Welding Tips and Tricks

I don't own a plasma cutter (yet).

What are the issues if one needs to use a plasma torch with the business end pointing up? This is a question regarding cutting a hole in the ceiling of a 20 ft shipping container when there's another container already on top of it. No choice but to cut from below.

I assume that since the thick steel floor ribs of the top container are about an inch away from the ceiling of the lower container that the torch won't really cut into those ribs due to the distance and properly dialed in the energy is used to just cut the relatively thin ceiling steel? Am I right?

What are the safety issues?

The ceiling of a shipping container isn't flat. It's corrugated for lack of a better term. I was thinking of cutting the lower part of the corrugation only on the first pass as that's the easiest part to get at. Then cut the tops of the corrugation in another pass, and finally cut the sides of the corrugation as a final pass. Comments?

Should I consider an alternate cutting tool? What?

A thin wheel on a 4-1/2 inch grinder might be another option. Depending on what's in the upper container the plasma or torch might get the floor of the upper one hot enough to warp / distort it or worse yet ignite the contents.

Any cutting on a shipping container compromises it's structural integrity. To limit the damage, one should NOT cut sharp corners as that's where stress cracks will appear. Even cutting a doorway or window opening should involve rounded corners of a minimum (IMHO) of about 2" radius. Therefore, cutting into a ceiling should be circular if possible.

Using an angle grinder to cut rounded corners or circles, especially when the material is undulating between the corrugations that are about 1" thick isn't really an option. I've cut circles with a plywood jig mounted jigsaw on a circle cutter attachment and that does work if you can guarantee that the stroke doesn't hit anything you're not intending to cut in the blind area. That was noisy, tiring (heavy jig saw plus plywood jig held overhead for a long time) and the end result wasn't that great as the undulations in the metal caused the blade to bend as the cut was made. That's why I'm thinking about a plasma alternative.

The wooden floor of a container is (I'm guessing) 9 inches above the bottom of the container. The space below the wood is comprised of heavy gauge steel floor joists that prudent use of plasma power should be unable to damage as it's at least an inch above the lower containers ceiling. The sparks and dross will hit the plywood, I presume, but I doubt it would set it alight. I'm not worried about it.

How about relieving the corners first with a large hole saw, and then finishing the cuts with the wheel on the angle grinder? I have some reservations about the upside down plasma cutter. And to be completely honest, they're just hunches, since I've never tried cutting overhead with mine.

Larry

I've cut over head and It went fine. I was only thinking that with only 1 inch clearance there would be heat issues. But if that could be resolved then plasma should work. I personally would probably use the hole saw / grinder option.

I was hoping to TIG weld the metal back in place after the repairs in the upper container are completed (plumbing). Cutting out 4 corners adds an extra level of clean up I'd rather avoid if possible.

Also, cutting with a disk means that at some point, the disk is 1" (at least) deep into the corrugation. By that I mean the ceiling is better though of a 2 levels 1 inch apart and undulating. When cutting the nearest level, that's fine. Then you have to traverse the slope that connects the two levels to start cutting the upper level and now the disk is at least 1" deep into the material. That's a recipe for binding up the disk and having the grinder go wild when it's over your head. Too risky IMHO.

I've had a grinder get caught on an edge and it ripped out of my hands. It was equipped with a cup brush and that brush walked up my chest and over my shoulder. As it was a grinder with an on/off switch, it danced on the ground till I unplugged the extension cord. I now only use paddle switch grinders.

Plasma is often used for overhead work. It will work fine. Jim Colt Hypertherm

jimcolt wrote:Plasma is often used for overhead work. It will work fine. Jim Colt Hypertherm

Agreed. I would add, in your application, to keep the current setting and air pressure just above the minimum required, and move as fast as those settings will allow. The heat blowing through the kerf will distort the metal above, and possibly blow through it at too high a setting.

Steve S