Subject: Early surface plates: How were they made?
From: (Chris Lindquist)
Date: 21/08/03 20:43 GMT Daylight Time
Message-id:
I've been trying to find out exactly what process was used for
originating a surface plate in the good old days. I expect it involved
careful hands and a large amount of scraping, but what provided a
reference for "flat"? And how flat was flat around the turn of the
century?
I'm not planning to try this at home, of course. My Chinese granite
plate is plenty flat enough, thanks. But I've been going through my
books looking for an answer to this chicken-and-egg question without
finding a solution.
Can anyone help?
During my apprenticeship I had the 'priviledge' of resurfacing a number of
4'x6' cast iron surface plates. (1 was standard, I was the bosses
'favorite').
*
We were able to get a good start by cutting below the patina of the badly
warped plates on a single point planer. Then they were left to soak and
settle for a couple months.
*
For the plates in better shape, we just started in. Using three plates
(five is better, I'm told) the program says that you blue one plate to
another, and scrape away the blue on one plate. Then take that plate and
blue it to the third plate and scrape that one. Then take the third and
blue it to the first one and scrape the first one. Then you turn one of
the plates 180 degrees and start all over. This continues until all three
blue evenly to each other in all orientations. Then you usually flake the
surface to break it up to keep it from being too smooth (and to give a
place for minute dirt to collect).
*
Now - this takes a long time. Especially when the boss loans out the power
scraper. It took two of us 6 weeks to hand scrape approximately .003. A
paper coffee cup of shavings was a good days work.
*
I still have the scrapers I made for all that. Drew out some 6150 to about
1" X 24" X ~ 3/16 and brazed some carbide on the end. Made about a half
dozen to get one with just the right feel.