You can determine the number of minutes represented by one turn of the index

crank by multiplying the number of degrees covered in one turn of the index

crank by 60:

9 x 60 = 540'

Therefore, one turn of the index crank will rotate the index head spindle

540 minutes.

(c)

The number of minutes (540) divided by the number of minutes in the

division desired, indicates the total number of holes required in the index

plate used. (Moving the index crank one hole will rotate the index spindle

through the desired number of minutes of the angle.)

This method of

indexing can be used only for approximate angles since ordinarily the

quotient will come out in mixed numbers, or in numbers for which no index

plate is available.

However, when the quotient is nearly equal to the

number of holes in an available index plate, the nearest number of holes can

be used and the error will be very small. For example, the calculation for

24 minutes would be:

or one hole on the 22.5-hole circle. Since there is no 22.5-hole circle on

the index plate, a 23-hole circle plate would be used.

(d)

If a quotient is not approximately equal to an available circle of

holes, multiply by any trial number which will give a product equal to the

number of holes in one of the available index circles. You can then move

the crank the required number of holes to give the desired division. For

example, the calculation for determining 54 minutes when an index plate that

has a 20-hole circle is available, is as follows:

or 2 holes on the 20-hole circle.

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