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Tom Snow
Chairman

Resistance Welding Is a Science

December 1, 2018

Low current values are displayed in the weld log.

In today’s pressure-packed just-in-time manufacturing environment, proper troubleshooting procedures for resistance-welding problems often are ignored in favor of low-cost attempts at quick fixes.

This, however, can lead to continued quality issues and containment costs that far exceed investing in a scientific, data-driven solution. In other words, resistance welding should be viewed as a science and not “black art.”

The Challenge

Miles Sonnenberg of HTT, Inc., Sheboygan, WI, is a great example of a manufacturing engineer who made the leap from guessing at solutions to resistance-welding-quality issues to quantifying and solving them properly.

Founded in 1986 as High Tech Tool & Die, HTT specializes in deep-drawn stampings. As a value-added secondary operation, the company attaches brackets to round parts, each with a single spot weld. Since it’s an automotive application, that lone weld must be strong.

However, HTT had significant weld-quality issues related to nugget size and strength, and adding to the challenges, these weld-quality issues varied between shifts. As a result, HTT’s customer required a time-consuming, 100-percent quality check of the parts supplied, hurting production and profit margins.

The Journey

Sonnenberg’s first step in solving the weld-quality issues was to identify and define the following possible causes:

• Poor tooling design or condition;
• Improper electrode (tip) dressing;
• Changing key weld parameters;
• Obsolete, 30-yr.-old welder control; and
• Absence of a way to monitor key weld quality variables, such as current and force.

After purchasing new tooling, Sonnenberg performed an electrode-tip-life study to address when the tips needed to be dressed or changed. This improved weld quality, but not enough to satisfy the customer, which continued to demand 100-percent quality checks.

At this point, Sonnenberg called in an experienced outside resistance-welding consultant, who recommended a new weld control with process monitoring and other features available in several brands of controls:

• Current monitoring. Generates a fault if the welding current falls outside of preset limits.
• Constant current. Automatically compensates for welding-current losses due to changes in the incoming line voltage or secondary circuit impedance.
• Differential pressure transducer. Monitors pressure at both ports of the air cylinder and does not allow the machine to fire until the proper weld pressure is reached.
• Electronic pressure control. Allows the weld force to be set from the weld control, thereby eliminating the manual regulator that operators could previously access.
• Weld log (see Weld Log table). Creates a date and time stamp for every weld sequence, including key parameters such as weld time, weld current and pressure/weld force.
• Error log. Generates a date and time stamp each time that a fault is generated.

In addition to purchasing the new, state-of-the-art welder control, HTT also invested in onsite installation and training by a certified resistance-welding technician who set up the initial weld schedules.

Extra Effort

Graph shows the results of a process study using a consistent weld pressure and various weld currents to find the optimum current setting.

At this point, Sonnenberg and his associates at HTT put in the extra effort to ensure long-term resistance-welding success. He educated himself on the resistance-welding process and learned how to use all aspects of the control.

Also, he performed a process study using a consistent weld pressure and various weld currents to find the optimum current setting (see Force vs. Amps graph).

In addition, machine operators no longer were allowed to adjust weld-schedule parameters. To compensate for tip wear, they could only select preset proven weld schedules.