This is a spool hub designed to have a 608Z bearing press fit into the inside hole. Two of these hubs are then put onto an M8 rod, and then bolted in place to hold in an ABS spool. You can manipulate the file to suit your own spool sizes. My spools happened to have a 45mm hole in the middle. As you can see below, this assembly is then being suspended by the frame(carcass?) of my old Mendel.

The bottom edge of the inside hole and the top edge of the outer cylinder have a 2.5mm fillet to make it easier to press in the bearing, as well as insert into the spool.

This was a very straightforward print and required almost no post-processing cleanup. Simply place your bearing on a flat, solid surface, and gently press down on the spool hub until it is securely in place.