Now I know the easiest way to do what I want is with a push broach and make them as two pieces, but I want to make some specialty hex drivers with a one piece body. That leaves me with rotary broaching. The thing is I am not going to make a bunch of them so buying a rotary broach holder or spending a day making one isn't all that wise of use of my time or money. However once I was setup to do it rotary broach inserts are a lot cheaper than push broaches.
Recently I had need to do some cross drilling that would have been easiest with a preset cross drill on the lathe. Its slow and tedious to center up and drill the part on the mill, but of course I can do it. I don't have to do it often so again making a cross drill for the lathe isn't good use of my time.
Then I had an aha moment. Make it fairly robust and use it for rotary broaching as well. Centering would be a little tedious, but getting my 1 degree back angle from center would be dead easy to do after that. Then lock down all the gibs not in use and advance the carriage. I doubt it would ever work on the mini lathe or the small lathe, but maybe the PM1440 could handle it.
If I can use the same tool for both applications it almost makes it worht making one. I might still know where to find it by the next time I need it. Driven with hand held drill when cross drilling and of course driven by the stock when rotary broaching.
I don't think I've ever cross tapped stock in the lathe, but if I don't see why I could not add enough functionality to do that too if I need to.
So what's wrong with it? I know rotary broaching takes a lot of power. The lathe is big, but its only 3HP. Flex in the compound, cross slide, and carriage could be an issue. If a spacer is used internally in the cross dril that rides on a thrust bearing I think the drill itself could take it. I also considered bronze bushing with a 1/2 shaft collet chuck because the bronze can take so much force.