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What to look for in a spot welder

Tips and insights to find the best spot welder for your shop’s needs
Thursday, May 19, 2016 - 07:00
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When my dad purchased his first personal computer, I remember him saying, “It has a one megabyte hard drive, more storage than I will ever need.” Now, we routinely send media text messages from our phones much larger than that.

A spot-weld cross section
CTR7 MMI showing weld results

While changes in computer technology are extreme, similarly changes in the automotive industry are not far behind. The breakthroughs in building vehicles lighter, more fuel efficient and safer means a whole host of changes for the body shop. Just as that old PC cannot keep up today, expecting your old spot welder to safely repair newer vehicles is also unrealistic. So what has changed and how does that affect the purchase of a new Squeeze Type Resistance Spot Welder (STRSW)?   

The basics of resistance spot welding have not changed from Elihu Thompson’s original discovery in 1885. When electric current runs through metal sheets that are tightly clamped together, the inherent resistance to that flow generates heat and creates the weld. The combination of these welding parameters — welding current, weld time and squeeze pressure — creates a molten pool that forms the weld nugget.

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With regard to welding parameters, one might assume more is always better, right? Actually, that is not the case. First, consider welding current. Too little current results in no fusion, while too much will overheat newer steels, taking the strength out of the weld. Second, consider squeeze pressure. Limited pressure will have sparks flying everywhere (expulsion); however, too much can limit the size of the weld nugget; the excessive pressure reduces resistance, meaning less heat generated. Third, consider weld time. The time and current work together to create the right amount of energy. If you have too little or too much energy, the weld suffers. That total amount of energy varies depending on the thickness and type of material. The three parameters must be combined correctly to create a proper weld. So how do we know the right combination? Welders can learn through practice and experience, and also get some added assistance from a smart welder.

What makes a welder a “smart welder?” At its simplest, we can think of it as fully automatic versus manually setting the weld parameters.

  • Manual – Traditional welders have just two dials. The technician sets the welding current and weld duration themselves. Without exact information from the OEM, the technician must perform sample welds and destroy them until they find the correct settings for the material they are welding.
  • Semi-Automatic – Testing is done and parameters are built into the welder. The technician determines minimal information, usually the material type and thickness, and the welder sets the actual parameters.
  • Fully Automatic – As part of the weld process, the spot welder determines the material type and thickness itself, then sets all of the parameters for the technician. Thus the concept of “Pull the trigger and weld.”

In practice, this means when welding a B-Pillar, the smart welder self-adjusts every time the stack up changes. Without this technology, the technician must recognize the change and set the welder manually for the new conditions.

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