A bellows assembly, custom designed by A & A Manufacturing Co, Inc, is providing effective protection for the height adjustment mechanism of a robot arm-mounted plasma torch head that makes fast, clean cuts in three-dimensional parts and irregular surfaces. The torch head of the Hypertherm® HyDefinition® HD3070 plasma system moves up and down to control both pierce-height and torch-to-work distances. The bellows assembly helps preserve accuracy and reliability by excluding fine dust from the precision slide of the mechanism that moves the torch head.

Protection Without Interference

Providing protection for the precision slide mechanism of the robotic lifter assembly was a complex challenge, since the bellows had to accommodate the range of torch motion without compromising its ability to reach into restricted areas. Hypertherm Inc, of Hanover, NH, a leading manufacturer of plasma arc cutting equipment, touts its pointed PAC184 torch design for its ability to fit the tight contours of most three-dimensional parts while cutting mild steel up to 1/4 inch (6mm) thick. The torch assembly, including its Command® THC, weighs just 6.3 pounds (2.8 kg), which is light enough for most robot arms; the plasma arc aligns with the center of the robot torch-mounting axis, simplifying the "teaching" operation. The torch system was designed to minimize dross and optimize cutting angles through proper positioning. The system's controller adjusts torch height (Z-axis) at up to 100 inches per minute (5.0 m/min) to maintain the proper distance from torch to workpiece.

Because torch positioning and cutting head motion are so important, Hypertherm's engineers needed a way to protect the Command THC slide mechanism from damage and wear caused by fine dust particles present in the plasma-cutting environment. They worked with A & A Manufacturing Co engineers, who designed a combination metal shroud and elastomer-impregnated bellows assembly that protects the slide without significantly limiting its motion.

Bellows Meets Application Needs

To accommodate the linear motion of the precision slide, a folded construction similar to that of a camera bellows was used. However, the rectangular bellows for this application was constructed from an elastomer-coated fabric material chosen for its durability. Flange adapters at each end were designed to mount to the slide mechanism on one end and a special adapter fabricated of anodized aluminum, which mounts to the torch end and protects the non-moving portions of the mechanism. Together, these units form an assembly that protects the torch mechanism against harmful elements and complements its appearance without impeding its motion.

According to Hypertherm, the slide can move through a maximum linear travel of one inch. Operating duty typically averages several hundred cycles per day, depending on the user's application. Hypertherm notes that the temperature of the unit may reach as high as 200ºF, and the atmosphere tends to be dusty.

Experience in the Field

After approximately two years of field experience, the bellows is performing to Hypertherm's satisfaction. A case in point is the plasma cutting system used by a New England-based metal fabricating firm. For one product, the company cuts and welds rectangular steel tubing robotically in work cells that accommodate varying lengths. A Hypertherm HD3070 plasma cutting system mounted to a Motoman® SK16 single arm robot cuts the tubing to the proper length. Afterward, the cut components are placed in a fixture and robotically welded.

Because the plasma cutter must cut 360º around the tubing, the torch must work in a variety of positions, from vertical up and down to horizontal. In each case, as the robot arm positions the torch, the final height adjustment from torch to workpiece is made by the system's Command Torch Height Control (THC).

"One of the most critical parameters of the HyDefinition Plasma Cutting process is the torch-to-workpiece distance," says Jim Colt, mechanized applications expert at Hypertherm. "This distance varies depending on the power level and material thickness, but it must be controlled to within plus or minus .005 in. If the torch-to-workpiece distance is not controlled within this tolerance range, the cut face can be affected in terms of bevel angle, dross (resolidified material) and ultimately, cut part accuracy."

According to Colt, the rectangular steel tubing used in the application above has manufacturing tolerances that may exceed plus or minus .005 in., so an accurate method of controlling the torch-to-workpiece height is a necessity. He adds that the Command Torch Height Control from Hypertherm was developed to maintain these tight tolerances in an environment that is full of metallic dust from welding and cutting applications.