Powered ball turner, or How to cut an ideal ball with end mill?

r3292c

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Typical radius/ball turning attachments we make for our lathes can't cut an ideal ball. They usually allow to turn the part of the ball surface and don't let you cut the ball off. The typical ball made with a typical radius turner usually has a threaded hole in the body, which was used to mount the piece of material on a bolt. Usually that's acceptable because the most of the balls we make for decorating the handles :) . So, they have to have the threaded holes.
But what if I need an ideal ball? OK, I have a couple of options here.
I can use a crown drill bit in the second spindle, aligned (almost) perpendicular to the first one. Or I can use a special radius cutter with a bar that was shaped for both turning and cutting-off. I don't really like the first option because I don't like the crown drill bits I've seen. And building a better radius cutter seemed easier for me. Here is the one I've recently built.


On the video above the wooden ball is close to completion. The finishing cut is being performed. One can see how the ball could be cut-off. Ball size is 2" diameter, the end mill is 5mm, or 3/16". Rotary spindle is 500W, 12500 RPMs at 50V supply. Lathe runs at 1000 RPMs on rough paths in the beginning and at 200 RPMs on final paths. Rotation of the lathe spindle is reversed, however I didn't notice a big difference between FWD and REV spindle directions.

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The cutter is made of 1/2" steel plate, 5/8" shank. Recess of 1/50" depth is machined on the upper plate to form a 3" slide bushing. A needle thrust bearing sits underneath the tightening nut. Rotary plate moves tight and smoothly.

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Plan to add an indexer for fixing the milling spindle in certain positions, and a fine radius adjustments.

Using this powered cutter on wood, but will definitely try it on aluminum soon.

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Really cool. I wonder how it will work on steel. Need a much slower spindle for metal I'm sure.
 
I saw people are using these spindles on CNC routers to cut aluminum. So, I'd try on aluminum. Don't think using it on steel is a good idea :)
 
Benchmarked wood cutting versus aluminum cutting.
The wood is being milled very smoothly, can be easily cut by 1/12" (2mm) per pass.

Milling aluminum isn't a fun. It works, but not as good as wood. A lot of aluminum chips is produced. These chips are hot and sharp as needles. Not really comfortable for manual control.
 
Finally made one more ball turner using the same rotary base.
This one uses 3/16" thick part-off blade re-sharpened to have a profile needed. Working with this attachment is really comfortable, no problems with chip needles. Fine adjustment of radius is available.
 
So, using a milling spindle in radius turner is perfect for wood. It works very fast, no preliminary corner smoothing needed. But on aluminum it provides tons of sharp and hot chips. I think it's uncomfortable and dangerous to use it on Al because these chips fly everywhere, and I happened to get some under the skin.
On aluminum I prefer using the one with a part-off blade. It works just fine and supports rough cuts with thick chips.
 
Nice projects. For a newb like me. How do you get the star inside the ball?
 
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