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Early surface plates: How were they made?
jim rozen
TreeMoose
Then you turn one of
the plates 180 degrees and start all over.
Jim
I always thought it had to be 90 degrees. A saddle
error will not resolve for a full 180 rotation,
right?
Well, the old, old texts don't have any answer.
In case anyone's still interested:
The accuracy of cast iron surface plates, based on the three-plate scraping
method, is not one person's invention. That it was employed by English
mechanics during the 1700s, before the time of identifying method to person, is
sure.
Its systematic use is identified with Henry Maudslay by Samuel Smiles (the
great nineteenth-century English biographer of early industry), through whom
Joseph W. Roe, the author of "English and American Tool Builders," took much of
his content.
In any case, some of the early info on surface-plate generation comes from
James Nasmyth, an apprentice of Maudslay, who wrote
"The importance of having Standard Planes caused him [i.e., Maudslay] to have
many of them placed on the benches beside his workmen, by means of which they
might at once conveniently test their work. Three of each were made at a time
so that by the mutual rubbing of each on each the projecting surfaces were
effaced. When the surfaces approached very near to the true plane, the still
projecting minute points were carefully reduced by hard steel scrapers, until
at last the standard plane surface was secured. When placed over each other
they would float upon the thin stratum of air between them until dislodged by
time and pressure. When they adhered closely to each other, they could only be
separated by sliding each off each. This art of producing absloutely plane
surfaces is, I believe, a very old mechanical 'dodge.'
Nothing as to plate rotation in the scraping sequence. (Obviuously not a
technical text, either)
I suspect Moore's method may have added some footnote, with its 90 deg.,
square plate rotation consideration.
As TreeMoose points out, at some point you need to scrub the accuracy of the CI
plate, and frost it for use.
Cast iron has long lost its status as an ultimate measurement medium, in any
case.
Frank Morrison
(who still has a IC surface plate)
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