Forging presses are the second group of forging machines regularly used in impression die and large open die forging. They are commonly classified as mechanical or hydraulic, based on the means used to deliver energy. Presses deliver energy more slowly than do hammers. They are used for all alloy groups, and are used in preference to hammers for alloys that require slow deformation rates, such as 7xxx series aluminum alloys and most of the magnesium alloys. As with hammers, they usually operate vertically. The upper die is attached to the ram, and the downward stroke of the ram exerts force on the workpiece.
Mechanical Presses Mechanical presses typically store energy in a rotating flywheel, which is driven by an electric motor. The flywheel is engaged and disengaged to a mechanical drive such as a crankshaft, eccentric shaft, eccentric gear or knuckle levers, which convert flywheel rotation to vertical motion. The stroke is of set speed, length and duration. Mechanical presses therefore develop consistent forging results, offer high productivity and accuracy, and do not require as high a degree of operator skill as do other types of forging machines. In impression die forging, the workpiece is usually struck once in each impression. Mechanical presses are not suitable for open die forging, where the length of stroke must be varied between strokes.
The applied force is maximum at the bottom of the work stroke, and the estimated load at a position just above this point is the basis for press rating capacity. Ratings typically range from 100 to 10,000 tons. A few special design large capacity presses with ratings up to 16,000 tons are in operation.
Recent developments in mechanical presses are focusing on increased stiffness of the press structure to improve forging accuracy, automation, and high speed (in terms of die-to-workpiece contact time). They are increasingly replacing hammers because of greater environmental compatibility, ease of automation and lower operating costs.
Screw Presses Screw presses are not as widely used in North America as mechanical presses, but unique screw press characteristics are driving an increase in their use. As the name suggests, this type of press uses a mechanical screw to translate rotational motion into vertical. Briefly, the ram acts as the nut on a rotating screw shaft moving up or down depending on screw rotation. Energy is either delivered from a flywheel, which is usually coupled with a torque-limiting (slipping) clutch, or by a direct drive reversing electric motor. The main advantage of screw presses over offset or crank-type mechanical presses is in the final thickness control when the dies impact each other.
Hydraulic Presses Hydraulic presses are operated by large pistons driven by high-pressure hydraulic or hydro-pneumatic systems. They are slow moving compared with mechanical and screw presses, and squeeze rather than impact the workpiece. In operation, hydraulic pressure is applied to the top of the piston, moving the ram downward. When the stroke is complete, pressure is applied to the opposite side of the piston to raise the ram.
Speeds and pressures can be closely controlled. In many presses, circuits provide for a compensation control or sequential control, e.g. rapid advance, followed by sequences with two or more pressing speeds. The press can also be regulated to dwell at the bottom of the stroke for a predetermined time, raised at a slow release speed, and accelerated until it reaches original position. When needed, hydraulic press speed can be increased considerably. In many cases, hydraulic presses used for open and some closed die forging presses use microprocessors or computers to control the press operation, for parameters such as ram speeds and positions.
Hydraulic forging presses are rated according to the maximum force they develop. Presses in use in North America for impression die forging currently range up to 50,000 tons; presses of 72,000 tons and 82,000 tons are in operation in France and Russia. Presses used for open die forging range from 200 tons to 10,000 tons.
Forging Machines (Upsetters) Forging machines are also called upsetters. They were originally developed to upset metal for bolt heads and similar shapes, and are sometimes referred to as "headers". They are currently used to gather or upset (laterally displace) material, either at the end of the feedstock, between the ends, or in several places. They may be used to gather metal prior to forging operations on other equipment, or to produce complex, finished configurations with precision such as gear blanks, bearing races and spindles.
Forging machines are basically double-acting mechanical presses operating in a horizontal plane. They employ a flywheel, air clutch and eccentric shaft to operate the slide (or heading ram). In operation, bar stock, either heated or room temperature, is placed against the stationary die. The grip die moves laterally against the stationary die, gripping the stock tightly. The heading die with its attached heading tool (die) then moves forward against the end of the workpiece and displaces stock into the die impressions. As the ram recedes, the grip die retracts and releases the workpiece, which is ready for subsequent forging operations. In some cases, the forging is punched or sheared off of the bar stock in the final step.
Forging machines are rated for size according to the maximum bar size for which they can provide an upset head. For example, a two-inch upsetter could theoretically head bolts or form features in sizes up to two inches stem diameter.
When a large volume of products are to be produced, such as automotive and bearing manufacturing, automatic, multiple-stage hot forging machines are increasingly being used. These machines are based on a combination of features familiar to cold headers and hot nut formers, and operate at very high production rates approaching 160 parts per minute. They can produce products with complex configurations at high production rates.