Thomas Built: Driving automation

With robotic work cells, conveyors, cranes and one extra large automatic guided vehicle, Thomas Built Buses took its assembly process back to school.

By Staff -- Modern Materials Handling, 3/1/2006

When Thomas Built Buses designed the Saf-T-Line C2 school bus, it also designed a new assembly process to manufacture the vehicle. In addition to an automated conveyor system wide enough to transport a 28-foot long bus body, overhead cranes and robotic work cells, Thomas also installed an automatic guided vehicle (Transbotics) in a $39 million, 275,000 square foot plant in High Point, N.C., to produce the new buses.

"This is the very first time we've used an AGV," says Anthony Goff, a manufacturing engineer at the new plant. "The fact that it automates a process that is still done manually in our other plant has been a big plus. We don't have to tie up an operator's time or build extra time into the operation to move the bus body between operations."

The first Thomas bus rolled off a manufacturing line in 1936. Today, the company is the leading manufacturer of school buses in North America, with a 37% share of the market.

When work began on the C2 model, Thomas set out to redefine the conventional school bus. The result was a vehicle designed to improve the durability, reliability, safety and life cycle cost of school buses.

Thomas incorporated specially designed adhesives on body joints combined with self-piercing rivets to increase strength and durability. The combination results in fewer fasteners for a sleeker look and fewer potential leaks.

To manufacture the C2, the company constructed its new facility with a ¾-mile long assembly line and 75 workstations. An electronically controlled conveyor system moves the buses through assembly while overhead cranes do the heavy lifting on the bus bodies. Robots have also been incorporated into the assembly and paint processes.

An AGV provides a flexible automated link between processes, delivering a bus body from the end of the trim line to one of two stations in the mounting area, where it is joined to the chassis. The duty cycle is about eight minutes. At the current rate of production, the AGV moves one load every hour. At full run rate, it will move a load every 20 minutes.

To accommodate a school bus body, the AGV measures 28 feet long and eight feet wide. Its deck sits two feet off the ground. A chain conveyor on the vehicle bed accepts the bus from a 28-foot wide assembly-line chain conveyor. Bus bodies are loaded from the side.

Finally, as a member of the Freightliner group, which is owned by DaimlerChrysler, Thomas incorporated lean, just-in-time assembly processes. To minimize inventory at the line, materials are delivered on custom carts to specific assembly points.

The facility currently produces eight buses during one shift. At full capacity, 192 employees will produce 22 buses per shift.

Using an AGV was a new concept for Thomas. In its other plant, a bus body is built on a cart that is pushed from station to station.

"That's why this is such an improvement," Goff says. "It can take up to four workers to push the body through the manufacturing process."

Thomas considered other automated solutions, but an AGV was the most flexible for the operation. "In this plant, we are able to drive the chassis into place so that a bus body can be mounted in place," says Goff. "That meant we couldn't have conveyor because you'd have to drive over top of it."

With conveyor out of the question, Thomas considered overhead materials handling, since cranes are used in other operations. However, the distance between the two lines seemed prohibitive for moving a large bus body.

The AGV seemed the best fit. "Since it uses laser navigation, there is no impediment in the floor," says Goff. "And because it's an automatic move, no manpower is required."

Now that the plant is up and running, Goff says the major hurdle was transitioning from traditional manufacturing processes to more modern automated processes.

The biggest piece of the learning curve was trusting that the automation would do its job when it was supposed to do it, Goff says.