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.

IMG_20190511_233351656.jpg
 

Jubil

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Really cool. I wonder how it will work on steel. Need a much slower spindle for metal I'm sure.
 

r3292c

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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 :)
 

r3292c

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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.
 

r3292c

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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.
 

r3292c

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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.
 

petertha

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That's cool.
Can you elaborate on your spindle motor? I've seen some Asian 500W brushless motor packages with speed control, power supply & ER collet. Is that what you are using? How are you finding the run time (in terms of heat). Do you have a sense of shaft runout & how the bearings are holding up?
 

r3292c

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Yes, this is a brushless 500W motor from China. I bought it on Aliexpress. That was a package including a motor, a driver with speed control, ER16 chuck (that was quite hard to find, typically, you can easily find ER11 one), and a mount for the motor. The blower is quite efficient, motor doesn't run really hot. Driver is OK, requires DC supply up to 48V. I used it with 24V first, then bought 48V, adjusted it to 50V. The driver is fault-protected, recovers from overload in 1-2 sec. I wouldn't recommend to use this motor on aluminum, it'd to weak for this. But on wood it works just fine. I didn't measure runout.
 

r3292c

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More things made on this ball cutter



And the rotary base can be used in a fixed position either, providing a fixed mount for the milling spindle

 

r3292c

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Here is a new brainteaser made on this setup. The Ball in a Ball with three nested levels.
Two inch diameter, black walnut and cherry wood.
IMG_20200405_153306906 (2).jpg

IMG_20200405_091117383_HDR (2).jpg

 

r3292c

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Here is a new video about making Star in a Ball, step by step
 
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