In article , Jim Wilkins
wrote:

"Joe Gwinn" wrote in message
...
In article , Jim Wilkins
wrote:

Could be, a kinematic fit may be good enough for the steady rest
since it doesn't slide and wear.

If you recall, I got a 70-degree included angle conical mill bit for
making Vee grooves to fit the Clausing. The resulting grooved
blocks are quite stable on the ways, not being in the least wobbly.

Joe Gwinn

A kinematic fit refers point or line contact, the minimun needed to
prevent wobble. A 3-leg stool is a good example. Unlike a large-area
contact they wear quickly and may deform under heavy load.

True, and often used in precision instruments. A common variant is
elastic kinematic mounts, where the line and point contacts are
flexures versus rolling or sliding.

The problem with non-flexure kinematic mounts is their sensitivity to
wear, which is why they are rare in machine tools. Scrape-to-fit
methods yield area contact (this being defined in practice by the
number of contact bumps per square inch). Such surfaces are orders of
magnitude more wear tolerant than the point and line contact of
rolling-element kinematic mounts.

Coming back to steady rests, the mystery rest does show signs of having
been scraped a bit. This may have been frosting (for oil retention) or
for looks, or for real - it's hard to say.

Joe Gwinn