In this article on joint preparation we are going to learn how to prepare metal for welding so that you can get the best weld possible.
When do you need to prepare a joint for welding?
A rule of thumb is to prepare the joint (bevel it) if the plate is 3/16 thick or more.
Some say 1/8 of an inch more.
In the Tee joint image above no beveling was needed. But regardless of the metal thickness you still have to prepare it by cleaning it and removing rust, oxides, mill scale etc.
Warning: When preparing aluminum (or aluminum alloys) do not use sodium hydroxide or any type of cleaner with a pH that is higher than 10 because these types of cleaners will have a chemical reaction. You should research what type of cleaning and or preparation method is proper for each type of metal you are going to work with because some react differently to others and can cause severe health hazards.
Side Bar: we are going to talk about beveling edges because that is the most common way to do joint preparation.
Joint preparation is a more technical term used in the welding industry to describe how you prepare metal for welding.
See, when you weld something you are usually welding two pieces of metal together to form a joint. And unfortunately many welders do very little if any joint preparation. It's very important because it's crucial to achieving a good weld.
If you don't do any joint preparation you will end up with 'tall' welds (meaning they are sticking up more than they are getting closer to being flush with both pieces of metal), and on a more serious note you may get poor fusion.
Side Bar: Ron Covell covers a really cool Plasma cutting technique for beveling edges on thicker material in Tig Welding Basics.
Fusion occurs when your filler metal fusses with your base metal. You get good fusion when the base metal side walls melt and fuse with the molten weld metal (filler metal) you are using to to weld with.
As you know by now, any product you weld together must have welded joints, otherwise it's not usable. So when you weld to pieces of metal together you are welding a joint.
The joints that you weld become what is called a 'weldment'. That's a technical term you don't really need to dwell upon, but it's good know know.
And when you weld a joint together you will use your wire or rod (which is your filler metal) to fuse the joint. By the way, the base metal is your metal that you are welding together (or cut).
As we have discussed, how good your joint that you are welding is depends not only on how well you weld it, but how well you prepare it.
Prepare metal well and it will reduce contraction and expansion from cooling and heating. If you do not prepare metal well heat that radiates to the base metal as you weld will be low (or lost) and your base metal will not fuse properly with your filler metal.
Conversely you do not want excessive heat either. There is a balance you are trying to achieve and joint preparation is crucial to achieving it.
The first step to preparing metal for welding is to remove all the impurities otherwise your weld will not be
a good one. So remove rust, mill scale, and oxides. If you do not do
this they will get into your weld and ruin it or make it ugly and weak.
Metal edge preparation is crucial in allowing the filler metal and metal edge walls to fuse without too much melting.
As we discussed in the article on the types of joints, there are 5 joints that you will be creating for your projects:
I just want to remind you to read 'the first step' above first...
You can prepare a butt
joint by using several different techniques including using a flame,
chipping, shearing, cutting, and more. For most metal preparation you will use a standard grinder.
For welding light sheep metal 3/8 of an inch to 1/2 inch plate you can use the single V groove joint or single U groove joint (in other words, the beveled edges look like a V or a U in a cross section view. See second image below)...
In the image above, look at B (the Double V Joint, upper right corner). This should be used in plate that is 1/2 of an inch up to 2 inches thick.
If you are going to weld plate that is 3/4 of an inch and up look at the lower right hand corner of the image above (D - Double U Joint). This is the preferred method of joint preparation for this thickness of material.
The reason you want to prepare your metal on both sides (double V or double U) is because it's:
Corner joints are either: Flush (and also referred to as Closed), Half open, or Full open:
An edge joint is one of the weaker welds. It is used most often on sheet, plate reinforcements, mufflers, and more.
As the name implies, your joint will be your base metal overlapping one another:
A Tee joint is another joint that looks the way it sounds. If you look at it from a cross sectional view it looks like the letter 'T'.
So it basically where two pieces of flat bar, plate, or whatever are at a right angle but not on the edge. If it was on the edge you would be making more of a corner joint.
As you can see in right hand side image above there is weld on either side of it. To prepare this Tee Joint you will bevel the edges, tack weld it on both sides, and then lay a bead on either side. But there are occasions where you will only weld one side as you can see in the left hand image.
Note that the beveled edges on either a single side or double side weld will be at an angle that is about 50% of what you would do on a Butt joint.
It is very important that you get good penetration. You want penetration to the root of your weld.
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