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Technology of watch production - part 31

inc reas es considerably, this being detrimental to the surface-finish quality .

It should be noted that the cros s - s e ctional a rea of the chip is small when

m achining watch parts , and the refore the depth of cut is usually not taken

into account when establishing the cutting conditions .

The cutting speeds and feeds recommended in Tables 4 and

are bas ed

on a sys temati zation of the up - to - date experience of watch plants .

TABLE 4

Cutting speeds on Swiss-type automatic screw machines

Steel UlOA
Steel U7A V
Steel El699

Metal

Brass LS63-3 . . . . . . . . . . . . . .

German silver MNTsS63-17-18-2

TABLE 5

Cutting speed, m/min

2 0-40
2 0-60

1 5- 30

30- 1 00
3 0-60

Feeds for U7A V steel worked on Swiss-tyjle automatic screw machines, mm/revolution

Process

Turning journals and chamfering pinions and shafts

Turning cylindrical, conical and contoured surfaces

Cut-off,

plunging, facing and recessing

•

Turning nonworking surfaces

•

Drilling, centering

•

7th

0.015

0.008

0.02 0

0.015

Surface-finish quality

8th

9th

l Oth

0 .01 0

0.008

0 . 005

0.010

0.008

0.005

0 .015

0 .01 0

The cutting conditions recommended are not optimal and the cutting

speeds and feeds can and should be increas ed as s oon as improvements in

the limiting elements make this feas ibl e .

When using U 1 0A o r EI6 9 9 s teel s , the feeds recommended above must

be multiplied by 0 . 9 and 0 . 8 , respectively. When using type LS6 3 - 3 brass
the feed s hould be multiplied by 1 . 2 .

CALCULATION OF THE SETUP

Setup calculation involves planning the s equence of all operations and idl e

movements of the cutting tools and working parts of the automatic machine

(allowing for possible overlappings ), and the es tablishment of dwell inter
vals between operations . The s etup must ensure that the finis hed part cor

responds to the specified dimensions , tole rances and s urfac e quality, and

that its machining requires a minimum of tim e .

I n order t o d raw the p rofiles o f the cams which control all the motions

of the working parts of the machine, it is necess ary to determine the values
of the headstock and tool motions , to s elect the cutting speed and feed, and

to calculate the angles of rotation of the camshaft for the various operations

and idle movem ents .

1 2 1

The cam curves are determined by two parameters : 1 ) the value o f the

central angle corresponding to the angle of rotation of the camshaft during

the time of the operation or the idle movement; and 2 ) the value o f the in

crement (or decrement) of cam radius (the value of the linear displacement
of the given working part of the machine ) .

The cam curves are c alculated so a s to complete the entire cycle of

operations and l.dle movements and to completely machine one part in one

camshaft revolution. Sometimes 2 to 3 parts are completed during one

revolution of the camshaft, as in the case of the p art shown in Figure 2 6 .

The machining o f this part is simpl e . The b a r already has the specified

diameter and the only operations carried out on the machine a re the cham-

FIGURE 26.

W ristwatch bridge pin

fering of both ends and the cut- off.

Three cycles are completed, and there

fore three parts p roduced, for one cam

s haft revolution.

The cam ris e for the operations and

the idle movements depends on the ratio

of lever arms s elected. This ratio

varies within certain limits for the head

stock and for the rocker, while for the

other mechanisms it is a c onstant mag

nitude (see Table 6 ).

Two headstock cams and three rocker

cams can be mounted on the camshaft. Several cams are installed in order
to speed up the process of changing over from machining one part to machin

ing another . A side from this , in complex s etups the p rofile of the cam curve

for s ome operations is split up into two or three p rofiles , thus creating the

possibility of increasing the accuracy of the cams .

The travel of the headstock o r the tool is defined as the difference be

tween its initial and final pos itions . Thus , the value of the heads tock travel

will be equal to the length of the p art plus the width of the cut- off tool .

The travel of cutters No . 2 (or 1 ), 3 , 4 , 5 is determined by the formula

C D - d

s = +

where

distance from the initial cutter position to the bar;

= bar diameter

machined diameter

= tool cross - ove r.

( 3 )

The distance

(at cut - off) is usually 0 . 5 m m when machining bars of

3 mm diameter and up . When bars of s maller diameter are machined the

cutters must be s et at a distanc e of 2 mm from the bar axis in order to

make sure of the free approach of the cutters in working position and the

free passage of the bar between the cutters . The value of

is 0 . 1 to 0 . 2 m m .

