An Intro to Plasma Pattern Cutting using Miller's Spectrum® 375 X-TREME™

(this article first appeared on thefabricator.com)

Perhaps the easiest way to perform artistic plasma cutting, or to cut identical pieces, is to use a CNC-driven plasma cutting table and a computerized art file. While tables from companies, such as Tracker CNC, can be purchased for about $20,000, if you don’t have one, all is not lost. This article offers some tips to maximize your hand-held plasma cutting efforts.

The design, of course, will be up to you. It is beyond the scope of this article to help you develop an artistic eye. Once you’ve developed your design, however, either by hand or using a graphics program, you need to transfer it to your work metal. There are various methods of accomplishing this, such as tracing a cardboard cutout or using carbon paper to transfer a sketch and then retrace it with a black marker so it can easily be seen while cutting. Carbon paper is still available in art supply stores.

Plasma cutter selection. If you haven’t already chosen a plasma cutter, there are a few considerations to keep in mind when doing so. Probably the most important is the machine’s cut rating, which is the thickness of material that may be cleanly cut when traveling at a rate of about 10 inches per minute (i.p.m.). The cut rating is in direct proportion to the output amperage of the machine.

You should choose a plasma cutter that cuts the thickness of metal with which you will most often be working. There is an exception, however. If you will begin a lot of your cuts by piercing the metal, as opposed to starting from an outside edge, a good rule of thumb is to use a machine capable of cutting twice the thickness with which you’ll be working. It’s important to quickly pierce the material or molten material can splash back into the torch tip and shorten tip life.

The choice of plasma cutter also affects the type of tips that can be used. For plasma cutters rated below 40 amps (about 5/8-in. cut rating), the cutting tip can rest directly on the metal when cutting. 40 amps and above, the cutting tip cannot rest on the metal, since double-arcing can occur and quickly damage the tip. Because of this, the operator must maintain a standoff distance of about 1/16 to 1/8 in. or use a drag shield that can rest on the surface and maintain proper standoff.

For machines rated below 40 amps, a cutting tip (left) can be placed directly on the work while cutting. 40-amps and above, the user must either manually maintain a standoff or use a drag shield (right). If you’re designing a template for repeatable cuts, you’ll have to account for the radius of the tip in your calculations.

For this demonstration, we chose a Miller Spectrum® 375 X-TREME™, a 27-amp, inverter-based plasma cutter with a cut rating of 3/8 in. at 14 i.p.m. It has more than enough power to quickly pierce the 1/8-in. steel used, and it allows placing the tip directly on the metal. Because the unit is about the size of a lunch box and weighs only 18 lbs., it also lends itself to a portable or easily storable set up.

	A lightweight setup, which can be easily stored when not in use. Blocks of metal are placed on the table to raise the workpiece and still maintain good electrical contact. The hole and fan draw fumes away from the operator, while an arrestor in the fan unit contains the sparks.

Templates. If you’re designing a piece to be used as a template, you need to account for the radius of the cutting tip or drag shield in your initial design. For instance, in to cut the circle (solid line) shown below from a piece of metal, your template would need to be drawn to the size of the dotted blue line.

To remove a circle the size of the solid line from a piece of metal, the template would be made to the size of the outer circle, which is the size of the inner circle plus the radius of the particular cutting tip being used.

In addition, the thickness of the template material should be approximately ¼ in. This is to provide a firm vertical surface for the side of the tip to press against without slipping.

Safety

Familiarize yourself with the machine’s owner’s manual, paying particular care to the safety section. Among the requirements are wearing gloves, long sleeves and an appropriate pair of shaded safety glasses.

Setup

The template and work piece are placed on metal blocks and securely clamped to the worktable. The ground clamp is secured to the table to provide a good electrical connection to the workpiece. Although a plasma cutter can cut through paint, the connection from the ground clamp to the workpiece they should be free of paint or other contaminants.

Because of the tight angles in the design, some of the cuts will need to be made by hand, so a marker is used to outline the design.

Starting the cut

When piercing the metal at the beginning of a cut, start in a piece outside the design and hold the torch at an angle, as shown here. Push the trigger and roll the gun until it’s perpendicular to the metal. Then move to the side of the template and being cutting.

Use your non-cutting hand as a brace to maintain the proper position. Rehearse the cut beforehand to ensure you can make the cut without changing position.

Once the template cutting is complete, the template is removed and areas too small for the template to accommodate are cut by hand, following the outline.

Clean up

The dross that remains after the cut can easily be knocked off using a piece of scrap metal.

A grinder is used to finish the piece, cleaning up any rough edges and provide another aesthetic element.

The final piece.

For more plasma cutting tips, resources and other articles, visit MillerWelds.com/resources/improving-your-skills/plasma/.