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How to Weld a Lap Joint



The Voice: Hey, Kevin. What are you doing?

Kevin Caron: I'm welding a lap joint. The other day I received an e-mail from one of my YouTube video subscribers, wanting to know more about lap joints. So, let's talk about lap joints and how to weld them.

Here I have two eighth-inch steel plates that I pulled out of the scrap bucket. Now this part would be a butt joint. (That's a different video altogether.) This, on the other hand, is a lap joint, where one piece overlaps the other.

A good rule of thumb to remember is: overlap twice the thickness of your metal. Example: I've got eighth-inch plate, so I want to overlap a minimum of a quarter-inch. I already drew my line on there, so I've got something to line up to.

Then I come over to my table, where I have a little piece of scrap metal that I cut to go underneath the edge. It's eighth-inch also, so now everything is more or less flat and I can get my clamps on.

The Voice: So, you're putting the clamps on the top piece?

Kevin Caron: I'm putting it on the top piece because I've got that little shim underneath it, which will keep the top piece flat. That'll push down on the bottom piece.

Everything is now grounded to the table; I've got my welder grounded to the table, giving me a complete circuit. Now I can go ahead and weld.

First, let me get my helmet, and you get yours, and we'll spark it up.

We've got the Miller 200 sine wave set up for stick welding. This is 6013 rod, eighth-inch diameter to go with the eighth-inch plate that I'm welding, so I should get a nice fillet running down there.

(welding) OK. Let's see what we got.

Using our chipping hammer and some eye protection... hey, it fell off all by itself. That's the slag. This is the flux that's on the metal rod; the flux turns into slag when you weld. It covers the weld while it's still molten and then when the weld cools, you chip this off or sometimes it just falls off like this piece did. And there's a weld underneath.

Now, this is called a single-fillet weld. It's OK for general purposes. It's not very strong for loads that would come from the front in. If this was on the outside and something hit it, it wouldn't be a very strong weld. From the back side, it would because it's stronger.

This is a single-fillet. If you flip it over and weld the other side, it would be a double-fillet. And that's a lap joint. Pretty easy. Pretty quick. Pretty simple.

Let's take it off the table and examine the weld. It's a nice-looking weld, with decent penetration on both sides. There's a little discoloration to the back where the heat was starting to come through. A decent weld, overall. It would probably stand up to a weld test.


Next time, we'll talk about butt joints.

I'll see you all later.

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