helical milling an aluminium lap

Cutting a helical groove into a piece of aluminium tubing which will be used as an ID lap. The work is done on my trusty Thiel Duplex 158 milling machine with a Walter UTA 200 dividing head. The x-axis of the table is connected to the dividing head by means of a gear train which determines the pitch of the helix. This was my first attempt doing this. At first I miscalculated the gear ratio and got an incorrect pitch as you can see in the video.