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Technology of watch production - part 49

Feed, mm

per blank

revolution

0,1 0

0.20

0.30

0 .45

0 .75

1 .35

TABLE 13

TABLE 14

Feed gearing

Hob-spindle speeds and cutting speeds

Gears

Hob-spindle

speed, rpm

500

700

1000

1400

2000

Technical data, model 530 machine

Maximum work diameter, mm

automatic operation

semiautomatic operation

· . . . .

Maximum hobbing length, mm

. • • • . . . . . • . .

Number of teeth hobbed

semiautomatic operation . . . . .

Maximum module, mm

. . . • • . . . . . . . . . . . .

Minimum module, mm

. . . . . . • . . . . . • . • • •

Hob diameter, mm

. . • . . • . . . . . • . • . . . . . .

Hob spindle tilt, degrees

. . . . . . . . • . . . . . . .

Electric motor pmver, kw

. . . . . . . . . • • . . . . •

Cutting speed,

m/min, for

cutter diameter

V =

24 mm

37 .8

52.8

75.5

1 05.5

150.8

6-100
6-300

0.05

25-30

Up to 3'

0.65

Speed of

electric

motor, rpm

1400

2800

Hob cutters.

Involute gears are cut by bobs having straight- sided rack

teeth according to GOST 3 0 5 8 - 4 5 . This tooth profile is called the b a s i c

r a c k (Figure 2 5

Watch- gearing p rofiles are bobbed, as previously m en

tioned , by special bobs designed for each wheel . Since this method is us ed

in one of the most advanced branches of precision- instrument manufacture ,

the design of bobs is a practical necessity, and we shall therefore dwell on

the subj ect in greater length.

FIGURE 25. Basic rack

1 9 3

Family of plane

curves

F(x.IJ.C)·O

FIGURE 26. Envelope of the family of plane

curves - mated profile

According to the general theory o f gearing, the active portions of the

profiles of a meshed pair must be conjugate in their relative motion. If

we imagine the gear

in contact with gear z2 , s o that their pitch circles

roll without sliding (Figure 2 6 , a ), then the successive positions of the gear
tooth p rofile generate a family of plane curves on the fixed plane fixed to

gear

z2 ,

(Figure 2 6 , b ) . Acco rding t o the general theo ry o f gearing, the

envelope of this family of curves is the mating gear-tooth profile , desired

s ince : 1 ) each point on the envelope i s a point o n one of the curves of the

family of curves , and ,

2 ) each point on the envelope is the limiting position of the intersection

of two adjacent curves of the family when they are drawn infinitely near to

one another.

A s the hob revolves (see Figure 2 1 ) its tooth p rofile generates

family

of plane curves on the fixed plane connected with the gear being milled.

The envelope of this family is the mating gear-tooth profile and the envelope

of the family of curves generated by the gear-tooth p rofile is the hob p rofile.

Therefore, the determination of the p rofile of the hob tooth reduces to find

ing the envelope of the family of c urves of the gear tooth when the pitch

circle of the gear rolls without sliding on the pitch curve of the rack.

Several plotting methods exist for obtaining the mating tooth p rofile of

the hob . The most widely used among them are the m ethod of graphical

generation and the graphical-analytical method.

The m ethod of graphical generation is the simplest and most easily vis

ualized of all the graphical methods . It not only makes it possible to plot

easily and rapidly the desired profile, but is suitable for checking the re

s·ults against the initial profil e .

FIGURE 27. Gear-tooth profile

The procedure is as follows : the tooth-space profile of the gear is drawn

to a large s cale on a sheet of pap er . The pitch circle is divided into s e veral

equal parts ,  and  the  points  of  division are numbered as  s hown in  F igure  2 7 .
Radial  straight lines  are  drawn  through  the  division points .  A  s traight line
is  drawn  on tracing paper (Figure  2 8 )  and taken as  the hob  pitch line .  This

straight line is divided into parts o f length e qual to that o f the arcs of the

pitch circle between the division points and the points obtained are numbered
in the same way as on the pitch circl e . Perpendiculars t o the s traight line

are drawn at the points of division. The tracing paper is then s uperimposed

on the sheet in s uch a way that the zero point and the perpendicular on the
tracing paper c oincide with the zero p oint and the radial straight line on the

sheet. The tooth - space profile is then drawn on the tracing paper. Next,

1 94

the corresponding points to the right and left on the two sheets are cons e

c utively b rought into coincidence, and the tooth space profile i s drawn each
time on the tracing paper.

