Automation: It's elementary

With robotic work cells, conveyors, cranes and an AGV, Thomas Built Buses changed its assembly process to build a new school bus design.

By Bob Trebilcock, Editor at Large -- Modern Materials Handling, 6/1/2005

When Thomas Built Buses, a member of the Freightliner LLC group, 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 starting last fall.

"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. That's freeing up our operators for more valuable work."

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 37% share of the school bus market.

When work began on the C2 model, Thomas set out to redefine the conventional school bus. The result was a new 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 manufacturing facility with a ¾-mile long assembly line and 75 workstations. An electronically controlled conveyor system moves the buses through the assembly line 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 that can measure up to 35 feet long 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 8 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 8 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.

"The AGV can move forward and backward," says Goff. "Or it can stay in one position and do a 360-degree turn."

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

The facility is currently producing 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, on the other hand, 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. When it receives a body, the AGV knows exactly where to take it."

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

"Robots and the AGV were new to Thomas Built," Goff says. "Our operators came here from our other plant, so they had a learning curve."

The biggest piece of that curve was learning to trust that the automation would do its job when it was supposed to do it.

"Now that our operators have been working with the AGV, it's just part of the atmosphere," Goff says. "They let the vehicle do its job and stay out of the way."

While the plant is operating with one AGV today, plans are already in the works to implement another vehicle later in 2005. "The AGV has proved itself in operation," says Goff. "But it's also so critical to the operation that it could shut the plant down if it required maintenance while the assembly line is running.

Click here to read more about AGVs.
(AGVs keep plastic bottle production in top shape - Oct. 2, 2004 - Casebook Directory)