A Treatise on Staff Making and Pivoting | Page 7

inch in diameter.
Now if 1/2500 of an inch is the proper side shake for a pivot measuring

12/2500 of an inch in diameter, it is certainly not sufficient for a pivot
which is one-third larger. Of course it is understood that side shakes do
not increase in proportion according as the pivot increases in size, for if
they did a six-inch shaft would require at this rate a side shake of 1/2
inch, or 1/4 inch on each side, which would be ridiculously out of all
proportion, as the 1/64 of an inch would be ample under any
circumstances. Neither can we arrive at the proper end shake for a pivot
by reducing in proportion from the end shake allowed on a six-inch
shaft, because if we followed out the same course of reasoning we
would arrive at a point where a pivot measuring 12/2500 of an inch
would require an end shake so infinitely small that it would require six
figures to express the denominator of the fraction, and the most minute
measuring instrument yet invented would be incapable of recording the
measurement. We must leave sufficient side shake, however, on the
smallest pivot and jewel for the globules of the oil to move freely, and
experiments have shown conclusively that 1/2500 of an inch or 1/5000
on each side of the pivot, is as little space as it is desirable to leave for
that purpose, as the globules of the best chronometer oil will refuse to
enter spaces that are very much more minute. But to return to our pivot
gauge.
[Illustration: Fig. 11.]
[Illustration: Fig. 12.]
Each division on the gauge represents 1/2500 of an inch, which is all
that we require. The diameter that the pivot should be, can be
ascertained by inserting a round pivot broach into the jewel and taking
the measurement with the pivot gauge, and then making the necessary
deduction for side shake. Slip the jewel on the broach as far as it will
go, as shown in Fig. 12, and then with the pivot gauge, take the size of
the broach, as close up to the jewel as you can measure, and the taper of
the broach will be about right for the side shake of the pivot. If,
however, you prefer to make the measurement still more accurate, you
can do so by dipping the broach into rouge before slipping on the jewel
and then remove the jewel and the place which is occupied on the
broach can be plainly discerned and the exact measurement taken and

an allowance of 1/2500 of an inch made for the side shake. Another
method, and one which is particularly applicable to Swiss watches,
where the jewel is burnished into the cock or plate, is to first slip on to
the broach a small flat piece of cork and as the broach enters the jewel
the cork is forced farther on to the broach, and when the jewel is
removed it marks the place on the broach which its inner side occupied,
and the measurement can then be taken with the gauge. If care is used
in the selection of a broach, that it be as nearly perfect in round and
taper as possible, by a little experiment you can soon ascertain just
what part of the length of the broach corresponds to one degree on the
gauge and by a repetition of the experiment the broach can then be
divided accurately, by very minute rings turned with a fine-pointed
graver, into sections, each representing one degree, or 1/2500 of an
inch, and the measurement will thus be simplified greatly.
[Illustration: Fig. 13.]
As before stated, much depends upon the condition of your gravers and
the manner of using them. It is of the utmost importance that they be
kept sharp, and as soon as they begin to show the slightest sign of
losing their keenness, you should sharpen them. The proper shape for
balance pivots was shown in Fig. 4. Now let us examine into the best
positions for holding the gravers. In Fig. 13 two ways of holding the
graver are shown, A representing the right and B representing the
wrong way. If the graver is applied to the work as shown at A, it will
cut a clean shaving, while if applied as shown at B it will simply scrape
the side of the pivot and ruin the point of the graver without materially
forwarding the work. Again, the holding of the graver as indicated at A
has its advantages, because the force of the cut is towards the hand
holding it, and should it catch from any cause the jar of the obstruction
will be conveyed immediately to the hand, and it will naturally give
and no harm will be done.