Bend without braking

Modern Machine Shop, May, 1992

Bending sheet metal into useful shapes without hard tools or dies sounds fantastic, if not futuristic. But, it is being done successfully by the Institute of Fundamental Technological Research at the Polish Academy of Sciences. Representatives of the Institute showed how, at the recent Precision Metalforming Association (PMA) show, in Nashville, Tennessee.

On display in their small booth were a number of rather elaborately formed workpieces. All of them were made using a process that forms sheet metal by applying precisely controlled heating and cooling to the surface of the workpiece. That's right, no rollers, dies, press brakes, tools or other mechanical forming method was used to Make their samples.

The two processes - heating followed by cooling - cause a push and pull within the structure of the sheet metal, permanently changing its shape. Using data gathered by experiment, the Institute's researchers can predict the amount of bend resulting from this heating and cooling. The information becomes the basis for a computer program. The programmed computer can, in turn, direct a machine to reproduce the desired shape in sheet metal - with no mechanical contact with the workpiece.

Of course, the movement of heated sheet metal is not a new phenomenon. Welding often produces deformation, usually unwanted, in the workpiece. And in practice, welders will cool the area around the weld with damp cloths or the like, to reduce the amount of warpage.

Likewise, in this forming process, the workpiece is repeatedly heated and cooled to produce what is, in essence, controlled warpage. The Institute's unique contribution is the degree of predictability and precise control they bring to an otherwise random process.

One big advantage of this process is the ability to produce short runs, one-offs or prototypes - when time does not permit building dies or tooling. Any process that reduces response time is valuable, as shops must search for better ways to respond to the demands of Just-In-Time (JIT) production schedules. Also important is this method's effectiveness on difficult-to-form metals. Brittle and hard metals, which resist traditional bending techniques, respond to this new method, says the institute.

In operation, a laser generates the heat. The Institute's prototype machine uses a standard VFA 2500 Photon Source laser that works best on sheet metal below eight millimeters thick. But the continuous addition of materials to the Institute's database, and their variety, creates the need for a more flexible laser source - one that generates a wider range of energy beams, perhaps from 0.5 to 20 kilowatts and more.

A programmed laser head directs the laser beam, enabling it to follow workpiece contours. A two-axis table and attached positioning unit maneuver the workpiece. Under computer control, these components move the workpiece back and forth beneath the laser beam and coolant nozzles.

The cooling device chills the heated sheet metal with water or a cold gas. It uses four nozzles to direct the coolant as it is applied behind the laser. Control, for the rate of flow of the individual coolant nozzles and the critical depth of chill, comes from the computer program.

Because the heating and cooling of the workpiece is so short in duration, little or no change in the structure of the metal occurs using the laser bending process. The Institute claims this method successfully bent even brittle materials, like cast iron, molybdenum alloys and others, a result not possible with traditional forming technologies.

The process is monitored constantly, using two separate systems. The first measures and records the temperature readings from four locations on the workpiece. The second system takes measurements of the shaped workpiece and compares the results with the expected preprogrammed values. Together, these monitors provide data on the process, in real-time, enabling the computer to make corrections as needed. The level of control afforded by the monitoring systems achieves a high degree of accuracy and repeatability.

The Institute continues to accumulate data on various metals, thicknesses, optimum laser power requirements and other parameters of the process. So far, databased programs for processing several kinds of steel, ranging in thickness from two to eight millimeters and up to one and a half meters in length for prismatic profiles are available for execution.

In addition to straight-line bending, production of other so-called developable shapes can start with this process. Cones, tubes, collars, necks and other shapes may all be formed using the laser-bending process.

A successful laser bending prototype machine is operated by the Institute. The machine is made entirely of commercially available components, with the exception of the cooling and monitoring devices. They were custom designed and built. Of course, the software is also custom.

The process, as yet unnamed, works well in the lab. So well in fact, that the Institute sent representatives here to promote the technology, hoping to drum up interest in commercial development of the process. They took warpage, a normally undesirable and unpredictable byproduct of heating metal, applied it and controlled it, creating what appears to be a viable new metal forming process.