Tech Directions - December 2008 - (Page 16)

Choosing the Right CC Welding Unit Student Success Depends on It By Neal Borchert NDERSTANDING the personality of a constant current (CC) dc welding machine can make the difference between a successful or unsuccessful weld test or between a student who pursues a career in welding and one who may quit in frustration. A true story: A young journeyman (we’ll call him “Mike”) was testing to qualify for a stick pipe job using unfamiliar equipment. Mike failed several tests in a row, something that had never happened before. Was it his fault? Did he not practice enough? U ● Stick welders that can TIG weld. An example of this type of welder would be a dc generator, an “8 pack” magnetic ampliﬁer-type unit or traditional welder with a big mechanical rheostat for amperage control. Older welders call these machines “droopers” because of the shape of their volt/amp curve. (See Fig. 1.) ● TIG welders that can stick weld have more of a “vertical” constant current output. (See Fig. 2.) An example of this type of welder would be a conventional TIG machine (this is the type of welder that gave Mike ﬁts with an E7018 rod). Fig. 1—The drooping volt/amp of a traditional stick unit (which can also TIG weld) enables the operator to control heat input and the shape of the weld bead by manipulating the electrode. No, Mike had the right stuff but the wrong machine. An instructor saw Mike struggling, put him on a different welding machine, and his next weld was ﬂawless. What Mike and many others don’t understand is that CC welders have two different “personalities.” You can learn a lot about a welder’s personality by looking at its volt/amp curve, the “birth certiﬁcate” of a welder: Neal Borchert, business development manager at Miller Electric Mfg. Co., works extensively with schools. There are also welders that stick and TIG weld equally well, which often use inverter or chopper technology. (Mike used an inverter to pass his weld test). Welders with “multiple personalities” may be able to produce both diagonal and vertical volt/amp curves (Fig. 3). Reading a Volt/Amp Curve To interpret a volt/amp curve, you must understand the relationship between arc length and voltage. A basic fact of the arc welding process is that as arc length increases, voltage goes up; as arc length decreases, voltage goes down. Manufacturers design stick welders in such a way that as the operator changes arc length, the amperage does the opposite of voltage. In other words, if the operator decreases arc length (lowers voltage), amperage increases. If the operator increases arc length (increases voltage), amperage decreases. In conjunction with this information, another basic fact that operators need to remember is that voltage is electrical pressure. Pressure (voltage) controls the height and width of the weld bead, while amperage controls penetration. Armed with this knowledge, let’s re-examine Fig. 1. Experienced stick welders know that “pulling a long arc” (increasing voltage while lowering amperage) enables them to create a ﬂat, wide bead with shallow penetration. Long arcing also causes the weld puddle to freeze faster because it lowers the total amount of energy available. Conversely, pushing the rod closer to the joint (increasing amperage while lowering voltage) creates a narrower weld bead, deeper penetration, and a more ﬂuid (hotter) weld puddle. Thus, stick welders with “drooper” characteristics allow the operator to control weld bead quality and appearance by manipulating the electrode. However, this same welder, while ideal for stick, is less than ideal for TIG. Imagine that you’re welding an outside corner on 18-gauge stainless steel. If, for whatever reason, you shorten the arc length (lower voltage), a “stick welder that can TIG weld” will attempt to maintain total 16 techdirections ◆ DECEMBER 2008

Table of Contents Feed for the Digital Edition of Tech Directions - December 2008

Tech Directions - December 2008 Contents Advertisers Index Technically Speaking Direct from Washington The Report Technology Today Technology’s Past Mastering Computers Digital Portfolios—Powerful Marketing Tool for Communication Students Choosing the Right CC Welding Unit—Student Success Depends on It Think Green—Increasing Awareness of Garbage and Recycling Think Green—Teach Students Smart Ways to Reduce Home Energy Use Go International! Build a Simplified Shortwave Receiver Mooresville Middle School Snags Web Site of the Month Gifts for Geeks More than Fun

Tech Directions - December 2008

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