Process
How does oxyacetylene cutting equipment perform the cutting process?
The oxyacetylene flame brings the steel at the beginning of the cut up to kindling temperature of 1600°F (871°C). At the kindling temperature, steel will readily burn in the presence of oxygen. When the oxygen lever is turned on, the pure oxygen stream with the steel at kindling temperature burns, this combination causes a chemical reaction called oxidation. The mixture of oxides of iron is called slag. This slag has a melting point much lower than the melting point of steel itself that is 2600°F (1427°C) and readily runs out of the cut or kerf. The force of the oxygen stream provides additional help to clear the kerf of molten oxides. In addition to the oxyacetylene preheat flame, the burning of the iron in the oxygen stream releases large amounts of heat. This aids cutting action particularly when cutting thick steel. Moving the torch across the work produces continuous cutting action; straight, curved or beveled cuts are readily made.
What is the kerf of a cut?
When cutting is performed material is removed, the width of the cut is the kerf; when flame cutting the oxidation of the metal along the line of the cut removes a thin strip of metal or kerf which is the thickness of the cut which is also the bore size of the cutting tip. In steel under two inches in thickness, it is possible to hold the kerf to about
Figure 2–7Kerf and drag in an oxy-fuel cut
What factors determine kerf size?
•Kerf size depends on the following:
•Torch oxygen bore (orifice) size
•Torch tip design
•Oxygen pressure and flow rate
•Preheat flame size
•Cutting speed
What is the proper size cutting tip to use for various material thicknesses?
Cutting tip bore drill sizing, like welding tip orifice drill sizing, numbering system is not standardized in the welding industry. The drill sizing is standard but the manufacturer’s numbers placed on the tips are not standard. One company may identify a number one tip for cutting one inch steel while the same bore drill size from another company may call for a number two tip. The American Welding Society (AWS) has been urging manufacturers to stamp cutting tips with material thickness size to eliminate confusion with the publication AWS C4.5 Uniform Designation System for Oxy-Fuel Nozzles. Compliance is not mandatory therefore manufacturers have not followed through with using this standard.
What does the bore drill size indicate?
Cutting drill bore size indicates cutting orifice size and material thickness which can be cut. See Table 2–2.
Bore Size for Oxy-Fuel Cutting
| Plate Thickness inches (mm) | Bore Drill Size inches (mm) |
| 1/4-1/2 (6.35-12.7) | 68-53 DR 0.031-0.059 (0.794-1.51) |
| 3/4(19.05) | 62-53 DR 0.038-0.059 (0.965-1.51) |
| 1(25.1) | 56-53 DR 0.046-0.059 (1.18-1.51) |
l | 51-46 DR 0.067-0.081 (1.70-2.06) |
| 3-5 (76.2-127.0) | 46-44 DR 0.081-0.086 (2.06-2.18) |
| 6-8(152.4-203.2) | 40-39 DR 0.098-0.010 (2.49-2.53) |
| 10 (254) | 39-35 DR 0.010-0.011 (2.53-2.94) |
Table 2–2 Material thickness to bore size for cutting tips
Why does kerf width grow larger with increasing steel thickness?
Cutting thicker steel requires more oxygen, which requires a larger oxygen orifice size, greater oxygen flow rates and a larger oxygen stream. These lead to a wider kerf.
How can the bore size be determined?
Using a tip cleaner find the round file which will fit snuggly into the bore then determine the bore size by the chart list on the tip cleaner’s container.
What is drag?
The distance between the cutting action at the top and bottom of the kerf is called drag. When the oxygen stream enters the top of the kerf and exits the bottom of the kerf directly below, the drag is said to be zero. If the cutting speed is increased (or the oxygen flow decreased), oxygen in the lower portion of the kerf decreases and the kinetic energy of the oxygen stream drops, slowing cutting action in the bottom of the cut. This causes the cutting action at the bottom of the kerf to lag behind the cutting action at the top. Drag may also be expressed as a percentage of the thickness of the cut. See Figure 2–6.
What are the effects of excessive drag?
Excessive drag can cause loss of cutting action in thick cuts and restarting the cutting action may cause the loss of a part being flame cut.
When can reverse drag occur?
Excessive oxygen flow, too slow a cut, or damaged orifices may cause reverse drag leading to rough cut edges and excessive slag adhesion.
What is the chemistry of the OAC process?
There are three principal reactions producing three different iron oxides. Notice that the second reaction releases the most heat that helps sustain the cutting action. The equations show the ratios of oxygen to fuel (iron) needed. To a chemist these equations indicate that about 104 ft3 of oxygen will oxidize 2.2 lb. of steel to Fe3O4.
Fe + O2 → FeO + heat of 267 Kj (Kilojoules)
3Fe + 2O2 → Fe3O4 + heat of 1120 Kj
2Fe + 1.5O2 → Fe2O3 + heat of 825 Kj
What are advantages of the OAC process?
•Low cost compared with machine tool cutting equipment.
•No external power required.
•Readily portable.
•Steels usually cut faster than by conventional machining process.
•Cutting direction may be changed easily.
•OAC is an economical method of plate edge preparation for groove and bevel weld joints.
•Large plates may be cut in place.
•Parts with unusual shapes and thickness variations hard to produce with conventional machinery are easily produced with OAC.
•Can be automated using tracks, patterns, or computers to guide the torch.
What are disadvantages of the OAC process?
•Dimensional tolerance of OAC is dramatically poorer than machine tool based cutting.
•OAC process is commercially limited to steel and cast steel.
•Both the preheat flame and the stream of molten slag present fire and burn hazards to plant and personnel.
•Proper fume