Welding Skills Chapter 25: Cutting Operations

Questions and Answers

What causes metal to rust?

Rust is the result of corroding steel after the iron (Fe) particles have been exposed to oxygen and moisture.

What principle makes it possible to cut metal by OFC?

If metal is heated to its ignition temperature, it oxidizes and rusts much faster, allowing for cutting.

How does a cutting torch differ from a gas welding torch?

A cutting torch has a central hole for oxygen and smaller pre-heating holes for fuel gas.

What determines the oxygen and acetylene pressure for cutting metal?

The type of metal, the manufacturer's tip number size, and the metal thickness. Signup and view all the answers

What aids can be used to facilitate an even cut when using OFC?

A straight piece of metal bar or angle iron with a straight edge. Signup and view all the answers

How can it be determined that the OFC cut is penetrating through the metal?

A shower of sparks falling from the underside of the cut indicates penetration is complete. Signup and view all the answers

What is the position of the torch when cutting round stock?

Start 90 degrees from the top edge and follow the contour of the bar. Signup and view all the answers

How is it possible to make a bevel cut with a cutting torch?

The torch tip must be held at the desired angle to the cut line rather than perpendicular to it. Signup and view all the answers

Describe the operation for piercing small holes with a cutting torch.

Hold the cutting tip above the surface, heat the spot until cherry red, then slowly press the oxygen lever. Signup and view all the answers

What type of flame is used for cutting cast iron?

A slightly carburizing preheating flame. Signup and view all the answers

How is the torch held when cutting cast iron?

Position the torch at a drag angle of 40 to 50 degrees and gradually straighten it to 65 to 70 degrees. Signup and view all the answers

What torch motion is used for cutting cast iron?

The torch movement is similar to inscribing semi-circles along the cutting line. Signup and view all the answers

What is meant by PAC?

PAC stands for Plasma Arc Cutting. Signup and view all the answers

What types of metals can be cut by PAC?

Any electrically conductive metals, typically up to 3 inches thick. Signup and view all the answers

What type of electrode is used in the CAC-A process?

DC copper-coated electrodes are most widely used. Signup and view all the answers

What causes the removal of molten metal when using CAC-A?

A high velocity blast of compressed air removes the molten metal. Signup and view all the answers

What does the term washing mean when using CAC-A?

Washing is a process of removing a layer of metal from large areas. Signup and view all the answers

What precautions should be observed before engaging in any cutting operation?

Only cut in designated areas, clear combustible materials, and maintain a fire watch. Signup and view all the answers

Study Notes

Rust Formation

Rust results from the corrosion of steel when iron (Fe) is exposed to oxygen and moisture.
Oxidation of iron leads to the formation of Fe⁺⁺ ions, releasing electrons that create hydroxyl ions (OH) upon reacting with oxygen.
The reaction between Fe⁺⁺ and hydroxyl ions produces hydrous iron oxide (FeOH), commonly known as rust.

Cutting with Oxygen-Fuel Cutting (OFC)

Metal oxidizes and rusts faster when heated to its ignition temperature, causing a melting action that is swept away by oxygen, enabling cutting.
A cutting torch has a central hole for oxygen to blow away molten metal, surrounded by pre-heating holes for the fuel gas.
Oxygen and acetylene pressures for cutting depend on the type of metal, the manufacturer's tip number size, and the thickness of the metal.

Techniques and Tools

A straight edge or metal bar aids in achieving an even cut with OFC.
Indication of successful penetration during cutting is evidenced by a shower of sparks from the underside of the cut.
To cut round stock, position the torch at a 90-degree angle initially, then follow the contour of the material.

Bevel Cuts and Piercing Holes

Bevel cuts are achieved by holding the torch at an angle rather than perpendicular, with steady movement to ensure uniform cuts.
Begin piercing small holes with a larger cutting tip held close to the metal surface, heating until cherry red before activating the cutting lever.

Flame Types and Adjustment

A slightly carburizing flame is recommended for cutting cast iron; proper adjustment can improve cutting effectiveness.
For cutting cast iron, position the torch at a drag angle of 40 to 50 degrees initially, then adjust to 65 to 70 degrees as cutting progresses.

Plasma Arc Cutting (PAC)

PAC can cut any electrically conductive metal, typically effective for materials up to 3 inches thick.
Nonferrous materials like aluminum and copper can also be cut using PAC, with increased speed for ferrous metals under 3 inches.

Air Carbon Arc Cutting (CAC-A)

CAC-A uses three types of electrodes: DC copper-coated, DC plain, and AC coated, each having specific characteristics for stability and service life.
Molten metal is removed through a high-velocity air blast that strikes the molten metal behind the arc.

Safety Precautions

Cutting operations must be performed in designated areas; clear a 35-foot radius of combustibles if moving the work is not possible.
Maintain fire watches and have a charged fire extinguisher on site; check for smoldering materials after work hours.
Always wear appropriate protective gear, including filtering lenses, gloves, and clothing made of materials that are not easily flammable.
Special ventilation is necessary for cutting materials that produce toxic fumes, and relevant safety measures should be consulted with an Occupational Safety Officer.

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Description

Explore the essential cutting operations in welding as detailed in Chapter 25. This quiz will enhance your understanding of how various factors, including moisture and oxygen, affect metal properties. Test your knowledge and skills related to rust formation and its implications in the welding process.