Welcome to the forums, Rich. A very timely question, I am about to perform the same operation on what may be the same or similar project. Did you by any chance download your plan from PlansNow? This is my first attempt at a mortise on the table; up to now I have usually just used a biscuit joint but I think the mortise and tenon is warranted for this project. At first I thought I would use a forstner bit on the drill press, but that sounds like a lot of work and the potential for errors...2 boxes x 4 posts x 4 mortises each = 32 mortises total. Here is how I plan to proceed: Setup the fence to land the mortise at the correct distance from the edge. Place stop blocks on the fence such that the start and end of the mortise coincide with my layout marks. While holding the piece against the fence and lower stop block, tilt the piece into the bit. Push the piece toward the opposite stop block and make the cut. Raise the bit 1/4" at a time and repeat until you reach the desired depth. Should make a nice, repeatable mortise. I plan to round the tenon to fit, or you can chisel the corners of the mortise square. Should be easy-peasy...just waiting on the lumber to dry out...
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