Hey guys, ya don't need 1/4" thick copper buss bar as a heat sink to buck your welds and you don’t want it in most cases. Remember that copper has about the best heat transfer of any of the metals so just increasing the area will help pull the access heat away from the back side of your joint. You definitely want 100% penetration on you weld so you don’t want to pull to much heat from the area.
I've welded for most of my adult life and much of that time as an instrument welder at an accelerator laboratory. Heat sinking very thin materials is common practice and quite often used to match up heat transfer when welding thin to thick materials. When tig, gas or light mig welding even light .020" flashing available at any sheet metal shop or building supply is adequate and easy to form to the backside of your seam. The only place that you may need a thicker sink would be if you have a wide gap and you trying to mig weld it closed and even then the copper pluming fittings flattened out is usually more than enough.
Once your copper has been heated it will be annealed and soft enough that you will be able to form it by hand to fit flush against the backside of your joint.
Here is just one example of a .020” flashing used to back up a fender roll tear that was gas welded. Just a little additional tip for the new welders out there, by butting a same thickness stamp up against the end of the seam you can keep from melting away the edge. Start your weld at the edge and work in to the joint.
http://rides.webshots.com/photo/2736188780098611668ujExJC A common problem area with the front of the rear fenders is where it is bolted to the running board. This area is often bent outward, stretched and torn around the edge of the boards. It was on mine and here is another area that was gas welded using a .020” copper-bucking sheet. To pull the lip back into shape and hold it while I welded the flanges I made maple form blocks:
http://rides.webshots.com/photo/2077758280098611668etvvIN Since this area is quite visible and is subject to some flexing, rather than use a plastic to fill the minor deviation due to the weld bead, it was cleaned and leaded. The lead filler, when properly tinned and applied is bonded to the base metal well enough that you could fold it in half and the lead still won’t separate from the base. Key here is clean, clean, clean:
http://rides.webshots.com/photo/2163757360098611668YCEOkw The finished lead filler, is an absolute minimum:
http://rides.webshots.com/photo/2582800830098611668wtgreJ Denny Graham
Sandwich, IL
