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Welding Butt Joints

The first weld joint you must learn is the butt joint. While the basic square butt joint is fairly easy to master with the right amount of practice, there are many other butt joints that you need to have in your repertoire. Those include pass square butt, joggle joint, single V butt, single V weld with backing, lap joint weld, and many others.

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. 2.1. 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.

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manyweldjointFig. 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. 

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.

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. Thenthe 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.


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.

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 fitment. Several techniques are employed to make welding these joints easier. One approach is making a joggle or flange joint used to fabricate propane tanks, fire extinguishers, and other thin sheet-metal vessel end-cap welds. With this approach, one edge of the joint is formed so the joining plate fits over the bent area. This provides back support for the weld, and it’s more tolerant of slight fit-up variations.

Offset lap joint is the official AWS Sheet Metal Code name for this type of joint. The weld itself is referred to as a flare-bevel weld. Whatever you call it, this is an excellent joint when welding a sheet-metal patch panel. Simple locking pliers are available with dies welded to the grip faces, from companies such as Eastwood, that can progressively form the edges, providing a backing for the subsequent weld. There are air-powered devices that provide the same progressive crimping and make the task for preparing the panel faster.


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.


A number of industries fabricate sheet-metal parts such as air ducts and tractor cabs. A number of joints have been developed to make it easier to weld specific types of sections. Some of these designs, which include flange joints, may be useful for specific street rod applications. These flange joints, as they are referred to, make welding easier and may require less heat input. Melting the edges of a flange butt weld, as shown in Figure 2.5, is easier than making a square butt weld in sheet metal. In addition, the edges can be easily clamped together and the joint tack welded prior to final welding of the seam.


The same fit-up and welding benefits exist for the flange corner weld. Backing a weld with another part, for example in a corner weld, adds strength and is more tolerant of less-than-precise fit-up.


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, like the corner weld shown, adds strength and allows less precise fit-up.


Written by Jerry Uttrachi and Posted with Permission of CarTech Books


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