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How To Cut With Oxyacetylene, Part 4

You’ve bought the right torch and set up your shop for it. You’ve done the safety checks and are ready to rock and roll. Now, It’s time to light the oxyacetylene system. As usual, we’re continuing to draw on technical expertise from a variety of sources to guide us through the process the correct way. They include Cal Shaw, a welding instructor who teaches at SIAST’s Palliser Campus, published text books and other training material.

As always, make a quick visual inspection to ensure the system is in good condition and nothing is damaged or leaking. If an oxyacetylene system is going to be left unused for any extended length of time, the regulators should be removed from the cylinders and the safety caps installed. So reconnect the system (see the previous segment in the March 7 issue for information on how to do that) and open the cylinder valves.

When opening cylinder valves, be sure you do it correctly. There are two different types of valves in common usage. Oxygen tanks use a double-seated valve that can be opened by hand. This type must always be opened fully in order to prevent leaks. Just as the valve seats internally when it is shut off, it also seats in the fully open position. If left in the part-open position it may leak.

Acetylene tank valves should only be opened one to one-and-a-half turns. No more. Some older acetylene valves require a wrench to open. If your acetylene tank has this type of valve, leave the wrench attached in case the gas flow needs to be shut off quickly.

Valves on both tanks must be opened slowly to prevent damage to the regulators. Open the oxygen valve a small amount until full tank pressure builds up in the regulator, then turn it open the rest of the way. Always stand to the side of cylinders opposite the regulator when opening valves; this will prevent injury in case the regulator bursts — and it does happen sometimes (see the picture).

Before opening the cylinder valves, make sure the regulator valves are completely closed. That means the adjuster screws have been turned out (counterclockwise).

Check the chart that was supplied by the torch manufacturer to obtain the correct torch tip and line pressure setting for the job you’re about to do. To set the line pressures, start with the oxygen regulator. Open the oxygen valve on the torch about a half turn and hold the cutting lever down while you set the oxygen line pressure (turn the adjuster screw clockwise until the line pressure gauge reads the correct pressure), then close the torch valves. Next, open the acetylene valve on the torch a half turn and dial in the adjuster screw on the acetylene pressure regulator until it’s correctly set. Then shut off the acetylene torch valve.

If you don’t open the torch valves when establishing regulator settings, actual line pressure will drop during cutting. That will interfere with efficient operation. And never set the acetylene pressure above 15 p.s.i. Acetylene cannot be used safely above that limit.

This process also purges the lines to ensure there is no mixing of gases. The gauges may show a slight increase in pressure shortly after the initial setting. As long as it is a small increase, that’s OK. But if it’s a big change, shut down the gas flow. It’s an indication the regulator is faulty.

Now it’s time for a flame. Pointing the nozzle tip in a safe direction, open the acetylene torch valve one-eighth to one-quarter turn. Place a proper spark lighter close to the nozzle and hold it at about a 45 degree angle to the tip. Squeeze the lighter and ignite the flame. Never light a torch with any other tool, especially cigarette lighters. You could end up with some serious burns.

Increase the acetylene flow with the torch valve until the smoke disappears from the flame. Open the oxygen valve on the torch and adjust for a neutral flame. As the oxygen flow increases, the red acetylene flame will turn purple and become smaller, inner preheat cones starts to form. The inner preheat cones are white and become even brighter as oxygen flow increases, eventually they develop well defined edges. When this happens, you have a neutral flame that burns quietly. Press the oxygen cutting lever while adjusting the flame.

If you use too much acetylene, the flame will be turbulent and make a lot of noise. It will also create a lot of smoke.

Now you’re ready to cut. Next time we’ll take a look at how to use a torch to test steel and determine its welding characteristics.


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About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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