By MARY KAY MOREL, staff writer/editor, Motoman Inc.

Edited by RICHARD MANDEL

A mid-frame segment for a Volvo grader undergoes welding in a skyhook positioner

Automated weld-up on the VMH positioner

Volvo Construction Equipment traces its history back to a Swedish company that began producing tools and industrial machines in 1832. Horse and steam powered the expanding company's first heavy-work vehicles, and production has been steady since, with constant updates to the manufacturing techniques.

Machines produced at the Volvo Motor Graders division in Goderich, Ontario, Canada, range in weight from 17 to 23 tons, and are used for road maintenance, forestry and construction site work. The company began incorporating robotic arc welding into its manufacturing processes in 1998, with four Motoman FabWorld systems operating on the line.

In December 2005, Volvo implemented a fifth robotic welding cell for grader production. The custom-designed system is using advanced tandem GMAW technology to perform heavy-deposition welding on a new grader model.

The Frame Cell
"The majority of the new grader was a redesign from the ground up," according to Al Menheere, a welding process specialist for Volvo Motor Graders. "All of the welds were designed specifically for robotic production."

Grader frames are made in three sections from mild steel plate between 0.75-in. and 3-in. (19.05 mm and 76.2 mm) thick. The subframes then are joined together manually to make a complete frame assembly. Depending on the model, rear frames weigh more than 4,410 lbs (2,000 kg) each; front frames weigh approximately 1,450 kg (3,197 lbs) and mid-section frames weigh about 3,308 lbs (1,500 kg). Each frame section requires 50 to 150 welds that range from 4 in. to 9 ft. (101.6 mm to 2,743.2 mm) in length.

Volvo's new Frame Cell consists of an extremely long-reach, six-axis HP50-20 robot from Motoman (www.motoman.com) that has a 44.1 lb (20 kg) payload, 123.3 in. (3,106 mm) horizontal reach and 219.9 in. (5,585 mm) vertical reach with a repeatability of ±0.006 in. (±0.15 mm). To provide a larger work envelope, the HP50-20 robot is mounted on a 2,205 lb (1,000-kg) capacity servo track with 16.4 ft. (5m) of travel.

"We had several goals for the new robot cell," Menheere says. "Improving productivity and shortening lead times were important factors, as well as compensating for the shrinking number of skilled welders available to us. We run seven different models of the grader on the Frame Cell. We build to a schedule, so we can have a run of 100 graders of the same model or 100 variations. Production always varies."

"One of our challenges for this project was that the ceiling heights in the plant are very low," Menheere explains. "Ideally, a gantry would have been the way to go, but we just didn't have the ceiling height necessary. We would have had to dig pits eight feet deep, and it would have been a safety nightmare. That's why we decided to go with the HP50-20 robot on a slide."

"The new model has never been welded manually," continues Menheere. "We use the Frame Cell to weld all three frame parts. Changeover from welding one model to another is not an issue, the operator just selects a different program and it's all done through the robot teach pendant.

"With the new robot cell," adds Menheere, "we can now build on two shifts what would have taken three shifts. The new grader model would have taken about 10 or 11 skilled manual welders, but with the robot cell, it only takes two operators. The frame line is no longer a bottleneck."

Tandem GMAW
The extended reach Motoman HP50-20 robot is equipped with a tandem arc GMAW torch package and TPS5000 digital welding power sources manufactured by Fronius (www.fronius.com), with a Fronius interface to the Motoman NX100 robot controller. In a tandem arc process, two wires feed through a single torch, allowing heavy-deposition welding with faster cycle times and at a lower capital equipment cost than using two robots with individual welding torch packages.

"Tandem arc with the Fronius equipment is not cheap, but we felt that having one robot with two power sources was more costeffective for us than buying a second robot," Menheere explains.

The frames are welded with Lincoln L56 0.045-in. steel wire (www.lincolnelectric.com). High-speed, wire-assist feeders deliver the wire to the robot from 1,000 lb. bulk reels located 50 ft. outside the cell. Shielding gas is a mix of 92 percent argon and 8 percent CO2. Welding peripherals include an automatic wire cutter and a tandem torch reamer.

