Excellent point, Charles...not so obvious until it's needed...
Are you using square tenons in the spreaders instead of round?That's white birch that's 30 years old I'm using it on Tom so yes. Once the points enter the wood the top isn't going to move. A dedicated mortiser will set you back around $500, the attachment was $130 I think, but 25 years ago roughly. BJ used to say why have another machine taking up room when you already have a DP. That was my sentiment too but now I realize that the DP has far greater range so to me that makes the attachment an even better deal. The attachment takes 5 minutes or less to mount BTW. There is a clamp ring on the yoke that fits on the base of the quill, above the chuck. Mine was a Delta attachment for a Delta DP so no issues with fit but it came with a bushing for a different size quill and it sounds like the two sizes are pretty standard.
Yes. I used a 1/2" chisel and overlapped the holes to about 3/4" long so I cut some rectangular stock to match. If it's a little tight it only takes a minute to hand chisel out one end. None have been too loose yet but if they are I use paper or cotton cloth to tighten them up if needed. Cotton and paper will both absorb the glue and become part of the joint.Are you using square tenons in the spreaders instead of round?