JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III MACHINE TOOL DESIGN Previous Question Paper (November/December - 2020 - R16)

Hydraulic transmission is widely used in machine tools because it provides smooth, stepless speed variation and constant force or torque characteristics, which are essential for high-quality machining. Unlike mechanical drives, hydraulic systems offer inherent overload protection, as the system pressure can be limited by relief valves, preventing damage to the machine components.

Key advantages include:

• Stepless speed control: Allows for precise adjustment of feed rates and cutting speeds to match specific material requirements.
• Flexibility in layout: Power can be transmitted to remote parts of the machine through flexible hoses, simplifying the design of complex mechanisms.
• Self-lubrication: The hydraulic fluid acts as a lubricant for all moving parts within the pump, valves, and actuators, reducing wear.
• Reversibility: Direction of motion can be changed easily and rapidly without mechanical gear shifting.

A basic hydraulic transmission system consists of a pump, a control valve, and a hydraulic cylinder or motor. The pump converts mechanical energy from a motor into hydraulic energy (pressure and flow). The control valve regulates the flow rate and direction of the fluid, which is then directed to the actuator (cylinder or motor) to perform the work. A relief valve is placed in the circuit to maintain constant pressure and protect the system from excessive load. When the control valve is actuated, fluid enters the cylinder, causing the piston to move, which in turn drives the machine tool slide or table. The return fluid is directed back to the reservoir, often passing through a filter to ensure system longevity.

To calculate the strokes per minute (N) for the shaper, we use the following parameters:

Length of workpiece (L) = 175 mm = 0.175 m

Approach and overshoot (A) = 5 mm = 0.005 m

Total length of stroke (S) = L + A = 0.175 + 0.005 = 0.18 m

Cutting speed (V) = 30 m/min

Ratio of cutting stroke time to return stroke time (m) = 0.84

The cutting speed is related to the stroke length and the number of strokes per minute by the formula:

However, since the velocity ratio is given as the ratio of cutting stroke velocity to return stroke velocity (or time ratio), we define the time ratio factor as 1/m. Given the ratio of cutting stroke to idle stroke velocity is 0.84, the time ratio (m) is 1/0.84 = 1.19. The formula for cutting speed is:

Substituting the values: