Cutting with Oxyfuel

Cutting with OxyFuel

MAIN IDEA: What are the principles and proper procedures used in cutting with oxyfuel?

Combustible gases have been used in welding processes for over 200 years. A study of gas heating, cutting and welding procedures is an important part of modern agricultural mechanics. The term oxyfuel refers to the combination of pure oxygen and a combustible gas to produce a flame. Oxyfuels are used for welding, brazing, cutting and heating.

GENERAL PRINCIPLES:

Oxygen and fuel gases are stored under pressure in tanks or cylinders. They are released as individual gases through carefully designed valves, regulators and hoses.

The gas flame from an oxyfuel burns with intense heat. The temperature may range from 5000 F to 6000 F. These temperatures are hot enough to melt most metals and permit cutting and fusion welding. Oxyfuel cutting is a process in which steel is heated to the point that it burns and is removed to leave a thin slit called a kerf.

Cutting torches are designed to send a forceful stream of oxygen into preheated cherry red steel. The oxygen supports the combustion. The force of the oxygen stream drives the slag out of the area and permits the heat from the torch to keep the steel burning. As a cutting torch moves forward, the oxygen stream pushes slag out to form the kerf.

CHARACTERISTICS OF OXYFUELS:

Oxygen is not a fuel and it will not burn. However, it combines with other substances and causes them to burn.

Acetylene is the fuel most suitable for welding. This gas produces a cleaner weld than most other fuel gases. It also produces a more controllable flame. Acetylene gas is unstable, so it is a hazardous fuel. Because acetylene gas is also more expensive than other oxyfuels, it is used only when its burning characteristics are specifically needed.

Propane gas is available in cylinders and tanks. Natural gas can be pumped into buildings through public utility lines. Both gases can be used for cutting. However, they consume large volumes of cylinder oxygen, which may offset other price advantages offered by these gases.

MAPP® gas is a formulated mixture of methylacetylene and propadiene gases. The gas has many of the advantages of acetylene but is formulated to be more stable and, therefore, safer. Its high temperature flame is suitable for cutting.

CUTTING STEEL WITH OXYFUELS:

A cutting torch must be adjusted to obtain a neutral flame both with and without the oxygen jet. The illustrations on another page show several adjustments of the flame.

The procedure for obtaining a neutral flame is outlined below:

1.  Turn on the oxygen tank valve. Several rounds are ok.

2.  Turn the valve on the acetylene tank one half turn.

3.  Adjust the regulator pressure for the acetylene and oxygen according to the following table:

CUTTING GUIDE
Metal
Thickness (in.) / Airco Cutting
Tip Size / Pressure PSI
Oxygen / Acetylene
1/8
1/4
3/8
1/2
3/4
1
1 1/2
2
3
4
5
6 / 00
0
1
1
2
2
3
4
5
5
6
6 / 30
30
30
40
40
50
45
50
45
60
50
55 / 1 1/2
3
3
3
3
3
3
3
4
4
5
5

4.  Open the acetylene valve on the torch 1/8th of a turn.

5.  Light the acetylene using a spark lighter.

6.  Slowly open the acetylene valve until the smoke from the burning acetylene goes away.

7.  Slowly open the oxygen valve on the torch until the inner flame disappears. The flame is now a neutral flame.

When cutting steel, you will need to position the work for cutting, light the flame, and obtain a neutral torch flame. To do this, place the metal to be cut over a slag box. The slag box is a metal container filled with water or sand that is used to catch hot slag and metal from the cutting process.

When making cuts longer than 2 inches, use both hands to grip the torch and slide over the metal. Wheeled trolleys can be used to maintain the correct distance.

For general cutting, mark the line of the cut with a soapstone. Place one gloved hand on the metal near the torch head to control the tip clearance. Use the other hand to control the handle and the oxygen lever.

To start the cut, hold the flame over the corner edge of the metal, holding the torch at a slight angle away from the edge. The cones of the preheat flames should not touch the metal. Hold the torch steadily until the edge of the metal turns cherry red.

Next, press the oxygen lever and move the torch across the metal at a steady rate. Keep the flame cones about 1/8 of an inch from the metal.

Other special devices, such as a rod with a sliding center point, can be attached to a torch to help make perfect circles. Another piece of steel can be used to guide the torch to make straight cuts.

The marks left behind after a cut can indicated how the cut was made, giving information about the preheat procedures, speed and pressure.

If the preheat flame is too hot or the torch travels too slowly, the surface melts before the metal is heated through, leaving a melted or rounded appearance along the top. To correct this problem, raise the tip slightly. Also, increasing the speed may correct the problem. The clearance is correct when the tips of the primary flames are almost level with the surface metal.

If the torch is moved too fast across the metal, it results in an incomplete cut and rough edges. A torch that is moved too slowly melts the top edge and leaves gouges where the cutting stream wandered.

Oxygen pressure that is too high leaves a dish shape in the kerf near the top. On the other hand, if the pressure is too low, the cut may not be complete at the bottom.

A correct cut is straight and square with a smooth face. Correct preheat, speed and pressure allow a clean, accurate and fast cut. Cutting requires a steady hand, which can be obtained by practicing. When a good cut is being made, there will be a smooth even sound and a steady stream of sparks from the bottom kerf.

To cut pipe up to 3 inches in diameter, pierce a hole in the top of the pipe. Next cut a kerf to the left side, followed by one to the right side. Rotate the pipe and repeat the process to cut the underside.

To cut large pipe, hold the torch at a right angle to the pipe. Next move the torch around the pipe to make the cut. The pipe can also be rotated in steps or even continuously.