A unique fixture with moveable tooling makes it possible to weld a complex truck seat frame in a single operation, eliminating a huge expenditure in capital equipment and reducing labor costs as well. Normally the frame components would be welded into subassemblies in a series of preliminary operations prior to final assembly. This approach requires multiple fixtures, multiple robot pairs, and an operator for each operation. Performing the welding as a single operation eliminates the preliminary steps, but greatly complicates the one fixture that is used. Designers at Stryver Manufacturing realized that the only way to make this seat frame fixture work was to make a large tooling package moveable, sliding it into place for welding and then out of the way for part removal. To ensure precise positioning of the tooling, designers chose crossed roller slides over conventional ball roller slides because the former are specially configured to provide a high degree of stability. This stability ensures even wear and low maintenance, and enables the crossed roller slides to achieve the 0.5 mm true positioning required for this operation.
Stryver Manufacturing, Inc. originally began supplying welding fixtures at the request of a robotic welding manufacturer. Over time, the fixture portion of Stryver's business grew and Stryver also began furnishing its customers complete, integrated, turnkey robotic systems. Stryver was hired to design and build a welding system for the Ford F150 truck seat by Johnson Controls, a tier one supplier that will be producing the seats for this vehicle. Johnson Controls will manufacture the entire seat assembly including the frame, all the motorized components such as the lumbar support and adjustable arm and head rests, the side air bags, and so on. All Ford will have to do is drop the seat assembly onto the studs sticking through the floorboard, fasten the seat to the vehicle, and make the electrical connections through snap connectors.
Complexity drives complexity
Johnson Controls asked Stryver to produce two welding fixtures, one for the metal frame of the seat back and one for the metal frame of the lower portion of the seat. These frames are highly complex because the seats themselves are so complex, having many of the "extras" previously found only in passenger car seats. The frames, therefore, must include additional features such as the attachment points for all the internal mechanisms. The frame for the seat back alone consists of 17 separate parts. The complexity of the seat frames translates into complexity in the welding fixtures because of the manufacturing process that Johnson Controls prefers to use. Rather than joining some parts into subassemblies and then bringing the subassemblies together into a final assembly, Johnson Controls prefers to put all the parts together at once. This approach reduces costs because instead of multiple operations with multiple fixtures, multiple robot pairs, and an operator for each station, there is only one fixture, one set of robots, and one operator. That one fixture becomes very complicated, however. In the case of the seat back frame, for instance, the fixture must hold all of the 17 parts in position while they are welded together.
Stryver faced several additional challenges in the design of these fixtures. Some were related to the fact that Ford had decided to attach the pulley for the seat belt to the seat itself rather than on the pillar behind the front door. From its anchor point on the floor, the seat belt comes up the rear of the seat and then goes over the top of occupant's shoulder. The automaker did this to improve access to the foldout seats in back, but doing so added certain requirements to frame since it is now part of the safety restraint system. In previous trucks that had the seat belt pulley mounted on the body pillar, the restraint system relied on the stability of the pillar, one of the reinforcements for the roll cage. In the new truck, the seat frame has to provide that stability. Also, in an accident, the pivot point made by the pulley at the top of the new seat will tend to pull the seat down toward the floor. To counter that, the frame includes five additional irregularly shaped metal stampings. Their odd shapes make them difficult to fixture. Also, the fact that the frame is now a safety component complicates the fixture even further by requiring very tight dimensional controls and more checks than normal. For example, Stryver was asked to incorporate sensors to ensure that parts are in position, clamps are closed, and so on.
One additional challenge came from the unique attachment of the headrest assembly to the back of the seat. Normally the headrest is mounted on rods that fit into mating holes within the frame. On this seat, there was an elaborate stamping in the seat back containing holes with flanges. The head rest rods fit into these holes. The stamping needed to be positioned very accurately so that its holes would line up with the rods, but the stamping didn't have easily located features that would assist in the positioning. Also, this stamping was quite large. Just getting into place within the fixture required jockeying it around other parts and moving the tooling out of the way. That presented Stryver with the challenge of sliding the tooling back into the precise position for the welding operation once the stamping was in place.
Rollers for stability
Designers first considered slides with ball bearing rollers to move the tooling but ruled them out for several reasons. These rollers typically use a set of four hardened and ground shafts that surround the balls at four points. The distance between the shafts is relatively narrow, however, and the designers wanted something that was wider for more stability. They also knew that these rollers experience the point contact of balls. Although the load of the tooling package was only 20 pounds, the loading came from different directions due to the odd shape of the stamping. The designers were concerned that ball rollers would not hold up to the heavy, constant use that Johnson Controls would subject them to in this fixture.
After evaluating alternatives, the designers opted to use crossed roller slides from Del-Tron Precision, Inc., Bethel, Connecticut. Crossed roller slides physically resemble ball slides except for the bearing design. Specifically, each slide is comprised of an aluminum carriage straddling an aluminum base. Using a bearing system containing cylindrical steel rollers, the carriage glides, almost friction-free, over the base. The rollers, alternately crisscrossed with each other, move between a set of four, partially flat, parallel, smooth rods on each side of the base. The rollers share a larger contact surface with the rods as compared to the point contact of steel balls. This bearing design allows crossed roller slides to carry larger loads and absorb greater load impacts than equivalent size ball slides. Also, they are configured to have a diameter greater than their length, allowing them to lie at 45 degrees to each other and provide the same load capacity in any direction or orientation.
The Del-Tron crossed roller slide they chose, which is four inches long, has rollers that are spaced three inches apart. This is a full inch wider than ball rollers. Between this configuration and the larger contact surface, the crossed rollers are able to withstand the uneven loading of this welding operation. In addition, the carriage and base is made of aluminum while hardened steel rods and rollers and stainless steel end caps are used. These materials, along with the inherent stability of the crossed roller design, should allow the rollers to perform repeatedly with little wear and low maintenance. At rated load capacity and moderate speeds, expected life is 10 million inches of travel. The expected life at one half the rated load is 100 million inches.
By going with crossed roller slides, Stryver also got the accuracy needed for precise positioning of the tooling package. Johnson Controls had specified 0.05 mm true positioning, with a tolerance of 0.020 mm. The crossed roller slides themselves provide straight-line accuracy of 0.0001 inch per inch of travel when measuring the line of travel to a master straight edge, using a gauge or indicator mounted on the slide. Positional repeatability of the slides is 0.0001 inch. The coefficient of friction of these devices is only 0.003.
Installing the crossed roller slides into the welding fixture was simple. Counter bored holes in the base permitted quick attachment to the assembly. The tooling components attached to the carriage in the existing threaded mounting holes. Stryver recently delivered the completed fixtures to Johnson Controls, which will soon begin using them to mass-produce the F150 seats. Preliminary tests by Stryver indicate that the crossed roller slides are extremely stable. This stability, combined with and Del-Tron's reputation for quality products, should enable the slides to perform reliably for many years in this application.
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