I think most folks that have an interest in fabrication have seen folks hotspot shafts, weldments, sheet metal, and other items as a means of, in most cases, straightening them or in some cases intentionally producing a controlled amount of bend into them. Usually this is done with a fuel/oxygen torch. I have never seen or heard of it being done with a TIG torch. Today I needed to straighten some welded foundry flasks I made a year or so ago that I use to cast my 48" straight edges. They were constructed mainly from 1/8" x 1 x 1 " angle and 1/8 X 3" flat bar for the cope and 1/8 x 1 x1" angle and 1/8 x 2" flat bar for the drag. You can see in the before photo that there was about 1/2" of bow on the length of the flask side with it bowing inward---concave. The angle and bar were originally tack welded every 3 to 4 inches with TIG so as to minimize expected warping. But because the assembly is asymmetric along the vertical axis, warpage in the x-direction in the x-y plane (right ot left in the "before' photo) was unavoidable. No appreciable warping occured in the y-z plane as the assembly was symmetric in that plane. The planes referenced are the same as those used on a common knee mill. I had tolerated the bowing of the flask sides as the flasks worked OK as shaped. But, today I had to lengthen them by sectioning the long sides of the flask and inserting 5 more inches of combined angle iron and flat bar so that they were stretched from their original 52" length to 57 inches. At the same time I wanted to see if I could take the 1/2" of bow out of each of the four long sides. I had originally thought of getting out my oxy/acetylene torch. But I already had the TIG torch setup and running. And heat is heat----right. So, I decided to try straightening with the TIG. It worked and was considerably faster and likely a lot cheaper in consumables than burning acetylene and O2. In fact I used a phone stopwatch to time straightening two sides and found it took 7 minutes total to straighten two side faces. I was able to slide the torch on 200 amps at a rate of an inch every 2 or so seconds. It was just causing the surface to bubble but was not deeply melting the metal. As the metal cooled you could see it straightening as you watched. I just sighted down the length and touched up spots with residual bend. In a few cases I overshot and had to heat the opposite side. But the net result was rapid straightening to near perfect---perfect enough for my purposes. Here is a sketch of the construction of a flask side that I show here as the photos may not be clear as to construction details with tack welds represented by the small black dots.: Here is the "before" picture where a piece of 1.5 x .125" flat bar has been placed near the inside edge of the top of the flask side to help illustrate the curve. And here is the after view showing that the side now lays pretty closely to the flat bar. You can see where the mill scale on the angle is disrupted by heating. And here is a shot just showing the general construciton of a flask side. Why bother to write this up? Just to suggest that this is a straightening method that might be useful. If this is commonly done already and this post is just useless noise, please just skip it but also realize my intentions were good. ;-) I do think this method has potentially broader application than just flask sides. [Side Note: This flask set when filled with molded sand weighs just under 400 pounds. The empty flask set weighs 50 pounds. Compared to using wood for iron casting, steel or aluminum is "the bomb."] Denis
I'd successfully done something similar by having a larger than normal air gap on a stick welder to get a wider zone of heating after welding brackets on some 100mm by 50mm by 4mm (3/16") wall rectangular hollow tubing. It was bent like a banana after welding brackets every 40cm or so. The resulting weld was fairly flat with not much filler rod compared to normal and I hit it with a wet towel as I'd heard that increases shrinkage.
Here is a superb resouce for those interestedin learning more about the fundamentals and application of heat straightening. The article is written from the fuel/oxygen heating poing of view. But the principles are independent of the method of heat application. https://www.boconline.co.uk/en/images/Fundamentals-of-Flame-Straightening_tcm410-113398.pdf Denis