Having determined the heads tock or tool travel, the c am ris e or drop is

determined, taking into account the ratio of the lever arms . Thus , if the
part length is 3 . 2 5 m m , the width of the cut- off tool 0 . 2 5 mm and the ratio

of the headstock lever arms is 2 : 1 , the stock-feed cam rise will be:

( 3 . 2 5 + 0 . 2 5 )2

7 m m .

1 2 2

The maximum heads tock lever ratio (3: 1 ) is s el ected to m1mmize the

influence of errors in the cam profile in those cas es where a narrow toler

ance must b e held on axial dimensions . It must be kept in mind that accord

ing to the design data the maximum allowable cam ris e is 5 0 mm.

The 2 : 1 lever ratio is recommended f o r the manufacture o f long parts ,

s ince the ris e curve co rresponding to the 3 : 1 ratio is s teep, especially for

small radii, and jamming of the cam could occur.

TABLE 6

Lever arm ratios and cam dimensions for the 1A10P automatic screw machine

Displacement mechanism

Headstock feed

•

Rocker

•

Tool slide No. 3 feed

. .

Tool slide No. 4 feed

•

Tool slide No. 5 feed

•

Centering-drilling attachment spindle

feed

•

Drilling-threading attachment spindle

feed

•

S lotting attachment lever feed

Lever arm

ratio

F l a t

c a m s

from 1 : 1

to 3 : 1

3 ; 1

and

1 0 : 1

1 : 1

2 : 1

B e l l

c a m s

1 : 1

Basic cam dimensions, mm

cam thick-

dtameter meter

ness

160

2 0A

120

2 0A

120

28A

120

7 0

28A

120

7 0

28A

cam height

1 00

100

The motions of the working parts of the automatic machine can be clas

sified as motions directly connected with the m achining of the bar, and
motions not directly connected with m achining. Motions of the first type

are called o p e r a t i o n s; motions of the second type - i d l e m o v e m e n t s .

Calculation of the idle movements . When blanks of watch parts are

machined on Swiss - type automatic screw machines , from 15 to 2 5 o/o ( 3 5 o/o
in some cases ) of the cycle time is taken up by idle movements which, ob

viously, s hould be kept at a minimum value in order to increas e the produc
tivity of the machine .

Som e examples of idle movements are: releasing the collet, retracting

the headstock, closing the collet, the approach and retreat of the tools ,

dwells, headstock advance when no external turning is necess ary o r when

the bar is fed for chasing, etc .

It is usual to express idle movements by the number of degrees corre

sponding to a 1 m m ris e o r drop on the cam .

The number of degrees corresponding to 1 mm idle ris e or d rop for the

same tool is a variable magnitude and depends on the production rate of the

m achine for the given part.

1 2 3

The higher the production rate, the more degrees are apportioned per

mm idle movement.

When the production rate is low (up to 6 pieces per minute ), the inertia

forces on the moving parts of the machine are small and have no influence
on the work, and can the refore be neglected. When the production rate is

high (2 5 pieces p e r minute and more), the inertia forces become consider

abl e , and if the idle movement curves are incorrectly calculated, the operation
of the machine can be disturbed. Let us assum e that the inertia forces in

the lever driven by a cam follower (Figure 2 7 ) are quite considerable and
exceed the tension of the lever spring. In that case the follower, on reach

ing the top point on the cam p rofile , will c ontinue to ris e b ecause of inertia
to some height h and in coming down will strike the cam . To avoid this ,

the ris e curve must b e made less steep or, in other words , the number of

degrees per m m ris e must b e increas ed and the follower velocity at the

end of the rise must be reduced to zero.

FIGURE 27. Headstock feed cam

The number of cam degrees apportioned to the idle m ovements is c.stab

lished on the basis of the m achine production rate, taking into account the

dynamic phenomena occurring at high camshaft speeds .

The optimal idle movements , in degrees , for the headstock and rocker

cam s , who s e operation bas ically determines the pro duction rate of the

s crew m achine, are given in Tables 7 - 1 4 .

I t can b e s een from the tables that the cam angle apportioned t o idle

movements increas es with a decreas e in cam radius . The reason for this
is that a small rotation angle at s m all radii gives a very s teep ris e curve,

which l eads to rapid cam and follower wear and possibly, as was mentioned

earlier, to j am ming of the cam .

1 24

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