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While oxy-fuel cutting is a straightforward process, it nonetheless requires experience to perfect. Professional metal fabricators who have mastered this cutting technique produce smooth and square cut surfaces, near-vertical lag lines, and parallel kerf walls. The ideal cut surface has a slightly rounded upper edge and little-to-no slag adhering to the bottom. If you’re still learning the ins and outs of oxy-fuel cutting, with blemished cut surfaces being a common result of your work, here are some factors that may be affecting cut quality:
Cutting Speed Is Too Low
If the cutting speed of the oxy-fuel machine is too low, it can heavily gouge the cut surface. Additionally, low cutting speeds produce heavy slag, which accumulates on the bottom edge. These deficiencies are caused by an excess of oxygen that destabilizes the cutting process.
Cutting Speed Is Too High
When cutting speed is too high, heavy lag occurs. The result is a surface that not only is marred by curving lag lines but also bends slightly inward or outward. Furthermore, due to severe drag, the cut may not go all the way through the material.
Nozzle Is Too Far From the Surface
For best performance, the nozzle of the oxy-fuel cutting machine should not be more than ¼” above the surface of the plate. If it is too far from the surface, it will produce dramatic rounding of the top edge.
Nozzle Is Too Close to the Surface
Conversely, if the cutting torch is too near the surface, the inner cones of the preheated flame become buried in the kerf, burning deep grooves into the cut surface and over-melting the top edge. This can also result in the flame popping and loss of workable material.
Oxygen Pressure Is Too High or Low
Nozzle sizes are designed for specific torch pressures. If the wrong size nozzle is used, oxygen pressure can either be too high or too low; both scenarios cause distortions in the oxygen stream, which affect cut quality.
Excessive Preheat Flame
A common mistake of novice metal fabricators is turning up the oxy-fuel cutter’s preheat flame to increase cutting speed. This strategy simply doesn’t work, as the heavy preheat flame will melt the plate surface and, in fact, reduce cutting speed. It also results in a huge waste of oxygen.
Nozzle Is Dirty
Maintenance is key to ensuring smooth oxy-fuel cutting operations. If the torch nozzle is allowed to get dirty, it can disrupt the oxygen stream’s parallel form, creating a host of blemishes—such as pitting, under-cutting, and heavy slag—on the cut surface.
Plate Is Not Suitable for Oxy-Fuel Cutting
In addition to the settings of an oxy-fuel cutting machine, the plate itself should be taken into consideration to ensure a perfect cut. For instance, there may be alloys in the material that are not easily oxidized, which can lead to irregularities in cut quality. Also, if the plate is oily or rusty, cutting speed may need to be reduced and the preheat flame increased to produce a workable result.
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