Quick Return Mechanism
The ram moves at a comparatively slower speed during the forward cutting stroke. During the return stroke, the mechanism is so designed to make the tool move at a faster rate to reduce the idle return time. This mechanism is known as quick return mechanism.
As the ram moves at a faster rate during return stroke, the time taken becomes less. The total machining time decreases and the rate of production increases. The following mechanisms are used for quick return of the ram.
- Crank and slotted link mechanism
- Hydraulic mechanism
- Whitworth mechanism
Crank and Slotted Link Mechanism
An electrical motor runs the driving pinion(S) at a uniform speed. This pinion makes the bull gear (M) to rotate at a uniform speed. Bull gear is a large gear fitted inside the column. The point ‘O’ is the centre of the bull gear. A slotted link having a long slot along its length is pivoted about the point ‘K’. A sliding block ‘N’ is fitted inside the slot and slides along the length of the slotted link. ‘P’ is the crank pin and ‘OP’ can be considered as a crank.
When the bull gear rotates, the sliding block also rotates in the crank pin circle. This arrangement provides a rocking movement to the rocker arm. As the top of the slotted link is connected to the ram, the ram reciprocates horizontally. So, bull gear rotation is converted into the reciprocating movement of the ram.
Quick Return Mechanism
As shown in the diagram, ‘KA’ indicates the starting point of the forward cutting stroke and ‘KB’ the end of the cutting stroke. The rotation of the crank ‘OP’ in clockwise direction through the angle P1RP2 refers to the forward cutting stroke. The rotation of the crank in the same direction through the angle P2LP1 refers to the return stroke. As the angle P2LP1 smaller than the angle P1RP2, the time taken for the return stroke is less than that of forward stroke. So, it is evident that the speed at which the ram travels during return stroke is more.
Time taken for forward cutting stroke angle P1RP2 2160 3
————————————————- = —————————- = ——————- = ——————
Time taken for the idle return stroke angle P2LP1 1440 2
In some machines this ratio can be set as 7/5.
The strokelength of a ram is the distance the ram moves forward or backward. It depends upon the distance between the centre of the bull gear and the centre of the sliding block. it is adjusted according to the length of the work.