Page 34 - MetalForming-Sep-2018-issue

Welding Well
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resistance, making the gun hotter and the arc less stable. Conversely, non- threaded tips feature a larger mating surface and all-copper conductive path. They run as
much as 40-percent cooler and increase arc stability to
35 percent. A
more stable
arc reduces
spatter, and cooler tips last longer because cooler metal stays harder and provides more resistance to wire abrasion.”
Lastly, after a nonthreaded tip starts to wear (keyhole), the operator can rotate the tip several times to create a fresh contact surface. Threaded tips either seat in the same spot or cam- style tips seat two spots 180-deg. apart; in either case, they will wear more quickly.
To assess arc stability, Hassan used a scatter plot graph that maps weld parameter (voltage and current) vari- ations on the X-Y axes.
“Plotted data from a nonthreaded tip shows up as more of a rifle shot because of the consistency,” says Has- san. “Threaded-tip results look more like a shotgun blast because of their variability.” As an aside, he notes that increasing use of data capture and analysis systems will continue to shed new light on actual versus perceived welding performance.
Johnson adds that, “Nonthreaded tips have a unique bore length that provides a much lower chance for microarcing within the bore, so there’s less chance of burnback.”
Further, nonthreaded bores have the smoothest bore ID possible because they are made on Swiss-style CNC turning machines. Drilled and swaged bores common with value- priced contact tips have a much rougher surface.
“My research proves that a smooth and controlled bore minimizes debris buildup and extends tip life,” states
[From top to bottom]: A medium-duty gun with threaded consumables, a medium-duty gun with nonthreaded consumables and a light-duty gun with nonthreaded consumables.
Hassan. Rougher bores catch more debris, typically drawing compound as well as dirt and metal filings caused by wire deformation from drive-roll pressure. Inevitably, the “snowplowing” effect of debris buildup restricts wire flow to the point where the arc stum- bles, leading to a burnback and the end-of-tip life.
Parameter Shift
Excessive heat is the root of all evil in welding-consumable performance, directly affecting the consumable life, overall weld performance and economy of a welding system. According to research conducted by NIST Boulder Laboratories, factors contributing to the heat input of a typical GMAW con- tact tip are approximately as follows: 3 percent from Ohmic heat (internal resistance of the GMAW gun system); 37 percent radiative heat from the molten metal and 60 percent resistive heat (voltage drop at the arc).
Welding current, voltage and electri- cal resistance follow Ohm’s law: V=I/R (V = voltage, I = current through a con- ductor measured in amps and R = resist- ance or impedance of electrical current). The electrical resistance of a system and current are inversely proportional to each other. High resistance increases voltage drop in the system and reduces current flow. To adjust the current, volt- age must be increased, all of which increases total wattage of the system, ultimately equated to higher energy loss
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MetalForming/September 2018
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