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Monitoring quality welds at GM.

Monitoring quality welds at GM

Thunk! The satisfying sound of a well-hung door depends on a lot of manufacturing variables, particularly on accurate MIG welds of door hinges to galvanized-steel door stampings.

Here at GM's Moraine Truck & Bus plant, a new arc-welding system is providing automatic, on-line tracking of door-hinge welds in real time. Currently installed on the right-hand-door S-10 pickup line and slated for the Blazer line this fall, the computer-based system helps reduce process deviation and rework/repairs by providing weld-to-weld data that aids in parameter optimization, analysis, and fault identification.

Tied to the four-torch, mechanized welding system is a quartet of arc-welding computers supplied by National-Standard's Archon Group, South Bend, IN. The Archon II units provide continuous analysis of crucial welding data via sensors measuring voltage, current, wire-feed speed, and gas flow.

The system produces four MIG welds on each of two truck-door hinges. Door and hinges are manually loaded onto the welding fixture, clamped, and welded using a water-cooled backup. Four 1000-lb wooden reels supply 0.062"-dia N-S-115 carbon-steel welding wire to the system. A fifth backup reel helps cut changeover time to only 4 min. After the first weld, the four torches index to complete the second weld. Total burn time for all eight welds is 2.6 sec, arc time is 1.3 sec. Shielding gas is 100 percent [CO.sub.2.].

When the Archon II computer senses an out-of-spec condition for current, total arc power, voltage, wire-feed speed, or gas flow; it feeds this data to a PLC that instructs the welding system to do a reweld. At the completion of the welding cycle, the door is manually unloaded and racked by the system operator.


welding errors

GM began collecting on-line data in August, 1988, and modifications were made to include a current- and voltage-threshold feature to trigger data recording, or for lack of current or voltage, note a fault. As luck would have it, a potentially serious production error was caught shortly after the arc-welding computers were brought on-line. When one of the four power supplies failed, it was replaced by a spare unit but with incorrect polarity and thus, the wrong weld current. When the new MIG system was installed and the Archon II data studied, it quickly revealed that a number of doors had been improperly welded.

As maintenance supervisor, Henry Taylor notes, "This fortunate coincidence made the value of this weld-monitoring system very clear to both plant and management-level personnel. Without this documentation, the error would not have been detected." Following the incident, power-supply stud size was modified and terminals color-coded.

Holding parameters

For optimum weld penetration without voids, the system must control the 3/8" nugget size within a narrow window, thus weld specs are critical. When input from any of the torches indicates a variation from these limits, the fault is noted and communicated to the system PLC. To simplify operator corrective action, an LED display indicates which welding parameter has been exceeded and by how much.

For certain out-of-spec conditions, the computer triggers a reweld via the PLC interface. After a set number of consecutive minor faults, the system registers a major fault and shuts down. Computer data then provides information on the fault for rapid identification and minimal downtime.

At present, GM is using a printer to log all major faults during each shift. Limits are indicated along with what actually occurred--minimum, maximum and average value, plus standard deviation, tabular data, and sample points. Studying this data indicated, for example, that replacing tips three times during each shift helped maintain the optimum volt/amp ratio. It also revealed the relationship between varying part fit-up and the need for an automatic reweld cycle when the gap caused erratic weld performance.

The four-unit system can sample up to 20,000 sample points/sec, and monitor up to 120 views of the welding process simultaneously. Data generated can be displayed in a variety of reports. For example, production people can review production costs per shift for variables such as total maintenance, power, wire and gas consumption etc. Qualitative and quantitative analysis of these reports can have an impact on fine-tuning system profitability.

A range of pre-engineering Archon software can be used to develop and refine arc-welding procedures, monitor arc time, prevent overwelding, control gas and filler-material consumption, store information, produce control charts, correlate data points, and produce documentation. Adding a phone modem makes possible sharing data with other plants worldwide. The system can interface with a wide range of welding operations, including GMAW, GTAW, pulse, AC/DC, and high-frequency start and continuous modes.

GM's Truck & Bus arc-welding system has been producing right-side S-10 pickup door-hinge welds since January, 1989, at the rate of 450 doors/shift, or 900/day without a hitch--over a 100,000 to date. As a result of this successful program, GM plans to install an identical system to weld left-side doors later this year.
COPYRIGHT 1989 Nelson Publishing
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Workman, Robert
Publication:Tooling & Production
Date:Sep 1, 1989
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