Various mechanical arrangements, and occasionally rather complicated automatic apparatus, have been attempted or employed to replace the index; with the view of diminishing or entirely avoiding, the risk of errors in using the division plate. The counting index fig. 125, is one of the more simple forms, and is sufficiently effective. It has two distinct springs, the two points of which are used at the same time, inserted in the same circle of holes, one above the other.

The front index is very similar to those already described, it has a triangular slide and screw below for its vertical adjustment, and a transverse binding screw to the ball. The second or back index, is mounted directly behind the first, and works somewhat like a hinge at its lower end upon the pin, by which the entire apparatus is carried in the usual manner, in the brass ball attached to the lathe head. The spring of the back index is made in two pieces, the upper, a, sliding vertically upon the lower, b. The lower half, is provided with two rectangular, steel sliding clamps, which surround both halves of the spring and are adjustable to, and may be fixed at any position upon the lower half, b. The clamps when fixed, still permit the upper half to slide, but they also catch against projections formed at either end of a, and therefore determine the vertical traverse of the point, in both the upward and downward directions, within prescribed limits.

Fig. 124.

Counting Index 400112

Fig. 125.

Counting Index 400113

In producing say, 90 divisions from the 360 circle, as an example, the counting index is used as follows. The point of the front index is first placed in the 360 hole, and both clamps being slackened, the point of the bach index is placed a little above it, in any hole of the same circle. It may be placed in hole 10. 12 or 14, the exact position not being material, so long as the point of the back, stands just above and clear of that of the front index. The two points remaining in their respective holes, the upper clamp, is brought up against the top projections upon the upper half of the back index, and is fixed there. This, prevents the point of the back index, when withdrawn from the division plate, from sliding any lower, than the position of the hole in which it has been placed.

The point of the back index is then alone withdrawn, slid upwards, and placed 4 holes higher on the circle, viz. in 14. 16 or 18. The lower clamp is then pushed down and fixed against the projections at the lower end of a, which prevents the point from sliding any higher, than the hole in which it is now placed. In other words, the clamps thus fixed, limit the vertical traverse of the point of the back index, to a distance equal to that of 4 holes of the 360 circle.

In taking the consecutive divisions upon the work with the counting index, the point of the front index is withdrawn and held just clear of the holes, by the thumb and forefinger of the right hand in the usual manner; the point of the back index, still remaining in the hole at the bottom of its traverse. The pulley is then turned away from the operator by the left hand, until it is checked, by the point of the back index having risen or traversed so far as the adjustment of the clamps permits, the interval of four holes. The point of the front index, is then dropped into the hole that is opposite to it, which is four holes in advance of its former position on the division plate. The front index point being inserted, it retains the mandrel and the work, in the required position for the divisions or marks to be made upon it; but, previously to the work being marked or cut, the point of the back index, in its turn, is withdrawn, slid downwards to the extent of its traverse, and replaced in the division plate, ready to measure the next division in the same manner. When the mandrel is turned towards the operator, the point of the back index pursues the reverse path, starting every time from the top of its traverse.

The following table shows the divisions of the circle, obtained from division plates containing the numbers 360. 192. 144. 120. 112 and 96; which numbers are usually found to give sufficient divisions for all ordinary purposes. The first column contains the number or division required, the other columns, the number of holes or divisions to be taken in the different circles.

Table Of Divisions Of The Circle To Be Obtained From The Numbers 360. 192. 144. 120. 112 And 96

Parts required.

Circle 360.

Circle 192.

Circle 144.

Circle 120.

Circle 112.

Circle 96.

2

180

96

72

60

56

48

3

120

64

48

40

• • •

32

4

90

48

36

30

28

24

5

72

• • •

• • •

24

• a •

• ♦ •

6

60

32

24

20

• • •

16

7

• • •

• • •

• • •

...

16

• • •

8

45

24

18

15

14

12

9

40

. • •

16

...

• • •

• • •

10

36

• • •

• • •

12

...

...

12

30

16

12

10

• • •

8

14

• •.

• • •

..

• • •

8

• • •

15

24

..

• • •

8

• « •

• • •

16

• • •

12

9

• • •

7

6

18

20

• • •

8

• • •

• • •

• • •

20

18

• • •

• • •

6

• a •

• • •

24

15

8

6

5

• • •

4

28

...

• • •

• • •

..

4

• • «

30

12

• • •

• • •

4

...

• • •

32

..

6

...

...

• • •

3

36

10

• • •

4

• • •

• • •

• • •

40

9

• • •

• • •

3

• • •

• • •

45

8

• • •

• • •

• • •

• • •

• • •

48

• • •

4

3

• • •

...

2

56

• • •

• •

• •

• •

2

• a •

60

6

....

• • •

2

...

....

64

• • •

3

• •

• • •

• • •

..

72

5

...

2

• • •

• • •

• • •

90

4

• • •

• • •

..

• • •

• • •

96

• • •

2

• • •

• • •

• • •

1

112

• • •

• •

« • •

• • •

1

• • •

120

3

• • •

...

1

...

• • •

144

• • •

• •

1

• • •

••

• a •

180

2

• • •

...

« • •

• • •

192

• . •

1

...

..

...

• • •

360

1

...

...

..

• • •

• • •

The circle having the smallest number of holes containing the required intersections, is usually but not invariably, selected for easier and more convenient reading. Thus, to divide the work into twenty four parts, the table gives a choice among circles 360. 192. 144. 120 and 96. The 120 would in this case be preferable, not as the lowest number, but, because the number five occurs and is marked off as a divisor of 120, and is therefore easier to read than 4 in 96.