I have a job in the future to make some perfect spheres and I could do it with a template and lathe tools but I have often thought I’d like to have a jig to swing the arc to cut spheres. so I decided to make one that will cut convex spheres and concave pockets. I had some sketched but then started looking at what was commercially available and found the Carter Perfect Sphere jig. I followed that design since they had put in all the engineering and all I had to do was determine the size and workings of the parts of the jig.
I used aluminum for the arm and fitted it with a ball bearing that was laying around the shop.
I made the center post adjustable for a 15” or 16” swing lathe. The top of the post is drilled out to hold the horizontal cutter arm with the center shaft that advances with the turn of the handwheel in the back. I could not see how Carter made their adjusting mechanism so I devised one with a ball on the screw and a hole in the cutter arm that works very well.
I made the base mechanism adjustable for different gaps in the bed similar to the Carter design. I scribed the width lines on the bottom instead of the top.
I made some support rings off center so I had a thicker area for the thread without going to a huge diameter.
All the screws that go against the steel shafts have a brass insert in the end to prevent galling.
The horizontal bar has a flat on top and one at 15 degree to either side for doing a shear scrape cut in either direction ( A Carter idea).
The handle on the swing arm is Rosewood.
I made a sample sphere out of a piece of mesquite just to try it out. It cuts very smooth. The cutter is tipped down at 13 degrees. I measured that from a still shot of the Carter tool on You Tube. I think that idea has merit in hollowers as well. The tip is a round Hunter carbide insert that comes with a 4 mm screw.
Here are some shots of the assembly and the parts layout. The steel round parts are coated with Gun Bluing so as to not add to the diameter and yet protect from rust.
Now I’ll need a box for it!!
I have started on 2 pair of wooden cups (2” and 4”) to hold a partial sphere in order to cut the ends round and make it a perfectly round piece. Finding exact center again will be a challenge, I think.
-- Jim Jakosh.....Practical Wood Products...........Learn something new every day!! Variety is the Spice of Life!!