At 220 lbs., brake drum castings for Class-8 trucks and buses are difficult to move from pallets into machining centers. Operators often tire quickly, causing production to drop as time passes. There had to be a better way to do this.

Webb Wheel Products' Webb transit business unit, a Marmon Highway Technologies Co., was established in 2004. It produces cast brake drums, hubs and rotors. It services customers as both a Tier 2 company and direct to the bus and motorcoach aftermarket.

Webb has its ductile iron castings produced by outside vendors and then shipped to its Cullman, Ala., plant for final machining. These products are then shipped directly to customers or distributors.

"The weight of the product is around 220 lbs. in the raw state," says Robert Kelley, plant manager. "So it's hard for the machinists to get the brake drums in and out of the machining centers. We knew that there had to be a better way to move the product. During IMTS in Chicago in 2006, we saw the Motoman robotics display. I contacted Motoman and we had a meeting and told them exactly what we wanted in a system that would feed our castings into the machining centers. Motoman then contacted its local integrator, Amtec Corp."

Originally Kelley wanted to develop one line that would run two 12-hr. shifts containing four machining centers and one robot. Then the company's president decided that another line was needed using one more robot and two additional machining centers that would use both robots more effectively. One problem, however, was that two of the machining centers couldn't be moved because of their foundation requirements. This meant that both lines had to be integrated within close proximity.

"With two lines, we're able to produce twice the number of parts with one person instead of three people per shift," says Kelley. "We can reduce the workers by two per shift, saving the labor of four people altogether."

Designs for the manufacturing system began when Kelley gave Amtec's Tony Lee, senior project engineer, the parts that they needed to machine and the cycle times that had to be met. Lee says Amtec took it from there.

"We worked with the information supplied by Kelley and Motoman for the correct robot," Lee says. "We also communicated on how the parts would be coming out of the machine and where they travel within the cell. One of the problems was to gauge each part. Webb gave us the opportunity to design and build a non-contact gauging station. Tolerances for some of the features were tight, in the ±0.002-in. range."

Amtec ended up designing a rotating gauge column that used a 700-lb. chuck that spins the part to check inside diameters and software that runs the gauge.

A unique manufacturing system
Although Webb calls its manufacturing system line one and two, it can be considered an integrated manufacturing cell that uses three vertical lathes, three milling machines, part buffer stations, a blow-off station that removes chips from each part, and a gauging station, along with the robots and overhead transporter.

Line one has four machines that include two vertical lathes and two milling machines. Line two has one vertical lathe and one mill that are fed by two conveyor systems and two Motoman HP200 six-axis robots. Each robot has a seventh axis used by its base rotation and one more axis used by the gauging station to spin the part. The key to this unique manufacturing system is the use of the two Motoman robots that run along the overhead I-beam called a Motoman MotoRail. The entire manufacturing cell is run by a man machine interface (MMI) that also manages both lines and the gauging station.

Todd Martin, programmer for Amtec, says that it uses software from National Instruments called LabView to control all the devices in the cell. It also uses an Allen Bradley compact logic PLC. This is integrated with the cell using a DeviceNet communications system and Bluetooth wireless stations for communication to the CNC machining centers, says Matt Cooper, controls and project engineer for Amtec. The PLC runs the entire cell along with handling the seven-axes robots and the axis of the gauge station.

Part challenges
"When we first looked at this system, the part OD was basically the same for a family of drums that had to be machined on the first line," Martin says. "When they asked us to look at adding another robot and additional equipment, they introduced more parts. The OD of the new part was much larger. We had to design a robot gripper that had enough travel to accommodate all the parts. We took a parallel grip made by Schunk and made an angular gripper from it. We didn't want to lose any of the original gripper integrity, though. It was a design challenge because a robot is only capable of holding so much weight [the weight of the gripper and part]. The gripper weighs 80 lbs. With these weight constraints, we had to make the gripper's arms from aluminum to make sure it was strong enough to work continuously without losing integrity. We also had to figure out how to save the gauging station from any possible damage because we had different brake drum IDs to check on, and the electronic components for measuring are small. So to package it properly to get accurate and consistent readings became a concern. Also, the integration and working out the priorities in the cell were a challenge."

