Movement No. 100 demonstrates the quick-return crank mechanism, a clever arrangement widely used in metal shaping machines and slotting machines. In this mechanism, a crank pin on a continuously rotating disk drives a slotted rocker arm. Because the crank pin’s circular path covers a larger arc during the forward (cutting) stroke and a smaller arc during the return stroke, the rocker arm — and the tool connected to it — moves slowly during the cutting stroke and rapidly snaps back during the return stroke. This asymmetry in timing is highly advantageous in machining: the cutting tool does useful work on the slow forward stroke, while the fast return minimizes idle time and increases productivity. The ratio of cutting time to return time can be adjusted by changing the crank radius or the pivot position of the rocker arm.
100. Quick return crank motion, applicable to shaping machines.