Quick Return Mechanisms

Slow forward and quick return mechanisms are to be found on some metal cutting machines, digital printing machines and digital scanners. The output gives a slow forward linear motion followed by a quick return to the start position along the same path.

Two of the most common, which are found in shaping, planing and slotting machines for metal cutting, are

1. Scotch yoke mechanism and
2. Whitworth quick return motion.

1. Scotch Yoke Mechanism

The Scotch yoke is also known as the slotted link mechanism. The slotted link mechanism is used to convert the linear motion of a slider into rotational motion or vice versa. This mechanism is generally found on shaping machines where a single-point cutting tool is mounted on the front of the slider or ram in a hinged tool post. The tool cuts on the slow forward stroke and lifts over the workpiece on the quick return stroke. The slotted link rocks from side to side, driven by the sliding block on the bull wheel.

Figure 1: Slotted link quick return mechanism (Scotch Yoke)

As shown in Figure 1 above, the bull wheel rotates at a constant speed. The angle at which it rotates on the forward stroke is greater than the angle at which it rotates on the return stroke. This imparts a slow forward and quick return motion to the slotted link and slider. The sliding block's distance from the bull wheel's center can be altered to vary the length of the slider's stroke. The scotch yoke mechanism can be understood visually by watching the animation below.

 

Animation 1: Scotch Yoke. Source: Wikimedia Commons

2. Whitworth Quick Return mechanism

The Whitworth quick return motion also has a slotted link and sliding block, as shown in Figure 2. This mechanism is used on significant planning machines and small slotting machines. With the slotting machines, a single-point tool is fixed to the front of the slider and is used for cutting fine grooves and keyways. With planning machines, the slider is the worktable on which the workpiece is secured. This moves with slow forward and quick return motion beneath a stationary single-point cutting tool.

Figure 2: Whitworth quick return mechanism

The driving gear, which contains the sliding block, rotates constantly. The sliding block causes the slotted link to rotate,e but because it has a different center of rotation, its speed is not constant. As shown in Figure 2 above, the angle at which the driving gear and slotted link rotate on the forward stroke is greater than the angle through which they rotate on the return stroke. This imparts the slow forward and quick return motion to the slider; it can be seen and understood visually from the animation.

Animation 2: Whitworth quick return mechanism