I had been using a commercially available slotting saw arbor (Enco part 370-9050) held in a 3-jaw chuck for cutting slots with my Unimat milling head (see Homemade Lathe Modifications ). I made a new slotting saw arbor that threads directly onto the spindle.
The arbor uses 12L14 steel hex bar that is 1.25” across the flats. One end has an internal thread of M12X1 to fit the Unimat spindle and the other end has a ½-20 internal thread for a flat head hex screw. The hex shape plus two holes for 4mm tommy bars allow easy setup and removal. I use mainly 1/32" and 1/16" thick HSS slotting saw blades with 0.875” hole and keyway slot. The nose of the arbor has a 0.8745” diameter step and a matching end cap that retains the slotting saw blade. I used part of a 0.070” dia. hardened pin from an old minicomputer paper tape punch for the blade retaining pin. Also removed 0.050” from the top of the flat head hex screw to allow it to seat below the end cap.
By the way, for the Unimat purists, the arbor and end cap could have been made completely on the Unimat like I did for the milling cutter holders (see Homemade Milling Cutter Holders ). However, this time I used my 12” swing geared-head lathe to machine the countersink on the end cap but the rest of the arbor was machined on the Unimat.