Hollow Form Reverse Jam Chucks - Which one to buy?

[robert baccus]

The device in the first pic gets around the diameter and rim/tailstock pressure problems. I use mine on large bowls and 20" deep vases. The cone has no pressure on it--it's just for keeping a vase straight. Maybe $2.00 homemade.

[Mike Nathal]

I too usually just use a wood cone in the chuck with a piece of drawer liner and tail stock pressure. But I was re-reading Ellsworth's book the other day and he showed using a bowl instead of cone. I tried this on a vase with a ~1 inch opening and like it much better. First, it greatly increases the amount of wood that is bearing the load, as you are going from a 1 inch diameter throat to a 3-5 inch diameter shoulder. Second, it puts the wood in compression rather than tension (the cone is wedging the wood in the throat outward).

[Robert Henrickson]

I too usually just use a wood cone in the chuck with a piece of drawer liner and tail stock pressure. But I was re-reading Ellsworth's book the other day and he showed using a bowl instead of cone. I tried this on a vase with a ~1 inch opening and like it much better. First, it greatly increases the amount of wood that is bearing the load, as you are going from a 1 inch diameter throat to a 3-5 inch diameter shoulder. Second, it puts the wood in compression rather than tension (the cone is wedging the wood in the throat outward).

I took a look at the book. I also have used this method. I have a number of homemade jam chucks of various diameters with concave ends of various depths. They can be used in finishing bowls or plates, in addition to the hollow forms.

[Reed Gray]

On the few hollow forms I do, when I reverse chuck it, I make a recess type chuck, rather than an expansion type one. Don't want to overexpand the opening and split it.

robo hippy

[Thomas Canfield]

One key point is to have the main pressure contact at the foot or end of the jam against the live center, and the cone/bowl or top support just to help prevent bobble. The pad at that point is to protect the surface and should not be a wedge force. The cone gives more flexibility with a smaller piece of wood and also works for a natural or irregular lip on opening. I have to say that I have never tri a "bowl" at top, but it should work well also for most uses.

[John Spitters]

I question the ability of a thin HF to handle enough tail stock pressure to keep it in place without imploding the darned thing.

When you use a vacuum chuck as a jamb chuck ( no different than what Elsworth does with a bowl ) you are holding your hollow form in compression, under compression the form is very strong even with thin walls. Think of this in terms of an egg, perhaps one of natures strongest forms in its simplicity.