The ETA/Unitas 6498 has a wheel thickness of 0.27mm-0.30mm (the reference movement seems to have some variation between wheels) which necessitates the need for very thin brass. CZ120 brass sheet does not come in sheets anywhere nearly thin enough. However a reliable solution to this problem was found.
An experiment was carried out to see if a very thin disc could be parted from 8mm CZ121 brass round bar (brass rod). This was performed on the Sherline lathe as a good parting tool was readily available for this.
A few issues had to be overcome. Firstly, on a three jaw chuck, despite it being very tight, the pressure of the parting tool pulled the work out of the chuck gradually, causing misalignment. A four jaw independent chuck solved this. Secondly, the center height of the parting tool must be absolutely aligned to the center. Too high and it rubs on the work and cuts poorly causing lots of heat, too low and the parting tool gets pulled under the work. The parting tool must also be extremely sharp. Finally, due to some flex in the parting tool and the thinness of the brass disc, the parting tool does push on and warp the disc being parted, for which the solution is given below. A final note, I found this to work best without any cutting lubrication.
The final parted discs were dome shaped due to flex in the parting tool as shown below.
The knub in the center of the domed disc was removed with a side cutter and a diamond stone was used to remove any remaining material. Then the disc was gently peened flat with a hammer on an anvil. Finally I gave it a quick cleanup (not to final quality) with lapping paper. In the image below, three parted discs are shown, two with the dome shape and one which has been finished according to this paragraph. Final disc shown on the right below.
The disc was checked and remains at 8mm in diameter after the process. As can be seen below, the disc is perfectly round and flat with a depth of 0.27mm. Success!
Due to the thinness of the part, I would pre-drill the center prior to parting (which would also avoid the need to remove the knub). When making the final wheel, I will also try cutting the teeth prior to parting to see if this is at all feasible without damaging the teeth.
