When I was making some rotary valves for the Nalon Vipers (see previous thread) I wondered about how to generate the fuel inlet cutout. The simplest thing I could come up with was a mill spindle mounted scriber. Think of a spring or pump centre, with far less axial movement, and as close to zero radial movement at the pointy end. The actual scriber was/is a piece of cheap (CTC) 5.00mm diameter HSS, with an approx 60 degree ground point, courtesy of a Dremel grinder mounted in the toolpost. The bores for the pointer, one at each end, were lapped/honed to give a nice smooth, play free movement. The overall length is about 100mm, there is a bush for the scriber at each end, as far apart as possible to reduce any error due to radial movement. The main body is a bit of that lovely ground steel rod extracted from a dead printer. I span the thing up to check that the point was concentric….it was, and marked the rotary valves. Then milled "to the line". Obviously too slow for production, but perfect for what I wanted!!
The Household Photographer is away on a field trip at the moment, otherwise I would attach some photos, after all a picture speaks a thousand words etc etc
cheers
Bill