Chrysler Implements Flexible Manufacturing
The Chrysler Group has retooled its Sterling Heights Assembly Plant (SHAP) to build multiple models on one production line
By Bruce Morey
Challenged by an increasingly niche-oriented automotive market, The Chrysler Group (Auburn Hills, MI) must increase the number of models it offers while decreasing its capital investment. The company plans to offer 50% more models in 2009 compared to 2004, according to John Felice, VP of manufacturing, technology and global enterprise for Chrysler. More importantly, they recognize that a market with unpredictable demand requires a rapid manufacturing response.
Surprisingly, Chrysler believes the key to flexibility lies in standardization. Intended as a model for all future plant upgrades, the Flexible Manufacturing System (FMS) relies in part on a standardized bill-of-processes (BoP) and, most importantly, a standard body-shop design that relies heavily on modern robotics. Since the rollout of Chrysler Group's FMS in 2000, it has been the template for their plant modernization. The Sterling Heights Assembly Plant, Sterling Stamping, and the Belvidere Assembly Plant have pioneered this strategy.
The FMS Bill-of-Process (BoP) provides a common manufacturing system for all of Chrysler's plants that's intended to drive manufacturing quality and efficient product design. Standardization includes common operational sequences, dimensional strategy, and guidelines for machines and tooling.
"We have the same key steps to build the car, to build the subassemblies, and the same insertion points on the car," says Dave Taylor, advanced manufacturing engineering process manager for the Chrysler Group, explaining the FMS BoP. "If you were to go into either the Belvidere assembly plant or the Sterling Heights Assembly Plant, they would look very similar."
To design a flexible body-shop model, another key element of FMS, Chrysler looked to robots. A body shop turns stamped panels into body-in-white shells, ready to integrate with powertrain, seats, and other components to complete the car. Previously, vehiclespecific tooling assembled the panels and welded them to tight tolerances. When you're only producing a single model, such tooling makes changeovers to new models expensive. The investment is only worthwhile for long production runs.
Since the 1990s, robots have increased their lift capability to a maximum capability of 700 kg, while the price of the type of robot used in automotive work has dropped by 30%, according to Chrysler. Other inventions, such as the special geometry-setting tool called a GEO end-effector, have also made robots a key enabler in flex body shops, according to Neil Willetts, a vice president for Comau Inc. (Southfield, MI), the body shop integrator for SHAP. "Only in the last few years has the GEO end effector become more accepted for mass application," he says.
"Perhaps the most important trend is the perception by customers that robots are less risky, accompanied by the development of features such as our company's breakaway technology, which protects the end effector from serious damage in the event of a collision," says Willetts.
Dave Taylor of Chrysler believes the costs of installing a robotic, flexible body shop are comparable to those associated with a more traditional lift-and-carry system. "The real cost savings comes in future models. We are putting future models in [flexible robotic plants] for 10% of the initial investment [in robotic equipment], and 10% of what it used to cost us to remove the old model's equipment. You only have to buy the end-of-arm tooling and the end-effector."
ABB Robotics (Auburn Hills, MI) provided the robots, the majority of which were their models IRB 4400, IRB 6600, and IRB 7600. According to ABB Robotics, the robots perform spot welding with servocontrolled spot guns, stud welding, material handling, wheel alignment, hemming, dispense sealants, and use vision systems to enable them to unrack parts and subassemblies.
"All spot-weld guns used at SHAP are electrical servo guns and are coordinated with the robot as a seventh axis by an S4Cplus robot controller," says Mike Calardo director, field operations ABB Inc. "Spot welding uses a mid-frequency DC [MFDC] power source control that is programmed and monitored from the robot control teach pendant. The combination of servocontrolled spot guns and MFDC welding provides faster cycle times and higher-quality welds."
Twenty-six SHAP robots use vision systems. The vision systems locate parts and coordinate robot motion for unracking material and parts. They recognize what the end-effector needs to do to pick up the part and place it correctly. Chrysler is investigating the expansion of its use of vision systems in flexible plant applications.
