SigmaNEST – How to improve the cut quality on a plasma machine

The cut quality on parts that are cut on a plasma machine can be exceptionally good. Sometimes, however, things can go wrong, and the quality of parts can be questionable.

What can one do to improve and what are the parameters to change to get a better quality?
The first thing to note about a plasma flame is that the flame shape is not straight, but tapers downward. The top part of the flame (just as it exits the nozzle) is much straighter than the bottom part, where it tapers quite heavily. The result of the tapered flame is that it always cuts a bevel into the part contours. The one side of the flame is also more tapered than the other side because of the spinning motion caused by the gasses exiting the nozzle. The result is that one side of the cut is bevelled more than the other side. This is the reason internal contours are cut counter clock-wise and external contours are cut clock-wise. To improve the cut so that it has the smallest bevel, the goal is to cut with the top part of the plasma flame.
Cut height
The first parameter to investigate is the cut height. If the cut height is too high, then the contours are being cut with the bottom of the flame which has a larger bevel. Lowering the cut height decreases the bevel but increases the risk of the head colliding with the sheet. There is some testing involved in getting the optimal cut height correct. The test involves trial and error cutting a contour, checking the cut height, then lowering the head. This must also be done for each material and thickness and on each one of the amperages to cut with. This is quite a time-consuming exercise, but the rewards of better parts might just be worth it. If the machine can receive cut height info from a CNC file, then SigmaNEST can output the cut height to the machine. This helps because each thickness and each material will have different cut heights. The cut height can then be added into the tech table in SigmaNEST, which will output the correct cut height depending on the material and thickness selected.
The second parameter that can be investigated is the amperage with which the material is being cut. The choice of the cutting amperage is critical in achieving the best cut quality. If the cutting amperage is too low, then the flame length is short, meaning that the job is being cut with the bottom part of the flame, increasing the bevel on the sides of the parts. The feedrate on the lower amperage might also be too slow, burning away too much of the material leading to poor cut quality. If the cutting amperage is too high then the heat on the material is also much higher, melting away much more of the material which then causes bad cut quality. Inside SigmaNEST it is very easy to select between different amperages for the same job. For instance, if a job is cut with 80 amps and it is noticed that the cut quality is poor, the cutting path can be cleared, the new amperage of 130 amps can be selected and the cutting path can be applied again before sending the program to the machine. SigmaNEST can do all this without re-nesting the job, which is a huge advantage.
What is the best amperage to cut with?
Looking closely at the cutting tables supplied by the plasma source supplier it can be noticed that you can cut the same thickness of material with two to three different amperages.

 The key is to select a thickness that falls in the centre of the table of the amperage. The thicknesses at the top and bottom of each table run into the limits of what that specific amperage can handle and therefore will give poor cut quality. If the thickness that must be cut, falls on the top part of the table, it would be better to choose a lower amperage for a cut. If the thickness falls on the bottom of the table, then a higher amperage will give better results. A key to note here is that these tables are determined in laboratory conditions with good quality material. The factory conditions are far from the laboratory conditions. These tables should be used as guidelines or start points for determining your best cut quality.
The correct feedrate to cut with is also a very important parameter in the quest for the best cut quality. If the feedrate is too fast, the cut might not go all the way through the material and can also cause a lot of slag. Also, to notice during a cut, is that the flame lags the nozzle resulting in the angled lines on the cut surface. If the feedrate is too fast, then the lag is quite significant. If the cut then goes around a corner, the lagged flame will prevent the corner from being straight and sharp. SigmaNEST has the ability to cut the contours at optimal speed and then to slow down the feedrate just before the corner to allow the flame to catch up to the nozzle and then to increase the feedrate again after exiting the corner. If the feedrate is too slow, then the flame burns away too much of the material. It can also cause the material to heat up too much in a specific region, melting away the material and causing very bad cut quality. A slower feedrate can also help in correcting the cut when there is an excessive bevel on the cut surface. Because slower feedrates melt away more material, lowering the feedrate will cause more of the bevel to be melted away. When the best feedrate is determined the feedrate can be added into the SigmaNEST tech table, with SigmaNEST then automating the output of the feedrate for future jobs.
Height sensing
Height sensing is also a very important parameter, especially for cutting holes. When cutting holes with height sensing on, the head tends to move up and down the entire time during the cut, causing a very bad cut quality. When cutting small holes, it is always better to switch off height sensing during the cut and switch the height sensing on again after the cut. SigmaNEST automates the switching on and off of the height sense when cutting small holes.
Tech Table
Once all the parameter values have been determined, these values can be added to the tech table inside SigmaNEST.

SigmaNEST then automates the output of these parameter values to the machine.
To get the best cut quality on a plasma machine, testing on all of the different thicknesses and materials needs to be carried out. There are also a few parameters to change during the testing. The good thing to know though, is that once the testing is complete and all the parameter values have been inserted into SigmaNEST’s tech table, SigmaNEST automates the output of these parameters to the plasma machine.