Technology of watch production - part 29

Small surfaces o r recesses are machined by plunging tool No . 2 while

the heads tock is s tationary .

The rocke r is so des igned that the feed motion of tool No . 2 and the

withd rawal  of  tool  No .  1  are p roduc ed by  the  rocke r  cam ,  while  the  with
d rawal  of  tool  No .  2  and  the  feed  motion  of  tool  No .  1  are  induced  by a
spring.  Tool  No .  1  is  not us e d  in plunging  ope rations ,  s ince  the  sp ring

p ressure could bend the part or break it. Ins tead of being cam - actuated,

tool No . 1 can also operate on a fixed s top , a p rocedure used fo r machin
ing the most accurate s urfac es .

Tool No . 1 can turn s e ve ral diam ete rs in succession. When one diameter

is finis hed, the tool retreats the neces sary dis tance while the headstock

dwells . When the tool is pos itione d for the s e cond

r-···

FIGURE 1 7 . Scheme of cut-off

with tool and headstock moving

simultaneously

diameter, the heads tock begins to advance .

F o r tap e r turning, tool N o . 1 and the head

s tock move  simultaneously and  uniformly.  Con
toured surfaces  of a�y  desired  shap e  are  ob
tained by  fe eding the tool and heads tock  simul
taneously at  s uitable  different  spe eds .

Rear - end tap e r turning is p e rformed by the

simultaneous  fee ding of the  heads tock  and  tool
No .  4 .   Cone fo rmation a t  cut- off  is  shown  in
Figure  1 7 .   The  front - end  cone  in  the  following
part  is  obtained by  suitably  grinding tool  No .  4 .

I n order t o obtain a sharp ap ex in the front- end

cone , the tool ove rruns the axis by 0 . 1 - 0 . 2 m m .

The s ketch of the blank given o n the operation

sheet specifies a blank length which allows fo r
the o ve rrun .

The rear cutting edge of the cutter has a

clearanc e angle of

to 5° in o rder to avoid rubbing the enti re cutting edge

along the cone surfac e . The cone surface obtained by s imultaneously feed

ing the cutter and heads tock is somewhat s e rrated as a res ult of tool traces .

Some watch pa rts have chamfe rs with dimens ions ranging f rom 0 . 0 5 mm X

X 4 5° to 0 . 1 5 mm X 4 5 ° . Such chamfe rs are m ade either by the combined m o

tion of cutter and heads tock o r b y a s uitably ground cutter cross f e d . In

the latter case slides No .

o r No. 5 can be us e d .

It is s ome times more expe di ent t o p roduce a chamfer b y us ing the c o m

bined motion of cutter and heads tock. Such a cas e is shown in Figure 3 1

( 9th  operation ),  where a  0 . 0 6 m m X 4 5 °  chamfer is  p roduc e d .   In this  cas e
cutte r  No .  1  has  a  continuous  backward  motion  during  the  form ation  of  the
shoulder  and  the  chamfer;  the  heads tock  advances  continuously from  the

beginning of the chamfering till the turning operation is completed. When

a s eparate tool is us ed fo r chamfering, additional tim e must be expended
on its approach and plunge , as can be s een from Figure 3 1 ( 1 6 th ope ration ) .

In addition, one of the tool s lides is occupied b y the chamfering tool .

The largest diameter of a p art is s ometim es not turned, s ince the bar

s tock can have the required diameter.

In cases where the part is subjected to heat treatment, howe ver, or

where the l argest diam eter is a mating surfac e , the bar mus t be turned on
the largest diameter in order to remove surface s c al e o r to attain a higher

degree of conc entricity, respe ctively.

1 1 3

Undercutting is usually p e rformed by tool slides N o . 4 o r No . 5 . The

cutte r is pos itioned fo r the operation by a cam . The headstock is then ad
vanced to the undercutting depth, and is withdrawn when the undercutting

is complete d . The cutter then retreats to its initial position. The s hape

of the undercut is the s am e as the shape of the cutting tool . Underc utting

can also be p e rformed by means of tools mounted in the horizontal slides
of an auxiliary attachment (Figure 1 8 ). Slides ( 1 ) and (2 ) are actuated

through the levers ( 5 ) and (6 ) by the bell cams ( 3 ) and (4 ) on the camshaft.

The slide carriage ( 8 ) is actuated by the flat cam (7 ) .

FlGL!I\E 18. Attachment 1vith two horizontal slides for undercutting

Back rec essing, especially of deep recesses , should not be attempted

on this type of automati c s c rew machine, as the cutting will take place at

a conside rable dis tance from the guide bushing, and the headstock, during

the plunge, is withdrawn by a s trong sp ring. This can lead to b roken cut

ting tools . Parts having a back recess must the refore be sta rted from the

recess side. On the s trength of thes e considerations the machining of the

central pinion is started at the sho rt end (see operation sheet on pp . 1 42 - 1 4 3 ) .

