By the aid of the pinion calipers and the old staff, the diameter of the
roller seat and the balance and hair-spring collet seats may be readily
taken, but it is perhaps better to gauge the holes, as the old staff may
not have been perfect in this respect. A round broach will answer
admirably for this purpose, and the size may be taken from the broach by
means of the calipers. In fitting our pivots, we can not be too exact;
and as yet no instrument has been placed upon the market for this purpose
which is moderate in price and yet thoroughly reliable. The majority of
watchmakers use what is termed the pivot-gauge, a neat little instrument
which accompanies the Jacot lathe, and which may be obtained from any
material house. This tool, which is shown in Fig. 11, is, however, open
to one objection in the measurement of pivots, and that is that it may be
pressed down at one time with greater force than at another, and
consequently will show a variation in two measurements of the same pivot.
Some of my readers may think that I am over-particular on this point, and
that the difference in measurement on two occasions is too trivial to be
worthy of attention, but I do not think that too much care can be
bestowed upon this part of the work, and neglect in this particular is,
I think, the cause of poor performance in many otherwise good
timepieces. The ordinarily accepted rule among watchmakers is that a
pivot should be made 1/2500 of an inch smaller than the hole in the jewel
to allow for the proper lubrication. I am acquainted with watchmakers,
and men who are termed good workmen, too, who invariably allow 1/2500 of
an inch side shake, no matter whether the pivot is 12/2500 or 16/2500 of
an 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. If, on the other hand, the graver should meet with
an obstruction while held in the position indicated at _B_, the force of
the cut will be in the direction of the arrow, downward and toward the
rest, and the rest being unlike the hand, or rather being rigid, it
cannot give, and the result is that the work, or graver, or both, are
ruined. In Fig. 14 two other methods of holding the graver are shown. The
general roughing out of a staff should be done with the graver held about
as shown at _A_, Fig. 13; but in finishing, the graver should be held so
that the cut is made diagonally, as indicated at _A_, Fig. 14. It is
rather dificult to explain in print just how the graver should be held,
but a little experiment will suffice to teach the proper position. The
best indication that a graver is doing its work properly, is the fact
that the chips come away in long spiral coils. Aim to see how light a cut
you can make rather than how heavy. Never use force in removing the
material, but depend entirely upon the keenness of the cutting edges.
Never use the point of the graver, except where you are compelled to, but
rather use the right or left hand cutting edges. By following out this
rule you will find that your work, when left by the graver, requires
little or no finishing up, except at the pivots. At _B_, Fig. 14, is
shown the correct manner of applying the graver when turning a pivot.
Hold the graver nearly on a line with the axis of the lathe and catching
a chip at the extreme end of the pivot with the back edge of the graver,
push slightly forward and at the same time roll the graver towards you
and it will give the pivot the desired conical form. By keeping the
graver on a line with the length of the pivot, all the force applied is
simply exerted in the direction of the chuck, and does not tend to spring
the pivot, as it would were the extreme point applied, as in Fig. 13.
When we come to such places as the shoulder of the back slope, the seat
for the roller, balance, etc., we must necessarily use the point of the
graver.
[Illustration: _Fig. 14._]