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Plasma Cutting with Ron Covell

This is the Miller Spectrum 375, a compact machine rated at 3/8” material thickness, but it can actually cut thicker stock.

©Ron Covell 2005

Building or modifying a car involves a lot of cutting and welding of metal. In this series of articles, we’ll look at some of the most modern equipment available for these tasks – this time we’ll focus on plasma cutting.

For many years, oxy-acetylene cutting was often the process of choice for quickly cutting through steel plate. Over the past few years plasma cutting has pretty much taken over, for some very good reasons – perhaps most importantly - a plasma cutter will cut through ANY metal that is electrically conductive. That means that one unit will cut steel, stainless, aluminum, copper, bronze, and brass, to name the metals that are most often found on street rods. Should you ever need to cut titanium, inconel, or monel, a plasma cutter will slice through them all with no complaint, while oxy-acetylene cutting is pretty much limited to steel.

Although beyond the rated capacity, the machine will make a decent cut in ½” steel plate.

Another great benefit of plasma cutting is that the equipment is lightweight and easy to move around. The smaller ones are as portable as a suitcase, although many builders put them on a wheeled cart to make them more convenient to move around the shop or garage.

The plasma jet that does the cutting is hotter and narrower than an oxy-acetylene flame, so the kerf width is smaller, and you can get cleaner cuts. This makes plasma cutting particularly well-suited for cutting sheet metal, a task the oxy-acetylene cutting torch is not particularly well-suited for since it leaves a lot of slag on the edges. The extremely tight focus of the plasma arc tends to minimize heat distortion in the cut parts, as well.

Plasma cutting is easy to learn, too. You can cut freehand, or use a simple guide to run the handpiece against. Because the heat is so localized, you can even use materials like Masonite or MDF as a guide for occasional use. To use the machine, you bring the cutting tip to the metal, pull the trigger, and move the cut through the metal. It simply couldn’t be any easier!

So what magic makes this process to work so well? Actually, it’s not magic at all, just controlled physics! Compressed air is fed into the machine, and when you pull the trigger, an electrical field in the ‘swirl chamber’ in the handpiece causes the air to ionize and become electrically conductive, and the temperature shoots up as high as 30,000 degrees, creating pressure which escapes as a jet of super-heated gas through a small hole in the chamber. This narrow, focused jet of plasma will cut through any metal up to the capacity of the machine, which can be ½” for even a small unit!

One of the neatest things about plasma cutting is that it does a great job on sheet metal, so it’s great for cutting body panels.

As with any tool that involves heat and electricity, there are some safety precautions to observe. You should use eye protection, usually a shade 5 welding lens, like you’d use for gas welding. Gloves should always be worn, along with appropriate clothing. (No synthetic fabrics or sandals, please!) Of course, you should be sure that there are no flammable materials on the ground close to where you’re cutting, since the sparks that come from the cut could ignite them!

There are some things you should check before you start - plasma cutting requires that the part be grounded, and it’s important that the ground clamp attaches to clean metal, since rust or paint can interfere with the cutting efficiency. Each time you use the machine, you should inspect the nozzle, since it may wear over time, and when worn, the arc is not focused as tightly, making it more difficult to cut with precision.

There is a broad range of machines available - it might be a good idea to discuss the kind of work you plan to do with your dealer so they can help you choose the best machine for your use. I chose a smaller, 115 volt machine for portability, but it can cut up to ½”, or even sever 5/8” in a pinch!

Take a look at the accompanying photos, and you will see how this little powerhouse of a machine can tackle a great deal of the cutting tasks required for building or modifying a car!

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