MILLING MACHINE OPERATIONS - OD1644 - LESSON 1/TASK 1
When counting holes, start with the first hole ahead of the index
The sector (figure 11 on the previous page) indicates the
next hole in which the pin is to be inserted and makes it unnecessary to
count the holes when moving the index crank after each cut. It consists of
two radial, beveled arms which can be set at any angle to each other and
then moved together around the center of the index plate. Assume that it is
desired to make a series of cuts, moving the index crank 1 1/4 turns after
each cut. Since the circle has 20 turns, the crank must be turned one full
turn plus 5 spaces after each cut.
Set the sector arms to include the
desired fractional part of a turn, or 5 spaces, between the beveled edges of
its arms. If the first cut is taken with the index pin against the left-
hand arm, to take the next cut, move the pin once around the circle and into
the hole against the right-hand arm of the sector. Before taking the second
cut, move the arms so that the left-hand arm is again against the pin; this
moves the right-hand arm another five spaces ahead of the pin.
the second cut; repeat the operation until all the cuts have been completed.
It is a good practice always to index clockwise on the plate.
The construction of some index heads permits the
worm to be disengaged from the worm wheel, making possible a quicker method
of indexing, called direct indexing. The index head is provided with a knob
which, when turned through part of a revolution, operates an eccentric and
disengages the worm. Direct indexing is accomplished by an additional index
plate fastened to the index head spindle. A stationary plunger in the index
head fits the holes in the index plate.
By moving the plate by hand to
index directly, the spindle and the workpiece rotate an equal distance.
Direct index plates usually have 24 holes and offer a quick means of milling
squares, hexagons, taps, etc. Any number of divisions which is a factor of
24 can be indexed quickly and conveniently by the direct indexing method.