Tim Wescott wrote:


I suspect that a hand-scraped way done with care is _way_ better than
a laser calibrated ground way done by someone who doesn't give a
****. I also suspect, however, that a good, careful guy with a
laser-guided grinding setup that's kept in good trim can at least turn
out stuff that's as good as hand-scraped and probably faster.

The hottest-**** setup would probably be a laser interferometer plus a
really bright, committed guy with a scraper, though.

That doesn't work on hardened beds, though! (Don't ask how I know. OK,
I'll tell you.
I did the closest I could think of to hand scraping a hardened Sheldon
bed. I used
a "test carriage" made to have the same shape as the Sheldon carriage on
the bottom,
just shorter along the bed (about 6"). I used a hand scraped
straightedge with marking
medium (spotting dye) and a Taylor Hobson Talyvel electronic level.
With the test
carriage and the straightedge, I identified local high spots, and the
Talyvel was used
to keep an eye on long-span variations along the bed. I used an air die
grinder and
Cratex rubberized and cloth-bonded polishing wheels to remove material
where needed.
I finally finished the surfaces with bench sharpening stones that had
been rubbed together
to obtain a nearly flat stone. These stones, when rubbed over the bed
with a heavy
application of light oil, would remove a tiny amount of metal from even
more localized
high spots, and leave a beautifully polished surface. But, it was
backbreaking work
(although more fun than working out on a Nautilus machine.)

Anyway, I finally got the 6 foot bed down to +/- .0005" or so over the
entire length.
I could have kept working, but I thought that was as good as I'd ever
need it. The tricky
part is to get the front/back motion of the carriage as it travels along
the bed down to
a minimal amount.

Jon