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The pushmi-pullyu (pronounced “push-me-pull-you”), popularized in the stories about Doctor Dolittle . was a gazelle-unicorn cross that had two heads, one at each end of its body. It would walk in either direction, pushing or pulling as the case might be. A reciprocating saw is a machine-powered saw that creates a cutting action through a push-and-pull (“reciprocating”) motion of the blade.
A reciprocating saw has a blade similar to a jigsaw blade, slender with jagged teeth on one side. It is connected to a long machine containing a rotating motor that moves a piston up and down. The piston, in turn, is attached at one end to a clamp that holds the blade. The saw generally has a handle designed to be used when cutting vertically through a horizontal surface. The foot of the saw, the cutting end, typically has a spring-loaded hinge to provide a cushion between the saw and the surface that is being cut.
The reciprocating motion of the blade is also referred to as a “back-and-forth” or oscillating motion . There are several ways of transforming the rotary power of the motor into this reciprocating motion; a crank or Scotch yoke type drive, a swash plate type, or a captive cam – sometimes known as an eccentric or barrel cam.
The problem with the crank and cam type of drives is that they are lopsided by nature, and they require counterweights to provide an offsetting balance to reduce the vibration in the machine. Unless the weights and counterweights are absolutely perfectly balanced, there will also be a vibration in the tool, especially at higher speeds. The vibrations make it difficult to control the cut of the blade and motion of the saw. This can lead to ragged cuts that wander all over the place.
Crank and cam type of drives are fairly well known, and the source of the vibrations is readily apparent to anyone who takes a look at them. A swash plate type of drive is different. There is almost no rotational out-of-balance condition to counteract so vibrations are cut nearly to zero. Any vibrations that remain are in line with the blade and therefore very easy to control.
A swash plate is a circular plate attached to the rotating shaft the comes out of the motor, but instead of being set perpendicular (90º) to the shaft, the plate is set at an angle, so it looks tilted or slanted in relation to the shaft.
As the shaft turns or rotates, the edge of the swash plate appears to move up and down. One point of the swash plate is attached through a linkage to the “up-and-down” shaft that drives the blade. The main shaft rotates, turning the swash plate. As the plate passes any particular point around the circle, call it “north”, the edge of the plate moves up and down at that point due to the slant in the plate. The linkage transfers this up-and-down movement to the secondary shaft which moves the blade back and forth across the surface that is being cut.
All of the motion is in line with the motion of the blade, resulting in little or no vibrations, making the saw easier to control. Therefore the cuts produced by a swash type drive are much smoother than the ones produced by a crank or cam type of drive.