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Bending metal and bend radius
Sheet metal enclosures are fabricated by 'cold forming', where the metal is clamped and bent in machines called 'press brakes'. As a result, metal typically cannot be formed into a true 90 deg. corner, but rather, the corners are curved as per the figure below (FIG 1). Different degrees of curvature can be achieved, and it is described by a parameter called 'bend radius' (see figure showing series of different bend radius FIG 1). Note that bend radius is specified as the radius of the INSIDE surface of the bend.
FIG 1 (A)
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FIG 1 (B)
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Consequences:
Sheet metal stretches during forming to some degree and the manufacturer must consider this when fabricating the enclosure. It is a function of tooling, metal thickness, bend radius, and metal composition. Metal composition varies somewhat with even the best quality metal from the best suppliers and as a result, there is always a certain tolerance on dimensions when bends are involved. Manufacturers will supply standard tolerances on request.
Generally, cutouts should stay a minimum distance from bend radii in order to avoid deformation and wrinkling of the enclosure during forming. See FIG 2 below. Strictly speaking the cutout need only avoid being on the bend, but in practice, an exclusion zone of 1/16" is suggested to allow for mfg tolerances.
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| FIG 2 |
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Special consideration must be given to the intersection of two bend lines, i.e. a corner of an enclosure. Specifically, extra material must be removed in order to prevent bends from interfering with each other and is called a corner relief. See FIG 3 below. This is done automatically in ProtoCase templates, but must be considered when designing enclosures from scratch.
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| FIG 3 |
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