The existing big chain ring was a 44 tooth one, but I never rode fast enough to shift up into that range. I found a 48 tooth gear in my parts bin, and decided to repurpose it.
Since I had access to a metal lathe at TechShop
, I decided that was the tool I wanted to use. I used the four jaw chuck on the metal lathe, since this crank set uses four spokes to locate the gears. It is important to center the part when using a four jaw chuck, hence the dial gauge on the left in the photo. Centering is done via small adjustments to each of the jaws, measuring the runout between adjustments. This was by far the longest step in the process.
Make sure the chain ring is held in tightly, but not too tight or the gear will flex out of round.