Inside Radius Cage

An article on using a lathe to rough out human figures lamented the inability to turn inside radii on non-continuous pieces. It's easy to turn inside radii on solid things like bowls, but it said there was no way to mount something like a spindle that needs a longitudinal hollow.

Not so, says I. The turning has to be treated as a faceplate mount, not an off-center spindle. And it requires a special armature.

The armature is a disc mounted to the faceplate; the disc's radius is something larger than what you want to turn. Arms (two or three, radially opposed) reach along the axis of the cylinder described by the disc, to where the tailstock would be, and are there attached to a ring. Now we have described a cylinder, but its sides are not complete.

To the inside surface of the arms, we mount our (hopefully balanced) workpieces. Reaching in through the tailstock end of the armature, we can engage the workpieces as they turn, and observe progress through the sides as the gaps pass. The three-arm design is especially nice for this, as a two- or four-piece armature will always have another side blocking your view unless you view from something of an angle.

Perhaps our workpieces aren't balanced. Certainly the stress of engaging the pieces won't be symmetrical. This could be disastrous, since there is not much angular rigidity to this contraption. It'll flex out of line, then start to whip, and beat itself to shreds. Not to fear! Devise a large-diameter center steady, like for working with very long pieces to prevent whipping, and thus hold the tailstock end firmly in place.

Obviously many risks exist, not least of which the possibility of catching the turning tool and flinging it, what with all the many gaps whizzing by. We all know that turning has the potential for such mishaps, and off-center turning even more so. Be damned careful if you choose to try to make one of these things.

I've drawn up the design but not made one. It looks perfectly feasible and I can't see any glaring showstoppers. If I can figure out how to upload the picture, I'll do it.