Tool School: A Quicker Way to Cut Steel

Building things from metal can seem intimidating—metal just feels so much more permanent than, say, wood, and with the all the sparks and pressurized cylinders, it seems like just a matter of time before you blow up your shop. But once you know your way around a few key tools, you’ll be amazed at how simple metalwork can be. Case in point: the plasma cutter. This small, relatively inexpensive machine has one dial, no cumbersome gas tanks, and can zip through any conductive material faster than a jigsaw through pine. It’s also basically a sci-fi machine made real (c’mon, it slices through steel with hafnium and air!). And since they’re for sale in most big-box home stores, you can even put one on your X-mas list. Master this, and your metal creations can take on just about any shape you want.

In a previous Tool School post, we looked at oxy-acetylene cutting. Oxy-fuel cutting is great for thicker materials. But when it comes to thin materials, especially 0.25-inch and thinner, the plasma cutter shines. For the record, plasma cutting shines on thicker materials as well, but requires a bigger, more expensive machine. For the shop on a budget (and who isn’t?), stick with oxy-fuel for the thick stuff and keep a small plasma cutter for thin materials. Also, while oxy-fuel cutting is limited to steel, plasma cutting can cut through any conductive material.

The plasma cutter works by using sophisticated electronics and a bit of hafnium, which releases electrons into the air very easily, to create a superhot plasma (ionized gas) at the tip and compressed air to blow that plasma through the material being cut. (Here’s a good video on how plasma cutting works from machine-maker Thermal Dynamics.) Because of that, the heat applied to the part being cut remains far more localized than with oxy-fuel cutting. In practical terms, this means that thin metal is less likely to warp when plasma cut than when cut with oxy-fuel methods.

Safety:

As with the oxy-acetylene Tool School post, we need to address safety first. Plasma cutting generates hot sparks and slag (hot bits of melted metal), which means that all of the apparel guidelines that apply to welding and gas cutting apply here as well.

1. Apparel: Wear a protective jacket designed for welding. Wear pants made of a natural fiber without cuffs. Wear gloves. Protect any part of your body you don’t want to touch hot bits of melted metal.

2. Eye Protection: Plasma cutting generates intense light. Wear shaded eye protection in the range of #7 to #9. Thicker materials call for a shade on the darker end of the range.

3. Environment: Hot sparks and slag will rain down from the cutting area. Make sure they can’t fall onto anything they can ignite.

4. Ventilation: Plasma cutting produces gasses and airborne dust; have a way to evacuate them from the work area in short order.

5. Remember that no quick post online can convey all of the safety points needed to operate complex and dangerous tools. It is your responsibility to do your own research and to educate yourself as to the safety best practices. What is provided above is only a primer. Always make use of the best tool that you have available: common sense.

Cutting:

Plasma cutters are far easier to setup and use than oxy-fuel setups. There are really only two variables and they are related: The power level (the single dial on the machine) and how fast you move the torch. Given the same thickness of material, you can cut at a lower power level if you move the torch slower. Conversely, the torch can move faster at a higher power level. Unless the situation calls for a very narrow kerf (the width of the actual cut), it’s generally easier to lean toward higher power settings, especially while still getting a feel for the equipment and the process.

I made a few sample cuts on 1/8-inch mild steel plate to show some common beginner problems. Comparing these cuts to yours should help to dial in your power settings and cutting speeds.

Plasma Cuts (Top)

1. The power level was set way too low for the material, especially at the cutting speed used. The cut did not even penetrate all the way through the material. Remedy: Turn up the power and/or move the torch more slowly (probably both).

2. The power level used here was the same as that in cut #1, but the torch was moved very slowly. This is still not a good cut. The power level is just too low for the thickness of the material, as indicated by the buildup of slag on top of the piece next to the cut. I didn’t put enough energy into the cut to blow the slag out the bottom. And slag on top of the cut is much harder to chip off than the slag on the bottom. Often it has to be ground off. Remedy: Turn up the juice.

3. The power level used here was higher. Technically speaking, it was too high for the material and cutting speed used. It works fine, but leaves a very wide kerf. The great thing about plasma cutting is that you don’t really need to be that precise with the dial—just vary the torch speed accordingly. Remedy: Either do nothing, or turn down the power slightly.

4. This is the cut you want. The power was set higher than on cut #1 and lower than on cut #3. Approximately the same cutting speed was used in cuts 1, 3, and 4. The kerf is a reasonable size and there is no slag on top of the cut. Once you have dialed in the amount of power and the cutting speed to this point, all that is left to do is practice your hand eye coordination and learn some of the finer details. (Experts may want to share some of the tricks they’ve picked up over the years in the comments below.)

5. This cut was made with worn out consumables—torch tips, electrodes, cups—in the torch head. The cut is a bit rougher and, although it is hard to see in this picture, is angled rather than straight through the metal. The plasma jet has a tendency to go wherever it wants as the torch tip begins to wear. Remedy: Replacing the worn out parts will bring the cut back to the point of cut #4.

Cleanup:

After cutting, there will be slag on the back side of the cut, as in the picture below.

Plasma Cuts (Bottom)

Resist the temptation to grind this off! If the cut was well executed, with the proper power settings and cutting speed, this slag should easily chip off with a cold chisel and hammer. Using the cold chisel will leave a far cleaner edge than grinding ever would and will also take far less time.

Cuts # 1-4 have been left untouched. The slag has been chipped from the back side of cut #5 to show the result. It took about 10 seconds.

While this may seem oversimplified, plasma cutting, like wire-feed welding, really is something you can pick up in an afternoon and start building your first project. Of course, like any DIY skill, there’s almost no limit to how good you can get with enough practice and, if you’re lucky enough to know some, tips from old pros. In the meantime, check out Miller’s extensive plasma-cutting resources to learn more.