I am building some furniture out of highly figured wood and had a lot of 1/4" mortises to do with the mortising machine. I had a problem with the chisel wanting to bend and follow the grain so I came up with a solution that I thought some of you may be able to use.
I took some of the stock of the same thickness and cut a 1/4" slot in the center 7/16 deep, then ripped that stock with the groove to 3/4". I laid that on top of the piece to be mortised, taped it in place on the clamp side so not to mess with center, transferred the marks up to this scrap piece and clamped the whole thing in the machine.
By starting out the mortise with the chisel riding in the slot the mortise stayed straight and did not wander with the grain any longer. I had three mortise sizes so I made one for each size and just moved it from piece to piece, taping it where I needed the mortise.
Worked for me............