Sheet-metal enclosure with correct flat patterns and GD&T drawings

Lumen Grid makes a monitoring unit that sits next to rooftop solar inverters. Its enclosure is 300 × 220 × 120 mm, made today from 11 pieces of 1.5 mm galvanised steel welded together and then powder-coated. The supplier charges ₹2,450 (about €27) per box, welding distorts the lid seat so the gasket leaks in two boxes out of ten, and the PCB mounting studs are often 1–1.5 mm out of place, so the board has to be forced onto them.

The supplier says the drawings are the problem: dimensions are given to inside faces on some views and outside faces on others, and nobody has stated the bend radius or how the flat pattern should be calculated. Lumen Grid wants a new design made by laser cutting and press-brake bending, with as little welding as possible.

The requirements are: - 1.5 mm galvanised steel (DX51D Z275 or similar), inside bend radius 1.5 mm; - a hinged lid with a continuous gasket seat flat to within 0.5 mm; - four self-clinching studs for a 160 × 110 mm PCB, positioned within ±0.2 mm of the PCB holes; - three M20 cable-gland holes in the bottom face; - louvres or a vent on the sides, with a drip edge so rain does not enter.

You will set the design rules, calculate bend allowance by hand, model the base and lid in a sheet-metal CAD tool, export flat patterns as DXF, and produce drawings with general tolerances to ISO 2768 and GD&T to ISO 1101 for the studs and the gasket seat. You will also run a tolerance stack-up to show the PCB will fit.

You need no workshop. FreeCAD with its SheetMetal workbench or Fusion 360 student covers all of it.

Done looks like a two-part enclosure with no structural welds, flat patterns that the bend calculation agrees with, drawings a laser shop could quote from, and a cost estimate at 500 units a year.