Welding Company Manufactures Safety-Related Devices for Automobiles
The following is an actual trip report documenting an actual consultation with one of WeldComputer Corporation’s customers. Names and addresses have been changed or removed for privacy purposes.
Company visited:
ABC Company
Somewhere, USA
Purpose of trip: ABC Company manufactures safety-related devices for automobiles. The company was destructively testing 50% of its production components due to inconsistent weld performance. ABC had replaced many parts on its welding machine in an attempt to achieve consistent welding performance, but these actions did not solve the problem. ABC requested an on-site visit from WeldComputer Corporation to determine the cause of the inconsistent welding performance and to provide recommendations on how to correct the problem and prevent bad welds from passing through production without being detected.
Summary: Within the scope of a one day consultation, WeldComputer Corporation 1) provided ABC with a short course on the physics of resistance welding, 2) used a WeldView® Monitor to identify problems with the machine and the process, 3) provided recommendations that enabled ABC to immediately correct those problems, 4) used the monitor to establish more robust weld schedule settings that allowed the operation to produce consistent high quality welds and 5) demonstrated how to set upper and lower monitor limits to non-destructively evaluate the integrity of every production weld.
Report:
This visit involved providing a short course on the physics of resistance welding and then connecting a portable WeldView® Monitor to a projection welding machine. The variables monitored included: combination work piece thickness, projection set-down profile, cylinder pressure, transformer secondary voltage, conductance, power and current.
A weld was produced utilizing the parameters set by ABC for production welding: 1000 pounds electrode force (set by a mechanical force gauge placed under the electrode), 84% heat applied for duration of 7 cycles. The ABC representative destructively tested the weld and reported that it was unacceptable.
The monitor reported that 68.25 milliseconds after the beginning of the weld the cylinder pressure rapidly dropped from 43.8 PSI to less than 20 PSI. The actual trace of pressure with respect to time is shown below.
On the next weld produced the monitor reported that 86.6 milliseconds after the beginning of the weld, the cylinder pressure rapidly dropped from 44.3 PSI to 21.75 PSI. Six more welds were made, and in each case the monitor reported that the pressure was widely variable throughout the weld and inconsistent from weld to weld. Especially in light of this observation ABC should be cautioned that, even though every other weld produced was being destructively tested, such action provided no information about the integrity of the welds that were not destroyed.
Examination of the monitor set-down information revealed that the projection was completely collapsed within the first two cycles of weld heat and that the remaining five cycles of weld heat were contributing nothing to the size of the weld. The actual monitor trace of the projection collapse profile with respect to time is shown below.
Based on this information the WeldComputer representative recommended that ABC reduce the weld heat to 73% and the duration to 3 cycles.
It was also discovered that a pneumatic valve actuating during the weld sequence was contributing to the cylinder pressure variations. Some adjustments were made that reduced the pressure variation, but did not eliminate the problem. Several welds were made with the new settings. ABC destructively tested all of these welds and reported that they were all consistent, good quality welds. It was also noted that the parts now had a superior cosmetic appearance because there were no signs of burnt surfaces or excessive surface discoloration that previously existed on each completed assembly. Based on the reduction in weld time, ABC can expect more than a 60% reduction in the power bill to operate this machine.
Shown below is one of the set-down profiles, typical of the welds produced, along with upper and lower acceptability limits that may be used to evaluate each weld (instead of continuing to perform destructive tests).
The monitor reported that these welds, which were produced at the new heat and cycle settings, had a consistent cylinder pressure of approximately 45.8 PSI. The actual pressure trace is shown below along with upper and lower limits that can alarm the operation and halt welding the instant an unacceptable pressure fluctuation is detected.
The weld quality assurance system provided by the WeldView® Monitor is more reliable than destructive testing because destructive testing provides no information about the welds that are not destroyed.
At this point the WeldComputer representative and the ABC representative visited the soda machine. Upon returning, another weld was made, and the monitor immediately reported a fault. Examination of the monitor display revealed that the cylinder pressure was now far outside of the acceptability limits established for this process. The actual pressure trace from the monitor (shown below) reported that the pressure was now 71.27 PSI.
The above information provided compelling evidence that the pressure regulator should be replaced, so the ABC personnel replaced the pressure regulator.
ABC personnel set up the machine by placing a mechanical force gauge under the electrode and adjusting the regulator until a force reading of 1000 pounds was achieved. While ABC performed this procedure the monitor reported that the cylinder pressure was approximately 60 PSI. This welding machine produces its electrode force by applying air pressure to a cylinder that has a piston area of 9.6 square inches; the cylinder rod transfers its force to the electrode via a class 3 lever with a mechanical advantage of approximately 0.7. The resultant electrode force, which can be computed by multiplying Pressure x Area x Mechanical Advantage, is found to be approximately 400 pounds. It can be concluded from this information that the force gauge used to set up this process has a 60% error and should no longer be used.
After replacing the pressure regulator, 14 welds were produced, and the monitor reported consistent pressure from weld to weld. After making some welds at different pressure settings and rendering a slight heat adjustment it was determined that the best results were achieved (out of the combinations tested) with the following settings: 48.2 PSI cylinder pressure, 3 cycles at 75% heat.
The WeldComputer representative identified a deficiency in the destructive evaluation system. Specifically, it was observed that one of the weld nuggets was cracked in half by the destructive tester. Close visual examination of this weld indicated that its diameter and penetration were comparable to those welds that tore the parent metal. Since the destructive tester is manually operated, the force vs. time function applied by the apparatus on the part can be widely variable; if the operating lever on the tester is pulled quickly then it can be expected that there will be a greater rate of force increase at the site of the weld than if the operating lever is pulled slowly and uniformly. The repeatability of the destructive tester can be improved by replacing the manually operated lever with a hydraulic cylinder that can be programmed to undergo a repeatable motion profile. If a manually operated destructive tester continues to be employed, then in instances when an insufficient amount of torn parent material is observed, the site of the nugget should be closely visually examined to determine if it is a fully formed nugget that was cracked by the apparatus or if it is indeed an undersized weld.
At this point it was past 5:00 PM and the WeldComputer representative had to pack up the monitor and race to the airport.
The following day the ABC representative telephoned WeldComputer Corporation to report that production proceeded throughout the night with every 16th part being destructively tested, and that all of the destructive tests passed.
Recommendations:
1) Monitor each welding process. Remove the guesswork from resistance welding.
2) Use the monitor to verify production welding parameters. Immediately detect an incorrect pressure setting or heat setting that would result in faulty welds.
3) Use the monitor to verify proper machine performance. Save time and money by avoiding unnecessary maintenance procedures and by knowing immediately when maintenance is needed.
4) Use the monitor to verify the quality of every production weld. Improve quality assurance by preventing sporadic bad welds that cannot be prevented with periodic destructive tests from being shipped to customers.
5) Eliminate destructive testing. Increase productivity and save money.