In 2001 Cadillac, General Motors Luxury Automobile and SUV division, began selecting suppliers for the 2004 SRX sport utility vehicle with an emphasis on new vehicle technology that demanded new manufacturing technologies and methods. Through this process, Novi, Michigan-based Tower Automotive was selected as a supplier of the vehicle's primary frame assemblies, the front engine cradles, and rear suspension cradles. Subsequent to its selection, the company initiated its equipment selection process. It was clear from the beginning that Tower was charged with a project that would require not only new manufacturing technologies and methods but a new way of thinking as well.
"The design of the SRX cradles, the specifications, and the sales predictions from Cadillac really drove our decision to depart from our traditional "bowling alley" assembly line approach to building frame assemblies to a flexible manufacturing cell method," says Jeff Funke, Tower's welding engineer.
That decision, in turn, drove the effort to depart from traditional hydraulic piercing for the necessary frame mounts to a laser cutting technology.
"Cadillac had very precise specifications for the suspension mounts. Hydraulic piercing was not possible in the cradle subassembly. If hydraulic piercing took place prior to the stamping process, we would have variations in the hole dimensions in the frame assembly process. Laser offered more flexibility," Funke says. "We also knew that laser cutting was going to be a challenge due to the physical constraints of the compact cradle."
Precitec's right angle cutting head before (a) and during (b) the suspension mount cutting process.
The physical constraints were compounded by the need of Tower-Lansing to have the suspension mounts cut from the inside out to eliminate rough mounting surfaces. (Outside in cuts produce dross, sworf, or expulsion that interfered with assembly and finishing processes downstream.) Given these challenges, selecting the best laser-cutting supplier quickly became a critically important decision for the success of the project.
"We needed a laser cutting supplier that would understand our unique requirements, develop and deliver the best solution for our application, and provide technical support when needed," Funke says.
After talking with laser cutting suppliers Tower selected Precitec (New Hudson, MI, and Gaggennau, Germany). Since 1971 the company has delivered laser cutting and welding technologies to OEMs worldwide.
The company emerged as the only laser cutting supplier that was willing to develop, deliver, and service the solution that we needed, when we needed it," Funke says.
In the first quarter of 2001 Tower awarded the project to Precitec (www.precitec.us) and the company began the product development process in conjunction with its technical center in Germany. Within six weeks it had developed and delivered a detailed rapid prototype cutting head in stereolithography to Tower, complete with system fittings and cable connections. The robotics integrator, Fanuc Robotics, used the prototype to verify its robotics process.
"The biggest challenge we faced in developing the cutting head was driven by Tower's material requirement for stainless steel for the body rather than the traditional use of aluminum," says Robert Borgstrom, Precitec's COO— technical director. "This required our engineers to significantly streamline the body design because of Fanuc's 9 kg weight requirement."
Once the weight hurdle was cleared cooling jackets were added to the body at Tower's request and a fully functional product for the prove-out process was delivered in early August. By early September, all finished product was delivered for pre-production trials.
The final package consisted of four Nd:YAG cutting heads for two work cells, including three right angle cutting heads and one straight cutting head with focal optics, collimating optics, a sensor circuit, and a drive motor. The cutting heads were then mounted to four Fanuc Robots and each head performs, on average, a dozen 14mm hole cuts to the cradle material, which is low carbon (mild) steel with an average thickness of 3.5 mm. The cuts are either specific to the suspension mounts or custom trim cuts to aid in the downstream assembly process.
"The decision to go with a right angle cutting head was driven by the physical constraints of the cradle design and the customer's requirement for smooth surfaces for the suspension mounts," says Cliff Ankersen of Precitec.
For this application, the cutting head nozzle acts as a sensor by measuring the field to work surface, which enables the operator to easily adjust and set the optimal focal point. This approach is unique because the focal point adjustments are made at the cutting head rather than at the robot, which makes it easier and quicker to set and fine tune.
The sensor provides analog output and input to the robot, which allows the robot to monitor any variations in stand-off. If the stand-off is beyond the system requirements for the optimal cutting distance, the robot will stop so the operator can make adjustments to the system before continuing.
"With this approach we were able to set and fine tune the stand-off distance using height sensing as a measurement tool anywhere from a range of 5mm to 1mm," Funke says. "This feature is much easier than other laser cutting systems that require stand-off adjustments be made with the robot."
The interface between the cutting head sensors and the robot also made it possible to program additional production features. For example, Funke decided during the launch process to code each cradle. Through a simple programming task, this was easily achieved, enabling Tower to trace all production by date, shift, and operator right down to the minute.
"Tracing our product by code was easily implemented with this system with a simple programming operation, which is another advantage of this new cell process and the use of Precitec's laser cutting technology," Funke says. "In a dedicated assembly line environment, a feature enhancement like that would have been more difficult and costly to implement."
Product quality is another advantage to the Tower approach. Initially they had purchased sensors for each manufacturing cell. The integrity of the laser cuts has been so good that the measurement devices were not needed.
The implications of their decision to implement a flexible manufacturing cell approach are significant to both manufacturers and the laser industry. Departing from dedicated lines and using flexible cells makes it more economical and practical to use laser technology.
The SRX rear suspension cradle emerges from Tower's laser cutting cell with suspension mounts cut to specification.
"Laser cutting and welding is more difficult to apply in fixed line applications due to safety and maintenance issues," Ankersen says. "It is much easier to integrate light tight barriers and maintain the equipment in a cell."
For the manufacturer, the cell-based approach is more cost effective, easier to maintain, and more flexible. While cells typically require more floor space and dedicated labor, these considerations are offset by the significantly higher capital costs associated with dedicated lines. From an equipment maintenance perspective, cells enable offline maintenance to one cell without shutting down all of the other cells. In a fixed line set up, the whole line shuts down if one operation needs maintenance. The added flexibility to make equipment adjustments is also a big advantage of this approach.
"If the customer comes back with a model year adjustment in their product, we can make the change much more easily than in a fixed setting," Funke says. "We can also use this equipment for other jobs in the future, which you cannot do with fixed lines. When the job is done, the line is done."
The SRX rear suspension cradle suspension mounts cut to specification.
"We are very excited with the success of this project because Tower showed a lot of confidence in our ability to deliver the right laser cutting solution for their application," Ankersen says. "It demonstrated that we are more than just a catalog laser cutting and welding head manufacturer and that we have the technical expertise to provide a unique solution for our customers and support that solution from product development through finished product and beyond."
Apparently Tower agrees because the next project for Precitec is to deliver its laser technology to Tower for a similar application for Cadillac's 2005 STS (Seville) front engine cradles and rear suspension cradles. And the bonus, the STS will launch on the same cell equipment as the SRX later this year with little or no interruption to the SRX production schedule. Try doing that on a dedicated line without laser cutting technology.
Kurt Ruecke is marketing communications associate for American Laser Management Company.