Reply by SqPeg

yes it will be a blind spline 1/4×2”. The frame sides are 25” long so. I have to hold them at a 45 degree angle under the head of the mortiser so the bit will go in perpendicular to the face of the miter. Not sure how or if it can be done and still move the stock so that the bit will cut the entire lenght of the spline.

I get the feeling a bench mortiser is limited to doing only do miters in butt joined parts