Fig. 1.8. A plasma cutting torch is similar in some ways to a TIG torch. An arc is formed between a non-consumable electrode and the workpiece. However, the arc is forced to go through a very small hole, which concentrates the heat and raises the arc temperature to more than 30,000 degrees F. That is more than five times hotter than an oxyacetylene flame, and therefore it can cut through most materials and thicknesses.
When air or nitrogen are used as the plasma gas, some nitrogen compounds form in a thin area near the surface of the cut material. If the cut edges are going to be welded, they should be ground to remove this thin layer.
The AWS designation for this process is PAC, for plasma arc cutting. However, few folks use that acronym. In this book I use the shortened form, plasma cutting.
Fig. 1.9. Bob Gage, working for the Linde Development Labs (my old employer), invented plasma cutting in 1957. Welding was mentioned in the patent, but its use for cutting has gained much more popularity. Initially, plasma cutting used nitrogen as the plasma gas. Today, however, manual systems mostly use compressed air as the plasma and cutting gas.
Hundreds of joint types are used in welded fabrication. Butt joints, tubular structural joints, and fillet welds are the most common. Complex joint designs are used for welding thick sections, and for these complex joints, J- and U-grooves are used to reduce the amount of filler metal required to complete a weld. Also, many joint types are used to weld sheet metal and tubular structures, which are employed in various industries. Fabricators developed many of these weld joints as an efficient method of achieving the fit-up needed to make consistent quality welds.
Various fabrication specifications exist that define specific welding procedures and detailed welding amps, volts, and travel speed ranges for these joints. This allows a fabricator to use specific joint types without the need to prove the joint can produce the required weld quality. A number of these weld joints are shown in this chapter and may provide ideas for their use in a specific street rod or race car project.
Fig. 2.1. Hundreds of joint types are used in welded fabrication. A number of more complex joint designs relate to welding thick sections, where J-grooves and U-grooves are employed to reduce the amount of weld metal needed. There are also many joints defined for use in sheet metal, such as for ductwork, that could be used in street rod applications.
Structural tubular joints are often used for race car chassis and roll bars. In addition, exhaust systems use thin-wall tubes that must be joined. A number of industries use tubular members for the fabrication of structures, such as building members and highway sign supports. These designs are subjected to varying loads. Designers use sophisticated stress analysis techniques to optimize the use of materials. Some of this design and welding information can be useful in race car and street rod fabrication.
Simple square butt welds are often used in automotive-type welding. Variations may be useful to provide increased strength. Welding from one side only can leave some of the root area unwelded. This leaves a stress concentration that can cause a crack to form in the weld. Where maximum strength is required, a full-penetration weld should be used. TIG can produce full-penetration joint welding from one side, but you need to carefully control the penetration and be sure the full joint is melted.
Fig. 2.2. Fabricators have developed many weld joint designs as efficient methods for achieving the fit-up needed to make consistent quality welds. A number of industries that use tubular structural components have developed design criteria and weld procedures, and these can be adapted to race car and street rod fabrication.
Fig. 2.3. A full-penetration weld should be used to achieve maximum strength. In the bottom left panel, the joint shown is very useful for somewhat thicker material. First, a weld is made in a single V-joint to achieve good penetration. Then the back side is gouged or ground into sound weld metal, making a U-groove. A second weld is then made in the U-groove to fully penetrate into the first deposit.
When you can weld from both sides of the joint, a full-penetration weld is easier to accomplish. For thin material, the edges can be butted together, a weld made on one side, and a weld made on the back side that fully penetrates into the first.
A single V-preparation for butt joints is a proven method accepted for a number of design specifications and can ensure a full-penetration weld is achieved. V-preparation used for a full-penetration joint is particularly useful for thicker materials, such as 3/16- and 1/4-inch plate. By first using a single V-preparation, leaving half the plate thickness as a land accomplishes two things. It ensures good penetration on the first weld and leaves a land under the V that prevents excess penetration where the fit-up is not perfectly tight.
The V can often be made with a grinder, but you must be careful to leave half of the surface as the land. The first weld is placed into the V-groove. It should be made with sufficient current and speed to penetrate about three-quarters of the plate thickness.
Fig. 2.4. These joints are suited for welding sheet metal. The upper left joint is commonly called a joggle joint or flange joint. The official AWS Sheet Metal Code name is an offset lap joint. Whatever it’s called, this is an excellent joint when welding a patch panel. Simple locking-type pliers are available that can progressively form the edges providing a backing for the subsequent weld.
Fig. 2.5. Fabricators, including those making air handling ducts and tractor cabs, weld sheet metal. They have developed a number of joints that make it easier to weld specific sections. Some are useful for specific automotive applications. Flange joints make welding easier and may require less heat input. Backing a weld, such as the corner weld shown, adds strength and allows less precise fit-up.
Then the back side is gouged or ground into sound weld metal by using a grinder held on its side or an air-powered chipper with the proper groove should go sufficiently deep, so the bottom reaches defect-free weld metal in the first-side weld, and it should result in a U-shaped joint.
A second weld is then placed in the U-groove with sufficient current, so it fuses into the groove on the first pass. The resulting weld should overlap about 20 percent of the joint thickness. This overlap eliminates any root defects that may have been created in the first weld.
A J-groove is essentially half a U-groove and can be employed where the edge of a thick plate butts to a vertical member, as might be encountered in a cross-brace attachment to a side frame rail. As with a U-groove, a J-groove minimizes the amount of weld metal and weld heat while still ensuring adequate penetration.
Square butt welds made in sheet metal require very close