In Robin's (Tandum) QHY11 thread there is some interesting discussion (to me) about making adaptors on a lathe.
So that this info that maybe of interest to others is not lost I have created this thread. Maybe some one that is more thread savvy than me can copy some of the original posts here as well.
Of the many ways of cutting threads on a lathe, the most common technique is to use a tool that is shaped to represent a cross section of the thread to be cut.
As most thread forms that we come across are roughly triangular with rounded peaks and troughs, the cutting tool also has a triangular shape with a rounded tip and is normally just advanced (plunged) with the cross slide deeper at right angles into the material
to be cut over a number of passes, getting deeper each time until the correct depth and shape is achieved.
Because of this triangular shape, the deeper we go (and closer to our finished thread) the more work this tool has to do on each face and because of the pressures on (and geometry of) the cutting faces to the material there is a tendency for the edges to tear resulting in a rough thread of poor form.
A modified method is to turn the tool and tool slide approx half the angle of the thread to be cut and advance the tool using the tool slide on this angle.
This makes the tool advance into the material almost parallel to one face of the thread taking a small amount off the trailing edge and doing most of the cutting on the leading edge.
In the QHY11 example the thread angle was 60 degrees and Mark (Marki) used a set over of 29 degrees.
This halves the work and material removed by the tool, improves the cutting geometry and results in a cleaner cut.
Sorry for no pictures or diagrams, but if you Goggle - set over screw cutting
On another note, a good workshop resource was Chris Heapy's page where he describes this and other techniques including mods to an LX200.
I think this is an archive of the original site
http://www.astronomiainumbria.org/ad...h/homepge2.htm
Let the discussion continue, Neil