In the last five issues, we’ve looked at the whole process of preparing to use an oxyacetylene system to cut steel. This time we get to work doing it. As usual, we’re drawing on technical expertise from a variety of sources. They include Cal Shaw, a welding instructor who teaches at SIAST’s Palliser Campus, published text books, and technical training material to guide us through the process.
After lighting the torch, set a neutral flame (see a previous instalment on how to do that properly). When a neutral flame is set correctly, the inner preheat cones are white and have well defined edges, the flame also burns quietly.
In order to cut, you must preheat the metal. So position the torch nozzle over the start point with the inner preheat flames about 1/16 to 1/8 inch above the metal’s surface. It is easier to start a cut at the edge of the metal. Starting in a centre position requires piercing the steel, which makes the process more difficult.
As soon as the starting point has been heated to a bright cherry red, press the cutting lever down to supply a jet of oxygen through the centre of the nozzle and begin cutting. This will cause rapid oxidization of the steel and blow metal away leaving a cut or “kerf.”
Hold the torch nozzle perpendicular to the steel or angle the flame slightly forward. Aiming it forward is best when cutting thinner materials (under one inch) that take less heat to oxidize. It allows for a faster cut that ends up using less gas.
The thinner the metal, the more severe the forward angle should be. Holding the nozzle at a steep angle effectively increases the thickness of the metal being cut. On thin metal, use the smallest tip possible and angle the tip up to 15 or 20 degrees. Sometimes, though, it may be possible to hold the tip nearly horizontal when cutting very thin sheet metal. The edge of the nozzle can be placed right on the base metal, but the preheat cones should still be kept just above the metal surface. And the nozzle must be moved very quickly to avoid excessive heat and an irregularly shaped kerf.
When cutting thicker materials, a perpendicular position works better. There may be some “drag” when making a cut. Drag is the horizontal distance between the entry and exit points of the cutting jet (see image). A little drag is good, and it ensures the proper amount of oxygen is consumed, resulting in a nice, even cut. If, however, the drag is excessive, it is an indication that the torch is being moved too quickly, the oxygen pressure is set too low or the steel hasn’t had enough preheat for an efficient cut. On very thick metals it may be necessary to change to a larger cutting tip to reduce the drag distance.
When a torch is used correctly, the edges of the kerf will be very smooth, with practise you can make a remarkably good job. Rough, uneven edges are the result of incorrect cutting speeds. The edges of a cut should have a machined appearance and be completely straight through the full depth of the kerf. If too much oxygen is applied, the cut can develop a bell shape removing too much material near the bottom and leaving a lot of slag. Practising is the only way to fine tune your skill.
In order to get a straight cut, you need to know how to hold the torch for best results. There are two methods that will get good results. The first way is to rest your left hand on the steel under the torch to keep it steady. Then as you make the cut, roll your wrist over as the cut progresses from right to left. Moving in this direction also makes it possible to get a good view of the developing cut.
The other way is to slide the torch straight and forward through your left hand, similar to shooting a pool cue. But when cutting this way, it is better to slide the torch away from you rather than toward you. Either of those two cutting techniques will give you good control over the movement of the nozzle tip. You also want to use a comfortable three-point stance when working.
Make sure the metal isn’t excessively dirty when making a cut. Impurities on the surface can slow the effective cutting speed and cause rough and irregular kerfs to develop.
And one last word about safety. Always wear protective equipment when using oxy-fuel systems. That includes full-face eye protection, a welder’s cap, non-flammable clothing (no petroleum-based fabrics like polyester) and high boots. There are special leather aprons and leggings available through welding supply stores that can provide increased protection. It’s better to buy them and not ever need them than think about how they could have protected you “that day.”
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