Depending o n the configuration of the part, the cutters can work i n a

diffe rent order as well . For instance, the cut- off may be p e rformed by

cutter No . 4 o r 5 , and the recessing by cutter No . 3 . The dis tribution of

wo rk among the cutte rs is explained below.

D r i

1 1

i n g . Holes are drilled by attachments mounted on the left part

of the bed in front of the tool head . The drill ed hole must be concentric

with the outer surfac e s , and accordingly the part must be accurately c ente r

drilled (c ente red ) before drilling.

1 1 4

The part is c entered either by one of the vertical cutters , by a c enter

drill mounted in a holder in a vertical slide (see Figure 1 9 ), or by a cente r

drill mounted in one of the spindles of the attachment. Centering with a
cutte r is more accurate , although it involves a greater time expenditure .

FIGURE 1 9 .

Holders :

a-for a center drill; b-for a carbide-tipped
tool; c-for a common cutting tool.

Drilling without first centering is acceptable only if the hole depth does

not exceed two diameters and if a s trict concentricity between the hole and

the outer surface is not required. The concentricity of the attachment and

heads tock spindles is very impo rtant when holes of a depth of more than two

diameters are drilled.

In o rder to inc reas e the p roductivity of the machine , c entering o r drilling

mus t be combined with other operations as much as poss ibl e . Drilling, fo r

instance , can be combined with turning. In drilling one must take into ac

count that the total feed of the d rill and the headstock must not exceed the

admissible feed magnitude .

C entering and drilling will be accomplished with the highest degree of

accuracy if the operation is started before the bar leaves the guide bushing.

A drilling ope ration should not begin with a cons iderable length of bar

protruding from the guide bushing as the blank is then less stable and the

machining m ight be inaccurate . It is not recomm ended to combine drilling

and radial- feed machining.

FIGURE

20. Layout of bell spindle feed cam for drill attachment, with recesses for retracting the drill

1 1 5

Stepped holes are d rilled using two - , three - , and four- spindle attachments .

The last spindle can b e us ed for facing. Stepped holes are best d rilled by
s eparate drills , and not by s tep drills , in o rder to achieve accurate dia

meters and depth for each step. Straight- flute drills are generally us ed

for drilling holes .

Blind holes and s tep holes with close depth tole ranc es are drilled using

the heads tock feed. Spindl e feed is used when a through hole of uniform
diameter is drilled.

In d rilling deep holes , the drill is retracted many times in order to

remove the chips and to cool the drill . The deeper the hol e , the more

frequent the retractions . The following relationships have been empirically

es tablished between the drill diameter, the hole depth, and the number of

retractions of the drill:

Retraction . . . . . . . First

Drilling depth . . . . . 3d

Second

Third
6 .5d

Fourth

.5d

Fifth
8 .5d

The bell cam which feeds the spindle of the drilling attachment has s uitable

recess es for retracting the spindle (Figure 2 0 ) . It is, however, more

expedient to retract the drill by means of a special cam mounted on the

back wall of the bed (see Figure 1 3

)

T h r e a d c h a s i n g is carried out on automatic s crew m achines using

the differential method, also called the o verrunning m ethod. The machine

spindle rotates at the same speed and in the s ame s ens e as in turning while

the spindle carrying the chasing die ro

tates in the s ame s ens e, but at a speed

�

which is higher than that of the machine

spindle when chasing a right- handed

thread, and lower when chasing a left

handed thread. The speed of thread chas
ing is equal to the difference between the

spindle speeds .

When the chasing of a right- handed

thread has been completed, the chas e r

begins t o rotate m o re slowly then the ma

chine spindle, and thus unscrews itself.

The s am e effect is achieved in chasing

a left- handed thread by reversing the

s equence.

FIGURE 21 . Thread-chasing attachment drive

The differential method of thread chas -

ing has many advantages over the m ethod

used in turret automatics , where the machine spindle is revers ed and

slowed down and the chasing die or tap is fixed in the turret head and has

only a translato ry motion. The s witch- over of the machine spindle takes

time and the design of the machine spindle ass embly is much more c omplex.

Left- hand threads are chased using the same attachm ent as for right

handed threads with only the pulleys on the machine m ain drive shaft chang

ing places (Figure 2 1

)

For thread chasing, the headstock must first advance

to 8 mm at high

speed, s o that the cutters will not p revent the chas e r from advancing along

1 1 6

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