Mechanical Forging Presses
Mechanical Forging Presses generally incorporate a ram that moves in a vertical direction to exert a squeezing action on the work piece, in contrast with the repeated blow characteristics of hammer forging. In general, presses can produce all of the same types of forgings produced on hammers and, in addition, can forge some alloys of moderate ductility that would shatter under the fast impact of the hammer die.
Driven by a motor and controlled with an air clutch, mechanical presses have a full eccentric type of crankshaft that imparts a constant length stroke to a vertically operating ram. Ram speed is greatest at the center of the stroke, but maximum force is not achieved until near the bottom of the stroke. Because the stroke is a fixed length, care must be taken to ensure that the closure allowed is not so small as to risk having the press “stick” at the bottom of the stroke, and not complete the stroke. Such an event can cause severe damage to the press or, in the least, substantial downtime to “burn” the dies apart to free the press.
Mechanical presses are best suited for low profile forgings and usually incorporate knockout/liftout pins in the dies which automatically eject the forging from the die allowing the die, and thus the forging, to be designed with less draft allowance. This can reduce weight and subsequent machining. Stresses in press dies are usually high, but there is very little impact load so harder die can be used without the risk of breakage that might be experienced on a power hammer.
Tooling costs are generally higher and the tool change and setup time is slower, so presses have been more cost effective on longer forging runs. As technology and systems change rapidly, this may not be true in the future. Higher production rates are possible on some part configurations with presses than with hammers. Many forging presses can deliver up to 70 strokes per minute. In general, presses require less operator skill than forging hammers.
Hydraulic Forging Presses
Hydraulic Forging Presses are not commonly used for conventional hot forging due to the extremely slow ram speed and high die contact time. They are, however, used extensively in open die forge applications and also for very large tonnage applications, primarily to forge materials other than steel. These machines are ideal for isothermal forging applications because of heir slow squeezing operation. The largest of these machines is rated at 50,000 tons with a die area of 12 feet by 32 feet. Generally, a water hydraulic system is used to drive these machines.
Die contact time of the various types of equipment is of interest to the forger, for the longer the contact time, the lower the die life tends to be. This is due to both the loss of heat in the work piece and the possible elevation of the die temperature above its design specifications. The contact times vary for the amount of deformation in particular forging operation. The heavier the deformation, the longer the contact time.
The Screw Press uses a friction, gear, electric, or hydraulic drive to accelerate the flywheel and screw assembly to convert angular kinetic energy into the linear energy available in the ram. In friction drive machines, vertically mounted drive wheels are rotated continuously. To make a downstroke, the drive wheels are shifted to enable one wheel to engage the main flywheel and accelerate the ram down. When the energy is completely used up as the stroke is made, the flywheel, screw, and ram come to a stop.
The drive wheels are then shifted to allow the flywheel to be reversed and return the ram to the top. In direct electric drive machines, a reversible electric motor is built directly onto the screw and frame. This design uses a screw which does not move vertically but is threaded into the ram/nut assembly. As with the friction drive machine, the flywheel must come to a complete stop and all energy used up in order to reverse the ram back to the top. A variation of the direct drive uses a gear drive and slipping clutch flywheel assembly in which the drive gears and screw are protected from overloading by the slipping clutch.
This design is used in larger capacity machines. Both electric and hydraulic drive motors can be used. The largest screw presses in operation (16,000 ton nominal rating) are based on this design. The total energy available in the screw press is determined by how much kinetic energy is input to the flywheel by the drive system. It is possible to control the force of each blow by controlling the speed of the flywheel. This can be accomplished by disengaging the drive from the flywheel at predetermined times to limit flywheel RPM and, thus, ram speed.
New design technology allows not only the force of each blow to be controlled, but also the stroke distance of the ram. The hydraulic clutch drive screw press design used in the machines in our Lebanon facility allows the operator to program individual blow control settings for both stroke and force. This feature means that no more energy than necessary is used to make a part, and also lets preformed “pancake” shapes to be made to consistent thickness. This design also allows the press to be cycled faster than conventional friction or direct drive machines. The flywheel is disengaged from the screw and ram assembly hydraulically, prior to completion of the stroke, and continues to rotate. The ram is then returned to the top position by auxiliary hydraulic cylinders, not by the main drive system. Because the flywheel can then be brought back to speed quickly, the press has its maximum energy and force available very shortly after the downstroke begins.
These features, plus the availability of using knockout/liftout pins to remove forgings from the dies, allows the screw press to take advantage of some of the most desirable traits of both hammers and mechanical presses. These include low die contact time, consistent and repeatable blow force, reduced draft angles to limit waste material, and easier die setting.
Upset forging, sometimes called Heading, is performed on a horizontal forging machine called an Upsetter. It is essentially a process for enlarging and reshaping certain sections of a bar or tube. In its simplest form, hot upset forging is accomplished by holding the heated stock between two half dies and applying pressure to the end of the stock in the direction of its axis with a heading tool, which upsets (spreads) the end by metal displacement.
Present day machines and tooling permit the use of multiple pass tooling that can produce complex shapes accurately and economically. The process is now widely used for producing shaft type parts, ranging in complexity from simple headed bolts to flanged shafts, cluster gears, and wrench sockets that require simultaneous upsetting and piercing. Forgings requiring center (not at the bar end) or offset deformation may also be produced.
Hot upsetting may be used to preform or prepare sock for another forging machine, such as a hammer or press, or as a finishing operation following forging such as upsetting a flange on the end of a crankshaft. In addition to upsetting, the heading tools are used for piercing, trimming, extrusion, and bending. In the upset forging process, the working stock is confined in the die cavities and the heading action creates the pressure required to fill all of the die impressions completely. Since the dies are split, a wide variety of shapes can be forged and easily removed from the tooling, which is primarily composed of three die elements – two gripper and cavity dies (one is stationary and one is fastened to the moving die slide), and the punch, which is fastened to the header slide (ram).
During the upset forging cycle the movable die slides to the stationary die to grip the stock. The punch that is fastened to the header (ram) advances forward and forces the stock into the cavities of the dies. When the punch retracts about 60% of its full stroke. The movable die slides to its open position allowing the forging to be released. The forging may then be shifted down to the next pass (die) where the cycle may be repeated. Many forgings require multiple passes (some as many as six) before completion. The stock may also be forged on one end and then slipped over (reversed) to be forged on the other end in one heat-cycle. After completion of the forging operation, the forgings are usually dropped through the throat of the machine to a conveyor that delivers the hot forgings into a metal tote box for cooling and transportation.