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Robots Weld Large Parts, Produce Sizable Savings When Systems Integrated

WHEN Iowa Mold Tooling (IMT), Garner, Iowa, paired the right robotic technology with the right systems integrator, the company cut labor costs more than 75% and boosted productivity more than 420%.

IMT manufactures contractor bodies, service bodies and tire service bodies, along with cranes for wallboard and material handling. It was in the crane-manufacturing unit that IMT was able to reap the benefits of robotic technology.

The manufacturing process involves welding heavy sections of low carbon steel, ranging from 1/2-inch to 1-inch thick and often reaching 18 feet long.

"This size generally is considered impossible for robotic welding," says Larry Snyder, manager of manufacturing operations. "New corporate management wanted to automate as much of the welding in the shop as possible to increase productivity and enhance quality. We thought that automating the arc welding process, if possible, would do that."

To meet this task, IMT chose Productivity Welding Inc (PWI) of Minneapolis, Minnesota, as the systems integrator. Panasonic Factory Automation (PFA), Franklin Park, Illinois, manufactures the welding and robotic equipment that IMT selected.

The Systems Integrator To begin, Snyder followed a detailed procedure to select a robotic vendor. He visited at least five of PWI's robotic welding installations, talking with the front-line people responsible for robotic welding. Of key interest were safety, ease of programming and operation, and the experiences of others in operating a robotic welding cell. Accompanying Snyder were many of IMT's manufacturing and welding personnel, so they could ask questions relating to their own specific concerns about beginning an automated welding operation.

PWI provides services to welding fabricators primarily in Minnesota, Wisconsin, Iowa, and Nebraska. The company has 45 working installations, plus many customers that receive service even though PWI did not do the original installations. The firm maintains a demonstration center equipped with an average of seven robots in four working cells on-site in Minneapolis, where customers routinely come to see test demonstrations of the robotic welding of custom parts. This approach proves that a particular system works for the customer's parts production before a commitment to purchase is made.

Demanding Criteria Among the criteria demanded by IMT was that the investment in robotic welding would have to pay for itself in less than three years. PWI's president and general manager, Mike Ross, submitted an analysis that included time standard studies and cost justification for four different boom-welding applications, to be used in subsequent investment justification.

Tim Nacey, PFA's assistant general manager, recognized that the boom welding job was difficult, but he committed PFA to the task. "I felt that we had solved the problems of adaptive and harmonic control required," he says. "We also had proved to IMT that the necessary arc stability was available in the HM-Series of dip-pulse inverter power sources."

As part of the preliminary evaluation process, a run-off was held at PWI's demo facility, and the largest and smallest booms were welded successfully, using PFA's AW-006AL robot and HM-500 amp power supply.

The result: IMT placed the order in February 1996, and the company added the welding of hydraulic cylinder cases and rods to the scope of the job.

Detailed System Design For the system design, PWI planned all fixturing, mechanical, electrical, and electronic systems in addition to the robot and power source designs. CAD drawings of the installation layout were created to permit IMT's engineering and welders to visualize the proposed handling and welding system. Detailed safety, maintenance, and operating instructions also were included.

"PWI handled it all-engineering, integration, documentation, welding, fixturing, training, human factors, installation, programming, and the entire flow in our facility," Snyder says. "When PFA and PWI were done, they knew our plant better than we did. They examined all involved parts to determine the potential variability in dimensions and location when in position for welding, the welding parameters required, the quality level we demanded, and the entire flow of parts and materials to and from the welding station."

Training Is Key Leo Liske, a nine-year robotic welding specialist from PWI, was the project engineer for the installation. He provided the training in all aspects of the set-up.

"Liske and our engineers and floor operators spent huge amounts of time together, so that his know-how was typically transferred on a one-to-one basis," PWI's Ross says. "It is a whole lot more effective than asking a welding operator to read an often-impenetrable operations manual. The close relationship between our operators and Leo made the difference. You can't get that kind of quality training when the people involved are long distances apart."

The Challenges To implement the project, the companies had to disprove the common perception that large parts cannot be welded with robots. Among the concerns:

* The boom size and complexity of the welds to be made.

* The productivity and quality requirements. This demanded state-of-the-art adaptive controls, touch sensing, through-arc seam tracking, and harmonic motion control.

* Controlling the motion of both the part and the robot.

"When the part is not a simple shape, as a simple straight line, the path of the robot in typical robotic systems must be programmed literally by plotting point by point," says Stewart Stevens, a PFA application engineer on the project. "This represents a substantial cost due to the time demand for a skilled operator, especially when the part is large, as in IMT's applications. Failing to plot large numbers of points in order to reduce costs means the arc will move in an irregular pattern, running the risk of missing the weld joint or operating at the wrong travel speed and, therefore, increasing defects."

With a large part, uncertainties arise regarding the location of the part. This is caused by the inevitable tolerances in part preparation. It is made even more difficult when welds ranging from 6- to 18-feet long must be made. In addition, the heat generated by the welding process causes distortion that changes the location of the part relative to the arc.

"In this situation, we knew that large deflections of as much as +/-1/2 inch existed, plus repeatability of part placement was an additional variability of +/-1/2 inch. This causes more uncertainty in arc location and motion control," Snyder says.

Finding Solutions To address these issues, IMT selected welding equipment that combines use of harmonized motion control, through-the-arc seam tracking, and touch-sensing options.

The equipment requires only that the arc's start location, stop location, and required travel speed be programmed. This option achieves synchronized and simultaneous three-axis control in concert with all axes, which includes servo-driven positioners for complex weldments.

In addition, the Panasonic equipment automatically coordinates the robot movement with the 25-ft-long track unit, and the two 2,000-kg (4,400-lb) Panasonic DICE headstock/tailstocks. As a result, the arc motion is maintained uniformly and on the taught welding path.

With PFA's harmonized software, the time and skill level is reduced significantly. Programming time is reduced from 30 to two minutes-a 93% reduction for each application-by adhering accurately to welding procedures.

Touch Sensing The AW-006AL robot installed at IMT employs high-speed touch sensing capable of working in one, two, or three dimensions. This allows the robot to learn where the required position is for arc initiation.

"Requirements change for each weld due to the uncertainties of preparation and location," says Snyder, "The technology of the AW-006AL was a must."

Since the seam tracking automatically maintains the robot in the correct position, knowing the previous three parameters is all that is required to track and weld any geometry. This enables IMT to weld a part of virtually any geometry.

Once the arc is started, a through-the-arc seam tracking system keeps the weld located correctly-regardless of whether the part is distorted, the work clamping is not set properly, or the piece is prepared wrong. This function is necessary due to the three-dimensional variations in arc location caused by uncertainties in the location of the part, deflections and shape changes due to heat, and varied fixturing.

Another sophistication of the Panasonic system is Root-Pass Memory to enable accurate multipass welds to be made. A 25-ft robot transport unit manufactured by Preston Eastin of Tulsa, Oklahoma, was used to increase the robotic work envelope effectively and accurately.

Impressive Results The installation at IMT's crane manufacturing operations in Garner occurred in June 1996. Once on-line, the efficiencies achieved in actual production far exceeded the job specification, according to IMT's Snyder. For example, manually welding an 18-ft boom required 11 manhours of effort. The PWI system reduced the labor consumed from the two manhours forecast to only 1.75, a further 12.5% reduction from forecast. Arc on-time experienced during a boom welding sequence exceeded 93%.

The table below, while not specific to IMT, provides a general method for comparing robotic and manual welding.

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