CHAPTER 4 GAS CUTTING

The common methods used in cutting metal are oxygas flame cutting, air carbon-arc cutting, and plasma-arc cutting. The method used depends on the type of metal to be cut and the availability of equipment. As a Steelworker, oxygas or air carbon-arc equipment is the most common type of equipment available for your use. Oxygas equipment is explained in this chapter and air carbon-arc cutting is covered in chapter 7.

The oxygas cutting torch has many uses in steel- work. At most naval activities, the Steelworker finds the cutting torch an excellent tool for cutting ferrous metals. This versatile tool is used for operations, such as beveling plate, cutting and beveling pipe, piercing holes in steel plate, and cutting wire rope.

When using the oxygas cutting process, you heat a spot on the metal to the kindling or ignition temperature (between 1400°F and 1600°F for steels). The term for this oxygas flame is the PREHEATING FLAME. Next, you direct a jet of pure oxygen at the heated metal by pressing a lever on the cutting torch. The oxygen causes a chemical reaction known as OXIDATION to take place rapidly. When oxidation occurs rapidly, it is called COMBUSTION or BURNING. When it occurs slowly, it is known as RUSTING.

When you use the oxygas torch method to cut metal, the oxidation of the metal is extremely rapid and part of the metal actually burns. The heat, liberated by the burning of the iron or steel, melts the iron oxide formed by the chemical reaction and accelerates the preheating of the object you are cutting. The molten material runs off as slag, exposing more iron or steel to the oxygen jet.

In oxygas cutting, only that portion of the metal that is in the direct path of the oxygen jet is oxidized. The narrow slit, formed in the metal as the cutting progresses, is called the kerf. Most of the material removed from the kerf is in the form of oxides (products of the oxidation reaction). The remainder of the material is molten metal that is blown or washed out of the kerf by the force of the oxygen jet.

The walls of the kerf formed by oxygas cutting of ferrous metals should be fairly smooth and parallel to each other. After developing your skills in handling the torch, you can keep the cut within close tolerances; guide the cut along straight, curved, or irregular lines; and cut bevels or other shapes that require holding the torch at an angle.

Partial oxidation of the metal is a vital part of the oxygas cutting process. Because of this, metals that do not oxidize readily are not suitable for oxygas cutting. Carbon steels are easily cut by the oxygas process, but special techniques (described later in this chapter) are required for the cutting of many other metals.

OXYGAS CUTTING EQUIPMENT

An oxygas cutting outfit usually consists of a cylinder of acetylene or MAPP gas, a cylinder of oxygen, two regulators, two lengths of hose with fittings, and a cutting torch with tips (fig. 4-1). An oxygas cutting outfit also is referred to as a cutting rig.

In addition to the basic equipment mentioned above, numerous types of auxiliary equipment are used in oxygas cutting. An important item is the spark igniter that is used to light the torch (fig. 4-2, view A). Another item you use is an apparatus wrench. It is similar in design to the one shown in figure 4-2, view B. The apparatus wrench is sometimes called a gang wrench because it fits all the connections on the cutting rig. Note that the wrench shown has a raised opening in the handle that serves as an acetylene tank key.

Other common accessories include tip cleaners, cylinder trucks, clamps, and holding jigs. Personal safety apparel, such as goggles, hand shields, gloves, leather aprons, sleeves, and leggings, are essential and should be worn as required for the job at hand. Information on safety apparel is also contained in chapter 3 of this text.

Oxygas cutting equipment can be stationary or portable. A portable oxygas outfit, such as the one shown in figure 4-3, is an advantage when it is necessary to move the equipment from one job to another.

To conduct your cutting requirements, you must be able to set up the cutting equipment and make the required adjustments needed to perform the cutting operation. For this reason it is important you understand the purpose and function of the basic pieces of equipment that make up the cutting outfit. But, before discussing the equipment, let's look at the gases most often used in cutting: acetylene, MAPP gas, and oxygen.