Production of an Original Surface Plate.
I will introduce here a description of the method of producing an
original surface plate.
The production of mechanically true planes by the method of scraping
was first suggested by Mr. Whitworth, and was brought to perfection
in his works. Having had and having improved the opportunity there to
study this system, and having employed it largely in the manufacture
of high-speed engines, it seems to me that an account of it should
find a place in these reminiscences, especially as the importance
of mechanical truth is coming to be more and more realized in this
country. I will therefore describe the process of producing an original
standard surface plate.
The first point, of course, is the design. The square form, 30 inches
square, has been found most suitable. I could not, however, use this
form myself, a long surface plate being required for the guide-bars and
steam-chest joints of my engine.
The plate must be incapable of deflection. To insure this it is ribbed
on the under side with ribs seven inches deep, and is supported at
three points, equidistant from each other and from the center, so that
its equal support cannot vary, whatever may be the surface on which it
stands. It is provided on two opposite sides with handles, by which it
can be lifted and rotated. The arrangement of the ribs and feet is here
shown.
It must be cast of hard and close-grained iron, which will possess the
most durable qualities, in a baked mold without a cope, so that the
gas shall escape most freely. As cast, the plate should be one inch
thick. About three eighths of an inch is planed off, removing all
dirt, and leaving a perfectly sound surface, with a thickness of about
five eighths of an inch. Three of these plates are made. After these
have been planed, their edges squared and steel handles put in they are
delivered to the fitter.
[Illustration: Surface Plate for Producing a True Plane.]
I will first describe the tool used in scraping. Originally this was
a hooked tool, and the scraping was done by a drawing motion. Two of
these tools were employed, one for the roughing work, in which the
hook projected downward about three quarters of an inch, and the other
for fine scraping. In the latter the hook projected downward only
about one quarter of an inch, and absolute freedom from vibration was
aimed at. These tools were used for a number of years, but afterwards
a radical change was made. The modern method is to employ a single
straight tool, like a carpenter’s chisel, about an inch and a quarter
wide and an eighth of an inch thick, with a square end. This end is
slightly curved, and its corners are rounded to avoid scratching the
plate. The scraping is done by a pushing motion.
This tool has been found preferable on all accounts. Projections
needing to be removed are in front of the tool, instead of being
covered by it. The tool is perfectly rigid, and can be inclined to
the surface at any desired angle. The cutting edge is a square angle,
and being well supported keeps sharp for a considerably longer time
than when it is an acute angle, and when ground or honed two edges are
formed. Moreover, the pushing motion is preferred.
Two of the plates only are first brought together. For disclosing the
high points, one of these is covered with a raddle made of finely
sifted red lead and oil. This is made quite stiff, and all of it that
can be removed by the palm of the hand is rubbed off, leaving only a
very thin uniform film on the surface. Any dust having been carefully
removed from both surfaces by a soft brush, one of these plates is
inverted on the other, and at one corner each plate is marked in the
edge with a prick-punch. The upper plate is then rubbed about on the
lower one for, say, half a minute. When lifted off, the high portions
of the surfaces are shown on one plate by the raddle put on, and on
the other by that rubbed off. The workman then gives to these parts
of the surfaces a general scraping, giving to his tool a long sweep,
say from four to six inches. This is repeated two or three times, the
stroke being shortened each time, and the upper plate being placed in
a position at right angles with its last one, which can be determined
by the prick-punch marks. This change of position is necessary to
avoid a cross-wind or spiral form. The scraping should now extend over
the entire surfaces, and these should have a general uniform bearing
on each other, with the points of contact uniformly distributed and
equally distinct. The work should be continued in the same way until
all these requirements are fulfilled.
Now appears the use of the third plate. The two surfaces thus formed
are sure to be, one of them convex and the other concave, in some
corresponding degree. The workman now numbers the plates, by numbers
stamped in the edges, these being marked Nos. 1 and 2, and the third
plate No. 3. No. 2 is now set aside, and No. 3 is scraped to fit No.
1. It is thus made a duplicate of No. 2. Next, No. 1 is set aside
and Nos. 2 and 3 are brought together. Supposing these to be convex,
they will bear together at the middle point, on which the upper plate
will rock, and the degree of their convexity will thus be shown. The
workman then in the same manner scrapes these plates equally to the
best of his judgment, until their entire surfaces are brought together,
with equal distribution of the points of contact. These two surfaces
will now again be, one convex and the other concave, though in a much
less degree. The next step is to apply No. 1, which is concave, to
either No. 2 or No. 3, and scrape it to fit. It is then applied to
the other, of which it has now been made a duplicate, and the same
process is repeated, until the three plates can be interchanged in any
way, and will have a uniform general bearing on each other, with equal
distribution and distinctness of the points of contact. We have thus,
in a general way, produced three demonstrated true planes, but the
surfaces are yet far from the desired approximation to absolute truth.
Now follows the fine scraping, which is not attempted until general
truth has thus been established. The object of this is to multiply the
points of contact and perfect their equal distribution and prominence.
For this operation no raddle is used, but the surfaces are rubbed
together dry. When the plates are separated, the points of contact
shine like stars. Here skill and care are pre-eminently required. The
scraping takes off only a dust. If too strong depressions may be made
deeper than before, and requiring the reduction of the entire surface.
The superiority of the modern tool is now especially shown. By lowering
the angle of the tool, the workman presents the slightly curved edge
to the surface in a position as nearly parallel with it as he desires.
Interchanges similar to the former ones are now repeated, until the
bright points are brought as close together as is desired, with uniform
distribution and distinctness. The tedious operation is now finished,
and these bright points remain as witnesses.
The three plates were necessary to the production of one. They have
also a permanent use. They are indispensable to the preservation of the
true plane, which it has cost so much patient labor to produce. The
date of their completion is stamped on their edges. Then plates 1 and
2 are put away in the store-room, their surfaces carefully protected
from rust or injury, which last is best avoided by inverting one on the
other, and No. 3 is put into use. A prominent use is for the production
of smaller plates or straight-edges adapted to special purposes. After
a while, perhaps in a little while, this plate loses its truth by
unequal wear. Indeed, speaking with absolute truth, it may be said that
the first time this plate is used it is ruined. But by taking pains to
use different parts of its surface as equally as possible, it may be
kept in fair condition for some time. It can at any time be restored to
its original condition by scraping it to No. 2, taking the same pains
to turn it one quarter way around at every rub. In the course of time
No. 2 will itself become worn unequally, when its truth can be restored
by rubbing it on No. 1. Finally the three plates can all be restored to
their original condition by rubbing them together interchangeably as at
first. Thus the true plane can be absolutely perpetuated.
The importance of this work can only be realized when we consider
that the true plane affords the only means by which true cylindrical
work also can be either produced or verified. It is thus seen to be
fundamental to all mechanical truth.