Forging update: warm forming is hot.
Charles W Frame, chief engineer of Chambersburg Engineering Co, says, "Process control is the key to profitable drop-forging production. Focusing on this objective, our new developments include computer-assisted design and hands-on instruction programs to help forge-shop people implement changes.
"Adaptive, self-regulating, and intelligent programmable controls are available for our forging hammers. Also, we can update existing board-drop and steam-drop hammers at affordable costs with Ceco-Drop air-powered conversion systems. Programmable hammers can integrate with CAM systems."
The firm offers a computer program, CADIF, to speed complex computations required for die design. Compatible with IBM PC, the program also enables a computer to estimate forging costs, the number of flash-forming die impressions, preforming die impressions, and total blows needed. It generates estimates of stock size, material yield, flashland geometry, and maximum die pressure, force, and energy requirements.
Major Chambersburg products include the Impacter, a horizontal automated drop-forging system, and the Die Forger, a basic vertical-hammer system that can strike up to 100 blows/min. This accelerated drop hammer is air operated with microprocessor-based programmable controls. Larger models have falling weights up to 13,000 lb.
The Die Forger operator can choose a continuous series of forging blows, or he can initiate a delay or "restart" between sequences. The hammer handles repetitive production of drop forgings in closed impression dies requiring frequent stock manipulation and involving forging blows of various striking energies.
Press update for labor competition
According to J R Blakeslee III, president of Ajax Mfg Co, "It goes without saying that the forging industry is experiencing some of the most severe foreign competition in its history. As a result, the American forger has recognized that most orders lost to overseas suppliers were those parts that are labor intensive. When comparing labor rates of $2/hr in Korea, for example, as opposed to $17/hr plus $5/hr fringes in US shops, it doesn't take a genius to recognize the US cannot compete man for man. Couple this factor with steel costs in the US versus foreign suppliers, and the situation becomes almost overwhelming.
"However, one thing US forgers can be proud of--they are not quitters. They have come to us and asked for our help." Their plea to Ajax is simple: "During the good years, we bought your equipment, but now we have excess capacity and can't afford new equipment. We must update what we have."
The US forger has two basic problems: Steel costs and labor costs. Mini-mill production and, sadly, off-shore steel purchases are beginning to solve the steel-cost problem, according to Blakeslee. "Labor costs, we must address. We must automate, or at least partially automate."
Ajax thus speaks for many companies building forging gear. In its case, the firm is answering the call by semiautomating or fully automating new and existing equipment, by building specialized equipment to increase productivity, and by retrofitting automatic push-button-controlled shut-height adjustment for presses.
Retrofit wedge packages are available for Ajax forging presses, and these include hydraulic bolster clamping, shallow wedges, push-button controls, and digital readouts. The operator does not have to unbolt bolsters and manually adjust wedges to change shut height. Installed on a 700-ton press, for example, one automatic system now lets a user make adjustments in less than 25 sec, where it formerly took hours manually.
Presses in a special series from 100 to 1000 tons each have four pitmans for ram stability and wide die space. These let the customer trim, pierce, and hot coin--all in one press. Interestingly, the firm is shipping some of its new presses to the Far East. Other products include tube upsetters, forging rolls, wire drawers, and cutoff machines.
Verson Allsteel Press Co, Chicago, IL, is North American representative for Eumuco AG hot-forge systems. The team sells forge rolls, which Verson usually makes locally to fill US orders. The company also makes the Eumuco wedge press. Eumuco-Verson die-forging presses range in capacity from 700 to 8800 tons, and such presses are "the successor to the drop hammer for high-volume forging accuracy." Verson also says that "die-forging presses are ecologically recognized as superior to forging hammers and screw presses because of recent advancements in automation and reduction in noise emission."
Many single-pitman die-forging presses must transmit force from eccentric shaft to ram via conrod and gudgeon pins. Here, force has a buckle flow, and off-center pressing forces exert a tilting moment on the ram. This in turn loads the guideways and reduces forging accuracy. Eumuco press uses twin-pitman design to transmit forces directly to the ram via twin conrods and pressure pads. Press force on left and right of machine center cannot generate any tilting moment. However, forces at front and rear of center can still be a problem.
Eumuco electronics can monitor bearing temperatures in bronze bushings near ball-bearing rings. Eight checking points, each with a resistance thermometer, are scanned at a rate of one per sec and digitized. If any check point exceeds a preset temperature limit, the control shuts down the machine to protect bearings. A pressing-force monitor also is available to help prevent press overload.
