In the plain indexing, the crank is to be turned by \(1 \frac{3}{7}\) turns, using Plate no 2 ∶ 21, 23, 27, 29, 31 and 33 holes. This can be done by: 

Concept:

Simple indexing mechanism

• A simple indexing mechanism consists of a 40 – tooth worm wheel fastened to the index head spindle, a single-cut worm, a crank for turning the worm shaft, and an index plate and sector.
• Since there are 40 teeth in the worm wheel, one turn of the index crank causes the worm, and consequently, the index head spindle to make 1/40 of a turn; so 40 turns of the index crank revolve the spindle one full turn.

The number of turns that the crank must be rotated for each indexing can be found from the formula

\(N = \frac{{40}}{Z}\)

Where,

Z = No of divisions or indexing needed on the work.

40 = No of teeth on the worm wheel attached to the indexing plate, since 40 turns of the index crank will turn the spindle to one full turn.

Calculation:

Given:

The rotation of the index crank for simple indexing

\(\frac{{40}}{Z} = 1\frac{3}{7}\;turns\)

\(i.e.means = 1\frac{{3 \times 3}}{{7 \times 3}} = 1\frac{9}{{21}}\;turns\)

1 full rotation + 9 holes in 21-hole circles,