On Sat, 13 Feb 2010 06:06:35 +0000, DoN. Nichols wrote:

On 2010-02-13, Michael Koblic wrote:
.... [re making a ring, 8" OD, 6.5" ID, from .25"-thick steel
plate, on a 9" lathe]

can you part the piece out by having the parting tool at 90 degrees to
the usual direction? Cutting into the piece at the right angles with
the tool lined up along the lathe bed? I have seen it done with a wood
lathe.

No! The blade of the parting tool would need to be curved like
a parenthesis on a radius to match the cut to be made. This is called a
trepaning tool. A standard parting tool would bind on the outside below
the cut.


Geometrically, a lot of other shapes would work, such as some
triangle and wedge cross sections. Of course, the fixed-radius
curved-cross-section trepanning tool that you mention probably
would be strongest and best for thin cuts. However, at radius
3.25" cutting .25" deep with a .5"-high cutoff blade, it would
work to grind a 5-degree[*] side relief (and 1 degree of back
relief, per http://yarchive.net/metal/parting_off.html toward
the end). Also, you might want to trepan to a slightly smaller
radius than the finish ID and then finish by boring, depending
on what gives a better or quicker result.
[*] For a trepanning blade of height h at radius r, the
interference at the outer bottom edge is about (h^2)/(2*r).
With a .5"-high blade, this comes to 0.038" at 3.25" radius
and to 0.042" at 3".

....
Of course if the plate which you are machining can tolerate some
holes in its surface -- perhaps in places which would be machined larger
later -- drill and tap for bolts to hold it to the faceplate..
....

I've snipped DoN's other comments; take them as predecessors
to following suggestion: If the ring is for a sun-dial, holes
on the back won't show, so you could drill holes .15" deep to
seat upon pins in a mandrel plate attached to the faceplate.
The pins would withstand shear forces, allowing much heavier
cuts than you could take when depending on glue or tape to
withstand those forces. One could tap the holes, use loctite
and studs, etc, but presumably not worthwhile to do so.

--
jiw