Converting NMTB40 to CAT40

Have a 60's era horizontal mill with a vertical head, both spindles are NMTB 40. My CNC has CAT40 tooling. Being able to share oddball tools as needed would be quite nice.

The PO of my mill made it sound like the drive lugs had already been swapped, or were capable from the factory of taking CAT40 tooling. A quick fit up the other night proved they are not.

I already have a piece of stressproof coming for longer drawbars (seemed like a reasonable material choice). So I am looking at what to do with the drive lugs.

My plan was to remove and keep the original lugs and simply make new "thinner" ones to accommodate the shallow notch ,regardless of the orientation, of the CAT 40 tooling. Suggestions on material? 4140 PH was mentioned elsewhere. I have some 4140 (Annealed) in the rack and could heat treat if I had to. Suggestions?

If it makes any difference, this is a 5HP machine and poised to do odd jobs, not heavy production machining. Read: I would rather err on the side of protecting the machine rather than ensuring I can run the the spindle at 125% load for hours on end.

Thanks

When I picked up a huge lot of CAT 50 tooling to use on my No.4 Cincinnati all I had to do was slightly modify one drive lug. Doesn't affect the use of NMTB stuff at all.

As for the drawbar mine was already about 6" longer than needed. It had two nuts back to back for NMTB and you could just move them back a bit for CAT 50 stuff. No clue on material but since you shouldn't be torquing it down a ton since it gets it's driving force from the lugs it shouldn't need to be something over the top.

I would just make the lugs from something soft, but tough. If you actually wear a lug out on it then maybe make a set and harden them. Or make them from a material you send out for HT and when you send a batch toss the lugs in?

I just can't see wearing out drive lugs on a manual mill like that in this age.

I pulled the little screw/pin alignment thing out of the spindle in my bridgeport a decade ago and have never spun a collet or otherwise missed that pin.

This machine also has the captive hex on the bar + a tension nut. However, it the bar is made just to length so it will not even start to engage the threads of a CAT40 even with the nut completely back. My plan is to retain this setup so I can use the longer bar for both NMTB and CAT, simply choke up with the nut when using the NMTB.

I am starting to wonder if simply making the lugs out of annealed 4140 would be sufficient? If there is a solid reason to go with pre-hard, I can make that happen to.

The advice so far has been well taken. Thank you.

I've posted this a few times before but it's still relevant here...

Machine Tool Shanks (Tapers)

the top nut is made a little smaller, so the wrench slips over it easyly. when you spin the wrench and the bar reaches full travel, it drops onto the lower nut and you are ready to tighten. (one of these days im going to make the top nut longer so there is less chanche of the wrench flying off.)