Using the expansion monitoring capabilities of the WeldComputer® Adaptive Control, aerospace companies have received government endorsement of plans to eliminate periodic destructive testing required by MIL-SPEC.
Electrode force – along with applied heat and time – is one of the key parameters of a successful resistance weld schedule. Variation in weld force results in variation in weld quality. It is easy to predict electrode force by using cylinder pressure calculations and it is relatively easy to statically measure electrode force by placing a force gage between the electrodes. Unfortunately, neither of those methods is adequate to assure proper weld force during welding.
Plant air pressure drops, cylinder leaks, and inadequate hold times can result in improper electrode force during welding. Binding in the welding head, cylinder “stiction”, or excessive head inertia are even more insidious problems since they are dynamic phenomena that only occur as welding takes place.
Continuous force monitoring is the only reliable method of assuring that the specified electrode force is actually being achieved when it is required: as the weld is being made.
While current is not a reliable method of determining weld quality, half-cycle by half-cycle monitoring of true RMS current can be very useful in diagnosing machine malfunctions.
For example, control failures such as incorrect number of cycles of heat, SCR failure, or inconsistent weld heat delivery are easy to detect by monitoring half-cycle by half-cycle current values. Benchmarking a machine’s “like-new” performance and monitoring current over time can provide useful information regarding periodic maintenance intervals. For operations with many welding centers producing the same components, monitoring current can be useful in terms of assuring consistent set-ups and schedules from machine to machine.
As an indicator of the quality of any specific weld, however, current monitoring will only detect the most disastrous conditions while allowing many bad welds to escape.
As welding machines age, conductance gradually decreases. This means that proportionally more of the power that was available for welding when the machine was new (or in good repair) is being dissipated throughout the welder in the form of heat. Monitoring the conductance of the secondary circuit of a welding machine can provide good information regarding periodic preventive maintenance cycles or requirements for machine rebuilding.
Monitoring the conductance of the workpiece itself can provide useful information about its suitability for resistance welding. Low conductance of the workpiece in comparison to expectations can be indicative of incorrect material, corrosion or other surface defect, dirt or other foreign substances, damaged or dirty electrodes, or improper parts fit-up. High conductance of the workpiece can be indicative of incorrect material, flattened electrodes, and shunting.
Continuous conductance monitoring can provide important information about the condition of the welding machine as well as the suitability of the workpiece for resistance welding.