section was purposely changed from “Oxyacetylene Welding” to “Oxyfuel Cutting.” The AWS term for the process is OFC (oxyfuel cutting). OAW is commonly used for welding because acetylene is the only gas that can be effectively used for welding, while a number of other fuel gases can be used for cutting. In fact, natural gas can be used for automatic cutting machines.
Fig. 1.7. Similar to the oxyacetylene welding torch, a cutting torch uses a special tip. Mixed oxygen and fuel gas exit multiple holes around the outer edge of the tip producing high-temperature flames. A large center hole in the cutting tip emits a high flow rate of pure oxygen, and this oxygen does the cutting. The flame preheats the steel and the oxidation of iron generates the heat to maintain the cut.
The process for cutting steel is shown in Figure 1.7. Similar to a welding torch, a cutting torch utilizes a special tip that has multiple small holes around the outside of a larger center hole. These small holes flow mixed oxygen and fuel gas and produce the high-temperature preheat flames. A larger center hole in the cutting tip has a high flow rate of pure oxygen only, and that oxygen does the cutting.
The hot outer flames preheat the steel, and the oxygen converts the hot iron to iron oxide. This chemical oxidation generates the heat to keep the cut going! Therefore, any fuel gas can be used to start the process, but depending on which fuel gas, it may take longer to start the cutting action. However, with a steady hand, the cut can be made.
Acetylene is often the preferred fuel gas. If you don’t have a steady hand, the cutting may stop with other fuel gases. When using the hotter, more intense acetylene flames even with slight hesitation it continues cutting, and that makes it easier to operate.
I found that out by experience when I was troubleshooting a welding job and needed to take a sample of a 1-inch-thick weld on the airplane. All of the weldors and cutters were at lunch so I offered to make the sample cut. I picked up the torch and proceeded. My experience had been cutting with oxyacetylene. This torch was using propane as a fuel gas. After a number of starts and stops, the cut was made, but it wasn’t pretty!
Bob Gage, working for the Linde Development Labs (my former employer), invented plasma cutting in 1957. Welding was discussed in the patent, but the process has gained much more popularity for cutting. Initially, the process used nitrogen cutting gas; today, manual systems mostly use compressed air as the plasma gas. The process creates an arc between a non-consumable electrode in the plasma torch and the workpiece. However, unlike TIG welding, the arc is forced to go through a very small hole that concentrates the heat and raises the temperature of the exiting plasma gas. The exiting gas in the center of the arc column reaches more than 30,000 degrees F, and that melts and blows away any conductive material. The innovative design and the rapidly swirling plasma gas in the nozzle throat allows the much-lower-melting-point copper nozzle to remain unharmed from a 30,000 degrees F arc coming through its small-diameter orifice.
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.
