Technology of watch production - part 50

CUTTING CONDITIONS FOR HOBBING

Hobbing is used, as mentioned earlier, for milling wheels and, some

times, pinions .

The m ounting and positioning of pinion and wheel blanks in the model 5 3 0 ,

and T - 84 machines i s similar to the m ounting and pos itioning i n the S - 4 0
and S - 5 3 machines (see Figure 7 ).

The cutting speeds and feed rates are es tablished as a function of the

mechanical p roperties of the material, the dimens ions of the blank and the
hob material . The feed rate in hobbing is expressed in millimeters per

revolution of the part being hobbed and is determined by the following for

mula:

Cs · M"zb

mm/ rev.

where

the feed in mm per blank revolution;

a factor depending on the mechanical prope rties of the material;

the module;

z =

the number of teeth on the wheel or pinion being hobbed.

The values of the factor

and the exponents

and

are given in Table 6 .

The cutting speed is determined by the formula

The values of the factor

Cf}

and the exponents

and y are given in Table 7 .

A s a tim e -saving m easure, the cutting speeds and feed rates recomm end

ed for hobbing watch- gearing wheels and pinions are given in Tables 1 6 - 2 0.

The hobbing (basic ) time Th is determined by the formula

L·z:p_

n·S0 ·k '

where

the numbe r of teeth on the hobbed part;

n =

the speed (rp m ) of the single- thread hob;

the numbe r of parts machined simultaneously;

the feed per blank revolutions;

l + [1 + [2 ,

the travel, where

the length of the hobbed part

(or of the s everal simultaneously machined parts ), [1

the cutter

approach (according to the table in Appendix 3 ) , [2

the over

travel ( 1 - 1 . 5 mm).

The value of [1 can be determined from the formula

[1 = cos �

h0 (d - h0)

l ,Stan�

(

h0) ,

where �

the hob lead angle.

In the case of hobs , the value of the angle � does not exceed 0°3 0 ' , as

s een in Figure 30, and therefore cos � � 1 , tan � :::: 0.

This reduces the formula to

h0 (d - h0)

(see formula ( 3 )]

The time Th , calculated as necessary for the manufacture of one part,

is compared with the available work-spindle speeds and the time per revolu

tion closest to Th is selected.

1 9 7

TABLE 16

Feed

s0 ,

mm/blank revolution, for hobbing wheels and pinions made of U7AV and U10A steels

Number of

Module,

wheel- or

pinion teeth

0.10

0.12

0.15

0,20

0.25

0.30

0.08

0.085

0.10

0.11

0.13

0.14

1 0

0.085

0.09

0.11

0.12

0.14

0.15

0.09

0.10

0.12

0.13

0.15

0.17

0.10

0.11

0.13

0.14

0.17

0.18

0.11

0.1

0.14

0.16

0.19

0.20

0.12

0.13

0.15

0.17

0.20

0.22

0 .13

0.14

0.16

0.19

0,21

0.23

0.14

0.15

0 .17

0 .20

0.22

0.24

0.145

0.16

0.18

0 .21

0.23

0 .25

0 .15

0.17

0.19

0.22

0.24

0.26

TABLE 17

Cutting speeds v , m/min, for hobbing wheels and pinions made of U7A V steel

Feed

•· ,

Module,

mm/blank

0.10

0.12

revolution

0.15

0,20

0.25

0,30

0. 05

128

118.6

108

86.7

80.6

0.08

106

97.5

78.4

71.4

0.10

88.4

80.3

71.2

64.7

0.12

74.1

65.7

59.6

55.2

0.15

68.5

60.5

0.20

59.4

52.7

47.8

44.3

0.25

63.8

54.2

43.4

40.4

0.30

58.7

54.3

49.7

43.8

0 .40

52.2

44.1

39.1

35.3

32.8

TABLE 18

Cutting speeds v , m/min, for hobbing wheels and pinions made of Ul.OA steel

Feed

sJ .