Other optional equipment includes automatic beam transfer on Eumuco-Verson forging presses up to 7000 tons, and a heavy-duty, robot-like manipulator. The electronically controlled, hydraulically powered manipulator can load workpieces into the press and transfer them from die to die inside the machine, then deposit them at predetermined points. Transfer tongs and tong motions are adapted to workpiece shapes and to the forming sequence.
Verson of course has a complete line of presses, and many on special order. For instance, you can get a Wheelon hydraulic model operating at 10,000 psi and rated at capacities up to 60,000 tons. Such a press can do jobs that formerly required staged drop-hammer work or multistage conventional presses.
To cast or weld
How do you build a really big press? Erie Press Systems believes that the high strength and rigidity of welded steel is best for smaller presses of 500 to 1300 tons capacity. As press sizes increase, however, the firm employs a cast-steel frame to provide high strength, rigidity, and also extra weight and mass for stable installation.
On presses of 6000 tons and up, single-piece frames are not feasible because of the extreme size and weight. Erie uses tie-rod construction on these larger presses up to 12,000 tons. In this design, four steel tie rods hold frame components in compression.
A Scotch yoke provides greater precision, higher production, and low operating and maintenance costs, according to Erie. A company spokesman says, "It's the most significant advance in forging-press design since the pitman arm." Comprised of a set of top and bottom sliding blocks wrapped around the shaft's eccentric portion, it's completely contained within the ram.
The Scotch yoke transmits forging forces equally across the entire ram face, resisting deflection and providing greater dimensional precision when loading is off center. It allows longer gibs and provides better ram guiding than a pitman arm, eliminating the need for leader pins.
Erie offers a P/H Head to retrofit existing steam or air forging hammers. The company says you'll get increased productivity, improved environment, more energy for less input power, and good performance with operators of less skill. P/H stands for pneumatic-hydraulic operation, and ram movement is achieved by shifting hydraulic valving. Pressurized oil lifts the hammer ram to a preprogrammed position measured by a cylinder transducer.
To release the hammer, a valve shifts to block the pump circuit and send oil back to the reservoir through a down-throttling valve. Working gas pressure on the cylinder piston, and weight of moving parts, accelerates the hammer. Ram position or travel distance determines hammer-blow energy. Controls range from manual to fully programmable. The system works with gravity-drop or double-acting hammers of 1000- to 50,000-lb capacity.
As the screw turns
Bemcor Inc offers the Ficep friction screw press for hot forging. The press and matching Ceptrol CNC control provide exacting, repeatable blow energy, preset forging strokes, and digital tonnage readout to take the guesswork and operator error out of the operation. The control also presets ejectors, die-cooling systems, and charge and discharge automation. Finally, it monitors machine status.
As with most of the computerized press controls available today, the Ceptrol CNC monitors malfunctions and provides both self-diagnostics and diagnostics of the forging press and its automation systems.
Bemcor says that the screw press has several advantages over hammers and other forge presses. Its high energy level allows it to do the same work in one or two blows that would take seven or eight blows on a hammer. The actual
forging speed of a screw press (0.7 m/sec) is close to optimum deformation speed. Yet, ram rebound after the blow ensures that the hot forging is in contact with the tooling for only a short time--thus boosting tool life.
The energy and shock of a screw press are confined in its closed-ring frame, so foundation requirements are reduced. Almost no shock transfers to the floor, and noise level is low. The high energy levels available result in a smaller screw press doing the same work of an eccentric press some 1.3 to 1.6 times larger.
Off-center loading with multiple dies is less troublesome, and, because it's an energy press, oversize parts won't damage the setup. A larger billet merely produces an oversize part. Likewise, screw presses overcome expansion problems as dies heat up.
The Weingarten screw press available from Cosa Corp employs a direct motor drive. In 1936, Mueller-Weingarten dropped the friction drive, believing it uneconomical. To avoid high slip at the beginning of the upward stroke, the firm developed multiple-wedge-section friction rollers. These eliminated the large slip losses and reduced drive-component wear rate.
After the introduction of motors that could stand up to higher switching frequencies without impaired reliability, the firm simplified the drive, introducing gearwheels for larger machines. The gearwheel drives employ a screw without motion in the axial direction, a feature that Weingarten believes is exclusive, and which is necessary for low wear and reliable operation.
