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  VW laser cuts hydroformed parts
Posted by: Börries Burkhardt on Saturday, March 23, 2002 - 09:53 AM
 
 
All Topics Hydroforming creates hollow metal structural parts from a tubular element that is three-dimensionally shaped inside a mold by fluid under pressure. Parts have very high stiffness, tensile strength properties, and structural integrity. The number of moldings required is reduced, and one part replaces several. Parts also are lighter and have less distortion problems, mainly because is no need for a lot of welds (mostly MIG). Engineers can also hydroform a variety of metals using the same mold.

The process has some weaknesses, though. Existing parts must be redesigned to make hydroforming acceptable, because many pieces have dimensions beyond the capacity of the present-day presses. Investments may be needed for the hydroforming presses, because their structural features differ from those of conventional presses. Moreover, preliminary bending of components is often necessary, which can lead to a reduction in the thickness of the tube wall in the area forming the outer radius of the bend.

Despite these disadvantages, the hydroforming technique is rapidly becoming more widespread in the automobile industry for the manufacture of load-bearing structural elements in cars, trucks and buses. Laser processing is entering the picture because the hydroformed tubes must be trimmed after shaping to remove the ends used to enable the hydroforming operations and to add openings in the tube to allow for various couplings with the component.

The laser's cutting edge

In one of the first laser-cutting applications of hydroformed pieces discussed at the ALAW 97 meeting (Novi, MI), Utilase (Detroit, MI) reported on the results of systems installed at Formet Ind. (St. Thomas, Ont., Canada) to cut components that form part of Chrysler truck structures. Two laser robots in cells operate simultaneously to perform face cuts and create the openings for subsequent couplings. Of concern with these cells was robot precision--amplified by inaccuracy in the positioning of the pieces on the benches and by the maintenance of the initial conditions during the parts' slow motion along the bench. Engineers also encountered difficulties related to the limited life duration of the optic fibers that carried the CW Nd:YAG beams to the working point.



A CO2 laser alternative to the above approach was shown in Germany at EuroBlech, where Prima Industrie (Torino, Italy) was processing three hydroformed structural parts for the Volkswagen Golf 4x4 automobile.

The split-cabin option

"With the introduction of the 'split cabin' version of our Rapido," said Alberto Delle Piane, Prima commercial manager, "we can now deal with large-scale production in niches of the mass production process, such as this application. In it, the cycle time provides for three pieces in two minutes, i.e. 90 pieces/hour or 700 pieces/shift. Other benefits include division of the volume and of the work, as well as equipment that allows loading/ unloading operations while the other process continues. These allow us to demonstrate how laser cutting of three-dimensional bodies is no longer just a prototyping tool, but a production tool--albeit still in niche applications with limited production volumes."

The working volume of the Rapido system is divided into two halves. In this arrangement, while the machine is working in total safety in a completely enclosed area, the other area can be loaded and unloaded. During the subsequent cycle, the roles of the two areas are reversed. To switch from one side to the other, the cross beam and cutting head travel by means of an open cantilever structure over a separating plate situated between the two sections. In less than 5 s, the cutting head can begin processing on the piece that the operator has just placed in position, while he unloads the part just cut.

At Volkswagen

Prima Industrie will deliver a split-cabin laser system to Volkswagen's plant in Braunschweig, Germany, where the hydroforming presses are already in operation. The equipment has two fixed benches that hold three supporting elements. It is equipped with a PRC (Landing, NJ) 2.2-kW CO2 laser source.

To enlarge click on the image

Processing involves trimming two ends of each piece and creating holes that couple the part with other structural components in the assembly linking the differential gearbox with the suspension block on the two drive wheels. Every piece is processed in one cycle without repositioning using a fixturing device. Part loading and unloading are done by hand. The important features of the various processing operations are high productivity (90 pieces/hour), high accuracy and repeatability, no problems with accessing certain areas, and easy removal of scrap.

At VW, some components the system will process include the primary supporting bar, two components called the wishbones, and a secondary supporting bar. To process the primary supporting bar, the two ends are trimmed and five holes (all on the same side) are created. The part is positioned with ends pointing down so scrap can drop easily into a box for collection. This upside-down position requires the characteristic penetration and dynamic features of the Rapido cutting head. Small-scale scrap is cut into two parts to avoid any remaining material from being trapped inside the component. This processing sequence takes 66 s.

Processing the wishbone involves the trimming of the two ends. The fixing assembly supports two pieces that are easily locked into the right position by means of spring devices. This allows cutting both ends without moving the piece. These components also have a 66-s processing time.

To laser cut the secondary bar, it is necessary to trim the two ends and make seven holes. The piece is positioned with the ends facing upward because there is no longer any risk of the cut parts getting stuck in the rest of the component. It is then tilted through 30° to allow cutting of a small hole situated on the bottom of the piece. The cycle time is 60 s. Because there are two wishbones and only one each of the other two components on each auto, the processing of this secondary bar is done in the same area as the primary bar, after the necessary changes to the fixing unit.

Article published at hydroforming.net with permission by Antoni Vendramini (e-mail: antonio_vendramini@iol.it) and Prima Industries:

 
 
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