In a shaper rotary movement of the drive is converted into reciprocating by mechanism within the machine. The ram gets the reciprocating movement. In standard shaper metal is removed in the forward stroke, while return stroke goes idle and no metal is removed during this period. To reduce the metal machining time it is necessary to reduce the tine of return stroke. So the shaper machines are designed such that the tool moves slowly during the forward stroke, whereas during the return stroke it consume less time. The mechanism is known as “Quick Return Mechanism”
The types of quick return mechanism are as below:
- Crank and slotted link mechanism.
- Whitworth quick return mechanism.
- Hydraulic shaper mechanism
Crank And Slotted Link Mechanism
The motion and power is transmitted to the bull gear through a pinion, which receives its motion from individual motor, or overhead line shaft through speed control mechanism. Speed of bull gear is changed by combination of gearing or shifting belt on step cone pulley. A redial slide which carries a sliding block is bolted to the center of bull gear. Crank pin is fitted into the sliding block. The sliding block is fitted into a slotted link, which is known as rocker arm. It is pivoted at it’s bottom end and attached to the frame of column. The upper end of the rocker arm is connected to the ram by a pin. Now as the bull gear rotates the crank pin also rotates. The rocker arm sliding block will also rotate on crank pin circle, and will move up and down. The slot in the slotted link gives it rocking movement, which is connected to ram. Thus rotary motion of the bull gear is converted into reciprocating motion of the ram.
The principle of crank slotted mechanism is shown in separate figure. When the link is at the position PM the ram will be at extreme backward position and at PN it reaches to the extreme forward position. The forward cutting stroke takes place through an angle C K D and the return stroke place when the crank rotates through an angle C L D. From the figure we can say that the angle C K D made by the forward stroke is grater then the angle C L D made by the return stroke. Therefore the return stroke completes within in a shorter time. So the mechanism is known as Quick return mechanism.
The ratio between cutting time and return time may be determined from the formula
CUTTING TIME = Angle C K D
RETURN TIME Angle C L D
The ratio usually varies between 2:1 and practical limit is 3:2. The only disadvantage of this mechanism is that the cutting speed and the return speed is not constant throughout the stroke.
Adjacent of length of stroke:
For getting better work by the shaper machine it is essential to adjust the length of stroke for this adjustment the bevel gear placed at center of bull gear. It may be rotated by a handle, causing another bevel gear to rotate, which is mounted upon the small lead screw. This lead screw passes through the sliding block. Thus rotation of the bevel gear causes the sliding block to be brought inward or outward. Maximum stroke will be obtained when the crank pin is shifted towards the farthest end of slide.
Whit worth quick return mechanism:
In this type of mechanism the bull gear is mounted on a large fixed pin upon which it is free to rotate. The crank plate pivoted upon the fixed pin. The sliding block fits into the slot provided on the crank plate. At another and of crank plate a pin connects a connecting rod. This rod is connected to the ram.
When the bull gear is rotated at constant speed, the crank pin with the sliding block will rotate on crank circle and the sliding block will cause the crank plate to rotate about the pivoted point with variable angular velocity. The pin at the other and of the crank plate will rotate in a circle. The rotary motion of this pin is converted into reciprocating motion by connecting rod mechanism.
When the pin of the slider is at point C the ram will be at extreme backward position. When the pin is at point B the ram is at extreme forward position. The angular velocity of crank pin is uniform. Now the time taken by the crank pin to travel through the angle C E B. which is grater then the time taken to move through the angle B D C. Thus the quick return motion is obtained. Shifting the pin at the other end of the crank plate changes the length of the stroke.
Hydraulic shaper mechanism:
In hydraulic shaper the ram is moved forward and backward by a piston in a cylinder placed in the ram. The piston rod is bolted to the ram. The oil under the high pressure is pumped from reservoir and made pass through valve chamber. The oil is exerting pressure on the piston from the right side. This is caused the ram to perform forward stroke. The oil at the left hand side of the piston is discharged to reservoir through the throttle valve. At the and of extreme forward stroke the shaper dog hits against the reversing level causing the valve to alter their position in valve chamber. Oil under high pressure is now pumped to the left side of the piston to perform the return stroke of the ram. Oil pressure at the right hand side is discharged to the reservoir.
The quick return motion is affected due to the difference in stroke volume of the cylinder at both the sides. The left hand is smaller due to piston rod As the pump is at constant discharge one in a fixed period. So the same amount of the oil will be pumped at the both side of cylinder simultaneously. This will indicate that same amount of oil will be packed within a smaller volume cause oil pressure rise automatically. So the return speed is increases.
The length and position of stroke is adjacent by shifting the position of the reversing dogs. Controlling the throttle valve may change the cutting speed.
Advantage of hydraulic shaper:
- The cutting and return speeds are practically constant throughout the stroke. This permits the tool to work uniformly during the cutting stroke.
- The reversal of ram is obtained quickly without any stoke as oil on the other end of the cylinder provides cushioning effect.
- Infinite numbers of cutting speeds may be obtained and the control is easier.
- With the high rate of return speed, a grater number of cutting stroke may be available within the range of cutting speed.
- The relief valves ensure safety to the tool and the machine when the machine is overloaded.