For small- and medium-sized screw presses, Weingarten developed a direct electromechanical drive that has been fitted to machines supplied since 1963. Torque from the drive motor goes directly to the screw without intermediate transmission gear, wear parts, or losses. The press can stroke every 3.4 sec, and it saves energy, because the motor is switched on only when the ram is in motion. There is no wasted idle running.
Weingarten's direct drive is enhanced by frequency-controlled electric motors. The motor and drive provide constant torque, avoiding current peaks when the motor switches on. At start of each stroke, motor voltage and frequency are low, increasing during acceleration, and resulting in measured slip at a constant 5 to 7 percent. Motor losses thus are held to a minimum.
After the forming blow and flywheel acceleration in opposite direction, the flywheel is braked during the upward stroke. While braking, the motor acts as a generator and feeds surplus return-stroke energy from the flywheel back into the power line. The mechanical brake comes into operation just before top stopping position, when the ram and flywheel are almost stationary.
On screw presses with frequency-controlled drive, energy capacity can be preset accurately over a wide range. Controls can change ram speed mid-stroke to provide preselected energy just as the die hits the workpiece, and they can tie into DNC computers. Hydraulic ejectors and locators in bed and ram can position and hold workpieces accurately for automated feed systems. They are ready for FMS.
The Model 300 Screwpres from National Machinery Co is another example of a direct-drive screw press. The stator of a polyphase induction motor fastens to the press bedframe, while the rotor, connected to the screw through a brakewheel, rotates freely. The screw, held in position between a bronze thrust bearing and top nut, is guided in the upper bedframe by a long bronze sleeve bearing. As the screw rotates, the ram is free to move axially along the thread length. The thread's nonlocking pitch won't allow the ram to stall at bottom of stroke.
A Screwpres and robot have been combined to make up a completely automatic forgin line including a two-box induction heater, automatic billet infeed mechanism, GMF robot with National beam-transfer grippers, and die-lube system with nozzles mounted on beam transfer. The Model 300 Screwpres includes bolsters, tooling, upper and lower hydraulic kickouts, and programmable controller. Its robot has five stations: one loading, three forging, and one trim.
Billets feed into a heater magazine, then through induction heating coils, followed by checkout with an optical pyrometer. The system rejects billets of incorrect temperature, placing proper billets in reach of the robot arm.
The robot picks up a billet at the feed station every other blow of the press, placing it in position for flattening, blocking, finishing, and trim. At the discharge station, the robot transfers flash to a separate chute, while the parts fall through the tooling and slide down another chute to a conveyor. Typical parts forged in this setup are 15" adjustable wrench jaws transferred in platters of two at about 450 platters/hr.
Warming up, or warming down?
The really hot news in forging is warm forming (WF). The process is a hybrid of hot and cold forming, extending the range of sizes and materials formerly handled by cold forging, but also taking work stepping down from hot forging.
Many cold-forming machines handle WF efficiently, and almost any hot-forging press could work warm, except for the economic drawback of overkill! Most conversions are from cold forming to warm forming, but some have switched from processes such as screw machining. WF creates a lot of smoke, so you need good ventilation and pollution control.
How warm? Definitions range from 100 F on up. National Machinery Co says the WF temperature range is 200 F to 1500 F, but puts the commercial range at 1000 F to 1330 F. The lower limit is blue brittleness, and the upper is recrystallization.
National provides a formal definition: Warm forming is an extension of cold forming metal that has been heated to any temperature below its recyrstallization temperature to improve its malleability. Hot forging always takes place at a higher than critical temperature (usually 2250 F), and cold forming has no heat intentionally added to the workpiece.
Horst Wilms of Siempelkamp Corp says the practical range is between 930 F and 1650 F, depending on the type of steel. Others agree, stating that microalloying can raise recyrstallization temperature above 1600 F.
"Depending on the chemical composition," says Wilms, "the blue brittle zone for steels is between 480 F and 930 F. In this range, forming is not possible because the strain rate is at its minimum and the flow stress is at its maximum. The steel is brittle. High-alloy steels and austenitic stainless steels are exceptions to this rule because they are not subject to blue brittleness. They can be formed in the range of 480 F to 930 F. Below 480 F, flow stresses are very high and any forming is cold forming. Self-heating of the workpiece may occur, however."
Horst Wilms points out several benefits of warm forging as opposed to hot forging. First, considerably lower energy is needed to heat a workpiece to 930 F to 1650 F than is needed to heat a workpiece to forging temperatures of 2000 F to 2250 F. Second, oxidation or scale formation is reduced. Third, depth of decarburization is reduced.
