Resistance Welding by the Numbers

About this book

Resistance welding joins metal without a flame, an arc or filler wire. Two sheets are squeezed between copper electrodes, a large current passes for a fraction of a second, and the metal at the interface melts and freezes into a nugget. It is fast, clean and easy to automate, which is why it holds together car bodies, appliances, cabinets, wire mesh and countless sheet-metal products. It is also unforgiving in a particular way: the weld is hidden between the sheets, so a bad one can look exactly like a good one.

This book is for the people who set up, specify, troubleshoot and cost resistance welds: sheet-metal fabricators, manufacturing and design engineers, production supervisors and quality staff. It assumes Ohm’s law, basic sheet-metal practice and simple algebra in SI units. It does not replace the welding equipment maker’s instructions or qualified training. It explains, with numbers, why a schedule works or fails, so that settings can be reasoned through and checked rather than copied.

Three books from 1918 to 1921 as a starting point

Electric resistance welding began with Elihu Thomson, who conceived the idea in 1877 and developed it in 1886. By the end of the First World War it was an established workshop process, and three books from that period are the historical starting point here. Electric Welding by Douglas T. Hamilton and Erik Oberg, the editor of Machinery’s Handbook, was published by the Industrial Press in 1918. Spot and Arc Welding by H. A. Hornor, published in 1920, records the heavy spot welding machines built to speed American shipbuilding. George W. Cravens’ Welding of 1921 surveys the processes for the American Technical Society.

Many of their observations still hold: that pressure, current and time must be set together, that faster welds use less energy, that a transformer can carry a heavy overload for seconds, and that copper electrodes are the weak point of the whole process. Some of their practices have been replaced, and one of them, the advice to reduce pressure and time for zinc-coated sheet, is the opposite of modern practice. Each chapter takes one of their themes, restates it in modern terms and works it through a modern case.

The running case

This is an illustration. A sheet-metal shop builds 40 steel electrical cabinets a day. Each cabinet brings together the four main resistance processes:

• a spot-welded body of 1.2 mm cold-rolled low carbon steel, with 64 spot welds;
• twelve projection-welded M8 weld nuts for mounting plates and hinges;
• a seam-welded drip tray, 600 × 400 mm, that must not leak;
• a flash-welded base ring of 40 × 5 mm flat bar.

Through the book the shop sets its schedules, sizes its machines, designs its joints, tests its welds and works out what each weld costs. The shop, the cabinet, its dimensions, settings, test results and costs are invented for teaching. They are not KEVOS® work, and the settings are not recommended values for any material or machine. Real schedules come from the equipment and electrode makers, the applicable standard and the shop’s own weld trials.

How to use the book

Chapters 1 to 4 build the tools: where the heat comes from, the welding window of current, time and force, the transformer and supply, and the electrodes. Chapter 5 designs spot-welded joints. Chapters 6 to 8 cover seam, projection, and butt and flash welding. Chapter 9 deals with coated steels, aluminium and other difficult materials, and Chapter 10 with testing, monitoring and cost. Every chapter ends with an exercise and its worked answer.

Limits and professional responsibility

This book is educational. Resistance welds in load-bearing, pressure-containing, automotive or other safety-critical products must follow the applicable standards, the customer’s specification and qualified welding procedures, with testing and approval by competent people. Resistance welding machines carry high currents, strong magnetic fields, pinch points and expelled molten metal; work must follow the machine maker’s instructions and Safe Work Australia’s model code of practice on welding processes as adopted in each jurisdiction. Welding coated steels produces fumes that need control. The 1918 to 1921 methods are described as history, not practice.

An original treatment

READOS covers how metals behave in welding in Weldability by the Numbers and repair of steel plate in Weld Repairs That Hold. This book concentrates on the resistance processes. The explanations, the cabinet case, the calculations, figures and exercises are original KEVOS® writing. The historical books are public-domain works in the United States; their facts are summarised, discussed and corrected in new words, and none of their text, tables or illustrations is reproduced.