Specific question about hollowing spheres...

[David DeCristoforo]

The post of my sphere became a great discussion about various methods. But I remain unclear on one point. So I wanted to ask this specifically (lifted from my response to Dale's last comment):

"OK. I'm gonna show how dense I can be. I totally get the donut chuck thing and the cutting a groove thing and even the turning between several centers thing. But the hollowing "between cups" bit is not sinking in. It seems to me that to mount the sphere between cups, one cup has got to be right where you are going to want to stick your tool, so to speak. So what am I missing?"

[David E Keller]

I don't think you can hollow between cups... You can shape the outside that way, but you can't hollow with something covering the center of rotation on the tail stock end of the piece.

[Wally Dickerman]

David, I suggest that you check out Christian Burchard's site. I've watched him turn and hollow spheres using a turned jam chuck. He cuts a square opening as a contrast to the round sphere. Then he mounts the piece off center and cuts patterns of deep grooves, or cuts them randomly. Since it's off center the grooves are cut only part way around the sphere. A very nice appearance.

Hey, Christian lives in Ashland, Or., just a short trip up the road from you. Christian is a true wood artist and a nice guy. (Tell him I said so)..give him a shout.

[Dale Miner]

David,

I don't think it possible to hollow between cups. As you said, the cup on the tailstock would cover the hole needed to hollow through. It would be possible to turn a cup jam chuck, and jam the sphere into the cup firmly, and use only a single cup to hold the sphere. On WOW, I posted a piece called 'Sorcerer's Flask' that has some in progress shots that would be similar to the cup jam chuck mounting described. This method of mounting for hollowing does not provide for an easy means of measuring wall thickness of the portion of the sphere in the cup chuck during the hollowing process.

In the method I described for hollowing a sphere, the hollowing is done while the sphere is still held by a tenon. After the outside and inside are finished, the piece is reversed onto a stub, the tenon turned to a small nub, the nub removed with a chisel or what ever, and the outside of the sphere turned perfectly true using cup chucks and the shadow line method. It is necessary to have the sphere very close to true before using the cup mounting method, or have sufficient wall thickness that will allow for clean up. I use a template similar to the one you posted a picture of, but cut mine from sheet metal to an accurate scribed line.

Dale

[Dick Strauss]

David,
How about if the outer (tailstock side) cup has a hole in the end and the two cup surfaces are joined by screws around the perimeter? If the two cups are screwed together around the perimeter then no tailstock pressure would be involved if you use screws to hold the sphere. The sphere can be jammed between the two cupped surfaces and you can hollow through the hole as normal.

[Curt Fuller]

David,
How about if the outer (tailstock side) cup has a hole in the end and the two cup surfaces are joined by screws around the perimeter? If the two cups are screwed together around the perimeter then no tailstock pressure would be involved if you use screws to hold the sphere. The sphere can be jammed between the two cupped surfaces and you can hollow through the hole as normal.
In David Springett's book "Woodturning Wizardry" he gives instructions on building jigs like what Dick has mentioned here. They're used for the complex sphere hollowing he also teaches for Chinese Balls, etc. Essentially they are cups held together with screws and held on the lathe in a chuck. The tough part of using them is getting the outside shape perfectly round and exactly the size of the cup so it holds it firmly enough to hollow with no movement.

[Faust M. Ruggiero]

David,
I am assuming you want to complete a sphere the turn it to a random position and hollow it. How about this idea. Start with a larger blank than you want to finish with. Mount between centers and make round, do the line around the middle thing that was mentioned earlier. Turn the blank 90 degrees and turn the other direction thereby creating your sphere. Now remount between centers at the angle you want. flatten off an area for a 6" face plate and mount it with the outside holes. Now reduce the diameter to the point where the eventual bottom of the form is within the area inside the screw line. You will have to make the sphere twice but you will be able to choose any grain line you want to be the axis. Meanwhile, though you will hollow the inside before the last small portion of the outside is shaped, you will have the security of holding the sphere on a faceplate for hollowing.
faust