Steve Zlotnicki Click image to enlargeby Steve Zlotnick

How to achieve consistent cutting results

Oxy-fuel torch cutting, or flame cutting, is by far the oldest cutting process for mild steel. It is generally viewed as a simple process, and the equipment and consumables are relatively inexpensive. An oxy-fuel torch can cut through very thick plate, limited primarily by the amount of oxygen that can be delivered. When adjusted properly, an oxy-fuel torch delivers a smooth, square cut surface. There is little slag on the bottom edge and the top edge is only slightly rounded from preheat flames. This surface is ideally suited for many applications without further treatment. Consistently achieving good cut quality with oxy-fuel requires understanding the factors that impact cutting results. While there are a number of factors to consider, the following are the root of most common cut quality issues.

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.

Cutting Speed Too High
An extremely high cutting speed results in heavy lag, indicated 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.

Nozzle Too Far From Surface
When the nozzle is too high above the work, excessive rounding of the top edge occurs. The cutting speed may have to be lowered. With the correct nozzle clearance, preheat flames should not be more than 1/4 in. (6.35 mm) above the top surface of the plate.

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. In addition, the flame becomes subject to popping and lost cuts may result.

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.

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. Moreover, oxygen and fuel gas are wasted.

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.

Being aware of the factors that can influence oxy-fuel cut quality and making the proper allowances for them can help machine operators consistently achieve high quality cutting results that rivals that of a machined surface. SMT

Steve Zlotnicki is product manager, cutting, with ESAB Welding & Cutting Products, Florence, SC .

 

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