The two welding power supplies share a master controller that coordinates wire feeding and current control between the two wires. This central control can control the wires independently with different feed rates or modes of transfer. The controller also can fine-tune each arc independently, or provide single-wire operation by turning off one of the power sources.

All frame components are manually pretacked in specially designed fixtures prior to robotic welding. The line workers at Volvo Motor Graders work hard to achieve good part fit-up, which allows the tandem GMAW process to weld at least 2.5 times faster than other welding processes. Additionally, close fitting assemblies reduce cycle time by eliminating the need for multi-passes on long, heavy-deposition welds.

The system supports touch-sensing and seam-tracking capabilities that are typically required on large welds. "These fixtures use common data points with the robotic holding fixtures, which results in improved joint location, repeatability and fit-up," says Menheere. "Basically, that minimizes how much touch-sensing we have to do (to find the welding joints). Right now we are not using touch-sensing, which improves our cycle time, but we will be adding a little bit.

"In terms of welding options, we use Motoman's ComArc seam-tracking functions on more than 90 percent of the welds, because everything is heavy-deposition," notes Menheere. "We plan to start using the Com-Arc touch-sensing function soon to help with one little oval weld on a vendorsupplied part that can vary by a few millimeters. It's on the outside of the machine where everybody sees it, too, so it is important to maintain quality.

"Our biggest challenge is aesthetics — making the welds on the outside of the grader frame look pristine. We don't want to have to do any grinding, we just want to be able to paint over it and have it look nice." Menheere says.

Positioners
On one side of the cell's single-axis weld robot's shuttle track, grader frame midsections are supported for welding by a Motoman MT1-3000 S2X two-axis servo-tilt/rotate skyhook positioner with a 6,615 lb (3,000 kg) payload capacity. A Motoman VMH-3000 S3X three-axis, two-station servo-powered positioner is located on the opposite side of the robot shuttle track. Each station on the VMH-3000 unit is a headstock/tailstock positioner with a 6,600 lb. (3,000 kg) payload capacity, mounted on a rotating base axis. Span between headstock/tailstock faceplates is 12.3 ft. (3.75m). Front frames are welded on one of the headstock/tailstocks and rear frames are welded on the other. The positioner configuration allows operators to change out the workpiece on one side of the cell while the robot welds the frame section on the other side.

The NX100 controller on the six-axis robot also controls the movement of all the cell's positioners, a total of 12 axes: that includes the robot, the single-axis track, the two axes on the skyhook positioner and the three axes on the rotating positioner with the two headstock/tailstocks. Software coordinates the motion between the robot and positioners during welding, rotating the workpiece through multiple planes, depending on the part's geometry. Turning the part during welding positions the joint into the most desirable flat orientation with respect to the weld torch, resulting in better welds and faster cycle times. A programmable logic control (PLC) is not required for overall cell control, but the cell is equipped with a small PLC for safety features, such as the light curtains.

Operations sequence
Operators use an overhead crane to bring frame parts to the positioners, and manually bolt the pieces into place. "There are no hydraulics, no pneumatics. We're not building 100 pieces per shift, so we don't need a fast changeover," says Menheere.

"Our ideal flow is to load a mid-section frame on the servo-tilt/servo-rotate skyhook positioner and have the robot weld on that, which takes about an hour. Meanwhile, we can load and unload parts from the two stations at the indexing headstock/tailstock positioner," Menheere says.

When the robot finishes welding on mid-section frames, it can rotate to the other positioner and weld front or rear frame sections. Menheere says, "Front frames take 20-30 minutes to weld, and rear frames take 40-50 minutes, depending on the model. While the robot welds the front and rear frame parts, we unload and reload the servo-tilt/servo-rotate skyhook positioner. One complete cycle through each positioner gives us one complete grader frame."

According to Menheere, the robots have improved productivity, quality, safety and profit, and Volvo Motor Graders is planning to add additional units in the coming years.

"We want to continue to build upon the success we've achieved in the Frame Cell by switching to similar tandem arc welding robots, as we replace existing equipment or add new cells in the future," he says.