Other advanced features include a robotic roller-hemming process used on doors, hoods, and rear deck lids, rather than fixed tooling, according to Calardo. Located at operator load stations, state-of-the-art safety light screens networked to safety PLC controls allow the operator to load parts directly onto a fixture carried by a robot.
While robots might be key to a flexible body shop, integrating them efficiently is also important in creating an effective body shop, according to Comau's Willetts. Five years ago, Comau conducted a benchmarking study in Europe and North America. What they found was that a purely robotic solution is not always the optimum design for a flexible body shop, according to Willetts. "You can use a lot of robots, but it's not always efficient."
"For example, on the body-side assembly you may need as many as five geometry-setting stations where you set the relationship of one assembly to another assembly. Using robots to weld, handle material, and exchange tools and fixtures makes for a complex system. At SHAP we used robots to weld and move material between stations, and used a flexible, indexing tool tray to move tools to the welding robots," says Willets. Boasting a weight capacity of up to 2500 lb (1135 kg), the tool tray provided by Comau indexes in five sec.
Although subassemblies are picked and transferred with robots, a pallet system is used in the framing line for station-to-station transfers. The pallet system supplied by Comau is a carrier that transfers the underbody, which then becomes a body-shell as it progresses through the body shop. Speed of transfer is vital. The Comau Geo-Pallet system used at SHAP transfers a fully loaded pallet 22' (6.7 m) in less than 5.5 sec. "When a part is moving it's not being welded," notes Willetts, emphasizing the need for fast transfers.
The pallet system at SHAP is equipped with a closed-loop system provided by Comau. That closed-loop system uses pallet and carrier IDs to replace barcode readers, reducing cycle time by approximately 15–20% over conventional open-loop control systems. Other advantages include eliminating a pallet lifter needed in other systems for respot welding, as well as providing a common transfer time for both light and heavy product pallets. This improves the reliability of the system, as there are no switches or lifter mechanisms required.
Comau learned some lessons in building a highly robotic body shop. "It took us longer than we estimated to develop the robot programs for all the robot hand-off systems. The combinations of sequences required to handle multiple models with combinations of fixture handling, welding, and material-handling functions creates thousands of programs. They must all be validated to ensure scenarios can't happen that might cause collisions," says Willetts. "As you introduce a new model, it's not just about programming for the new model, it's also making sure that the programs for the existing models are not changed. Each time you add a new model you have to be aware of the existing models. This situation results in four times more programming than one would do with a simple linear system."
For Chrysler's strategy to be viable, their manufacturing plants must minimize the cost of changeover. "The challenge was to create a 'zero-downtime' changeover," explains Chrysler's Taylor. The robotic flex factory concept allows for parallel work efforts during model changeover. "The modules of work are small enough, and the robots flexible enough, that I can program a robot for a fourth model and still let it run the first three models." The target is to changeover SHAP for a new model within a two-week window while not interrupting current model production.
To realize the vision of FMS, an information system to ensure the right parts go on each model is just as important to a flexible factory as robots. The complexity can be impressive. "Combinations on the single flexible line now include left hand vs. right hand drives, sunroofs, convertibles, and two doors versus four doors," explains Taylor.
"To solve this problem we developed a scheduling sequencing system—a sophisticated kanban pull system." The software links data from machine controllers, including individual PLCs, so that as subassemblies are created they have a particular car associated with it. This software system forms the basis of all flexible plants built on the FMS model.
The Chrysler Group started building this kanban system two years in advance of the launch, reviewing scenarios where the system could get out of sequence, and ensuring that the integration system accounted for those scenarios.
The FMS strategy for flexible manufacturing is well on its way at Chrysler. "We've converted Belvidere, SHAP, and Brampton," says Frank Ewasyshyn, executive vice president—manufacturing for the Chrysler Group. "We are in the process of converting Windsor, St. Louis South, and St. Louis North.
We are getting close to being fully flexible."
