Internal spherical turning

I have the need to make a what is in effect a clamp with a spherical hole in it rather than a cylindrical hole in it.
How should I go about it?
I have seen ball turning attachments but can't get my head round how to go about using one to turn what is in effect a spherical cavity.
The clamp will be 3/4 inch thick and the diameter of the sphere it need to clamp is 1 1/2 inch.
I have done a bit of Google searching to no avail, probably I am not using the correct terms.
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You can use a ball turner just have the cutting tool projecting beyond the pivot point just like this
http://img.photobucket.com/albums/v156/jasonballamy/Fowler%20construction/PICT0053.jpg #
If its just a clamp you could just use angled internals as a full sphere would not be needed
Jason
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wrote:

+Thanks Jason. However, sorry I don't appear to have made my self clear. Your photo illustrates a concave surface on the outer surface of a shaft. I am wanting to produce a concave surface in side a cylinder. The cylinder is in effect 3/4 inch long and the spherical radius is also 3/4 inch. The item I am trying to make is like a rose joint. The bit I referred to as a clamp is ,in effect, the outer part of the rose joint.
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Schoolboy geometry. The centre of the ball turning attachement simply needs to be moved to the c/l of the lathe.
Simples
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http://www.holdridgemfg.com /
You should be able to figure out the necessary tool motion with a pencil and a teacup.
jsw
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Can you cut it with a ball end mill?
jsw
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You just need to use a boring tool possible with a hook shaped end in a ball turner and provided the pivot point is in the right place you will get your concave bore.
Another option assuming you will have to split the cylinder to get it over the ball is to machine it as two half rounds in the 4jaw with teh cut facing out and you could then use a ball turner across the face.
Jason
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