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Shaper Mechanism IIST Trivandrum 09562275100 Kinematics and Dynamics 1/31/2012
This project report is a part of our mini-project in Kinematics and Dynamics. Shaping machine is a based on the concept of quick return mechanism which is a real life mechanism and our project is based on the working and application of it.
A brief Introduction . . . First of all we should know what exactly is shaping whenever we refer to it. Shaping is a material removal process in which a cutting tool takes mass and shapes a stationary object to produce a sculpted or plane surface and a shaping machine is used to machine surfaces. It can cut curves, angles and many other shapes. It is a popular machine in a workshop because its movement is very simple although it can produce a variety of work. Now, here comes the role of our mechanism. A shaping machine may use: 1. Whitworth quick return mechanism 2. Crank and slotted link mechanism
3. Hydraulic shaper mechanism
But, we would restrict our discussion to the first mechanism only. Equivalent linkages present in QR mechanism.
Purpose and Role of mechanism In a shaper, rotary movement of the drive is converted into reciprocating movement contained within the column of the machine. The ram holding the tool gets the reciprocating movement. In a standard shaper metal is removed in the forward cutting stroke, while the return stroke goes idle and no metal is removed during this period. To reduce the total machining time it is necessary to reduce the time taken by the return stroke. Thus the shaper should be designed that it can allow the ram holding the tool to move at a comparatively slower speed during the forward cutting stroke, the cutting speed depending upon the type of material and machining condition, whereas during the return stroke it can allow the ram to move faster to reduce the idle return time and so the mechanism is known as QUICK RETURN MECHANISM.
Elements of Whitworth QR mechanism The Whitworth quick return mechanism is shown as below. The bull gear is mounted on a large fixed pin A upon which it is free to rotate. The crank plate 4 is pivoted eccentrically upon the fixed pin at 5. Fitted on the face of the bull gear is the crank pin 2 on the top of which is mounted the sliding block 3. Sliding block 3 fits into the slot provided on the crank plate 4. At the other end of the crank plate 4, a connecting rod 6 connects the crank plate by a pin 9 and the ram 8 by a pin 7.
The parts shown in the above figure are officially labeled as: 1. Driving pinion 2. Crank pin 3. Sliding block 4. Crank plate 5. Pivot for crank plate 6. Connecting rod 7. Connecting pin for ram 8. Ram 9. Pin A. Fixed pin Now, we would focus on the working of our mechanism.
Working Description Principle:
The principle of quick return motion is illustrated in the figure below.
Principle of quick return mechanism When the link is in position PM, the ram will be at extreme backward position of its stroke, and when it is at PN the extreme forward position of the ram have been reached. PM and PN are shown tangent to the crank pin circle. The forward cutting stroke, therefore, takes place when the crank rotates through the angle C1 and KC2 and the return stroke place when the crank rotates through the angle C 2LC1. It is evident that the angle C1 K C2 made by the forward or cutting stroke is greater than the angle C2 LC1 described by the return stroke. The angular velocity of the crank pin being constant the return stroke is, therefore completed within a shorter time for which it is known as quick return mechanism. The ratio between the cutting time and return time may be defined as:
When the bull gear will rotate at a constant speed the crank pin 2 with the sliding block 3 will rotate on a crank circle of radius A2 and sliding block 3 will cause the crank plate to rotate about the point 5 with a variable angular velocity. Pin 9 fitted on the other end of the crank plate 4 will rotate in a circle and the rotary motion of the pin 9 will be converted into the reciprocating motion of the ram similar to the crank and connecting rod mechanism. The axis of reciprocating of the ram passes through the pin 5 and is normal to the line A5. When pin 2 is at position C the ram will be at the extreme backward position but when the pin is at position B, the extreme forward position of the ram will have been reached. When the pin 2 travels from C to B the crank pin 9 passes through the backward position to the forward position in the cutting stroke, and the return stroke is completed when the pin 2 travels from B to C or the pin 9 passes from the forward position to the backward position. As the angular velocity of the crank pin is uniform, the time taken by the crank pin 2 to travel through an arc covering CEB is greater than the time taken to move through an arc covering BDC. Thus the quick return motion is obtained by the mechanism. The length of the stroke of the ram may be changed by shifting the position of pin 9 closer or away from the pivot 5. The position of stroke may be altered by shifting the position of pin 7 on the ram. Below is the figure showing a shaping machine and its parts.
The typical size of the links used in a QR mechanism depends on the application and we could not restrain the size. Anyway, we have an image showing typical sizes used in simulating a QR mechanism.
Advantages of this method… 1:funcation simple so manfucturing cost is less, 2:less costly than lathe or milling machine,
Disvantages of method: 1:circular objects can not be finshed; 2:-cutting only half of time,