Lean Transformation at SHAP
Chrysler and the United Auto Workers established a new organizational model for SHAP while launching the Sebring convertible and upgrading the facility for flexible manufacturing. Building on some of the Lean Manufacturing principles already in place at SHAP, the new model—dubbed 'Smart Manufacturing'—incorporates key missing elements, in particular a team structure for line workers.
"It's a huge change for the operation, it's a big change for management, it's a big change for the folks on the line," says Frank Ewasyshyn, executive vice president–manufacturing for the Chrysler Group. The team model has long been a hallmark of lean manufacturing systems, a fact alluded to by Ewasyshyn. Its introduction at SHAP seems to have unlocked some latent energy in the workers.
"Before, we would have one job to do," explains Patrice Lance, a Team Leader in closures assembly at SHAP. "Now we have seven jobs that each of us on the team rotate through." This team concept requires a higher level of expertise on the part of each worker. The higher level of expertise provides the facility with the flexibility required to respond to peaks in demand or unexpected absences, improving throughput.
Besides using teams, the new approach at SHAP uses more visible metrics and introduced problem boards. Metrics that measure Safety, Quality, Delivery, Cost, and Morale (SQDCM) are displayed prominently and discussed by each team in 5-min meetings. "Problem boards make sure that you get the attention you need," says Ron Hicks, a UAW launch leader. According to Hicks, even vice chairman and president Thomas W. LaSorda, who was then CEO, visited the problem board at one point in the launch preparations for the Sebring convertible. "Without this problem board, we would still be struggling," he states.
What was the outcome after all this work? "This was one of the smoothest launches we ever had," says Dave Taylor, advanced manufacturing engineering process manager for the Chrysler Group, referring to the Sebring convertible launch announced on June 4.
Karmann Spider Fixture
The Chrysler Sebring convertible uses a three-piece architecture for its convertible top. While offering many advantages, such as conserving space, the architecture makes it particularly difficult to assemble with a quality fit.
"This is a complex and sensitive system of parts. While meeting individual tolerances might not be difficult, we need to control the tolerance stackup," says Hendrik Loetter, vice president, Karmann USA (Plymouth, MI), the convertible-top supplier to Chrysler. He notes that there are approximately 400 parts for the convertible top, with a production target of up to 50,000 units per year. A solution had to be fast and repeatable.
Enter the Karmann Spider Fixture. "The Spider Fixture is an integration tool between Karmann and Chrysler," says Volker Rodeck, vice president and director–manufacturing for Karmann Manufacturing LLC (Plymouth, MI). "We use it to control the dimensions at the interface points between the convertible top and the car body." There is one Spider Fixture at the convertible top manufacturing facility and a mirror image of that fixture at the Chrysler SHAP facility. Lowered onto the build fixture, the Spider Fixture at the Karmann facility sets the parts to their nominal accuracy while they are fastened into place. The fixture maintains rigidity over the span of the assembly while the parts are fastened, thus controlling tolerance stackup. Karmann built the fixtures used at both the Karmann facility and Chrysler, including all software.
Workers use the mirror-image Spider Fixture at SHAP as a master gage, ensuring that the body interface is in tolerance before mating the convertible tops. Lasers measure individual points with a precision of 0.001 mm to ensure the fixture tolerance is within ±0.2 mm. It gives the line workers red/green indications. If indicators are red, workers can make adjustments on the body shell to ensure that the glass, main pivot bracket, and the floating plate interfaces are in tolerance before mating with the top. When adjustments have brought these interfaces into tolerance the indicators on the fixture turn green.
"The Spider Fixture here at SHAP is a 'master' top," explains Ron Hicks, a UAW launch leader at SHAP. "At the next station we put on the actual top.
"We don't have a stop station. This car goes from the beginning to end with no stops," says Hicks. Another feature of the fixture is that it stores data for up to six months, so that Chrysler engineers can review the data and adjust processes and tolerances as necessary.
This article was first published in the September 2007 edition of Manufacturing Engineering magazine.