tablesaw. Benchtop mortising machines are pretty good for what they do. Better yet, they're affordable compared to most woodworking machines. It's not uncommon to see them on the used market either, Good thing, I can’t imagine making a set chairs or anything with a plethora of M&T joints without one.
Here goes. Usually one would make the mortises first then fit the tenons. When you have a mortising machine, you will have very consistant sized mortises. So, what you do is, make a sample block that has each size of your mortising bits like this, and store it with your dado stack.
Now, with a dado installed, measure the length of your tenon, and get the blade height close to what you need to take off each side. Test on a scrap. Preferably a cutoff from the actual stock your using to ensure its exactly the same thickness. Slowly raise the blade until you get a snug fit. Cut all your long shoulders this way.
Sidenote: if this is going to be through tenon, leave it fairly over-sized. You’ll want to plane or sand it to final thickness so you don’t see the ugly dado grooves.
Raise the blade to desired hight for the shoulders on the short edge of each board. It goes without saying that your miter sled must be deadnuts 90 degrees to your fence and blade.
Now you take your tenons and mark just the length of the mortise. See, you are using the actual tenon to mark the mortise in each location.
The width, depth, and centered location will be taken care of by the mortising machine.
With the correct bit installed in the mortising machine, take time to get it centered on the stock.
Drill the mortise staying between the marks.
There you have it.
Any further fitting gets done with these guys.
You can crank out a whole lotta M&T joints in a hurry this way. I just snapped shots while I was making some doors. Even with machine setup time, I took these shots and made four doors (16 m&t joints) in less than an hour. But I have a good little helper bangin the pieces together for me:)
Hope it helps,