Troubleshooting CNC plasma cutting, Part II

Oct 14, 2024

Editor’s Note: Part I, which appeared in the March/April 2015 issue, discussed seven ways to improve hole quality with a CNC plasma cutting system. Part II addresses dross, edge angularity, warpage, and edge metallurgy.

The plasma cutting process superheats a high-speed column of ionized gas. The plasma jet’s high temperature melts the material being cut, while its velocity removes the molten metal from the bottom of the plate. If ferrous metals are cut with an air or oxygen plasma torch, the oxygen also creates an exothermic boost that helps to provide an oxidizing effect to improve speed and cut quality.

Cut quality is essential in plasma cutting. The four most common cut quality issues for fabricators are dross, edge angularity, material warpage, and metallurgy of the plasma-cut edge. Your ability to achieve the best results depends on the system, torch, and consumables you are using, as well as the accurate control of such highly critical parameters as pierce height, cut height, amperage, gas type and flow rate, and cut speed.

1 Dross is the resolidified metal that adheres to the top and bottom edge of the material being cut (see Figure 1). Sometimes referred to as slag or a burr, dross is a fairly common problem that has several causes and cures.

Slow cutting speed. This is the most common cause of bottom dross. Inexperienced plasma machine operators tend to slow things down when they encounter a cut quality issue, but in fact they should do the opposite. Typically there is a speed on a particular material thickness at which low-speed dross appears; as you accelerate to higher speeds, this dross is eliminated. If you accelerate too much, however, high-speed dross can occur. The speed range between low- and high-speed dross is called the dross-free zone (DFZ). The wider the DFZ, the better the cut quality.

Incorrect cut height. If the torch is too close to the material, bottom dross will appear; if it is too far away, dross or spatter will form at the top of the cut.

Incorrect system power level. The power level needs to be matched appropriately to the material you are cutting. Using too little power in terms of amperage and nozzle selection will result in a nice edge quality, but no DFZ.

Worn-out consumables. Consumable wear is normal, but it is exacerbated by incorrect piercing and power levels.

Material and surface conditions. Certain material and surface conditions make the DFZ very narrow or nonexistent. Shot- or sandblasted surfaces on the bottom of the plate provide a rough texture for the dross to adhere to. If your material has a shotblasted finish on one side, make sure that side is facing up during cutting. Rusty, oily, and painted surfaces should face down during cutting to minimize dross. Some steels with high carbon, silicon, or manganese content also will produce a narrow DFZ.

Slow CNC machine acceleration. If dross is present only in corners and on fine features of the part being cut, your machine might not be maintaining high enough speeds to stay above the low-speed dross limits.

FIGURE 1 Dross is the resolidified metal that adheres to the top and bottom edge of the materialbeing cut.

Close proximity of cuts. If you have a lot of detailed cut lines very close to each other, the material might overheat, causing dross. If your CAM software can apply cut paths based on thermal buildup, use that feature to allow areas to cool before the next nearby cut starts.

So what’s the bottom line on dross? Follow the specs in the plasma torch manufacturer’s manual for the cutting parameters recommended for various materials and thicknesses. The manufacturers design the torches and consumables and spend months in their labs getting the processes dialed in, so they are most likely to have the best answers.

Edge angularity is generally measured as a positive or negative angle in relation to 90 degrees from the surface of the plate. Keeping in mind that plasma cutters always have some edge angularity, positive, small edge angles generally are most desirable. Negative angles are labeled as undercut.

Plasma process engineers work hard to design minimal angularity into the torch and consumable designs and to ensure that the angularity is consistent around the perimeter of the part. Cutting slowly generally minimizes edge angularity, so plasma system manufacturers typically recommend optimal cut speeds that are the lowest at which you can cut without causing low-speed dross.

Optimum angularity always is achieved at the lowest power level that is specified in your cut charts for the material thickness. If better angularity is absolutely necessary, slow down even more, but you can expect a wider kerf and more dross if you do.

How can you achieve the best edge angularity?

Match the consumables and power level to the material thickness. Lower power and speed will produce less edge angularity.

Make sure the consumables are in good condition. A damaged nozzle or shield orifice is the first thing to look for if your edge angle varies dramatically around the perimeter of a cut. The nozzle orifice shapes the arc, so if there is a nick or crater affecting the roundness of the orifice, expect the arc, and consequently the cut, to be affected. The biggest killer of nozzle orifices is incorrect piercing.

Use the correct cut height after the pierce and through the entire cut. A high-definition plasma cutter needs to stay within 0.005 inch of the recommended cut height; a conventional or air plasma cutter should be within about ± 0.010 in. Make sure the height control is accurate and maintains constant height with no diving.

Consistent and minimal edge angularity can be achieved by using good consumable parts in good condition, employing a good plasma system, and following the manufacturer’s recommended cutting specifications.

Using suggested power and speed levels can help you control material warpage during plasma cutting, to some extent. Higher speeds impart less heat into the material, which generally produces less heat-induced material warpage. Here are some other suggestions:

All materials cut by plasma will show metallurgical effects on the edges. By choosing the right gas mixes and process power levels, you usually can minimize these effects. Often the edge metallurgy is affected by the localized high temperature of the cutting process, but atmospheric gases around the cut edge also have an effect.

Oxygen plasma produces the best edge metallurgy on most carbon steels. Some systems use an oxygen plasma/oxygen shield gas for cutting holes less than 2.5 inches in diameter. This produces hole edges that are almost unaffected by the cutting process and, in fact, often suitable for thread tapping. Oxygen-cut edges are 100 percent weldable and machinable, and they tend not to crack during forming operations.

Air plasma or nitrogen plasma will cause some edge hardening and nitriding on most steels. The nitriding can make the edges brittle and create porosity in some welding processes. Generally, the nitride layer is 0.006 to 0.010 in. thick and thus easy to remove.

Most abrasion-resistant wear plate will show some slight softening, usually less than 0.010 in. from the edge, with both air and oxygen plasma cutters. Stainless steel less than 0.250 in. thick will cut with a very pure edge using a mix of 5 percent hydrogen/95 percent nitrogen as a plasma gas and nitrogen as a shield gas. Thicker stainless sections will have a pure, weldable edge when cut using a 35 percent hydrogen/65 percent argon mix with a nitrogen shield gas. In addition, if you cut stainless steel under water using nitrogen plasma and nitrogen shield gases, you can eliminate the oxide layer that forms when cutting in ambient air.

Before trying any gas combination that your equipment might not be designed for, however, contact the manufacturer of your plasma cutting equipment. Incorrect gas combinations certainly can damage equipment and cause injuries.