flame cut small square hole

I had the need to cut some square holes in 1/8 steel this summer. It was to accomodate lag bolts to fasten together some railings.

I tried to make these with my cutting torch, but by the time I got to the fourth corner the steel was so hot it generally started to melt everywhere and I ended up with some really sloppy looking holes. Any suggestions on cutting small square holes in 1/8 or 3/16 steel?



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Use a small tip.

Use 2# acetylene and 6# oxygen.

Practice on scrap first.

Hold the torch still to get a red color. Have the tip perpendicular to the metal. Blow a hole in the center of what will be the center. Work FAST to go out and cut the square. Do NOT stop. All in one motion.

You are probably running too slow or too hot.

Lots of people do not realize it is the oxygen that does the cutting and blowing away of metal, and have too much acetylene and therefore heat going into the metal.

When you find the right mix of gases, and the right tip, it should be an easy thing. It just sounds like you are going waaay too hot and going waaaay too slow.

Let us know how it works out.

I found out that it takes very little more acetylene to cut one inch plate than it does to cut quarter inch plate. The oxygen pressure is all that needs to be adjusted.

Every wonder why the red zone on an acetylene gauge is only around eight pounds pressure? And the oxygen gauge goes all the way up almost to infinity?


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I'm going to step out on a limb and guess you don't have an ironworker with a square punch or you wouldn't have asked. But there certainly are square punches.

If I had to do it, I'd drill 4 small (1/16"?) holes tangent to the inside of your square, and another larger hole exactly at the center of your square so it nips the edges of the small holes. Then use a small file to clean up. A sharp file works fast in that stuff.


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Grant Erwin

Once you're done the things mentioned earlier, get yourself a piece of aluminum or steel and cut out a template that keeps the torch in the right place and lets you focus on speed rather than keeping inside the lines. Make it thick enough so the tip rides on the template and space it away from your work just a bit. You'll be amazed with how pretty you can make your cuts!

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carl mciver

What everyone else mentioned: use a template

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Good stuff Steve, if I may I'll go further on one point- the heat for a torch cut comes primarily from the cutting oxygen burning the steel, so with a square hole in thin stuff you're not only continuing to heat the steel in the area of the hole with your preheat flame, you're adding the heat from the burning of the steel as you cut also. It's a little tricky cutting holes like you need, works best if you can get the hole finished in one try.. be prepared to be happy with less than perfect 'till you get the hang of it.

Make sure your steel is clean, hitting a bit of rust two-thirds of the way around, after putting all that heat into that one area, will cause your oxygen to wash out and dig all around, ruining your nice hole. Hit it with the sander so you have a fresh surface to work with.

Cut the corners a bit farther out than you have them marked, they'll tend to fill again so doing this helps to make them end up right. Sort of curve the path away from the center going into and out of each corner, you'll see what I mean once you get doing it.

A template may not be a bad idea, I've never done it but it might work well.


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You did not mention the diameter of the hole. If it is one inch or smaller try this, drill a hole the size of the diameter you need , heat the surrounding area red hot with your torch and drive a square tapered punch into the hole, use a piece of pipe or the prichel hole one your anvil as a backer. Perfect holes every time and you are almost a Blacksmith !


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This is a totally different approach.

Why did you need square holes for lag bolts? I surmise that you were using carriage bolts and wanted the square hole to hold the carriage. I have always done this with the proper size hole. Use a caliper to get the diagonal size of the square. Use a steel cutting bit one size under. The carriage will cut its own square hole as you tighten the nut. The square is slightly tapered.

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