Movement No. 178 demonstrates a variable-speed mechanism for slotting and shaping machines that automatically varies the speed of the cutting tool slide throughout each revolution. A driving shaft rotates through an opening in a fixed disk. The fixed disk contains a circular slot positioned eccentrically relative to the shaft center. At the end of the driving shaft is a slotted crank. A sliding block fits simultaneously in the slot of the crank and in the eccentric circular slot of the fixed disk. A connecting-rod is attached to the outer end of this sliding block and drives the cutting tool slide. As the crank rotates, the eccentric circular slot guides the sliding block, forcing it to move inward and outward along the crank slot. When the block slides inward, the effective crank radius decreases, slowing the connecting-rod and cutting tool — this slow-down occurs at the cutting stroke where greater force and precision are needed. When the block slides outward, the crank radius increases, speeding up the return stroke. This elegant automatic variation in effective crank length produces the characteristic quick-return, slow-cut behavior essential to efficient shaping machine operation.
178. Contrivance for varying the speed of the slide carrying the cutting tool in slotting and shaping machines, etc. The driving-shaft works through an opening in a fixed disk, in which is a circular slot. At the end of the said shaft is a slotted crank. A slide fits in the slot of the crank and in the circular slot; and to the outward extremity of this slide is attached the connecting-rod which works the slide carrying the cutting tool. When the driving-shaft rotates the crank is carried round, and the slide carrying the end of the connecting-rod is guided by the circular slot, which is placed eccentrically to the shaft; therefore, as the slide approaches the bottom, the length of the crank is shortened and the speed of the connecting-rod is diminished.