A Guide to Oxyacetylene Cutting

Oxyacetylene cutting (OFC-A) was one of the first successful cutting processes that used a flame produced by the chemical combustion of acetylene and oxygen.

Oxyacetylene cutting (OFC-A) was one of the first successful cutting processes that used a flame produced by the chemical combustion of acetylene and oxygen.

Acetylene is a compound containing carbon and hydrogen. It is slightly lighter than air and unstable at pressures of more than 15 lb/in.² (100 kPa). All acetylene regulators show a maximum of 15 lb/in.² on the working pressure gauge. At more than 15 lb/in.², the scale of the gauge is red as a safety indicator.

Oxygen of high purity (99.5%) is normally stored in high-pressure steel cylinders of various sizes. It is compressed to about 2200 lb/in.² (15,000 kPa). All oxygen regulators show a maximum of 150 lb/in.² (1000 kPa) on the working pressure gauge.

A combination cutting torch consists of a torch handle with a cutting attachment (Photo 1). The valve to control the preheat acetylene is on the torch handle. The oxygen valve on the torch handle is fully open with the oxygen valve on the cutting attachment controlling the preheat oxygen.

Combination cutting torch. (This figure is a combination of Figs. 38 [p. 32] and 106 [p. 66] from AWS C4.9/C4.9M:20XX, Recommended Practices for Oxyacetylene Cutting of Steel; all figures in this article are courtesy of The Ohio State University Extension and the AWS C4 Committee on Oxyfuel Gas Welding & Cutting.)

Safety First

Before performing oxyacetylene cutting, industrial-quality eye protection and a protective face shield with a filter shade must be worn. Clothing must be free from oil contamination and made from durable, flame-resistant materials. Hands and arms must always be protected by wearing gauntlet leather welding gloves. Hard-soled footwear with leather uppers should be worn. While cutting, always have enough surrounding atmospheric air to ventilate exhaust fumes away from the cutting torch.

Setup

First, open the acetylene cylinder valve. Then, open the oxygen cylinder valve. The cylinder valve on the acetylene cylinder may be opened in a counterclockwise direction with a hand wheel or a special tank wrench. If the acetylene cylinder requires a special tank wrench, leave the wrench in place so the acetylene cylinder can be turned off quickly. The acetylene cylinder valve should be opened slowly, at least one full turn, but never more than one-and-a-half turns. The tank pressure gauge on the acetylene regulator shows the tank pressure.

The hand wheel on the high-pressure oxygen cylinder valve shall be turned slowly in a counterclockwise direction to open the valve and allow pressure gradually into the

regulator. When the tank pressure gauge on the oxygen regulator rises to its highest point, the oxygen cylinder valve should be opened fully to prevent leaks.

There is now pressure on each cylinder pressure gauge but no pressure on the working pressure gauge. Check to be sure that a flashback arrestor has been installed on the output side of each regulator.

Based on the thickness of the steel to be cut, use a manufacturer’s chart to select the cutting tip size along with the acetylene and oxygen working pressure.

First, turn the acetylene regulator adjusting screw clockwise until the working pressure gauge moves to the level required for cutting. The acetylene working pressure gauge now shows static pressure; no acetylene gas is leaving the cutting tip.

Next, open the acetylene torch valve on the torch handle by a 1/4 turn. Acetylene gas should be coming out of the preheat holes of the cutting tip with the acetylene working pressure gauge showing dynamic pressure as the acetylene gas leaves the cutting tip. Close the acetylene torch valve on the torch handle.

Turn the oxygen regulator adjusting screw clockwise until the working pressure gauge moves to the level required for cutting. The oxygen working pressure gauge now shows static pressure; no oxygen is leaving the cutting tip.

Next, fully open the oxygen torch valve on the torch handle. Then open the oxygen cutting attachment valve by a 1/2 turn. Oxygen should now be coming out of the preheat holes of the cutting tip with the oxygen working pressure gauge showing dynamic pressure as oxygen leaves the cutting tip. Close the oxygen torch valve on the cutting attachment.

Lighting the Torch

In a safe, well-ventilated area away from personnel and anything flammable, the cutter should now turn on the acetylene gas about 1/4 of a turn. Light an acetylene flame with a friction lighter.

Lighting a cutting torch. (Found in AWS C4.9/C4.9M:20XX, Recommended Practices for Oxyacetylene Cutting of Steel, Fig. 124, p. 77.)

Next, turn on the oxygen with the oxygen cutting attachment valve until an extended envelope of flame can be seen coming from the preheat holes.

Adjust the gases to a neutral flame, then push down on the cutting oxygen lever. The preheat flames will change from a neutral flame to a slightly carburizing flame with the cutting oxygen lever depressed. Readjust the preheat oxygen valve until the preheat flames are neutral again.

Basic flames of oxyacetylene cutting. (Found in AWS C4.9/C4.9M:20XX, Recommended Practices for Oxyacetylene Cutting of Steel, Fig. 131, p. 79.)

Making a Cut

The steel being cut should be clean. A cut is started by lowering the inner cones of the cutting tip to a point about 1/8 in. (3 mm) above the steel being cut and heating the edge to a red color (i.e., 1700°F [925°C]). The preheat flames bring steel to its kindling temperature. The oxygen lever is then depressed with a single stream of oxygen coming out the end of the cutting orifice, vaporizing the steel as dross. This ignition/burning action releases additional heat that raises the temperature of the steel being cut to further melt the steel in the cut. The pressure of the oxygen leaving the cutting tip blows away the oxidized metal, making the cut kerf.

Height of torch tip. (AWS C4.9/C4.9M:20XX, Recommended Practices for Oxyacetylene Cutting of Steel, Fig. 134, p. 81.)

As the cutting progresses, the cutting tip is about 90 deg to the line of travel, and the work angle is 90 deg with respect to the steel being cut.

As the cut comes to the end, the combination cutting torch is slowly raised, allowing the drag lines to form a clean, quality cut with the outer envelope of flame removed from the cut.

Conclusion

AWS C4.9/C4.9M, Recommended Practices for Oxyacetylene Cutting of Steel, is a soon-to-be-released publication with basic and advanced techniques presented and reinforced with illustrations provided by The Ohio State University Extension. Whether cutting in a fabrication shop or a training facility, C4.9 will have modules that can be used individually or as a complete resource to help make quality cuts quickly and safely.

This article was written by Lane L. Liston Jr. (chair of the AWS C4 Committee on Oxyfuel Gas Welding & Cutting) for the American Welding Society.