These factors permit forging of precisely shaped parts with close dimensional tolerances and above-average surface qualities. Because of good finish, WF can do without the energy-intensive thermal and chemical in-process treatments such as phosphate coating, removal of phosphate, and annealing now required for cold-formed parts.
Secondary benefits include reduced draft angles, reduction or elimination of flash, material savings, less machining, reduced thermal contractions and distortions, and simplification or elimination of post-forging heat treatment. Furthermore, workpieces heated in an induction heater with a controlled inert-gas atmosphere (argon or nitrogen) come out of the WF operation with virtually no scale formation or decarburization--providing precision net-shape or near-net-shape parts that require little or no machining.
Parts benefiting from WF include turbine blades, jet-engine blades, and transmission components such as bevel gears and synchronizing rings. It's hard to find WF equipment big enough to handle larger workpieces, and WF requires more expensive raw material than hot forging. All aspects of process control for WF require more detailed attention than required for hot forging, or else there may be no gain at all.
WF is practical in cases where cold forming would be penalized with excessive tool costs (greater number of forming steps), where necessary in-process annealing slows production and increases costs, and where cold forming is simply impossible because of part size, weight, geometry, and material-flow characteristics.
Some WF parameters
If 30 percent of hot-forging work were converted to warm forming, the nation would save 5 trillion Btu per year, according to one estimate. And, compared to cold forming, WF can save 40 percent in total energy consumption, and this estimate doesn't even consider the normalizing or hardening treatments often required for cold-formed parts.
User reports are less optimistic. In some cases, it is still necessary to normalize after a WF operation. Whether the WF process provides easy workability still depends on alloy content of the workpiece, and quality of the die design. Die life is supposed to be good with WF, but that's not always true. Proper die design is still vital.
Of course, the potential is there for longer WF die life coming from either direction. The process avoids the high temperature and scale of hot forging, and the severe impact to the die in cold forming. There should be lower flow stress and less fracture--if materials are chosen correctly. Lubricant, often graphite, must be chosen to match the job--and applied properly. Do you lubricate the billet as well as the die? Do you use precoated billets?
What about the dies? Carbide is three times more expensive than HSS tooling, but lasts three to ten times longer. Yet, it is a poor heat conductor, and die design and setup must account for this.
WF equipment is costly and dedicated. You might try cold forming first, if you have the equipment at hand. You might go from hot forging to warm for the sake of near-net shape, but not unless you have a lot of work for the new WF machine. Stick with hot for occasional jobs.
National Machinery makes all types of cold and hot forming equipment, and they have modified a standard three-blow, two-die header for warm forming small workpieces. This Warm Former is a complete system for making parts such as bearing races. The firm also has proved the technique with larger parts. For example, a 6-4 hot former makes a thick-walled, bottle-shaped part from 1340 bar at a rate of 70 strokes/min. Estimated production on a vertical forging press was only 10 parts/min. Larger presses serve for longer parts such as CV joints and bevel gears.
There are many other firms making forging equipment along similar lines. Verson Allsteel Press Co, for example, makes Impact Machining Systems for cold heading that, in fact, also do warm forging. A universal-joint spider is a typical product so made. Many Verson single-point eccentric gear presses also serve for WF operations.
Schuler Inc makes the Polymaster automated forming machine, suitable for both warm and hot forming. It's a multistage press that can add a fully automatic tool-change system. The machine provides up to five tool stations and operates under program control for tool setting when using the Changemaster system. Cycle times range from 32 to 130 strokes/min, handling short and long billets. The firm also makes SMG warm-extrusion presses, and these often serve with a robot manipulator under micro-processor control.
If you need information on research into tooling for warm forging, especially as it relates to parts that might have been cold formed, contact The Schurmann Machinery Co. This firm develops systems for warm-flow forming, precision size coining, die hobbing, etc. They'll help you get the most from WF equipment by using proper process development for intricate near-net-shape parts. They want to reduce your investment risk as you enter the world of advanced warm forming.
The Forging Industry Association can help you learn about forging basics as well as the state of the art. Their free booklet Forging: How--Why--Where? defines forging terms, gives tooling examples, and shows typical forging applications. The organization has just published a Forging Handbook covering primary processes, die design, materials selection, automation, CAD/CAM, and more. The 345-pg book costs $64. Their 1985-86 Forging Capability Chart is free, listing forge facilities in the US and Canada.
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|Author:||Miller, Paul C.|
|Publication:||Tooling & Production|
|Date:||Dec 1, 1985|
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