Ball Turning Attachment

[Roger B]

A while ago I saw a good offer on a Proxxon ball turning attachment and bought one. As I need to make a 2mm radius hemisphere with a M1.4 thread for my new diaphragm pump I thought it was time to try it.

There was a selection of fixing holes and I found a pair that matched the existing tapped holes in my cross slide. There are three possible positions for the tool holder to fit to the swivelling table to different radius concave and convex cuts. The tool height is easy to set with a jacking screw and a large clamping screw.

Using a point held in the chuck I set the centre for the pivot and the setback for the tool. Is there a clever way of doing this?

I then took a length of 4mm brass hex for the first trial. This needed more overhang than I liked but there was not much clearance between the swivelling table and the chuck jaws. Is this a normal problem?

The first attempt was cut by swinging the tool and feeding the carriage forward. Is this the best way?

The result is not very smooth but nothing jammed or broke 

[GWRdriver]

Roger,
(RTA = Radius Turning Attachment)
I start by locating the center of the sphere (the pivot point of the RTA), by finding the OD and end point of the stock.  Somewhere within of those two points lies your sphere center and RTA pivot point.

Once your RTA pivot position is set, and some rough metal removal is done to get to a ball shape, the cross slide and carriage shouldn't have to be moved as neither have an effect on the radius of the sphere, only on the location of the RTA pivot point.

If you are doing a hemisphere, and the finished radius is set, the carriage advance toward the headstock could be used to get to fresh metal and a finished surface, but once you pivot the RTA beyond 90° (ie, beyond turning a hemisphere) any carriage advance would ruin the sphere.

My RAT (home-built) has a threaded advance (in-feed) and the final few finishing passes are done with that.

I'm not sure if this helps but I hope so.

And yes, from time to time chuck clearance can be a problem.

[Roger B]

Thank you Harry 

I had another attempt running at a higher speed which gave a better finish. The result was 4.3mm diameter so I used the cross slide to move the tool foreword 0.15mm and got a result quite close to 4mm.