quality it is essential to have very good fit-up with minimum gaps. Time spent in joint preparation saves time in welding and produces the best quality structure. In Chapter 4, examples are shown of both proper and improper joint fit-up. In some instances small grinding wheels or abrasive cartridge rolls may be employed to achieve the desired maximum gaps of about .010 inch for thin tube walls such as .040 inch. For .062 and thicker wall tubes, .020-inch maximum gaps should produce satisfactory welds.
Fig. 2.7. The AWS Specification for Automotive Weld Quality—Arc Welding of Steel, defines the names for this series of sheet-metal weld joints. Arc and plug welds are commonly used for street rod fabrication. A plug weld is made through a premade drilled or punched hole. For thicker materials, a fillet weld can be made in an elongated slot.
Gussets can be used on tube joint intersections to stiffen the assembly. They are particularly useful for some high-strength materials such as 4130 chrome-moly, where a somewhat lower-strength, more ductile welding rod and smaller weld size can be offset with the added strength supplied by a gusset. An example of the use of a gusset is shown on a NASCAR roll cage in the lower right of Figure 2.2.
A fillet weld is a triangular-shaped deposit commonly used for many joints where two materials to be joined intersect at angles. In instances where two flat plates are joined there is little joint fit-up required. However, for fillet welding intersecting tubes, the complex joint geometry must be properly cut and matched to achieve the needed maximum gap of .010 to .020 inch. It is also important to ensure the bottom of the fillet weld, at the intersection of the shapes being joined, is melted and fused. It is possible to make a fillet weld having a good surface appearance that is not properly fused in this bottom area, called the root of the fillet.
Fillet welds are often partial-penetration welds and require a reduction in allowable loads because of the gap left at the weld root. The reduction factors depend on the exact joint, the amount of penetration, the type of loads involved, and design specification requirements.
Fig. 2.8. Fillet welds are considered partial-penetration welds and require a reduction in allowable loads because of the unwelded area at the root. The amount of reduction depends on specification requirements. A single fillet has the highest stress concentration because of the loading. The double fillet is better to use, and the full-penetration double-fillet weld in which the root gap is eliminated is best.
A single fillet has the highest stress concentration because of the loading. If loaded so that the joint is bent toward the weld, the stress at the root of the weld is significantly increased.
A double fillet weld is better because, although an unwelded area exists, when a side load is applied the stresses are shared by the two fillet welds and the root stress concentration is not as high as with a single fillet.
A full-penetration double fillet is the best joint because there is no unwelded area.
The commercial automotive industry has developed its own standards for the types of welded joints. These sheet-metal weld joints are defined in the AWS Specification for Automotive Weld Quality—Arc Welding of Steel. They include plug and spot welds, which are commonly used for street rod welding.
The difference between these two welds is that a plug weld is made through a premade drilled or punched hole while a spot weld relies on the arc melting through the top sheet and into the bottom sheet. This works well for thin sheet metal and quality can be ensured by having accurate times along with control of amps and volts.
The timing for a spot weld should start after an arc is established by having the welding machine start the timing sequence only when voltage and amperage are detected. If more strength is needed, for example on heavier top sheet materials, welds can be made in an elongated slot.
In Chapter 1, I mentioned oxyacetylene welding as an ideal process to learn and practice in order to gain the fundamental knowledge of fusion welding. Melting of the base material occurs slowly and is easy to observe. Filler metal is added separately and the two-hand technique used is similar to what is employed for TIG welding. Since it is a slower process than TIG, the manual skills are easier to practice. Maintaining a fixed distance from the welding tip to the work is not as critical as with TIG, which also helps develop manual skills.
In addition, the equipment needed is relatively inexpensive and can be used for welding, brazing, cutting, and heating. With the proper attachments, it is ideal for cutting steel of any thickness or heating metal for bending.
Therefore, although it may not be widely used in automotive work for general welding, it is good to review the basics before discussing TIG welding (see Chapter 4). It may be the proper tool to weld a very lightweight chassis made from small-diameter 4130 chrome-moly tubing (see Chapter 7). In addition, there are some unique joining applications, such as repairing a crack in cast iron, when braze welding with oxyacetylene is often preferred. The simplicity and relative low cost of oxyacetylene welding is another reason to consider its use for a number of applications.
Fig. 3.1. Oxyacetylene welding is an ideal process to learn and practice to gain the fundamental knowledge of fusion welding. The equipment is inexpensive and flexible to use. With the proper attachment, it is ideal for cutting steel of any thickness or heating metal for bending.
Fig. 3.2. Two cylinders, one containing oxygen and the other acetylene, supply these gases through long hoses to the torch. The gases are mixed before exiting through a small hole in the torch tip and ignited. For welding, acetylene is the only practical gas to use. (Figure adapted from ESAB’s Oxyacetylene Handbook with sketch by Walter Hood)
An oxyacetylene welding rig consists of two cylinders with regulators and hoses attached to a torch.
One cylinder contains oxygen and the other acetylene; they supply these gases to the torch through long hoses. Valves in the torch control the flow of gases. The gases mix before exiting through a small hole in the tip and are ignited. Different-size tips are available for welding various thicknesses of materials. For welding, acetylene is the only practical gas to use. It has the hottest inner cone temperature of any fuel gas (5,720 degrees F). Other fuel gases are acceptable for cutting because the hottest flame is not necessary.
Oxygen Regulator—Avoiding Explosions
