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How to improve oxy-fuel cut quality
September 9, 2013
7 Common Faults of Oxy-Fuel Machine Cutting
Although oxy-fuel cutting is generally viewed as a simple process, most machine operators realize that making it perform properly is not as simple as it looks. Experienced operators can achieve cut quality that rivals a machined surface, and do it in a fraction of the time cost of hard tooling. But consistently getting that quality requires understanding the factors that affect quality, and how they interact with each other.
Correct Cutting Technique
When everything is adjusted properly, the cut surface is smooth and square, and the kerf walls are parallel. The lag lines are almost vertical. There is little slag adhering to the bottom edge. The top edge is slightly rounded when the preheat flames are properly adjusted. This surface is ideally suited for many applications without further treatment.
1. Cutting Speed Too Low
An abnormally low cutting speed results in heavy gouging of the cut surface and slag adhering in large globules. Under this condition, oxygen and fuel gas are being wasted..
2. Cutting Speed Too High
An extremely high cutting speed results in heavy lag, as shown by the curved lag lines on the cut surface. The face is reasonably smooth but somewhat concave. Slag will adhere during cutting, but it may be removed with ease. Heavy lag cutting is recommended for straight line cuts only.
3. Nozzle Too Far From Surface
When the nozzle is too high above the work, excessive rounding of the top edge occurs. Also, the cutting speed may have to be lowered. With the correct nozzle clearance, the preheat flames should not be more than 1/4” above the top surface of the plate.
4. Nozzle Too Near Surface
When the nozzle is too low, part of the preheat flame’s inner cones become buried in the cutting kerf. This produces grooves in the cut face and excessive melting of the top edge. Also, the flame becomes subject to popping and lost cuts may result.
5. Excess Cutting Oxygen
If the cutting oxygen pressure is too high or the nozzle size too large, a reduction in cut quality will result. Nozzles are made to operate within a limited range of torch pressures. Therefore, excessive oxygen pressure causes distortions in the oxygen stream once it leaves the nozzle.
6. Excess Preheat Flame
Inexperienced operators often try to increase cutting speeds by using a heavy preheat flame. Excessive preheat causes melting of the top edge and may actually lower the speed of cutting. In addition, oxygen and fuel gas are wasted.
7. Dirty Nozzle
If the nozzle has been fouled, it may cause the oxygen stream to lose its parallel form. The cut surface will not be smooth and square, and there may be pitting, under- cutting, heavy slag or scale. The nozzle should be cleaned with care, so as not to distort, or bell-mouth, the cutting oxygen bore.
There are other factors that will affect cut quality also, such as oily plate, plate chemistry, or rusty/scaly plate. But the 7 factors discussed above will be the root of most common cut quality issues.