The greatwheel arbor is made of 5/16" drill rod thinned to 1/4" where it goes through the holes in the barrel, then thinned again to go through the frames. It's basically a straight-forward machining job, but there are some points to make.
The winding square on the end of the arbor is made with a file. While I do have a milling machine, this is one of those jobs which is easier, faster, and of equal quality to a milling job.
In the lathe, I cut a V-shaped groove near the end of the arbor. This is the limit of the square. Then, I clamp the arbor in the vice and file some of one facet. Then I rotate the arbor 90 degrees and file some of the next facet, and so on, until there are facets on all four sides -- the vice acts as an indexing device. You judge how much you've filed by keeping an eye on the "lands" -- the uncut round areas -- on the edges of the facets. Don't try to cut all of one facet at a time -- keep rotating the arbor in the vice. Try to keep the lands the same size and parallel-sided. If you're careful, you can cut the arbor perfectly square withing a few thou, and do it in fifteen minutes.
The part of the arbor beyond the V-groove is protected from file damage by a sleeve of brass tubing (or washers) which comes up to the center of the groove.
The greatwheel and arbor are meshed with the center arbor (what's driven) in the depthing tool, and marks are made on the frame with the depthing tool points, then center-punched, as described before.
Drills are fickle, and the drill can shift when drilling a larger hole. So before drilling I scribe a small circle around the center-punch mark on the frame to act as a reference. Then I drill a small hole (about 1/16" first), then a larger, and so on, up to the correct size. Should any shifting occur, it can be corrected with a file before the next size hole up is drilled.
Today's links:
The arbor.
Filing the winding square.
Square with lands.
In depthing tool.
Arbor hole outlined.
Wheels planted in frames.