Content .. 86 87 88 89 ..

Technology of watch production - part 88

The processing method selected and the sequence of operations depend

on the surface condition resulting from the preceding mechanical operations

on the metal, and on the type of finish to be applied.

Tumbling

Tumbling in revolving barrels is used for removing from the metal

surface burrs, rust, oxides and contamination in general . The parts are

loaded into the barrel in batches together with abrasives such as emery,

broken glass , hardwood s awdust, quartz sand, gravel, etc. If a more tho

rough cleaning is required, leather s craps and soap solution are added.

Barrels are round or polyhedral, and some des igns permit them to be tilted

to allow a more intense agitation . Tumbling is continued for s e veral hours
and as the parts rub against each other, their sharp angles are blunted,

burrs are removed, and the surface s are cleaned. The barrel revolves at

speeds between 15 and 60 rpm .

FIGURE 1 9 . S-205 machine

for

tumbling small-size parts

Barrels are loaded and unloaded through hermetically sealed lids .

The fine parts of pocket- and wris twatches are tumbled in the S - 2 0 5 ma

chine {Figure 1 9 ), in which two double barrels are set oscillating by a crank
mechanism placed in the machine base .

The barrels are removed from their housings for loading. The barrel

diameter is 75 mm and their capacity 1 liter. The numbe r of complete
s trokes per minute is between 3 5 5 and 5 0 0 . The tumbling duration is b e

tween 1 0 and 3 0 minutes and is controlled b y a time relay .

Most blanked parts are subjected to tumbling for burr

,....

removal

and many

parts , such as the pocket-watch bow, are polished by this process, since

their complex s hape makes grinding and polishing by any other method dif

ficult.

349

Polishing Brass and Nickel - silver Parts

Polishing as a preparatory operation to electroplating is applied mai

�

to the nonferrous parts of watche s : cases and dials . The parts are pol

ished on S-24 machines by felt wheels , 4 0- 5 0 mm thick and of 4 0 0 mm

maximum diameter (Figure 2 0 ) . Abrasive powder is glued to the surface

of the wheels .

The last mechanical operation before plating is polishing and this ope r

ation gives the parts the gloss necessary for protective - decorative finishes .

FIGURE 20. S-42A machine for rough- and
finish-polishing nonferrous parts

S-42A machines are used with textile

fabric wheels and the abrasive is chrom

ium -oxide paste . The peripheral wheel

speed for preliminary rough polishing

averages 25 to 3 5 m

sec and the final

polishing speeds are between 3 0 and

40 m

s e c .

Emery of various grain sizes, and

chromium - oxide pastes are used for

polishing (see Table 2 ).

Abrasive materials are classified

into three groups according to grain

size (GOST 3 2 3 8 - 4 6 and 3647 - 4 7 ), grains ,

powders, and flours . Dimensions are
given in Table 1 0.

Coarse grains are glued to wheels by

joiner ' s glue . Fine grains are bonded

using stearin, paraffin, oleic acid,

beeswax, etc .

After finishing (coating) S-42A ma

chines are used with s teel, brass, bristle or horse hair and vegetable-fiber

brushes to give the coatings a uniform hue or metallic gloss and to remove

any remaining dirt or oxides . This operation is called scratch-brushing.

TABLE

Grains

Powders

Flours

Grain size,

No.

f.L

No.

f.L

No.

f.L

No.

Jl·

1 0

2300-2000

600-500

100

150-125

M-28

28-20

2000-1700

420- 355

120

125-105

M-20

2 0- 14

1700-1400

355-300

150

1 05-85

M-14

14- 10

1400-1200

300-250

180

85- 75

M-10

10- 7

2 0

1200- 1000

7 0

250-210

220

75-63

M-7

7-5

850- 700

210-180

240

63-53

M-5

5-3.5

700- 600

180-150

280

53-42

M-3.5

3 .5-1.5

320

42-28

The brush wires must be elastic, and are accordingly given a corrugated

s hape . The finer the wire, the better the surface finish obtained . The

brush wires are usually of 0 . 08 to O . l O mm diameter and the liquid used

during scratch-brushing can be soapy water, F rench chalk, chalk, etc.

3 5 0

Dial surfaces are subjected to scratch - brushing before and after the

silver-plating in order to give them an even shade (instead of sand-blast

proces sing).

Chemical Degreasing and Pickling

Parts sent for electroplating are usually contaminated by fats and the first

operation is the refore chemical or electrochemical degreasing. Alkaline

solutions, alkaline salts , and special organic solvents are used in chemical

de greasing.

The fats which contaminate parts are classified as saponifiable and non

saponifiable fat s . Vegetable and animal fats, such as lins eed oil, colza oil ,

stearin, tallow, olein, etc . , belong to the first group. These fats break

up under the action of alkaline solutions and produce soaps which are soluble

in water . Mineral fats , such as mineral oils, vas eline, paraffin, grease,

etc . , are in the second category and they do not break up under the action

of alkaline solutions , but form emulsions (mixtures of liquids insoluble in

one another).

