Power presses

POWER PRESSES

Forging of parts by presses involve slow squeezing of plastic metal in closed impression dies instead of applying repeated severe blows by hammers. The production of forgings using press is preferred because the whole operation is completed in a single squeezing action.

There are number of power presses available based on the mechanism and structure of the machine. The important types of presses are described here.

1. Screw Press

In screw presses, the upper ram and die are connected to a large vertical screw that can be rotated by a flywheel. So, the ram can move up and down relative to the fixed die in the bed of the machine. The ram has a limited amount of energy for each stroke. Thus, multiple blows are usually employed similar to hammers. Screw presses are available in ratings from 0.63 MN to 63 MN (i.e. 63 to 6300 tons).

Features of screw press:

(a) Energy is limited.

(b) It is suitable for small production quantities

(c) It can manufacture thin parts with high precision.

2. Hydraulic Press

The power presses are mostly a hydraulic type. The moving die is attached to a ram actuated by a large hydraulic cylinder as shown in Figure 3.21. Various strokes, forces and closing speeds can be obtained on hydraulic presses. The pressurized oil from the pump is delivered to the cylinder through accumulator and distributor. The ram is forced down upon the material to be forged in the forward stroke. For the return stroke of the press, relatively lower oil pressure is required with a large volume of oil per unit time to accelerate the ram. The presses ranging from 300 × 103 to 1000 × 103 kg is used for very large capacities. Hydraulic presses are used for heavy work and mechanical presses are for light work.

Features of hydraulic press:

(a) These presses are operated at constant speed.

(b) It has limited load capacity.

(c) Initial cost of the press is high.

(d) It is relatively slow in speed.

(e) Low maintenance is required.

3. Mechanical Presses

Mechanical presses belong to a class of machine tools that encompass a wide range of different machine types. Primarily, the mechanical press transforms the rotational force of a motor into a translational force vector that performs the pressing action. Therefore, the energy in a mechanical press comes from the motor.

These types of presses are generally faster than hydraulic or screw presses. Actually, the screw press may also be classified as a mechanical press. Unlike some presses, in a mechanical press, the application of force varies in both speed and magnitude throughout the distance of the stroke. When performing a manufacturing operation using a mechanical press, the correct range of the stroke is essential.

In mechanical presses, a crank, eccentric drive, knuckle joint, scotch yoke or rack and pinion mechanism are used to apply a vertical squeezing motion between upper moving die and lower fixed die as shown in Figure 3.22.

Features of mechanical presses:

(a) It produces high force at the end of stroke.

(b) It has limited stroke length.

(c) High production rate is possible.

(d) It is easy to automate mechanical presses.

(e) Lower operator skill is required.

4. Difference between Forging Hammers and Forging Presses