Sheet Metal Parts by the Numbers

About this book

A folded sheet metal part starts life flat. It is cut from a sheet by laser, punch or shear, then bent on a press brake, one fold at a time, into a bracket, a cover, a channel or an enclosure. Everything about it depends on what happens in the bends: the metal stretches on the outside and compresses on the inside, the bent region takes up a length that is neither the inside nor the outside dimension, the bend springs back when the punch lifts, and any hole too close to the bend is pulled out of shape. A designer who knows those numbers draws parts that fold right first time. One who does not draws parts that fold nearly right, and the difference is found at the press brake.

This book works through the numbers in the order a part is designed: the bend radius and what limits it, the neutral axis and K-factor, the flat pattern, springback, features near bends, flanges and hems, the forces the press brake and punch must supply, tolerances that a folded part can hold, and the detail drawing that brings them together. Each chapter calculates with one part and shows a plausible mistake and how it was found.

It suits mechanical designers and drafters who design sheet metal parts, fabrication engineers who prepare flat patterns, and buyers who must judge a supplier’s queries. It assumes school algebra and trigonometry and a working knowledge of engineering drawing. The Library’s collection chapter on designing sheet metal parts for economical manufacture gives a shorter overview; this book goes into the calculations behind it.

The running case

Every chapter follows one fictional part. This is an illustration. A machinery builder needs a bracket to mount a variable speed drive on a conveyor frame. It is a C-shaped channel folded from 2.0 mm cold-rolled mild steel: a base 160 mm wide, two legs 60 mm high and two return flanges 15 mm wide, all as outside dimensions, and 300 mm long along the bends. Four Ø7 holes in the base take the drive’s M6 screws, and each leg has two 9 × 20 mm slots for M8 bolts to the frame. All four bends are 90° with an inside radius of 2 mm, and the part is powder coated after folding. Its folding supplier has a press brake and a turret punch, and its first K-factor for this material and tooling, from trial strips, is 0.40.

The business, the supplier and the mistakes are invented, and the material properties are illustrations rather than a grade’s specified values. None describes KEVOS® work or a real supplier.

Source limits and professional responsibility

The geometry in this book is exact, and every calculation was checked by running it. The process relationships, for minimum radius, springback and force, are published models that describe trends and give estimates; real values depend on the material batch, the tooling and the machine, and the book shows how to calibrate them with trials. Published rules of thumb for features near bends and minimum flanges vary between suppliers, and the book quotes one supplier’s guide as an example only.

Press brakes and punches are dangerous machines, and force estimates in this book are for planning and quoting, not for setting a machine. Machine capacities, tooling limits and guarding are matters for the machine’s manufacturer, the operator and Australian work health and safety requirements. The parts in this book are examples of design method, not designs for a particular duty.