Stiffening Sheet Metal by the Numbers

About this book

Thin sheet metal is cheap, light and easy to cut and fold, and it is also floppy. A flat panel of 1.2 mm steel the size of a small door bows visibly when air pressure acts on it, pops in and out when someone leans on it, buckles when the sun warms one side of an enclosure, and drums when a fan or motor nearby runs at the wrong speed. The answer is rarely just thicker sheet. Stiffness grows with the cube of thickness but so does nothing else useful, and a thicker panel can still drum. The better answers are shape: flanges and hems at the edges, wired and curled rims, rolled beads, and light stiffeners attached where they do the most work.

This book puts numbers on each of those choices. It starts with the flat panel, its deflection, stress and the point where the simple formulas stop applying, then explains buckling and oil-canning, then works through edge treatments, beads and attached stiffeners. It ends with vibration, because a panel that passes a static check can still fail by drumming, and with the tests that confirm a finished panel does what the numbers said.

It suits mechanical designers and drafters who design enclosures, guards, covers, doors and ductwork, and fabrication engineers who must explain why one panel rattles and another does not. It assumes school algebra, the idea of a beam’s second moment of area, and a working knowledge of sheet metal folding. Sheet Metal Parts by the Numbers covers bends, flat patterns and hems as folding operations; this book is about what those features do for stiffness.

The running case

Every chapter follows one fictional panel. This is an illustration. A small machinery builder makes an enclosure for a dust-producing machine. The enclosure’s side panel, called SP-90 here, is 900 mm high and 600 mm wide, cut from 1.2 mm zinc-coated steel sheet and screwed at 150 mm pitch to a light frame. An extraction fan holds the inside of the enclosure about 250 Pa below the workshop. The fan runs at 1,450 rpm and the machine’s main motor at 2,880 rpm.

Customers have complained that the side panel bows inwards when the extraction runs, oil-cans with a bang when leaned on, and hums. The builder sets three targets for a redesign: a deflection of no more than 3 mm under 250 Pa, no panel mode within 25% of the fan or motor frequency, and no visible bowing when the enclosure stands in the sun. The business, the customers, the complaints and the mistakes are invented, and the material properties are illustrations, not a grade’s specified values. None describes KEVOS® work or a real product.

Source limits and professional responsibility

The plate, beam and vibration formulas in this book are classical results for ideal edge conditions and small deflections, and every calculation was checked by running it. Real panels sit between simply supported and clamped, carry residual stress from cutting, folding and welding, and are rarely perfectly flat, so the numbers are estimates that a test must confirm. Where a formula’s limits matter, the chapter says so.

The book is educational. Panels that guard people from moving parts, contain pressure, carry structural load or control noise exposure have their own Australian standards and work health and safety duties, and the design and acceptance of such panels is a matter for a competent engineer and the applicable requirements. The 1918 trade text listed in the references was used only as a map of edge treatments; nothing from it is reproduced.