Module,

mm/blank

0.10

0.12

0.15

0.20

0.25

0.30

revolution

Cutting speed

tl,

m/min

0.05

117

1 08

98.5

87.3

73.5

0.08

88.6

71.4

60.2

0.10

80.5

73.2

64.2

54.6

0.12

73.8

67.5

59.8

54.4

50.3

0.15

68.3

62.3

55.1

50.1

46.4

0.20

64.2

59.2

54.1

43.6

40.4

0.25

58 .2

53.8

49.3

43.6

39.6

36.8

0.30

53.5

49.4

45.2

36 .4

33.7

0.40

47.5

43.8

40.2

35.6

32.2

29.9

1 9 8

TABLE 19

Feed

s0,

mm/rev, for hobbing wheels and pinions made of LS63-3-0T brass

Number

Module , mm

of wheel-

0.1 0

o r pinion

0.12

0.15

0.20

0.25

0.40

0.50

teeth

0.15

0.17

0.19

0.22

0.26

0.28

0.33

0.36

0.17

0.19

0.21

0.24

0.28

0.30

0.36

0.40

0.1 9

0.21

0.23

0.26

0.30

0.33

0.40

0.45

0.21

0.23

0.26

0.28

0.33

0.36

0.45

0.50

0.23

0.26

0.28

0.30

0.36

0.40

0.50

0.55

0.26

0.28

0.30

0.33

0 .40

0.45

0.55

0.60

0.28

0.30

0.33

0.36

0.45

0.50

0.60

0.65

0.30

0.33

0.36

0.40

0.50

0.55

0.65

0.70

0.33

0.36

0.40

0.45

0.55

0.60

0.70

0.80

TABLE 20

Cutting speeds

for hobbing wheels and pinions made of LS63-3-0T brass

Feed

So ,

mm/blank revolution

0.1

0.15

0.2

0.3

0 .4

0.5

0.6

v .

m/min

•

205

1 91

181

172

164

158

153

As an example , we will calculate the cutting conditions and the p roduction

rate for a specific case on the m odel 5 3 0 s emiautomatic machine .

It is required to hob the teeth of a barrel with

80, module 0.105 mm and material - LS63-3 brass

(Figure 31).

FIGURE 31. Wristwatch barrel

The number of parts machined simultaneously is

k =

We begin by writing down the data 1vhich are to be used in the calculation. We select from Table 19

the feed

0.33 mm/rev. , and from Table 20, the cutting speed

t1 =

172 m/min. On the basis of the

machine data we select the hob speed

n =

2300 rpm. The length of theo machined parts is (from Figure 31)

L =

1 .8

10.8 mm. The cutter approach is, from Appendix 3 : /1

2.3 mm. We fix the overtravel as

Lz =

0, since the overtravel is automatically provided by the turned-down diameter on the last part.

The total travel is

1 0.8

2.3

13.1 mm.

The hobbing time corresponding to the selected conditions will be

L · t p =

1 3.1 · 80

· So ·

2300 · 0.33·6

0·23

mm .

The additional time necessary for changing the mandrel, starting the machine, and other similar opera

tions is, according to the standards, 0.42 min, or 0.07 min per piece.

The calculated production rate for the machine per shift will be

480

min.

600 .

0.23 + 0.07

pleces .

1 9 9

We next compare the calculated data with the machine capabilities. The nearest value for

(from Table 13) 0.30 mm/rev. , the nearest value of

is (from Table 14) 150.8 m/min at

2000 rpm.

13.1· 80

The actual value of T h will then

2000. 0 30•6

0.30 min, instead of the calculated value of Th

•

480

0.23 min. The actual production rate will be

0.30

0.07

1300 pieces per shift.

C omparison of the Form - milling and Generation

(Robbing) Methods

The following conclus ions were drawn on the basis of inves tigations ,

conducted in watch production, of the accuracy and productivity of the two
methods of milling teeth on wheels and pinions :

1 . The p roductivity of the generation m ethod is 1 . 5 to 2 times higher

than that of the form -milling m ethod, The generation method is inherently

free of idle movements and no loss of time is incurred in indexing, so that

the milling process is c ontinuous . The basic milling time for milling the

teeth on the center wheel of the "Pobeda " brand wristwatches is 0 . 1 2 3 min

using the generation m ethod and 0 . 2 7 8 min using the form - milling method

(in two passes ). The piece times are 0 . 1 9 min and 0 . 38 min, respectively.

The following are the time rates fo r the manufacture of formed cutters

and hob cutters , and their respective service live s , according to data s up

plied by s everal watch plants :

1 . Formed cutter

2. Hob cutter

Time rate

1 hour

6- 7 hours, 50"7o taken by

form-relieving

Life (total)

10-12 shifts
20-25 shifts

The higher cost of hob cutters is compensated for by their longer life

and the highe r productivity of the generation method its elf.

The manufacturing time rates for involute hob cutters are about 2 0 o/o less

than those for watch- gearing p rofile hobs .

The diffulty in manufacturing watch- gearing profile bobs lies both in the

need to attain the specified accuracy of lead, profile and face run- out, and

in the need to obtain a tooth space with minimum blunting radius . The teeth

of hobbed wheels present a characteristic blunting of the tooth tip (Figure 32 ).

2 . The surface quality obtained using the generation method is class 8

o r 9 and is better by 1 or 2 classes than that obtained by the form -milling

method.

Every roughness on the cutting edge of a formed cutter tooth is repro

duced (in the negative ) on the wheel - tooth profile . In the generation method,

only thos e roughnes s es are reproduced which appear on the line of action

during generation. The surface quality, in both methods , may vary within

the limits of one class , depending on the quality of sharpening.

3 . Among the original machine errors the facto r which has the largest

influence on the accuracy of the completed gear wheel is the error in the

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