McGrath
12-07-2003, 03:23 AM
Since Ryan wants Tech posts, and I know more about Fabrication and Welding than I do about Suspensions and Drivetrains, I’ll make my Tech post about Welding Sheetmetal.
Sheetmetal is obviously one of the hardest things to Weld without warpage. I can offer a few tips on controlling warpage for the non-professional welders on this board. It’s easier to keep warpage out in the first place, than it is to fix it later.
To start with, tacking correctly is one of the best ways to control a piece of sheet metal. Once you get a couple of tacks to hold the piece in place, you need to start worrying about keeping both sides of the joint even. If you can reach the backside of the Weld this is best done by Dollying each tack as you go because that also “stress relieves” the tacks. If it’s a blind joint, control the relationship between the two pieces with a scraper blade wedged into the gap. You can then manipulate the two pieces of metal by prying them up or down until they match. I tack about every half inch when warpage is an issue, and also cool as I go with a wet rag.
Fit of the joint is especially important too because you want an even gap all the way around. On most automotive sheet metal I will try for an even 1/16” gap on the joints. When a Weld cools, it draws the metal towards it. An even gap makes it easier to achieve full penetration, plus allows the weld to pull evenly all the way across the joint. Full penetration is important because as you grind the weld down flush with the surface, you are taking away the strength of the joint. Full penetration leaves you with a weld joint that’s the same thickness as the surrounding metal after it is finished.
If you are a beginner, it is also important to practice on a couple pieces of scrap that are the same thickness as your project. Keep both penetration, and travel speed in mind as you adjust your amperage. The quicker you travel while welding, the less heat gets into the surrounding metal, but you still need the full penetration. If the Welder is set too hot, you run the risk of blowing through the metal. If it is too cold, you will be grinding all the weld off when you finish it out because you lack penetration. If you use lower amperage, but a slower travel speed to achieve full penetration, you aren’t any better off because you are letting more heat get into the surrounding metal.
Keeping the heat from getting into the surrounding metal is the number one way to prevent warpage from happening in the first place. You have to remember that in a formed panel, like a car body, the metal is stressed into shape. Getting too much heat into that stressed metal will cause it to relax. That is why its also important to do all your welding at an even speed and fill rate, so the entire joint gets the same amount of heat.
After I have a panel tacked in place, I will start welding about an inch at a time, skipping around and cooling between each weld. Once the piece is entirely welded, it is again a good idea to stress relieve it with hammer and Dolly if possible and I do this before doing any grinding. That keeps the hammer strokes on the Bead instead of the panel, and this is also important because the Bead is what’s doing the pulling. As you smack the bead between the hammer and Dolly, it expands what the bead has pulled, if you get my drift. It is possible to over-do it though and make the joint grow, so take it easy.
When rough grinding the joint, I suggest doing it with a Flap Disc. This looks like a Grinding wheel, but is made up of sandpaper flaps. 60 Grit is what I use most of the time because it gives you a good removal rate without getting too aggressive. A finer grit could be used, but it will put more friction heat into the metal because it is spinning at the same rate, but not removing material as fast. Frequent cooling with that wet rag is important here too. If you end up with a low spot or two, go ahead and get that area a little hotter before you cool it and the wet rag will help to draw the low spot back up.
Another tip for keeping heat out of the surrounding metal is to use a “Heat Sink”. I use strips of 1/8” Copper. It is easy to make the soft Copper strips conform to the shape of the panel you are working on, and Copper conducts heat much faster than steel. Clamped to the backside of a weld joint, it also keeps you from blowing through, and acts as a Gas Trap, which keeps porosity out of the backside of the Weld and enhances the strength of the Bead. Copper will also allow you to fill holes much easier because on top of drawing away the excess heat, it will not stick to the filler metal. Just make damn sure you don’t touch the Copper right after welding. It transfers heat so fast that even a couple of spot welds will make the copper hot enough to burn you.
Sheetmetal is obviously one of the hardest things to Weld without warpage. I can offer a few tips on controlling warpage for the non-professional welders on this board. It’s easier to keep warpage out in the first place, than it is to fix it later.
To start with, tacking correctly is one of the best ways to control a piece of sheet metal. Once you get a couple of tacks to hold the piece in place, you need to start worrying about keeping both sides of the joint even. If you can reach the backside of the Weld this is best done by Dollying each tack as you go because that also “stress relieves” the tacks. If it’s a blind joint, control the relationship between the two pieces with a scraper blade wedged into the gap. You can then manipulate the two pieces of metal by prying them up or down until they match. I tack about every half inch when warpage is an issue, and also cool as I go with a wet rag.
Fit of the joint is especially important too because you want an even gap all the way around. On most automotive sheet metal I will try for an even 1/16” gap on the joints. When a Weld cools, it draws the metal towards it. An even gap makes it easier to achieve full penetration, plus allows the weld to pull evenly all the way across the joint. Full penetration is important because as you grind the weld down flush with the surface, you are taking away the strength of the joint. Full penetration leaves you with a weld joint that’s the same thickness as the surrounding metal after it is finished.
If you are a beginner, it is also important to practice on a couple pieces of scrap that are the same thickness as your project. Keep both penetration, and travel speed in mind as you adjust your amperage. The quicker you travel while welding, the less heat gets into the surrounding metal, but you still need the full penetration. If the Welder is set too hot, you run the risk of blowing through the metal. If it is too cold, you will be grinding all the weld off when you finish it out because you lack penetration. If you use lower amperage, but a slower travel speed to achieve full penetration, you aren’t any better off because you are letting more heat get into the surrounding metal.
Keeping the heat from getting into the surrounding metal is the number one way to prevent warpage from happening in the first place. You have to remember that in a formed panel, like a car body, the metal is stressed into shape. Getting too much heat into that stressed metal will cause it to relax. That is why its also important to do all your welding at an even speed and fill rate, so the entire joint gets the same amount of heat.
After I have a panel tacked in place, I will start welding about an inch at a time, skipping around and cooling between each weld. Once the piece is entirely welded, it is again a good idea to stress relieve it with hammer and Dolly if possible and I do this before doing any grinding. That keeps the hammer strokes on the Bead instead of the panel, and this is also important because the Bead is what’s doing the pulling. As you smack the bead between the hammer and Dolly, it expands what the bead has pulled, if you get my drift. It is possible to over-do it though and make the joint grow, so take it easy.
When rough grinding the joint, I suggest doing it with a Flap Disc. This looks like a Grinding wheel, but is made up of sandpaper flaps. 60 Grit is what I use most of the time because it gives you a good removal rate without getting too aggressive. A finer grit could be used, but it will put more friction heat into the metal because it is spinning at the same rate, but not removing material as fast. Frequent cooling with that wet rag is important here too. If you end up with a low spot or two, go ahead and get that area a little hotter before you cool it and the wet rag will help to draw the low spot back up.
Another tip for keeping heat out of the surrounding metal is to use a “Heat Sink”. I use strips of 1/8” Copper. It is easy to make the soft Copper strips conform to the shape of the panel you are working on, and Copper conducts heat much faster than steel. Clamped to the backside of a weld joint, it also keeps you from blowing through, and acts as a Gas Trap, which keeps porosity out of the backside of the Weld and enhances the strength of the Bead. Copper will also allow you to fill holes much easier because on top of drawing away the excess heat, it will not stick to the filler metal. Just make damn sure you don’t touch the Copper right after welding. It transfers heat so fast that even a couple of spot welds will make the copper hot enough to burn you.