Page 90 - MetalForming-Oct-2018-issue

Welding Well
By Tom Snow
Resistance Welding Machines Require Respect
One of my father’s favorite stories from his 50-year career selling resistance welders involved an emergency service call to a customer’s plant. Upon arrival after a 7-hr. drive, he found that the welder was filthy and not something he wanted to work on in its current state. Having skipped lunch to respond quickly to the emergency, he asked the customer to have the welder cleaned while he got something to eat.
The customer did what my father asked, following his instructions to take all of the welding machine’s secondary electrical connections apart, from the transformer all the way out to the tips, and to polish the copper mating sur- faces before reassembly.
When my father returned to the plant, the customer had the machine running like new and said that a service call was no longer required. My father, of course, explained that his travel expenses still applied.
The lesson here: Simple preventive maintenance often is key to avoiding resistance-welder breakdowns. In other words, respect the equipment.
Defeat the Resistance
Corrosion had built up in this welder’s secondary electrical connec- tions due to many years of neglect. The added resistance eventually prevented
TomSnowisCEOofT.J. Snow Co., Chattanooga, TN, a supplier of resist- ance-welding machines, supplies, service and training. Snow, the imme- diate past chairman of the Resistance Welding Manufacturing Alliance, a standing committee of the American Welding
Society, shares his resistance-welding insights in MetalForming magazine’s Welding Well column every-other month.
Tom Snow, CEO
T. J. Snow Co. tomsnow@tjsnow.com
Fig. 1—Remachining surfaces may be required to restore full power to the welding machine.
the proper flow of welding current from the transformer through the machine’s secondary circuit and to the spot-weld- ing tips. With the welding current out- put typically transmitted from the trans- formertothepartatonly6to8V, almost any resistance in the circuit is too much.
As a nonengineer, I always think of a resistance welding machine’s ability to deliver welding amps in terms of a garden hose. Secondary voltage oper- ates like water pressure and pushes the welding amps out to the tips. Sim- ilarly, water volume is like
welding amps in that more
pressure equals more flow. Continuing our garden-
hose analogy, a kink in the
hose will restrict or com-
pletely cut off the flow.
erates the problem.
Loose connections are common and
eventually will lead to arcing between the mating copper surfaces, which causes pitting, reduces the contact area and limits welding current. Remachin- ing these surfaces may be required to restore the welding machine to full power. (Fig. 1)
We recommend breaking down a machine’s secondary electrical con- nections at least once per year for cleaning and polishing. Some users even silver plate the connections to improve conductivity.
If replaced during maintenance of a welder’s secondary connections, be sure that bolts are nonmagnetic, such as brass or stainless steel.
May the Force be with You
Because resistance welding is a forg- ing process similar to an old-time blacksmith hammering red-hot metal together until it fuses at the joint, prop- er operation of the welder’s force-deliv- ery system is critical to weld quality.
To form a strong weld nugget at the interface of the welded part, one or both of the opposing electrodes must move minutely at the instant the metal achieves a molten state. This
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Causes of current-flow problems include loose connections, oxidation, dirt, grease and other con- tamination. In addition, as the secondary circuit dete- riorates and generates more resistance and heat, it feeds on itself and accel-
Fig. 2—Accumulation of contaminants within water pas- sages can cause a welding machine to overheat.