Hob-tooth profile

Basic profile of cutter

FIGURE

28.

Hob-tooth profile

The envelope of the family of curves obtained is the required hob-tooth

p rofile . The plotting of the required p rofile reduces to s electing the arc

radii for each portion of the envelop e . The correctness of the profile ob

tained is checked by a revers e generation. The pitch circle of the gear is

dr.awn on the tracing paper, divided into the s am e number of parts as in the

preceding cas e , and the tracing paper is then s uperimposed on the sheet.

By drawing the profile of the hob tooth in every pos ition one obtains a family
of c urves whos e envelope must correspond to the original gear tooth-space

p rofil e . The results are checked by s uperimpos ing the tracing paper on

the initial p rofile drawn on the sheet. The method of graphical generation

has a shortcoming in that it is difficult to draw the pitch circle on the sheet

to a large scale. Figure 29 shows the profile of a hob tooth, designed by

the m ethod of graphical generation, for hobbing a pinion with

z =

and

m =

0 . 09 9 m m .

T h e g r a p h i c a l - a n a l y t i c a l m e t h o d i s us ed in cas es where it

is necessary to plot the p rofiles to a large s cale . I t consists basically in

plotting the pitch circle of the gear as points whose coordinates are cal

culated analytically, and drawing the p rofile graphically from the given
radius . The mating hob-tooth profile is plotted by the m ethod of graphical

generation.

A typical hob design for hob bing wheels of module 0. 05 to 0 . 3 mm is

given in Figure 30. It s hould be noted that the lead angle is 0° 1 5 ' to 0°30 ' .

The p rofile o f the tooth in the normal plane can therefore be considered

identical to its profile in the axial plane . This s ubs titution will be unaccept

able for lead angles above 0°3 0 ' ' as the differenc e in profiles will then be

considerable .

1 9 5

FIGURE 29. Hob-tooth profile for hobbing pinions with

z =

m =

0.099 mm

FIGURE 30. Dimensions of a watch-gearing

profile hob

The following maximum manufacturing e rrors are allowed according to

the specifications for hobs of module 0 . 0 5 to 0 . 3 mm (Table 1 5 ).

TABLE 1 5

Tolerances and deviations i n hob manufacture

Cutter errors

Outside-diameter run-out

. • • . • • • • • . . . •

Tooth-face run-out

• • • •

• • • • •

• • • •

Lead variation per turn

•

• • • • • • • • • • • •

Total lead variation for five turns

. . • . . • . •

Profile deviations:

a) module smaller than 0.15 mm

. . .

b) module between 0.15 and 0.3mm

. .

Tooth-space bottom blunting radius

. .

Permissible errors, mm

Class A

Class B

0.01

0.02

0.005

0.010

±0.002

± 0 .003

±0.005

± 0,008

0.003

0.005

0.004

0.007

0.01

0.015

Hobs are form -relieved in special machines . Since watch- gearing mo

dules have calculated values with three decimal figures (thus , the module

of the fourth wheel of the "Pobeda" b rand watches is m

0 . 0 9 9 mm ), the

pitch adjustment of the machine is realized by means of a special device

with a reading accuracy of 0 . 0 0 1 mm . The form- relieving is performed

in three operations : initial roughing, final roughing, and finishing. The

allowance for finish form -relieving is 0 . 01 mm per side .

The time norm for the m anufacture of a hob is 6 to 7 hours , with 3 . 5 to

4 hours of this time taken by form - r elieving.

The life of a hob b efore regrinding is , when milling brass gears , 8 to

1 0 milling hours .

1 9 6

Content .. 47 48 49 50 ..