Upon emulsification, the alkaline solutions break the films of fat on the

metal surface and form fine fat drop s . The drops remain in the liquid in

a suspended state and the soap and emulsion obtained as a result of the

chemical degreasing are easily removed from the surface by cold or hot

water . The rate of degreasing is increased by heating the bath to 9 0° and

agitating it periodically . Water glass , soap and other emulsifiers are added

to the bath in order to speed up the process . Both saponifiable and nonsa

ponifiable fats are removed by sol vents such as kerosene, gasoline, toluene,

carbon tetrachloride, dichlorethane, and trichlorethylene . The three last

mentioned sol vents differ from the first three in that they are not inflammable
allowing the degreasing to be conducted in conjunction with heating. Their

shortcoming is their toxicity, which makes good ventilation mandatory when

working with them .

Alkaline solutions of the following composition are used at a temperature

of 70 to g ooc for the chemical degreasing of steel and brass parts with un

polished surface s : 2 0 to 3 0 g

1 caustic soda, 2 5 to 3 0 g

1 sodium phosphate

or carbonate and 3 to 1 0 g

1 water glass or soap . Degreasing takes 2 0 to

30 min and is followed by rinsing in very hot water and drying. The de

greasing quality is determined by wetting the surface with water . The wetted

part is pos itioned vertically, and its water film is observed. No dis conti

nuities will be observed in the film if the degreasing is satisfactory. The

water film will break down in those places were fat or oil spots remain .

The next operation, after degreasing, is pickling which consis ts in the

removal of oxides from the metal surface by immers ing the part in acid,

alkaline or acid- salt solutions . Pickling can be chemical or electrochemical .

F errous metals are chemically pickled using sulfuric, hydrochloric,

phosphoric or nitric acid or mixtures of them . The process is more rapid

in hydrochloric acid than in sulfuric acids and the pickling can be speeded

up by heating the bath to 3 0 - 40°C . Phosphoric - or nitric - acid pickling forms

insoluble iron phosphates on the surface which aid in the satisfactory ad

herence of paints etc . Phosphoric - acid pickling is slower than sulfuric-

3 5 1

or hydrochloric - acid pickling. Nitric acid is used relatively rarely for

pickling ferrous metal s , and is used in a mixture with hydrochloric acid.

Chromium steels resis t sulfuric acid and dissolve in pure hydrochloric

acid or in hydrochloric acid mixed with other acids .

A mixture of nitric , hydrochloric and sulfuric acids in various propor

tions i s used for pickling brass and other nonferrous metals and alloys .
B efore the protective finish is applied, the parts are pickled in a s olution

of nitric and sulfuric acids (in equal proportions by volum e ) with a small

amount of common salt ( 2 0 g per liter of mixture). This operation imparts
a glossy surface to nonferrous metals , and is accordingly sometimes con

sidered a final operation.

Normally, howeve r, the parts are finally pickled for several s econds in

a bath of the following composition after the preliminary pickling:

HN03 (specific weight 1 .38)

. .

• .

. . . . . . . . .

1 liter

H2so4 (specific weight 1 .84) . . . . . . . . . . . . . . . . . .

1 "

HCl (specific weight 1 .17)

. . . . . . . . . . . . . •

. . . 2 cm3

Dutch soot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 g

Solution temperature

. . . . • .

. .

•

. . . . . . . . . . 50-6o•c

The pickled parts are rinsed in hot and cold water, or in cold water only .

Electrochemical Degreasing and Pickling

Degreasing with electric current in an alkaline solution is more effective

than o rdinary chemical degreasing. The electrolytes used are caustic soda,

potass ium carbonate , sodium carbonate , or s odium phosphate , etc . Emul

sifiers such as s oap or water glass are sometimes use d . The process of

electrochemical degreasing cons ists in emulsifying the fats and oils with the

aid of the hydrogen bubbles liberated at the cathod e . The gas bubbles pene

trate below the film of fat and b reak it up , and fine fat droplets are formed

as a result .

Degreasing proceeds more quickly when the electrolyte is heated to 6 0°

or 7 0°C and agitated . Steel parts such as springs , plate s , etc . are liable

to be saturated with hydrogen which, liberated in great quantity at the cathode,
penetrates the metal and makes it b rittl e . The adherence of electric platings

to hydrogen- saturated metal is poor. Parts made of high - carbon steel are

accordingly degreased by anodic degreasing.

Alkaline solutions and electrochemical degreasing conditions

F o r s t e e l a n d c a s t i r o n ( c a t h o d i c a n d a n o d i c d e g r e a s i n g )

Caustic soda . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Sodium phosphate or carbonate . . . . . . . . . . . . . . . . . . . . . . . . .

Water glass

•

· · · · · · · ·

Solution temperature, •c

•

Current density, amp/dm2 . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Duration of de greasing, min :

cathodic

•

anodic after cathodic . . . . . . . . . . . . . . . . . . . . . . . . .

3 5 2

10--20 g/1
25-50 g/1

3-5 g/1

70--80

3-10

2-3
1-2

Content .. 86 87 88 89 ..