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Should You Use a Lap Joint or a Butt Joint When Welding?



A viewer, Steve, asked why you would use a lap joint instead of a butt joint.

A butt joint is simply two pieces of steel - or wood or whatever you're working - put together end to end or edge to edge. When you weld them together, they come out flat.

With thick metal, Kevin chamfers the edges of the metal to get a strong joint. When he's done, he grinds the metal smooth on both sides to make the two pieces of metal become one. You won't even be able to tell where the joint is when it's done.

With a lap joint - it should probably be called an "overlap joint" because you have one piece of material laying over the top of another - you weld the metal along one side, then flip it over and weld along the other side.

So instead of just having two pieces joined, where you're relying on your weld to hold everything together through whatever stress you might put it through, you've have over laid your two pieces of metal so you have a good weld on both sides. It's also welded to thicker metal.

What would you use a lap joint for? Kevin has seen them used to build big steel storage tanks on tank farms. Lap joints are for any place where you need that extra strength and aren't concerned with what it looks like.

It's time to put the talk into action! Kevin is going to chamfer the edges of the metal he's going to weld, then weld a butt joint and a lap joint so you can see how they differ.

Kevin chamfers the edges of the metal. Because it's 1/4" plate steel, he grinds about 1/8" out and about 1/16" deep on both sides. That leaves 1/16" of vertical metal of the two pieces of metal to butt up against one another, with a nice groove in the middle to lay the weld in.

He starts with the butt joint, tacking the metal on both ends. He then MIG weldes the joint with the Everlast 253DPi, Everlast's dual pulse machine.

Next Kevin tackles the lap joint. He tacks the metal and then starts welding the lap joint. Kevin points out that you will need more welding wire for a lap joint as you weave back and forth across the two pieces of metal to make sure you are getting both edges. Under normal circumstances, he would also weld the joint on the other side of the two pieces of metal.

Kevin shows the basic butt joint and the lap joint he just welded. He hopes that answers Steve's question.

He's going back to work, but you might want to stick around for another moment to see Kevin Caron discuss another type of joint ....


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