Preamble :
Since there are countless ways of “perfecting”
watch parts, a Manufacture seeking perfection will not merely
make do with functional trimming, but will instead take time to
polish the rims of the drilled holes.
An
unpolished drilled hole (photo 1)
After machining, the rims of the holes are irregular, with burrs
and sharp edges that are both unattractive and detrimental to
the smooth running of the watch.
The
chamfer (photo 2) :
Representing a purely functional finish, it is a rounded angle
intended to eliminate sharp edges, as well as any remaining burrs
and turnings of material left after machining. All drilled holes,
whether visible or not, need to be chamfered.
The
recess (photo 3)
Refers to the part that is countersunk in a bridge or plate intended
to house a screw, a pinion or a stud. Depending on the desired
state of the surface, it may be polished by hand.
The
boss
Also called sink, polished concave chamfer or jewel-sink. It is
a deep concave chamfer (photo 4) generally polished (photo
5) crafted around a jewel to countersink and highlight it.
In addition to its significant aesthetic purpose, polishing
the surface also prevents oil from coming up and spreading around.
The
setting (photo 6)
This is a metal ring (generally in gold) in which the jewel
is fixed.
PS : various stories have been told about settings, but
the most probable is as follows. Historically, the gems used were
natural and therefore rare and expensive. So as to highlight them,
the gem was encircled by a setting which reflected it like a mirror,
thereby giving the impression that it was larger.
Currently, natural rubies have been replaced by more functional
and far more affordable synthetic gems. Nonetheless, in order
to perpetuate watchmaking tradition, certain prestigious brands
have decided to continue using settings, which in some cases even
become genuine decorative elements.(photo 7)
The
useful purpose of finishing the rims of drilled holes :
When the holes are drilled, tools leave particles of matter
on the edges. These burrs adhere with varying degrees of strength
and as they come off may fall onto delicate organs of the watch,
causing it to stop or engendering other disturbances. Chamfering
the sharp edges is therefore a functional necessity. The same
is true of polishing, since the correct spreading of the oils
largely depends on the quality of the state of surface finishes.
Nonetheless, let us bear in mind that the aesthetic aspect is
far from negligible, particularly in fine quality movements.
Chamfer,
recess, boss or polished concave chamfer:
The method
The metal
is pierced by a drill, leaving an irregular rim with sharp edges
and burrs, which must therefore be corrected.
There are two ways of achieving this :
- The modern or mechanical method
- The manual or traditional method
The modern
or mechanical method : (photo 8)
Initially, it is vital to correct all the holes in order to adjust
their dimensions and geometry with a reamer, or even better with
a reaming-head.
The
chamfer :
The work is then completed by rounding the angle using a 90 to
120 degree bevelling milling-cutter (photo 9) which will
form a chamfer (photo 9A) or recess with a pleasantly shiny
appearance, which can be further enhanced by chemical or manual
polishing.
Jewel-sinks
or bosses
With cuprous metals such as gold, jewel-sinks can also be entirely
made mechanically : this operation is called diamond-polishing
a diamond milling cutter (photo 11) is used, enabling one
to achieve a black or mirror-polished surface (photo 11A).
With regard to ferrous metals (steel), a ball-shaped milling-cutter
is used, since the diamond tends to be ground down on steel. This
method systematically calls for corrections, since the surface
state achieved is imperfect (photo 10A).
There are two available options :
- Chemical or tumbling barrel polishing for medium-range watches
- Hand polishing with pegwood for high-end watches.
The
traditional or manual method :
As we have seen, regarding the finish of drilling rims for
cuprous metals and gold, the mechanical method gives very satisfactory
results (photo 17).
Nonetheless, in certain cases manual correction is recommended
or even necessary :
- to correct a machining problem
- to polish the state of surface of steel parts after machining
and thermal treatments (oxidation) (photo 10A)
The
various hand tools:
