THE WORK OF THE MACHINES
reciprocating because the cutting only occurs on one stroke. The return
stroke simply brings the work or the tool back to its original starting-point
in readiness for another trip.
The common planer is a machine for general purposes. It takes any
work within its capacity. The functions of the shaper and the slotter are
extremely limited, since they only deal with small surfaces. The portable
machines are employed to perform their functions on massive articles in
situ or on floor plates on work that cannot be set on machines. The key-
grooving and the broaching machines are specialized designs that cut narrow
Fig. 61.—Motor-driven Planer
slots in bores and elsewhere. In some of their functions they resemble the
slotting machines, but they deal with lengths impossible on the slotter, and
produce sections at one stroke that could only be done much more slowly
on this machine.
The principal improvements in the later planers have been the following:
(i) An increase in cutting speeds, and provision for effecting several changes
in rates suitable for different metals and alloys. (2) A rapid rate of return.
(3) The cushioning of the reversal with springs to absorb and give out power
on the return stroke. (4) The employment of a light aluminium alloy for
the driving pulleys to lessen the inertia at reverse. (5) Driving at high
speeds with narrow belts, using separate fast and loose pulleys for driving