Caterpillar creates innovative technologies to boost welding performance.

By Leslie Gordon, associate editor

A robot welds together parts for end loaders and hydraulic excavators at a Caterpillar manufacturing plant.

Caterpillar's weld-quality system monitors an electrical current that incorporates a computer and a power source.

The monitoring software's user interface shows a disturbance in the current, which is reflected in the PSD reading. The red line indicates the PSD limit.

A disturbance in the current is too short in duration to stop the weld.

Caterpillar charts test welds for varying wirefeed rates and voltages. PSD readings above the blue line show porosity in the weld.

In today's global economy, companies who constantly innovate stay ahead of the game. And one of the main players is Caterpillar Inc., a leading heavy-equipment manufacturer headquartered in Peroria, Ill. In addition to other process improvements, Caterpillar engineers devise patented welding technologies that help build the iron and steel backbones of the company's big yellow machines. Two recent examples include a patented tab-and-slot technology that eliminates fixturing and weld-quality monitoring technology that may allow companies and construction sites to automate operations requiring high-quality welds.

Tab-and-slot eliminates fixturing
Dedicated welding operations typically use jigs, fixtures, and other equipment to hold metal components in place during welding. But these devices must be designed for a specific job, validated, and then maintained over the life of the products. To tackle these problems, engineers at Caterpillar's Peoria, Ill. facility developed a tab-and-slot temporary-fastening technology that uses the raw-material metal sheets themselves as temporary fastening mechanisms. They claim to have aligned sheetmetal — used to fabricate vehicle frames and other heavy-duty components — to within 0.005 in.

Caterpillar cuts a pattern of tabs and slots into the edges of these sheets. The tabs have radiused cams and align with the slots.

After inserting the tabs into the slots, operators twist them to engage the radiuses and lock the pieces together. Once in, the twist tabs extend out of the slots. Caterpillar personnel then weld the assembly and remove the tabs from the sheets by twisting them off with a tool.

The tabs have a design point to fatigue at the same location every time. This ensures the required torque is constant, regardless of the plate thickness.

"Depending on the application, we save up to 50% setup time compared to traditional fixturing techniques," says Keith Egland, a metallurgical engineer and engineering supervisor at Caterpillar's Manufacturing R&D group. "We use this technology to weld components in Caterpillar products such as mining, earthmoving, and forestry equipment."

Tab-and-slot
England reports that Caterpillar has used the technology on various internal and external component sections. "Non-planar pieces are possible but have more limited applicability than perpendicular pieces," he says.

Other applications include situations requiring welding in remote locations rather than in dedicated welding shops. "If you happen to be somewhere where you don't have dedicated jigs or fixtures, you could still align and orient parts using the tab-and-slot technology," explains Egland.

According to Caterpillar engineers, the tab-and-slot method greatly reduces the misalignment problems associated with other welding techniques, thus improving weld quality. As for the claims of 0.005-in. tolerances, that is for two plates in one dimension. A structure would involve a stack-up tolerance that would be bigger, notes Egland.

In addition, the technology's mechanical simplicity and ease of use increases efficiency to further reduce production costs.

Algorithms automate weld quality
Caterpillar's other recent innovation automates weld-quality monitoring. As Alan Kilty, senior research engineer of weld-quality monitoring and advanced production technology at Caterpillar's Center of Manufacturing Excellence, Mossville, Ill., explains, "Without reliable real-time technology-based feedback, welders can't know if they are leaving porous welds, convex welds, or welds that burn through to the other side of the workpiece."

To overcome these difficulties, he, Howard Ludewig, a Six Sigma Black Belt, and their team of engineers developed the new monitoring technology.

Because monitoring welding parameters is so critical to efficient welding, companies typically monitor power source currents and voltages, using the results to track weld seams, adjust fill volumes, or detect shielding-gas contaminants in pulse welding.

Monitoring these factors is particularly important at Caterpillar, which annually uses about 20-million-lb of weld wire to weld the frames, truck bodies, and bulldozer blades that make up its earthmoving equipment. "We are close to getting the Six Sigma ranking on robotic welding. This means we have less than 3.4 mm of defect per one million mm of weld," explains Kilty.

Caterpillar's new monitoring technology differs from other systems in how it keeps track of welding current, voltage, and wire speed. Kilty and his team created software algorithms that monitor these welding parameters using the current's standard deviation or its power spectrum densities (PSD).

When the operator is in a metal-transfer mode with welding parameters that don't exactly match the system's predetermined limits, the system automatically compensates and decides what limits to use.

The patented technology samples the welding current and voltage signals to determine weld quality based on information embedded in those signals. "In the lab, we carefully measured welding current's power spectrum density and found that porosity had the most disturbance from 20 to 100 Hz, which gave us numbers to work with when creating the new technology," explains Kilty.

"A change in signals indicate a change in porosity. And this technology correctly detects sub-porosity 96% of the time. A change in signals also indicates whether the tip is worn and needs replacing."

Caterpillar also monitors the water-cooled power cable for fatigue. This lets the company replace the cable during scheduled downtime — before it causes poor weld quality.

Sampling the current
With the standard-deviation algorithm, the monitoring technology samples the arc current, determining its standard deviation. It then compares this standard deviation to a predetermined limit and uses the results to assign a value to a variable, which becomes the quality parameter.

The system then compares this quality parameter with a second predetermined limit. If the weld quality is unacceptable according to the company's Six Sigma standards, the technology terminates the welding process and alerts the operator via the computer's user interface.

With the power-spectrum-density algorithm, the system's computer sums up the current's power spectrum densities between two predetermined frequencies.

The system compares the average PSD sum with a limit. If the sum is unacceptably greater than the limit, this technology also terminates the welding process and alerts the operator.

The system calculates PSD every half of a second. And the weld-quality algorithm employs an adaptive-limit method to recalculate the limits equally fast. "This provides a robust method for tracking weld quality with widely varying weld parameters," adds Kilty.

Kilty reports that Caterpillar uses the welding technology with both mild and high-strength steels. The weld-quality-monitoring system may work on other metals, but Caterpillar has not yet tested this theory.Caterpillar has implemented two monitoring systems with the standard-deviation algorithm in production, at two Illinois plants. To date, the company has only tested the power spectrum algorithm in its weld lab.

The monitoring technology can also operate with a proprietary vision system, which obtains positional data points for defining a weld-bead profile, also patented by Caterpillar.

 

Tab-and-slot technology from Caterpillar involves placing the fabrication piece's tab into another piece's slot.	After completing the weld, the operator twists off the tab.
	Potential applications Potential applications for tab-and-slot technology include: Aerospace structures Automotive components Framework of steel and metal buildings Heavy equipment The idea of modifying the sheet edges into tabs and slots came to inventor and Caterpillar employee Gordon Kelly when putting together a metal dollhouse for his daughter.
Once in place, the welder rotates the tab, which locks the two pieces together.

 

Potential applications Potential applications for the weld-quality monitoring technology include: Pipelines and bridges Framework of steel buildings Earth-moving equipment Heavy machinery Truck frames Forestry equipment Ship building Amusement-park rides