!/2" router collet -> 6" cut-off disk

Will that brand new router last as long as the new set of bearings for your current spindle? How much labor and down time will each router replacement cost you vs. rebuilding the current setup and letting it run a few decades before needing attention again? How about brush replacement on a router motor vs. none on the current 3ph motor?

Make a mandrel if you have a lathe. Even a cheap crappy lathe like mine. For something like this you don't even have to be too perfect. I was awfully proud of the first few mandrels I made for special applications, but now I do it as a matter of course. A 7/8 bolt with a short unthreaded shoulder will work nicely. If its just right a nut, lock washer, flat washer to lock the disc in place, but that's not likely to happen. A threaded coupler half drilled out is more likely. After drilling it out half way chase it with a tap and substitute for the nut. Turn off the first half of the threads on your bolt to fit the collet on the router. DO NOT BE TEMPTED to round the head of the bolt. Leave it hex so you can get a wrench on it. You will need to turn the back side of the head flat and square to the shank of the bolt so your disc will set nicely against it.

Or you could see if there is an off the shelf solution. There probably is one.

I use Hitachi routers for some operations. They are cheap and last about 6 months. The barden bearings in the spindle last about the same. We can change out a router in less than 5 minutes, the bearing change takes about an hour. The routers usually die before they need brushes.

How many hours under load? If I got 6 months of 40-50% load time out of a router I would be absolutely thrilled.

I would guess about half that. We slot a Beech block 1-1/2" wide x