Evaluating Plasma Cut Quality
Plasma cutting is a tremendously capable technology, but the quality of your cut can vary.
With minimal training, an operator can employ a plasma cutting system for the accurate and efficient fabrication of any electrically conductive material. In most cases, the cut quality produced by modern plasma cutting systems is very good.
There are, however, a great number of variables which when properly understood and addressed will yield the optimum quality cut.
Don’t worry, we’ll cover the variables you can use to evaluate the quality of your cut based on your plasma cutting equipment.
Evaluating Kerf Width of Industrial Plasma Cutting Equipment
Kerf is the material that is removed from the workpiece by the cutting process. In plasma cutting, kerf width is primarily determined by the amperage of the cut process. Lower amperages will produce a narrower kerf. Higher amperages will produce a wider kerf. To achieve the best cut quality in terms of kerf width, the lowest amperage should be selected that allows for a complete cut of any given thickness of a material.
Kerf width in plasma cutting can be as narrow as .4mm or as wide as 10mm depending on the amperage of the process. When nesting parts on a CNC machine, kerf width can add up quickly and make a difference in the utilization percentage of the material.
Bevel is an Important Factor in Cut Quality as well
Bevel is a top to bottom variation from perpendicular on the cut edge. Positive bevel indicates that the bottom edge of the cut is protruding. Negative bevel indicates that the top edge of the cut is protruding. In plasma cutting, the most common cause of unintended bevel is improper cut height. When the cut height is too high, a positive bevel will appear. When the cut height is too low, a negative bevel will appear. When a plasma cutting system is operating optimally, bevel on the “good side” of a cut may be less than 1°.
If cut parameters are off, or if there is a problem with the system, bevel may exceed 6°. If you are experiencing an extreme bevel, you should ensure that your torch is moving in the correct direction relative to the part being cut. The “good side” of the cut will be the right side of the direction the torch is moving.
When CNC cutting, the torch must also be square to the plate to avoid unintended bevel. Similar to bevel, a condition called undercut may also develop when cut height is too low. Undercut is indicated by a concave edge on the cut part, usually closest to the top edge.
Don’t Forget to Evaluate the Quality of Your Edge Rounding
Edge rounding is the slight melting that occurs at the edge of a cut part. It is similar to bevel, but takes a rounded shape instead of a sharp one.
Top edge rounding occurs most commonly when cut height is too high. While some top edge rounding is normal when plasma cutting, it can become excessive due to a number of factors including worn consumables, improper cut height, and incorrect gas pressures. Arc density also has a large effect on top edge rounding. The more dense the plasma arc is, the less likely top edge rounding is to occur.
Sometimes, rounding can occur on both the top and bottom edges of the material. This usually occurs when too much current (amperage) has been applied and can be remedied by selecting a lower current process.
Dross / Spatter is an Important Quality-determining Factor for Industrial Cutting Equipment
Dross is re-solidified metal that accumulates at the edge of a cut. There are two major varieties: High speed dross and low speed dross. High speed dross is hard and light and accumulates on the top edge of the material. Low speed dross is thick and bubbly and accumulates at the bottom edge of the material.
As their names imply, both of these types of dross are largely developed as a result of cut speed. When cut speed is too low, the plasma arc begins to widen and molten material is no longer completely discharged from the cut path. This type of dross is undesirable but fairly easy to remove. When cut speed is too high, the arc begins to lag behind and leaves a trail of material which has rolled over the top edge of the cut in the form of high speed dross. High-speed dross is much more difficult to remove and will usually require a secondary operation such as grinding.
Besides dross, the plasma cutting process also causes spatter. Spatter is when the molten material is ejected from the cut due to the swirling of gas that lands on top of the workpiece or torch. Spatter is easily removed once it cools. The use of an anti-spatter solution on the workpiece or torch can prevent spatter from adhering and make it even easier to remove if it sticks.
Look at Lag Lines in Your Cuts
Lag lines are small vertical ridges on the cut edge. They indicate the path of the plasma arc as it moves through the material from top to bottom. When cutting with air plasma, the lines should be nearly vertical (perpendicular to the surface). When cutting with oxygen, the lag line should lead slightly. When cutting with nitrogen or argon/hydrogen, the lag lines should trail slightly. If cut-speed is too fast, lag lines will take on an “S” shape. Lag lines are a great indicator of whether or not your cut speed is set appropriately.
Finally, Evaluate The Surface Finish
Surface finish of plasma cut parts can be highly variable. Some cuts will be smooth and glossy. Others may be rough, jagged, or inconsistent. Surface finish can be affected by the type of material being cut, the gases being used in the cutting process, or the cutting process itself. In CNC cutting, there are two categories of induced roughness: Those induced by the process (worn or damaged consumables, improper gas flow, etc.) and those induced by the machine (dirty rails, worn bearings, improper alignment, etc.). If cut edge irregularities are consistent through the process, you may have a process deficiency. If cut edge irregularities only appear on one axis, you may have a machine deficiency.
It is important to note that cut quality is subjective. What one person may consider to be a defective cut, another may deem perfectly acceptable for its intended application. Cut quality should be weighed in the balance against cut speed, intended use, and the potential of the equipment being used. After all, a perfect cut edge would mean little if the parts are not produced in time to be used.
If this guide was helpful, we wrote plenty of blog posts with in-depth information about plasma cutting, just for you! For example, we recently wrote a blog to help you avoid unintended plasma cutting issues.