How to Use an Oxy Acetylene Torch: Complete Guide for Cutting, Welding, Heating, Brazing, and Torch Tip Sizes

In the machinery manufacturing and repair industries, welding and cutting of metal plates or other metal components are frequently required. There are two main types of metal welding: electric welding and gas welding. “Gas welding” refers to the process of using a combustible gas—typically acetylene—combined with oxygen to generate intense heat through combustion. This heat melts the edges of the workpieces and the filler rod, allowing them to fuse together. Once solidified, the parts form a strong joint.

The shape, temperature, and quality of an oxy-acetylene flame depend on the composition of the combustible gases, specifically the ratio of acetylene to oxygen. By adjusting this ratio, three types of flames can be achieved: neutral flame, oxidizing flame, and carburizing flame. For standard welding, a neutral flame is used, where the oxygen-to-acetylene ratio is theoretically 1:1. Neutral flames provide high temperatures and strong welds. In practice, a slightly higher oxygen supply ensures complete combustion.

Since only part of the oxygen is supplied from the oxygen cylinder during gas welding, additional oxygen from the air also participates in the combustion of the acetylene flame. A typical neutral flame consists of three distinct zones: the inner cone, the intermediate (acetylene) flame, and the outer envelope. The inner cone is made of unburned mixed gas from the torch tip, where combustion begins. In this zone, incomplete combustion occurs using only cylinder-supplied oxygen, producing a reducing atmosphere rich in CO and H₂. These gases help deoxidize the molten metal, preventing pores and oxide inclusions. At the flame’s outer edge, residual CO and H₂ combust completely with atmospheric oxygen.

How to Use an Oxy-Acetylene Torch for Metal Welding

1. Preparation Before Cutting:

Before gas cutting, clean the metal surface thoroughly—remove any rust, dirt, or grease. The workpiece should be supported on a stable surface to aid heat dissipation and slag removal. Never cut directly on concrete, as high temperatures can cause it to crack or explode. Specific steps include:

① Ensure the workspace meets safety standards. Inspect the torch, oxygen and acetylene cylinders (or generator and flashback arrestors), hoses, and regulators. Connect the cutting equipment according to proper procedures.

② Elevate and level the workpiece, leaving a gap beneath it to allow for molten slag removal. Use shielding if necessary to prevent burns from flying slag.

③ Adjust the oxygen pressure. For injector-type torches, check suction ability by removing and folding the acetylene hose, then opening the acetylene and preheating oxygen valves. Place a finger over the acetylene inlet—if suction is felt, the torch is functioning correctly.

④ Test the cutting oxygen stream (“jet line”) by igniting and adjusting the preheat flame. Then open the cutting oxygen valve and observe the stream—it should form a straight, clear cylinder of appropriate length. Irregular jets require cleaning the torch tip with a tip cleaner.

Use a neutral flame for preheating and adjust its intensity based on material thickness.

2. Igniting, Adjusting, and Extinguishing the Flame:

2.1 Ignition:
Open the oxygen valve slightly, then the fuel gas valve. Let the gases mix and ignite with a spark or flame.

• If the oxygen valve is opened too far, you’ll hear popping sounds and may experience flame blowback.
• If the flame produces black smoke after ignition, there is insufficient oxygen. Adjust the valve until the flame burns cleanly.

2.2 Adjustment:
Once lit, gradually open the fuel and oxygen valves to achieve the required flame size and properties. Typical pressures:

• Acetylene: 0.05–0.1 MPa
• Oxygen: 0.3–0.4 MPa

2.3 Extinguishing:
To shut off the torch:

1. Close the oxygen valve slightly.
2. Close the fuel gas valve.
3. Fully close the oxygen valve.
4. Briefly open the oxygen valve again to purge the torch and confirm the flame is off.

3. Flame Heating Position and Angle:

Direct the tip of the inner cone (2–4 mm) at the starting point of the weld. For equal thicknesses, aim at the joint; for unequal thicknesses, favor the thicker side. Begin with a steep torch angle to form a molten pool quickly, then adjust angle and speed to maintain consistent pool size during welding.

4. Initiating the Weld:

Don’t begin welding until the molten pool is fully formed. If the pool is small and surrounding temperatures are low, move the flame in small circles to raise the local temperature. When a properly sized pool forms, insert the filler rod and begin moving the torch forward.

5. Adding Filler Rod During Welding:

5.1 Monitor the weld pool closely. Dip the end of the rod into the inner cone of the flame—never the outer envelope—to melt it and let droplets fall into the pool. Move the torch steadily to keep the pool uniform and avoid oxidation.

5.2 For thin plates, use the filler rod to shield the flame and prevent burn-through. If using flux or encountering slag, stir the pool with the rod to help impurities rise and escape.

6. Torch and Filler Rod Movement Techniques:

6.1 Forward Movement:
The primary motion along the joint to advance the molten pool and form the weld.

6.2 Vertical “Dabbing” Motion:
Torch and rod move up and down to control pool temperature and droplet transfer rate, helping ensure uniform bead height.

6.3 Lateral Motion (Side-to-Side or Circular):
Used to widen the pool and ensure proper fusion of joint edges. Also aids in mixing the pool with the filler material.

7. Weld Starts and Ends:

7.1 Starting Overlaps (Tie-Ins):
When restarting a weld, begin on the existing bead and form a proper pool before continuing. Overlap the new weld 8–10 mm into the old one.

7.2 Finishing the Weld:
At the end of the weld, reduced heat dissipation can cause the pool to enlarge and burn through. To prevent this:

• Decrease the torch angle,
• Increase speed,
• Add filler quickly,
• Raise the flame to slow solidification and allow gases to escape.

Final tip: end with a shallow angle, faster movement, quick filler addition, and slowly withdraw the torch.

Choosing the Right Torch Tip Size:

🔧 8. Torch Tip Size Selection Guide

🔢 Understanding Torch Tip Numbers

Torch tips are usually numbered (e.g., #0, #1, #2…#8), indicating the size of the orifice and the volume of gas flow. Smaller numbers are used for fine work; larger numbers for heavy-duty cutting or welding.

🛠️ Recommended Tip Sizes by Application

Note: Pressure settings and tip selection may vary by manufacturer.

🔍 Factors Affecting Tip Selection

• Metal thickness: Thicker metals require larger tips and higher gas flow.
• Desired flame intensity: High-energy processes like cutting need higher oxygen pressure.
• Process type: Brazing, welding, and heating all require different flow rates and flame types.
• Material type: Stainless steel, aluminum, and carbon steel behave differently under heat.

❓ 9. Frequently Asked Questions (FAQ)

🔸 What is the 1:7 Rule in Oxy-Acetylene?

The 1:7 rule states that acetylene cylinder pressure should never exceed 1/7 of its total capacity during use, to prevent unstable gas release and risk of explosion. Exceeding this rate can cause acetone to leave the cylinder, leading to dangerous conditions.

🔸 What if the Torch Won’t Light?

• Ensure acetylene is slightly open before attempting ignition
• Use a proper spark lighter (not a match or lighter)
• Check for leaks or blockages in the tip or hoses
• Make sure regulators are correctly set and hoses are not kinked

🔸 What to Do if Flame Backfires or Pops?

• Immediately close the torch valves
• Check for loose tip, overheated nozzle, or blocked orifice
• Make sure flashback arrestors are installed
• Avoid using too low a gas pressure
• Re-light with proper technique after cooldown

source：Welding Journal, Issue 5, 2023