June 12, 2003
Like the products it manufactures, CNH Global N.V. has to stay on the move—technologically.
The company's Wichita, Kan., facility uses robotic weldingto help produce Case- and New Holland-brand skid steer loaders.Powerful yet quick, skid steers can spin on a dime and move heavy loads of material in a jiffy. Skid steers are sold to a variety of industries and to many municipalities for use in construction, road and bridge building, landscaping, and agriculture applications.
To produce these material-moving machines and achieve critical fit-up requirements on the large frame components, the company implemented robotic automation, including extended-reach manipulators and heavy-duty positioners. By using a combination of skilled manual welders and welding robots to produce its skid steer loaders, the company has been able to increase productivity and meet its quality goals while improving safety.
Currently the facility has 17 robotic welding workcells in production, 10 of which the company added in 2001."The Case Compact XT® skid loader frame welding line includes six of these newer welding cells, along with some manual tacking and welding stations," said David Hoffman, welding engineering supervisor at the Wichita facility. "Individual cells are grouped in such a way that product flows from one cell to the next."
The final welding cell on the Case frame line—called OP80—was installed in August 2001. In it, two six-axis robots with 20-kg payloads are invert-mounted from individual servo-controlled swing beams. These provide a seventh axis of movement for each robot and provide the extended reach and part access required on the large frame parts. Several major frame components are welded together in this cell to form the main frame assembly, so fit-up is critical.
"The extended reach and overhead mount give us better access to the part than a floor-mounted robot would be able to give," Hoffman said. "Plus, it allows us to create a good flow. The robots weld at one station while operators load and unload parts at the other station with the help of an overhead crane."
In this cell, dual robots weld three different frame models. Each frame weighs between 1,700 and 2,000 pounds and is made of mild steel from 3/16 to 3/8 inch thick. The two robots weld simultaneously on the part to reduce cycle time.Each frame requires 75 to 80 welds from 2 to 12 in. long. Most are single-pass fillet welds, but some welds require weaving. The robots rely extensively on their high-speed touch-sensing function to find the welds and automatically offset the robot program. In other cells, seam tracking also is used to follow deviated weld seams.
Each robot has a water-cooled torch package and a 450-amp power source. Bulk dereelers dispense 0.045-in.-diameter welding wire.
Each robot also has its own automatic wire clippers and reaming, or torch-tending, station. Weld shielding gas is a mix of 92 percent argon and 8 percent carbon dioxide used throughout the plant. The company mixes its own weld gas on-site and plumbs it to various workcells in the main building to improve efficiency and reduce the cost of gas, at the same time reducing shop floor clutter.
Tapered locating pins are used to mount individual frames onto one of two heavy-duty, servo-controlled headstock-tailstock positioners. Each positioner can handle up to 11,020 lbs. Manual jog switches on the operator stations near each positioner allow the operator to jog the large, heavy parts to facilitate loading and unloading while the robot is welding on the opposite side of the positioner.
A programmable logic controller regulates hydraulic part clamping. Changeover of tooling between models requires about two minutes.
In manufacturing skid steer loaders to customer satisfaction, CNH finds that a number of quality considerations are critical, including welding, fit-up, and aesthetics, Hoffman said.
"Weld consistency and quality are important to us, and so are weld aesthetics. Quite a few of the robotic welds are visible on the final product, so we are looking for a good-looking, attractive weld," Hoffman said. Fit-up especially is crucial, he said, and relates directly to customer quality perceptions.
"Robotic welding has allowed us to meet our stringent goal of 1.33 CPK relating to critical fit-up characteristics between the skid loader frame and lift arm mechanism," Hoffman said. "Our customers' quality perceptions relate to the flatness of the bucket to the ground and how centered the loader arm is on the frame."
According to Hoffman, robotic welding has improved productivity by allowing the company to use its direct labor resources more efficiently.
"Using the robots, we get about a 40 percent reduction in direct labor time versus doing all the welding manually. The robots don't weld that much faster than a person. How we really gain the time savings and productivity improvement is that the robots do the welding while the operators do other work, such as loading another station and doing manual welding or pretacking," Hoffman said.
In addition, the robots improve productivity while also improving safety by keeping workers away from potentially hazardous welding fumes.
"We have a ventilation system in the plant, but the robots also help improve safety by basically taking the operators out of the fume zone, which helps with their breathing. Performing a large percentage of the welds on these frames robotically has helped reduce problems welders get when squeezing the trigger on manual guns for an extended amount of time," he said.The workcell also has a comprehensive safety package that includes wire fencing, arc screen curtains, sensors, and proximity laser scanners.
Most of the cells use floor-mounted welding robots, which Hoffman said makes programming straightforward. The OP80 cell is more challenging to program because its invert-mounted robots and servo-controlled swing beams also are controlled by the robot's XRC controller.
The company sent two welding engineers and four lead men from the weld shop to Motoman for formal programming training."Currently we have a robot technician on each shift who handles the bulk of the programming. On the XRC [newer robot controller] cells, we've trained probably 20 or 30 operators to take care of some of the minor situations they might encounter. This includes checking tool-center point and changing out gun parts. The operators also are trained to recover from minor robot stops," he said.
"With robots, there are more opportunities for things to go wrong than there are with manual welders, but reliability with these robots has actually been good, and we haven't had very much downtime with these cells."
CNH Wichita Plant, P.O. Box 9228, Wichita, KS 67277, 316-941-2320, fax 316-941-3422, www.cnh.com.
Motoman Inc., 805 Liberty Lane, West Carrollton, OH 45449, 937-847-6200, fax 937-847-6277, www.motoman.com.
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