Hand Forming Sheet Metal by the Numbers

About this book

Most sheet metal is formed by machines with tooling: press brakes, rolls, presses and spinning lathes. Some of it is still formed by hand, with hammers, mallets, stakes, sandbags, shrinkers and an English wheel. Hand forming makes the one-off and the few-off: the replacement panel for a restored machine, the prototype cover before tooling is cut, the repair patch shaped to an existing curve, the short run that would never pay for a die. It is often treated as pure craft, learned by feel. The feel matters, but much of what a skilled worker knows by experience can be calculated beforehand: how big the blank must be, how much the metal must stretch or shrink, how thin it will get, how many courses a dome needs, how long the job will take, and how much noise the worker will absorb doing it.

This book puts those numbers in front of the hammer. It starts with the three things hand forming does to sheet, stretching, shrinking and bending, and how to count them as strain. It sizes a blank for a dome three ways, plans raising and sinking, shrinking and stretching of flanges, wheeling of a crowned panel and wiring of an edge by hand, and ends with checking the finished work and managing noise.

It suits sheet metal workers, restorers, prototype makers and the engineers and estimators who plan and quote their work. It assumes school algebra and geometry and some workshop experience. Sheet Metal Patterns by the Numbers covers developed patterns for parts that are bent but not stretched; this book is about the parts that cannot be developed flat.

The running case

Every chapter follows one fictional job. This is an illustration. A small restoration workshop is asked to make two replacement fan housings for a pair of heritage stationary engines, job HF-01. Each housing has three hand-formed parts in 1.6 mm aluminium sheet of a soft, annealed, non-heat-treatable grade:

• a dome, a spherical cap 320 mm across its rim and 80 mm high;
• a mounting ring, a band 30 mm high and 320 mm inside diameter, with a 20 mm flange turned inwards at the top for the dome to sit on, and a wired bottom edge;
• a side access panel, 600 × 300 mm, crowned to match the original, with a radius of 1,200 mm along its length and 900 mm across it.

The customer’s drawing asks for a thickness of at least 1.3 mm anywhere on the formed parts, a dome profile within ±2 mm of a template, and a finish free of visible hammer marks. The workshop, the customer, the engines, the material certificate’s values and every mistake are invented. None describes KEVOS® work or a real business.

Source limits and professional responsibility

The geometry and strain arithmetic in this book are exact, and every calculation was checked by running it. The process figures, such as how much a course of raising reduces a rim or how much a shrinker bite takes up, depend on the worker, the tools and the batch of metal; they are given as illustrations from trials, and the book shows how to measure them for a real job. Material properties and annealing practice must come from the metal supplier’s data for the actual alloy and temper.

Hand forming is loud, repetitive and hard on hands and hearing. The noise figures in Chapter 8 are illustrations and the method follows Safe Work Australia’s material, but managing exposure is a duty of the business and its workers under work health and safety law. Hand-formed parts that carry load, contain pressure or form part of an aircraft, vehicle or pressure vessel are subject to their own approvals and must be designed and accepted by people competent to do so. The 1918 trade text listed in the references was used only as a map of traditional processes; nothing from it is reproduced.