Good sheet metal enclosure design starts from three questions: what goes inside, where the box will live, and how it opens. Answer those and the material, the construction, and the bends mostly decide themselves.
Most enclosure drawings we see start the other way round: someone models a neat box, then tries to fit the electronics, the gland plate, and a lid that seals. That's how you end up with corners that can't be welded, flanges too short to bend, and a lid that needs masking in six places. Below is the order we'd work in, with the rules of thumb that keep the box easy to make.
Start with the inside, then add clearance
List everything that has to fit: the DIN rail, the board, the battery, the cable entries, the hand that has to reach in and wire it. Get the real sizes of each component and the clearances they need.
Then add space for the things people forget:
- Bend radii. Every inside corner is rounded, not square. A component that's exactly the inside width of the box won't drop in.
- Flange returns. The lip around a door opening or the return on a lid takes up space inside.
- Cable bend radius. Cables need room to curve into the glands without being forced.
- Heat. Electronics in a sealed steel box in a South African summer get hot. Leave air space, or plan for ventilation.
Dimension the enclosure to the inside where internal fit matters, and say so on the drawing. Dimension to the outside where it has to fit into a cabinet, a wall recess, or a bracket.
Choose material and thickness for the environment
The environment drives the material far more than the contents do.
- Indoors, dry: powder-coated mild steel is the usual answer. Cheap, strong, easy to bend and weld.
- Outdoors inland: coated mild steel works with good pretreatment and a polyester powder. 3CR12 under a coat is a step up for wet or industrial sites.
- Coastal or washdown: 304 or 316 stainless, depending on how close you are to the sea and chemicals. 316 is the safer choice for anything exposed near the coast.
- Weight matters: aluminium, usually 5052 or 5083, which bend well and resist corrosion.
For thickness, think about stiffness across the largest flat panel rather than strength. A big door in thin sheet will feel flimsy and drum when you tap it. Bending a return flange around the edge of a panel stiffens it a lot without going up a thickness. Our guide to bending aluminium without cracking is worth a read if you're going the aluminium route.
Pick a construction
There are a few classic ways to make a five-sided box from sheet. Each trades off cost, sealing, and looks.
| Construction | How it's made | Corners | Sealing | Best for |
|---|---|---|---|---|
| One-piece folded | One blank, four sides folded up | Open corners with relief, or welded | Needs corners welded or sealed | Small to medium boxes, trays |
| Wrapper plus two ends | A U-shaped wrapper, with two end panels | Seams along two edges | Welded seams or gasketed joints | Long or deep enclosures |
| Folded and welded | Folded body, corners fully welded and ground | Closed, smooth | Good, once welded | Outdoor or sealed boxes |
| Bolted or riveted panels | Separate folded panels fastened together | Overlapping flanges | Needs gaskets or sealant | Large cabinets, kit-form, field assembly |
One-piece folded boxes are the cheapest when the size allows: fewer parts, fewer joins. As the box gets bigger or deeper, a wrapper-and-ends construction is often easier to bend and handle. For the trade-offs between joining methods, see welding vs bolting vs riveting.
Corners, reliefs, and sealing
Where two folded sides meet at a corner, both bend lines end at the same point. That corner needs relief, or the metal tears. Decide early whether the corner will be left open, welded closed, or sealed, because that changes the relief and the flat pattern. Our post on bend relief covers the corner options.
If the box has to keep out dust or water, it helps to use the same language the electrical world uses. IP ratings come from the IEC 60529 standard: the first digit rates protection against solid objects and dust, from 0 to 6, and the second rates protection against water, from 0 to 9. An IP54 box, for example, is dust-protected and resists splashing water from any direction. The rating applies to the finished enclosure, gasket, glands and all, and it's established by testing, so don't promise one on the drawing without a plan for proving it.
For the sheet metal side, sealing comes down to a few habits:
- Weld or seal the body corners on any box that must keep water out.
- Put a return flange around the door opening so the gasket has a flat surface to sit on.
- Make the lid overlap the body, with a drip edge on outdoor boxes so water runs off rather than into the joint.
- Keep the gasket face free of coating build-up, or mask it.
Doors, lids, and mounting
How the box opens drives a lot of detail.
- Lift-off lids are simplest: a folded lid with a lip that drops over the body, held by screws.
- Hinged doors need a stiff door panel (add return flanges), clearance for the hinge, and a stop so the door doesn't swing into something.
- Internal mounting plates are best as a separate folded plate that bolts inside. It can be coated separately or left bare for earthing.
Plan where fasteners go before the drawing is final. Holes for hinges and latches need to sit clear of the bends, and flanges carrying fixings need to be long enough to bend; our guide to minimum flange length gives starting figures.
Cable entries and vents are easy to add while the part is flat. Gland holes and ventilation slots can be laser cut into the blank, which is much cleaner than drilling them after bending.
Coating and masking
Most steel enclosures end up powder coated. A typical single coat is around 60 to 80 µm thick, which is enough to make threads tight, foul hinge pins, and stop an earth bond from making contact.
Mark on your drawing:
- Earth points that need bare metal
- Threaded holes or studs that must stay clean
- Gasket faces where coating build-up would stop a seal
- Colour, by RAL code or name, and the finish (gloss, satin, textured)
Our guide to masking for powder coating explains what can be masked and how to mark it.
Sending an enclosure for a quote
The best files are a STEP model of each part (body, lid, door, mounting plate) and a simple assembly drawing or PDF showing how they fit. Add material, thickness, quantity, coating colour, and any areas to mask. If you're unsure about material for the environment, describe where it's going and we'll help pick.
Our CNC bending service folds the body and lid, and welding, assembly, and powder coating can go on the same order. Start a quote for CNC bending when you're ready.