tool in the machinistís kit is more often referred to than the
Upon it he depends in making all ordinary measurements and for
the setting of his calipers and dividers. In first image below is shown a
standard steel rule or scale. They are made in various lengths,
from one to forty-eight inches, with any desired gradation, no
exceeding 1-100 of an inch in fineness.
For shop work the gradation most used is eights, sixteenths,
thirty-seconds and sixty-fourths, on the four corners of the
scale. A scale graduated sixteenths, thirty-seconds,
sixty-fourths and one-hundredths is convenient, and the 100ths
graduations will serve for measurements when required in
Where all the work is measured by fractions, however, the former
is the safest ruling to use, as there is then no danger of
inadvertently mistaking a tenth for an eighth division, etc.
this reason, rules graduated in twelfths and fourteenths should
not find their way into the machinistís tool box, as he will not
have occasion to use these divisions, and their presence will
call for greater care in selecting the proper division, with the
loss of time incident to changing ends with and turning over the
rule in order to get at the division required.
The end graduation, as shown below, is frequently very
convenient in taking measurements in recesses where the length
of the scale would prevent the use of the regular graduation. Standard steel rules, as made by
our leading makers, are remarkably accurate.
To be sure, the
length varies slightly with the changes in temperature, but
these changes are not ordinarily great, and the material
measured is usually affected about the same amount in the same
direction; so we may feel assured that any errors arising from
this source are well within the limit of the personal error of
the operator in making a measurement.
The late refinements in the
manufacture of steel rules have enabled the production of very
accurate tempered ones on comparatively thin steel. What we
usually know as the standard rule, however, is graduated on
thick steel, and is not tempered. The tempered rules possess the
decided advantage of resistance to wear.
An untempered rule is easily
mutilated, and soon rounds its corners through wear, making its
ends unfit for reference. The tempered rules may be classed as
heavy, tempered, semi-flexible and flexible. The heavy
are about about one-tenth; the tempered, one-twentieth;
the semi-flexible, one-fortieth, and the flexible one-eightieth
of an inch thick. For general work the heavy or tempered will be
found best suited. The flexible are graduated on one side only,
and are of value in measuring curved or irregular outlines.
In this connection it will be well
to call attention to the gear rule show below.
Its application is in the sizing of
gear blanks and where, by rule of thumb, two diameter pitches
are added to the pitch diameter of the gear in obtaining the
whole diameter. Thus, if the pitch is No. 7 diametrical
and the number of teeth 34, then the pitch diameter = 34-7=4-6/7
inches, and the blank or whole diameter = 4-6/7 +2/7=5- 1/7
inches, which can be taken directly from the scale. This
rule is of great convenience where many blanks for varying
pitches and numbers of teeth are to be sized.
In the image to the left is shown a neat
kink which will be found of value in taking measurement similar
to the ones shown in the figure, as well as for setting inside
calipers. The hook may be quickly removed. It is
hardened, and in connection with a tempered rule forms a
In the image below is shown a
standard steel square. This tool is made of cast steel, tempered
and accurately finished.
The two sides of the angle are
called the blade and the beam, the length of the blade being
measured from inside the beam. The form shown above is preferable
when the length of the blade exceeds eighteen inches, as it can
be more readily repaired in case of accident.
Either of the methods shown in
Figs. I and II may be used by the machinist in testing the
accuracy of a square, the latter being the most exact of the
In Fig. I, A is a plane plate of
iron with one edge, BC, perfectly straight. The surface A
should be smooth and true. The square is first applied as shown
at F and a file line DE scribed along its edge. It is then
reversed to position G, when, if the edge exactly lines with the
ruling DE, the square is correct. If, however, the edge and the
line do coincide, the square is inaccurate by one-half the
variation as shown. Since this method depends for its accuracy
on the eye of the operator it cannot be called and exact one.
The method shown in Fig. II reduced
the amount of the personal error, since the eye detects readily
the ray of light that passes through and exceedingly small
opening. In the figure the cylinder A, which has been
ground accurately parallel and faced on the lower end slightly
dishing, rests on a place surface, a standard surface plate
being best suited to this purpose. As the surface of the
cylinder is at right angles to the surface of the plate the
square can be compared with this angle by placing it as shown in