Welding is a process that joins two metals pieces together to create structures. These structures can include bridges, cars, pipelines, and even the chairs that people sits on. Welds that are created with care will be able to stand for many decades, even if the welded metal is under stress.
A bad weld, however, will fail at the worst possible time. The secret to good welding does not usually involve the welding machine or the welding rods that is used to create the weld. Instead, the secret to welding involves the joint that is selected prior to striking the metal with the welding arc.
If the joint is created properly, the weld will take care of itself. Conversely, if the wrong joint is created, there will be trouble in creating a good weld. Every fabricator who works with metal eventually learns of five basic joint types that can be used in metal fabrication.
These five joint types accounts for nearly all of the structures that are built with metal. By learning these five joint types, a fabricator can read the blueprint for the structures that are to be fabricated, and understand how the metal pieces will fit together before a single square is grabbed from a toolbox. Each joint type differs from the other in ways that are both subtle and important to those who watch the joints crack over time.
The Five Essential Welding Joint Types
1. Butt Joints
Butt joints are the first type of joint that many individuals thinks of when they hear of welding. A butt joint is formed by placing two metal pieces edge to edge so that the two faces of the metal pieces touch with one another. When formed correctly, the weld will be flush with the metal and will not interrupt the metals surface.
Butt joints are used when the weld must penetrate the metal pieces by beveling the edges of the metal. However, alignment of the two metal pieces is difficult when using this type of joint. If the metal pieces are not aligned perfect with one another, it is necessary to use grinder wheels on the joint prior to welding, and it is likely that more welding metal will be needed than is planned for the joint.
2. Lap Joints
A lap joint is formed in a different way than a butt joint. The lap joint is formed by sliding one metal piece over another. This joint type permit for the weld to have a larger surface area of the metal pieces.
Consequently, a lap joint allow for more forgiveness in the fit-up of the metal pieces. Because alignment is not critical to creating a good lap joint, this type of joint is commonly used in fabricating sheet metal pieces or performing metal repairs. However, the geometry of the joint may cause a problem for the joint.
Because the metal pieces are offset from one another when forming the joint, the joint may fail if it is placed under tension. However, if both sides of the overlap are welded, the joint becomes strong enough to handle the forces that may be placed on it.
3. T-Joints
A T-joint is formed when one metal piece is placed perpendicularly to another metal piece. An example of this type of joint is the joint between the tabletop and the tabletop leg. T-joints are used because they have good resistance against bending. Both sides of the upright metal piece are welded to form fillet welds.
The advantage to the use of T-joints is that the fillet welds require no beveling of the metal edges. However, if the metal pieces are of different thicknesses, the heat from the first fillet weld may distort the thin metal piece before the second fillet weld counterbalances the heat from the first weld.
4. Corner Joints
A corner joint is formed by placing two metal pieces together in such a way that they form an L shape. Corner joints are seen in many metal structures. The advantage to a corner joint is that the outside of the joint is readily accessible for welding. Consequently, a fillet weld can be created along the outside of the corner joint.
If more strength is required of the joint, the inside of the corner joint can be opened to allow for a groove weld. However, one potential problem with corner joints is the potential for burn-through of thin metal pieces. The metal joint burns too quickly due to the thin metal pieces.
Consequently, care must be taken in creating a corner joint so it is strong but does not burn through the thin metal. If the corner joint is created correctly, it will feel as solid as one metal piece; if not, there will be metal patching to perform later that day.
5. Edge Joints
Edge joints are created when two metal pieces are aligned with one another so that their edges touch, and then fusing those two metal pieces together while using minimal welding metal. Edge joints are fast to create, use minimal metal, and have no visible weld bead. Consequently, edge joints are used in the construction of food processing equipment or decorative metal railings. The disadvantage to edge joints is that they have little area to distribute the force that is placed onto the joint.
Consequently, edge joints are only used in locations where there is little metal stress. The use of an edge joint where metal stress is common is not effective. An edge joint may look nice, but it will fail when under serious tension.
Each of the five joint types has a place within metal fabrication. Each joint has been tested over the decades to find weaknesses and strong points. Butt joints require a high degree of humility regarding alignment.
Lap joints allow for speed in fabrication even if the fit-up to the metal joint is not perfect. T-joints and corner joints require welders to understand the effects of heat on metal structures. Edge joints require metal fabricators to understand that the smallest joint may perform the most important function.
Furthermore, an experienced metal fabricator who walks past a pile of metal metal structures will unconsciously form the shapes of the five joint types with one of the fabricators fingers. This demonstrates the importance of each type of joint to metal fabricators. The next time that you have two metal pieces that you would like to join together, take a few seconds to think about the type of load that may be placed upon the joint, the accessibility to the joint, and the potential for distortion in the metal piece.
These few seconds will allow you to consider the correct type of joint to create between the metal pieces. By selecting the proper metal joint, the metal will begin to work with you rather than against you. That is the ultimate goal of welders of metal joints.
