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Adjustable jam chuck for spheres

Lou Jacobs

Lou Jacobs

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Today I was working on larger pieces of ash I got at the beginning of the week. I made a hollow form from one and wanted it to be a pretty perfect sphere, or at least as close as I could reasonably get. First step of the mathematical model is to turn an octagon. Here I’m using the combination square to check the 45° angle of the octagon. When the angle is right the ruler will be perpendicular to the lathe bed. Overall this turning is 9 1/4” in diameter, and a hair less in height, accounting for the flat base.
 

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Donna Frazier

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robo hippy said:
In "theory" you should be able to get a "perfect" sphere in one set of rotations. Yesterday, I got one that was close to billiard ball round after rotating through the 3 axis pencil lines maybe 10 times. It seems that it takes a couple of revolutions of the different rotations before I get to the point where I don't take out any of the pencil lines.

The skate board wheel idea seems pretty good. May try it some day, but haven't been on anything like them in 30 years. I am too old and brittle....

robo hippy
Click to expand...
A wooden jamb chuck allows the best access in truing uo a sphere without having the tailstock in the way.Yes, it requires skill to reshape it for each sphere you are wanting to true up, but since it mostly grips on the outer circumference it does not require too much wood removal in making these adjustments.
There is an excellent YouTube with Stuart Batty where he shows and explains the process while making a spherical box.
View: https://youtu.be/MHvAa5Q73Iw?si=OYpGS3gqwuvPLT9C
 
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Donna Frazier

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If your sphere is too large to jamb chuck.... you might try using a light to cast a shadow. Use a compass to draw the circle out on paper and then cast the shadow onto it inorder to know where you need to make your refinement cuts on your sphere.
 
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robo hippy

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Well, the aim is to get billiard ball round for the final turning. I am sure there is a way.... If I had known how deep this rabbit hole was going to be, I would still have gone down it... I did get one that could have been sanded starting at 220. Getting closer. I keep thinking about some flexible PVC tubing that I can use inside extended jaws. Make the plastic with a cut out in it so size can vary, and I would probably need a bunch of them.....

robo hippy
 
hockenbery

hockenbery

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robo hippy said:
PVC tubing that I can use inside extended jaws.
Click to expand...
Reed - if all of your spheres are close in size you may be able to use pvc couplings.
Fred Holder who has passed, had an article in June 2011 journal on Chinese balls. He describes making a chuck in the article.
Fred used this chuck in demo he did for our club to hold the Springette Chinese balls.

You can adapt this to hold larger balls and allow a bit more to project.

I did a screen shot from the article - making it. And using it
IMG_1357.jpeg. IMG_1358.jpeg
 
Bob Brown

Bob Brown

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hockenbery said:
Reed - if all of your spheres are close in size you may be able to use pvc couplings.
Fred Holder who has passed, had an article in June 2011 journal on Chinese balls. He describes making a chuck in the article.
Fred used this chuck in demo he did for our club to hold the Springette Chinese balls.

You can adapt this to hold larger balls and allow a bit more to project.

I did a screen shot from the article - making it. And using it
View attachment 61826. View attachment 61827
Click to expand...
You might contact Charles Waggoner at Conestoga. I can't seem to get return call but maybe you can. He has simplified the whole process, but maybe he can help
 
Matthew Kilareski

Matthew Kilareski

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I also use skateboard wheels. The higher durometer (hardness level) ones seem to work the best for me. I also use ones made from clear urethane to keep from marking the wood. For the tailstock side, I was able to tap the inner hard plastic core to thread onto my Robust live center.
 
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