"Arch" wrote in message
...
Thanks for responding, George. An orphaned musing is a terrible thing to
lose. A differing opinion is a good thing,
dogma isn't. I only meant to share and provoke other ways to think about
the cuts we make. We won't change the world as we know it, but future
generations of woodturners will be saved.

Semantics aside, we aren't far apart. I said, "... but firmly held
tool edge...." and "....in shearing...,a tool edge _if not controlled_
can produce a scalloped surface...." (emphasis added)


Don't know how far. For my examples, I use wood cut across the grain.

I view the work as the most flexible part of the turning combination. Any
tool presentation which applies more of a force outward - more or less
perpendicular to the surface - rather than downward toward the bottom, where
it cuts as the wood wishes to be cut, can cause chatter. This can be
because of the angle of the tool, or because the turner "rides" the bevel
into alternately harder/softer wood. If the turner is foolish enough to
give leverage to the piece, tool flex can begin to affect the process as
well.

What I consider the worst possible presentation is the one where force is
radial - stuffing the edge of the tool into the rotating work at close to
90/90. This is the classic scraping presentation, where the two portions of
grain are lifted and/or torn and the piece departs the holding device if a
bit too thick a scrape is attempted. If a gouge edge, which is
wedge-shaped, and therefore drawn into such a cut, enters at a scraping
angle, it's called a catch.

To me cutting is what drops a shaving, scraping or stabbing what throw one,
and Newton's third law applies. It is, of course, an elastic "collision,"
(Leo) with the piece deforming.

As to shearing, consider the gouge. I have to use slashes, though it is
curved in length and in width. After the edge lifts the shaving to begin,
the gouge is rotated so the bevel can rub at a shear angle ( \ ) if looking
directly in toward bottom, and (\) to cut, if you could look through the
piece from above. It then takes a shaving along its engaged length as the
wood is transported past the edge. As we know from other woodworking, even
a sharp edge benefits from shear. Plane end grain without shear, or try
cutting straight ahead versus drawing a knife along the piece if you need to
refresh your memory. Shear is the optimum presentation for cutting wood
because it exerts the least force which might develop into chatter or
"scalloping."

I'll suggest another possible analogy. Not as specific advice for good
woodturning, just as a brief respite from Jet vs Delta. I know (ok,
believe) from experience that cutting metal below center draws the bit
inward and produces a rougher surface. Cutting metal above center pushes
the bit outward and can avoid a rough interrupted cut.


My bet is on compression of the material by the bit. Same as scraping.

If the analogy is partially valid, are there any useful parallels or
opposites for turning wood? I realize that this tortuous analogy is
only loose and general, since wood is cut and scraped with much more
latitude and with different cutting tools.

Are you referring to spindles? There the work squirms up in wood cut below
center, and is burnished down from above. It's that elastic collision
again. The best spindle cut, as we know, is produced by a tool named by the
angle at which its edge is presented - a skew.


Who will be the first to post that a little knowledge is a dangerous
thing?


Alexander Pope