Technology of watch production - part 20

A shortcoming of these dies is that the whole s et has to be dismantled

for regrinding. In other models the dies and punches are ground in the

shoes . Generally, s tandard crank presses are us ed for shaving operations .
P recision parts , in particular springs of the type shown in Figure 1 0, are

sha ved on vibration presses .

Bending.

When a part is bent, the external fibers of the metal are

s tretched and, if the bending radius is small and the part thickness large ,

failure occurs in the external fibers .

The values of the smallest bendin g radius permissible depends on the

properties of the m etal, the strip thickness and the relative directions of

the axis of the bend and the grain. The value of

is determined from the

formula

r = k · s,

where

= s trip thicknes s , mm;

= a factor depending o n the material , and having the following

values for various metals :

Soft brass

Semihard

Soft steel

Semihard

Stainless

Material

brass

steel

0 .25

0 . 35

0.6

A s suming that the length of the neutral axis remains unchanged in bend-

ing, and that the inside fibers are compres s ed and the outside fibers extend

ed, the blank length (Figure 3 1 ) is determined from the following fo rmula:

where

and

= lengths of the straight sections , mm;

= bending radius , mm;

= stock thickne s s , mm;

= bend angle, degrees ;

= a coefficient depending on ratio !:..,having the following values :

0 . 1

0 .25

0 .5

0 .18

0 .26

0 . 33

0 .39

0 .44

0 .47

0 . 50

Elastic deformation is always present in bending, and the part tends to

"spring back" , that is , partially returns to its original s hape . The actual

final bend angle is therefore smaller than the die bend angle .

The magnitude of the spring-back depends on many factors and it is

frequently necessary to determine the punch and die angles of bending dies

empirically. Figure 32 is a nomogram of the spring-back angle

as a

function of the bend angle a nd the ratio !.._, The dispos ition of the axis of

the bend relative to the grain of the material is very important .

A s mentioned earlier, rolled materials , i . e . m aterials having a definite

direction of grain, s hould not be bent about an axis parallel to this direction

as the extended outside fibe rs are liable to fail. The angle between the axis

7 7

FIGURE 31.

Bent plate

FIGURE

32. Dependence of the spring-back

<lc<

orr the ratio ¢

FIGURE

3 3 . Bending die

7 8

of bending and the direction of the grain should therefore preferably be 9 0°,

but shoul d

•

not be reduced below 30°.

Bending dies are relatively simple in design. The shapes of the punches

and dies correspond to that of the part to be bent , allowing fo r spring- back .
Open- type dies are used for simple bending operations , while more complex

operations are performed on dies provided with knock- outs and special

blank holders .

The design of a bending die for alarm - clock detents is shown in Figure 3 3 .

Both ends o f the detent are bent simultaneously. The die (6 ) with the guide

pins (4 ) and ( 5 ) is mounted on the support ( 7 ) . The support can move with

the slide (8 ) in the base (9 ). The punch (2 ) and the p ressure pad ( 3 ) are

fastened to the shank ( 1 )

When the p ress ram descends , the pad ( 3 ) presses

the part against the die fac e . As the punch (2 ) continues to descend, the

right-hand end of' the detent is bent by 1 05°. The bending radius

is about

2 . 5 mm, or

4 s.

DRAWING

Hollow parts are d rawn from flat blanks on single- action o r doubl e - action

pres ses .

Drawing can be defined as a process in which a punch forces a flat blank

into a die hole and caus es it to assume the shape of the punch (Figure 34 ).

FIG UI<E 34 . Schematic dia
gram of the dra1ving process

1 - punch; 2 -die; 3-blank.

FIGURE 35 Schematic di agram of
dra�;ing '''ith a blank holder

1 - punch; 2- die; 3-blank holder.

Drawing operations are o f two types : 1 ) drawing without changing the

initial s tock thicknes s (the thickness of the walls and of the bottom of the
product are equal to the blank thicknes s ); 2 ) drawing with a reduction in

the s tock thickness (the wall thicknes s of the product is smaller than the

blank thicknes s ). If the difference between the diameter of the p roduct and

that of the blank is s m all , drawing is performed without using a blank holder .

If the difference in diameters is considerable, a blank holder is us ed (Fig

ure 3 5 ), as otherwise wrinkles m ay form (Figure 3 4 ) . A blank holder is

us ed in all cases where the blank thicknes s does not exceed 1 / 7 0 of its dia

mete r .

I f

the thickness of the blank i s larger than 1 / 5 0 o f its diameter, a

7 9

blank holder is not necessary. If the thickness is s omewhere in between ,

either s ystem can be us ed. The press force required increases when a

blank holder is used.

Deep shells are drawn in two, three or more s tages with intermediate

annealings to eliminate the work- hardening effect. Alarm - clock cases and

air- and s eaborne watch cases are examples of drawn watch parts .

Blank

dimensions .

Drawing does not change the volume of the metal .

If the wall thicknes s of the drawn p roduct is the same as that of the blank,

FIGURE 36 . Development of
a hollow cylinder :

1 -blank; 2 -product; 3-de-

velopment .

the blank area

will be equal to the surface area

Fz of the drawn p roduct. If the height of the pro duct

does not exceed half its diameter and its wall is

not larger than 1 mm,

will be the external sur

face of the p roduct':' .

The diameter

of the circular blank from which

a hollow cylinder of height h and diameter

(Fig

ure 3 6 ) is to be drawn is found from the relation-

. "D2

"d2

. D

s h1p

h; we obtam:

(

+ h).

An allowance of 3 to 5 o/o is added for trimming

the edges which, after drawing, will be une ven.

When a complex part is drawn in s everal stages ,

the dim ensions of the blank are either computed or

experimentally determined .

Figure 3 7 gives the formulas for calculating the

blank diameter for various drawn shapes.

Number of stages i n drawing.

Deep s hells cannot

be pro duced in one s ingle draw from a flat blank as

the extensive deformation involved would caus e a

failure. Deep shells are therefore drawn in several

s tages , the depth being increas ed in each s tage .

for each draw is

The ratio of product diam eter to blank diam eter

expressed as the draw coefficient:

where

the blank diam eter;

m1, m2 , m,.

draw coefficients;

d,.

m,. = d,.-1 '

d1, d2 , d,._1

diameters obtained in the successive draws;

d,.

diam eter of the final p roduct.

The draw coefficient

depends on the technological properties of the

material as determined by the Erichsen test, on its surface condition and

thickness, on the drawing method us ed (with or without a blank holder), on
the pro duct s hape, etc .

Empirical average values of draw coefficients for cylindrical products

are given in Table 4 .

The higher values given i n each cas e s hould b e used f o r s tock not thicker

than 1 . 5 mm and drawn at high speed while the lower values are meant for

thicke r s tock, drawn at lower speeds .

•

For dra\v ing irregularly shaped products of complex shape, the calculations given in

V . P .

Romanovskii's

book " Spravochnik po l<holodnoi shtampovke" ( Handbook ofCold Stamping) Mashgiz,

1954,

will be useful.

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