Cutting and Gouging by the Numbers

About this book

Before almost anything is welded, something is cut, and before almost any weld is repaired, something is removed. Plate is cut to shape, edges are bevelled, holes are pierced, risers are cut from castings, cracked welds are gouged out, roots are back-gouged, and worn equipment is cut apart for rebuilding. The methods range from the oldest, melting metal away with a carbon arc, through air carbon arc gouging and plasma, to fibre lasers, abrasive waterjets and saws, with the oxygen lance for the heaviest work. Each removes metal at a different rate, to a different accuracy, at a different cost, and with its own fume, noise and hazards.

This book is for fabricators, repair welders, steel service centres and maintenance engineers who cut and gouge metal. It explains, with numbers, how each method removes metal, how fast and how accurately it does so, what it costs per metre or per kilogram removed, and how its hazards are controlled. Oxygen cutting, the subject of Oxy-Fuel by the Numbers, appears here only for comparison.

Early manuals as a starting point

Cutting with the electric arc is as old as arc welding. Hamilton and Oberg’s Electric Welding of 1918 described the carbon arc’s first and most important use as cutting or burning off iron and steel: cutting scrap into handling sizes, removing risers from castings and preparing broken parts for repair, with currents of hundreds of amps and cuts somewhat wider than the electrode and rough at the edges. They noted that gas torches cut structural steel more neatly and cheaply, and that the carbon arc suited small amounts of cutting where an arc welding set was already on hand. Viall’s Gas Torch and Thermit Welding of 1921 recorded an underwater cutting torch used by a navy diver, its flame protected by a hood and compressed air, and a calculation of the oxygen theoretically needed to burn away a kerf, against the much larger amount used in practice. The Imperial handbook of 1916 described burning carbon deposits out of engines with oxygen alone.

Those observations, on what each method is good for, how wide and rough its cut is, and how much of its energy or gas does useful work, are still the right questions. Each chapter takes one of them and works it through with modern methods.

The running case

This is an illustration. A steel service and fabrication business in a mining region supplies cut plate and profiles to local fabricators and repairs heavy mining and earthmoving equipment. It has a CNC plasma table with an oxy-fuel torch, air carbon arc gouging and plasma gouging in its repair bay, band saws, and access to a subcontractor’s waterjet. Its manager is deciding whether to buy a fibre laser, wants to cut the cost and time of gouging in repairs, and needs better fume and noise controls in the repair bay.

The business, its customers, jobs, prices and results are invented for teaching. They are not KEVOS® work. Cutting and gouging equipment must be used under the makers’ instructions and the work health and safety laws.

How to use the book

Chapter 1 compares the methods. Chapter 2 covers the carbon arc, and Chapter 3 air carbon arc gouging. Chapter 4 deals with back-gouging and defect removal. Chapter 5 explains plasma cutting, and Chapter 6 its cost per metre against oxy-fuel. Chapter 7 covers laser cutting, and Chapter 8 waterjets and mechanical cutting. Chapter 9 deals with oxygen and thermic lances and underwater cutting, and Chapter 10 with fume, noise, safety and the cutting record. Every chapter ends with an exercise and its worked answer.

Limits and professional responsibility

This book is educational. Its speeds, rates and costs are illustrations; real values come from the equipment makers’ data and the shop’s own trials. Air carbon arc gouging and plasma cutting produce intense noise, fume and radiation; thermic lancing produces extreme heat and fume; lasers and waterjets have their own serious hazards. All must be controlled under the makers’ instructions, the work health and safety laws and Safe Work Australia’s codes. Gouging and cutting of parts that carry loads, hold pressure or are under warranty follow their own repair procedures.

An original treatment

READOS covers oxygen cutting in Oxy-Fuel by the Numbers, weld repair in Weld Repairs That Hold, and arc welding equipment in Arc Welding by the Numbers; this book links to them rather than repeating them. The explanations, the business case, the calculations, figures and exercises are original KEVOS® writing. The historical sources are public-domain works in the United States; their observations are summarised in new words, and none of their text, tables or illustrations is reproduced.