Ultrahigh-pressure NC press cold bends heavy plate in one pass.
Many of the plant's metalcutting machines are computer controlled. We have been able to apply this concept to bending heavy carbon-steel and stainless-steel plate as well. The result is the only programmable, computer-controlled vertical press for bending heavy plate in the U.S. Figure 1.
This ASEA-Carbox press, a product of ASEA Pressure Systems Inc, Columbus, OH, is used to form cylindrical shells for reactor vessels from plate up to 5" thick. Shell heights may be up to 12 ft.
All forming is done cold, with the plate in the as-received condition except for cutting to size. Typically, radical deviation of the formed shell is less than 1/8".
The press forms plate vertically. The machine's structure, Figure 2, consists of two massive vertical beams that fit within two horizontal press frames. Each frame is wound with miles of high-tensile-strength steel wire, which exerts such high compressive stresses that even when the machine is operating at maximum rated tonnage, the frame is still under compression. The machine is thus able to contain forming forces safely, without danger of fatigue failure under repeated cycling.
Other benefits of the wire-wound concept are compact design, lightweight, and low foundation costs. The lower frame fits snugly into a 5-ft-deep pit. There is no need for access below floor level after the press is installed. The upper frame swings out of the way for loading and unloading workpieces by crane. How it works
Only three tools are required for bending the plate, Figure 3. A fixed tool with a suitable radius is mounted in the inner beam. This tool serves as a bending mandrel. It doesn't move. The other two tools are mounted on a moving tool block actuated by 12 hydraulic cylinders mounted on the outer beam of the press. The spacing between these two tools is adjustable.
During the bending operation, the moving tools are fed in to bend the plate around the fixed tool. Forming is done in a series of steps as the plate advances in increments of 4" to 6". Generally, the closer the steps, the more accurate the radius of curvature.
The same set of tools can be used for shells having different plate thicknesses and diameters. Diameter depends on forming depth and feed distance between steps. Required forming force depends on parameters such as plate thickness and yield strength.
A powered set of rollers feeds the plate into position for each forming step. The lower edges of the incoming plate and the partially formed shell are mounted on "roller skates" so they glide smoothly across the floor as forming proceeds. Since the weight of the workpeice is always on the floor, it doesn't influence the operation. The feed rollers eliminate the need for a crane during forming.
Forming is accomplished by applying force slowly (usually over a 2-min cycle) and evenly, allowing the material time to flow. To achieve uniform curvature of the shell and symmetrical loads in the press, a method called Tango bending is used. Here, steps are formed in modified numerical sequence. After forming Step 1, for example, Step 2 is skipped and Step 3 is formed. Feed then is reversed and Step 2 is formed. One backward step is taken for every three forward steps.
When half of the shell has been formed, the remaining straight portion is fed through the press and the direction of feed is reversed. Forming then proceeds (from the opposite end) until the shell is completely shaped. The accuracy of the operation ensures that the two ends of the plate meet within a radial tolerance of 0.03".
The shell is then removed from the press and the two ends are joined by arc welding (both inside and outside). Next, the workpiece is returned to the press for reforming of the weld zone. This is accomplished in two or three steps.
Shells are stacked and welded to each other to produce the reactor vessels, Figure 4. Of course, high accuracy is essential for good fit up prior to welding. Accuracy of shells produced on the vertical press in a single series of forming steps is more than adequate to meet requirements. This is in sharp contrast to roll bending, an alternate process, which generally requires five or six passes to attain the required roundness. Considerable operator experience and skill are necessary in roll bending, particularly when compensating for springback.
The fact that bending is done cold also helps to improve accuracy. When the material must be heated -- the case when forming heavy plate on conventional plate-forming presses and press brakes -- distortion may occur upon cooling and the metallurgical properties of the workpiece may be adversely affected. Heating also consumes large amounts of energy, increasing shell-forming costs. It may be necessary to reheat the plate several times, which reduces productivity of conventional bending. Again, a high degree of operator experience and skill is required to form accurate shells.
The accuracy of the press is attributable to its extremely rigid design and to the fact that forming forces are uniformly distributed by its 12 hydraulic forming cylinders. Because the upper and lower frames are wire wound in compression, and because of the inherent rigidity of the vertical beams, deflections are minimal, even under the full 5500-ton forming force. The uniform distribution of forces by the cylinders ensures that the shell's wall will be straight, thus precluding hourglass or barrel shapes. PC press programming
Programming can be accomplished on the console of the press; however, for convenience, programming is usually handled on a personal computer (PC) in the engineering office. Programming is in the conversational mode. Initially, the PC asks the programmer for the diameter and length of the required shell, the plate's thickness, and the yield strength, tensile strength, and percentage of elongation for the material.
Given this input, the PC makes all the calculations necessary to form the shell with a particular set of tools. It then prints full operator instructions and (if desired) a control tape.
One of the major calculations is the forming depth (i.e., forming stroke length) required to produce an accurate shell, which includes compensation for springback. The press is programmed to overform the plate by a calculated amount based on the work material's mechanical characteristics. After forming, the plate springs back to the correct curvature.
Position transducers in the tools measure curvature at three points during the forming cycle. When correct curvature is attained (including compensation for springback), a feedback system stops the operation automatically. If the measured curvature isn't within tolerance limits, a second stroke at that point is initiated by the press control. Resolution of the transducers is 0.0004". The forming stroke stops within 0.004".
Another feature that enhances press accuracy is that the two moving dies are always parallel, thereby maintaining uniform forming pressure. Linear position transducers measure the position of the tool block continuously during the forming operation. If tilt is detected, a feedback system adjusts the pressure exerted by individual actuating cylinders. Parallelism of the two dies is automatically maintained to within 0.004" or better.
Accuracy of plate feeding from step to step is measured by a digital shaft encoder driven by a measuring wheel that rolls on the plate. To make doubly sure that feeding is accurate, we lay out the plate in advance of forming. Numbered chalk marks on the workpiece indicate the correct position for each forming step. The operator controls the feed rolls manually to inch the plate into correct position. For fully automatic operation, a photocell could be used to detect the chalk marks.
Although we've successfully operated the press under NC, it usually operates in a semiautomatic mode. The operator follows a program printed out by the PC when setting up the job, transferring information to the press console. Forming and feeding are fully automatic, but each forming step is initiated by the operator.
This mode is fast and efficient. A 120"dia shell requiring 30 steps, for example, can be formed in about 1 hr.
Since correct curvature is automatically attained, there's no need for measurements by the operator. His primary role is to monitor the operation, interceding only when necessary. If a physical check of the radius is necessary, however, it can be quickly accomplished with a template.
While the high-pressure press was purchased primarily to form shells for nuclear reactors, we are now using it for other jobs, including forming thermal shields and steam-generator and heat-exchanger shells. In addition to forming heavy carbon-steel and stainless-steel plate, the press is used to form HY-80 armor plate. Tooling and handling equipment have been added to form relatively thin (1/4" thick) plate, too.
Since we are broadening the product mix at the Pensacola plant, other capabilities of the machine may eventually be utilized. These include forming cones and forming beams with square sections.
For more information from ASEA about high-pressure forming technologies, circle E33.
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|Author:||Steiger, John R.|
|Publication:||Tooling & Production|
|Date:||May 1, 1984|
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