Mortising on the drill press
The problem with these adapters is that they extend the quill of the drill press way down. The quill on most drill presses has a bit of play to begin with, and to extend it this far makes for a less than rigid arrangement.
The bracket that I'm showing here is actually no longer usable - it's what's left of one of these adapters after I experimented with building a hollow chisel mortiser
I'm going to use a spiral up-cut router bit (an Onsrud router bit, to be exact) as a very short drill to hog out the material. I mount this bit as far into the chuck as I can so that it barely extends out far enough to cut the mortises.
I clamp and un-clamp the work piece for each hole.
This method is not the fastest, and it's not the cleanest. Though I think the result is quite comparable to what you would produce with a hollow chisel mortiser. But unlike hollow chisel mortiser mortises, the bottom of this mortise is square and flat.
On the right, a 3/8" brad point bit, and a regular 3/8" drill bit. The drills on the right are not suitable, especially the furthest to the right. The drills are too long and are likely to flex or cause flexion in the drill press. The pointy front of the last drill is especially prone to following previously made cuts.
A square drill for round holes
Cheap fortsner bits: Low quality but they drill ok (video only)