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Q. I have read countless books on hot rod chassis building, and have come to the realization that these books have a fundamental shortcoming: They don't provide any information on weld geometry when components are being welded into place.
This illustration shows the size of a weld bead required to give a welded joint the same s
I am considering building a chassis from various sizes of rectangular tubing, and I'm curious how the pros do it. I have my ideas, but I'd like the same peace of mind that you get when the "big shops" put their name on a product. For example, if two pieces of rectangular tubing make a tee joint, should the intersecting end be fully beveled, or beveled with a land, and should a gap be left between the members to get full weld penetration, or simply left flat and fillet welded all around?
Joining or splicing tubing is another concern, such as when kicking the 'rails up over the rear axle. Should I bevel the edges of the tubing before welding? Which gives better strength: using fish plates on the outside of the tubing, or using backing plates inside the tubing with rosette welds to join them?
Where can I find more information on welding joint geometry?
Via the Internet
A. You have posed some very good questions! When a joint has a "T" configuration, beveling is not generally required. You'll want the weld to be as strong as the base metal, and if the weld bead is as wide as the base metal's thickness (or the thinner member, if you have two different thicknesses) the joint should be sufficiently strong, assuming you make a good quality weld that has adequate penetration with no undercutting or porosity. I don't recommend leaving a gap on joints like this.
When you are butting material together (either plate or tubing) beveling is recommended for materials 1/8 inch or thicker. It's good practice to leave a little flat (land) on the beveled edge. For 1/8- or 3/16-inch wall tubing, I'd leave a flat about 1/16 inch on the bevel. When welding joints of this configuration, you need to use enough heat to melt through the land. It's generally better to fit the joint tightly, with no gap, as long as you are sure you are getting full penetration. Leaving a gap will make it easier to get the penetration, but you'll get a lot more distortion.
Fishplates are not always required on butted joints, but they will certainly add extra strength if you think you need it. You will get the greatest strength from a plate on the outside of a tube, since it can be welded around the perimeter. Plates held inside the tubing with rosette welds can add some strength, but not nearly as much as what you'd get with an external plate, since welds on the perimeter will be much stronger than welds spaced in from the edges. You can go to any library and find a wealth of books on industrial welding that cover these subjects in as much depth as you want!
Q. I have been practicing my TIG welding on 1/8-inch aluminum after viewing your TIG DVD. Sometimes, after welding, the aluminum is dull and grainy instead of bright and smooth. I'm having particular trouble at the end of the weld. I have tried putting a spot weld on the panel to stop at, but I still overheat the metal and melt the heck out of it.
Via the Internet
A. All of your symptoms suggest overheating. As you weld across a workpiece, it takes a lot of heat to start, since normally the metal is at room temperature. As you progress a few inches from the start, the metal around the weld has gained some heat, so less "foot pedal" is needed to maintain the puddle at the optimum size. As you approach the end of the workpiece, there is a smaller "escape path" for all the heat that you've been pouring into the metal, so you need to "let up on the foot pedal" even more as you are approaching the end.
With lots of practice this becomes automatic-but as you are learning, pay special attention to the width of the weld puddle-that's a good indication of how much heat you're putting into the metal.