Technology of watch production - part 87
The diamond concentration of the wheels varies and depends on the char
acter of the work to be performed by them (concentration is the amount of
diamonds , in carats, per 1 cm 3 of bond calculated as a percentage ). The
concentration is taken as 1 0 0 % if there are 4 . 6 carats of diamond ( 1 carat =
= 0 . 2 g) per cm 3 of bond of spe cific weight 1 . 2 5 gjcm 3 . The concentrations
used in diamond wheels are 1 00, 50, 40, and 25 o/o . The lower- concentration
wheels are used with low speeds and low feed rates . Higher concentrations
are necessary when the part form (profile ) has to be pres erved .
Diamond is, as is well known, the hardest substance o n earth. I t s hard
nes s is 1 0 , 060 units (in kg
/
mm2 ) on the hardness s cale for minerals pro
posed by Prof. M. M. Khrushchev, while the mineral occupying the s econd
place - corundum - has a hardness of only 2 0 6 0 units . Diamond surpasses
by far all other abrasives in cutting speed, accuracy, and surface quality
obtained . The abrasive capacity of one carat of diamond i n sharpening a
carbide tool i s equivalent to that of 2 0 0 g of boron carbid e . The chemical
composition of diamond is pure crystalline carbon of specific weight 3 . 5 .
The surface quality obtained i s given in Table 8 a s a function o f the dia
m ond grain size .
The wheels must be well balanced and water-cooled. The run- out toler
ances on the working surfaces are indicated in the diamond - wheel drawings .
Machines using diamond whe els must be free of vibrations .
Bakelite - bonded wheels are dressed by a pumice lump or a cutter in a
lathe. C e rmet-bonded wheels are dress e d by corundum or carborundum
disks with wate r .
The working speeds f o r diamond wheels a r e between 1 2 a n d 2 0 m
/
sec,
and the feeds are 0 . 0 0 1 to 0 . 0 1 mm . Boron-carbide paste is used as a dia
m ond sub stitute for tool s ha rpening. The use of boron-carbide abrasive in
bonded wheels is m ade difficult by the fact that it is held poorly in a bakelite
bond, while in a cermet bond it is rapidly oxidized during baking and is
covered by an oxide film (at 5 0 0° ) which reduces its abrasive capacity.
The introduction of carbide wheels has made it possible to dispense with
the formerly widely use d m ethod of grinding and polishing with abrasive
wheels, powders and pastes (in the nonbonded state ).
Grinding and Polishing Keyless Wheels
The surface of the keyless wheels of pocket- and wristwatches undergo
various decorative grinding and polishing finishing operations .
Differently finished keyless wheels are shown in Figure 1 6 . Figure 1 6 ,a
is a keyless wheel having a polished groove (2 ) on its face , and a central
rim ( 1 ) finished with radial rays . The surface of the wheel teeth has a
ground chamfer ( 3 ) .
The s urface o f such wheels i s finished i n three operations: texturing the
central rim ( 1 ), polishing the spherical groove ( 2 ), and polishing the cham
fer ( 3 ).
T e x t u r i n g consists in making small grooves on the outer surface of the
wheels with the aid of emery paste or chromium - oxide paste .
The wheel planes are ground first . The ray- pattern texture is applied
on the S- 34 machine (Figure 1 7 ).
3 4 5