To make sure a part doesn't slip in the robot's grip during the manufacturing process, each robot gripper has four pads, two on each side made from tool steel. Each pad undergoes a Carbinite process that gives it a rough surface for a positive grip and long life.

How it works
Parts are loaded through two in-feed conveyors that have 10 stations and built-in controls so that each part will not contact the one in front or behind. One conveyor loads line one and the other conveyor feeds line two. The robots service both lines. Line one has two vertical lathes and two machining centers. Two are required for the cycle time that's needed on drums that have a long machining time. Line two's parts have a much shorter cycle time, and only one vertical lathe and one machining center are needed. Machines are loaded simultaneously, and holding stations are used to keep parts in queue.

The machining centers are integrated so that when a part is placed for machining or is finished, the CNC notifies the MMI. Then, the robot can replace or remove the part into or from the machine and place it into the next operation.

Also, if parts are trending out of tolerance, as noted by the gauging system integrated into the MMI, the operator can change the offsets on the fly in each machining center to get the part's tolerances to within specifications. This operation has helped Webb reduce scrap.

One brake drum is a high-volume part that has six variations and different overall lengths. Line two runs lower-volume parts and handles one part that has six different variations.

After finish-machining and gauging, if the part is within tolerance, it's placed on a conveyor that exits for shipping. If the part fails, then the robot places it on a conveyor next to the in-feed conveyor so that the operator can either reject it or put it back through the line for re-machining.

It's absolutely critical that the operator knows which family the part belongs to because that determines how the robot picks it up and drops it off.

Part flow and travel within the cell is handled by the PLC program. Robot motions are also separately programmed. The manufacturing cell uses a vision system placed at the end of the i-feed conveyor to orient the part so that the robot can pick it up properly. If the part isn't oriented correctly, the workholding fixture won't line up properly with the part. The part has to be critically oriented all the way through the line. If it isn't, it can cause a crash.

The Motoman robots running on the same rail present a crashing hazard, notes Martin. "Once the two robots tool center points were taught to them using a teach pendant, we could teach the tool center points to each robot and calibrate the two robots together," he says. "Therefore, we can't physically run one robot into the other. The Motoman robots' internal software and controller tracks both of them on the rail, along with their carriages and arm positions so that the carriage and arms won't crash. This is a nice feature for us because with it we don't have the headache of making sure the robots won't crash into each other."

The robot's tolerance for each part is within two degrees of rotational placement. This is the only tolerance that was important to Amtec because the robots' repeatability is under 1 mm, well within the system's needs.

Motoman's HP 200 features a 104.4-in. reach and can handle up to a 441-lb. payload. Motoman's MotoRail 7 overhead lineal system allows invert-mounted Motoman robots on servo rails with lengths that vary from 13 ft. to 105 ft.

Motoman's robots and MotoRail system can also be used to provide material handling to all types of fabricating equipment, such as waterjets, lasers, plasma systems and press brakes. It can also be used for welding operations where the robots perform the welding or hold the part to be welded.

"We had other people bidding on the project besides Amtec," says Kelley. "But Amtec sold us on Motoman. One thing that made the Motoman MotoRail so attractive to us was that the two Omega CNC lathes were mounted to the floor and couldn't be moved because of their base foundation. A rail was a perfect solution to feed both lines, even though we had to move the other machines into the manufacturing cell."

Amtec is a certified solution provider for Motoman and is a privately owned and operated small business founded in 1988. Historically, its principal activity consisted of providing technical support to the U.S. Army. In 2003, however, Amtec began providing expertise in custom automation and established the Special Equipment Group, which has grown from a staff of two to a staff of 25. It provides mechanical and electrical design, custom assembly and test equipment, robotic automation, fabrication, and installation and maintenance for customers across the United States and Canada. FFJ