Whenever you run into a snag on a project, it’s a pretty safe bet that somewhere there is a grizzled old man who’s solved it many years before you and will be happy to tell you it. That’s why I never miss the chance to chat up the old guys working at or just hanging around lumber yards, machine shops and scrapyards. I always walk away smarter.
One recent example: While helping PopSci’s John Carnett with his Green Dream house, we had to make a number of bolt holes in thick structural steel. (I would have preferred that the beams had come from the steel yard properly cut and drilled, but sometimes things don’t work out as we’d like.) The drill just wasn’t cutting it, so I turned to the oxy-acetylene cutting torch. It would easily pierce the thick steel, but I wasn’t sure it’d cut clean holes. Then I remembered a great trick for burning accurate holes that I learned from an old-timer at the structural steel yard.
When cutting with a gas or plasma torch, the intensely bright light and shaded goggles can make it difficult to see the outline to follow. Even the trusty soapstone, a kind of caulk-like stone used by weldors for marking that will not burn off in the heat generated by welding, comes up a bit short. I’ve tried silver colored paint pens as well; the visibility is good at first, but the paint burns off as I cut. It is something of a problem, but one I had long since accepted as inevitable.
That is where the wisdom of someone who has spent a lifetime in the field comes in handy. The simple process I learned is great for marking the outline of anything that needs to be torch or plasma cut freehand with a degree of accuracy. Your chances of accurately hitting the marks improve greatly when you can actually see them.
First, mark the line to be cut. A sharpie works well for this. In my case, I was marking what would become holes for 1/2-inch bolts, so I traced the outline of a socket with a diameter of approximately 9/16-inch. Next, using a center punch and a hammer, mark that outline with small divots every 1/8-inch or so along the line. Finally, trace again over the now punch-marked line with soapstone.
When cutting, the divot marks along the line stand out sharply. Following the outline becomes a simple case of connect-the-dots. The difference in the quality of the bolt holes I cut immediately before and after learning this method is striking. The same technique can be applied to any shape or pattern being cut, though of course it becomes more time consuming when applied to longer cuts.
For the basics on oxy-acetylene cutting, have a look at Tool School: Oxy-Acetylene Cutting.