Rotary motion of the bull gear is converted into a reciprocating motion of the ram through the crank and arm. The cutting is done in the forward stroke and the return stroke is idle. The time of return stroke is reduced to compress the total machining time which is known as quick return mechanism. The quick return mechanism may be attained by any of the methods: Whitworth quick return mechanism, crank and slotted arm mechanism, and hydraulic mechanism. In this text, crank and slotted arm mechanism is discussed as it is very simple in construction and operation.
The schematic diagram of crank and slotted arm mechanism is shown in Figure 19.26 (a). The driving pinion, which receives power from electric motor directly or over head line shaft, drives the bull gear either directly or through back gears. The crank pin may be adjusted by a hand wheel for specified length of stroke. The slotted link is provided with a fulcrum at its lower end, while the upper fork end is connected with the moving block. Crank pin is connected between the crank gear and slotted link. The rotary motion of the crank gear transmits the rocking movements to the slotted link by the crank pin within the slot. This rocking movement of the slotted link is communicated to the reciprocating movement of the ram through the moving block which is locked with ram screw.
Figure 19.26 Crank and Slotted Arm Quick Return Mechanism
It is seen from the block diagram in Figure 19.26 (b) that the path travelled by the crank pin during forward motion is much more than the backward motion. Since
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