How To Make A Sheet Metal Box
Content Menu
Introduction
Sheet metal boxes show up in more places than most people realize. Walk through any factory floor and you’ll spot them holding control boards, guarding wiring, or storing spare parts. Open the back of a vending machine or the panel on a generator and there’s another one. They look simple, but getting the corners square, the lid to fit without gaps, and the whole thing strong enough to survive daily use takes planning and the right sequence of operations.
Years ago in a small job shop, a customer brought in a sketch for a 12 × 8 × 6 inch electrical enclosure. The print called for 16-gauge galvanized steel, four mounting ears, and a hinged lid. We laid out the flat pattern by hand, sheared the blanks, bent them on an old mechanical brake, and spot-welded the seams. The first box came out warped because we forgot to allow for material thickness on the inside flanges. That single mistake cost two hours of rework and a valuable lesson: every step from material choice to final assembly affects the outcome.
This article walks through the complete process of building a sheet metal box from start to finish. The methods apply whether you run a CNC shop or work out of a garage with basic tools. Along the way, real examples from actual jobs illustrate what works and what to watch out for.
Materials Selection
Material choice sets the limits for strength, weight, corrosion resistance, and cost. Most boxes fall between 14 gauge (0.0747 in) and 22 gauge (0.0299 in). Thicker material handles heavier loads; thinner keeps weight down.
Mild Steel
Cold-rolled mild steel remains the default for indoor tool cabinets and machine guards. A 20-gauge sheet bends easily on a 30-ton press brake and welds with basic MIG equipment. One shop I worked in produced hundreds of battery trays from 18-gauge CRS. Primer and gray enamel gave them a clean look and decent rust protection inside a warehouse.
Galvanized Steel
For outdoor junction boxes or anywhere moisture is present, galvanized steel saves the step of painting. The zinc coating survives years of rain. A solar contractor once ordered fifty 24 × 18 × 10 inch NEMA 3R enclosures from G90 galvanized. We sheared, punched louvers for ventilation, formed the pans, and sealed the seams with silicone. Ten years later those boxes still look new.
Stainless Steel
Food plants and chemical facilities demand 304 or 316 stainless. An 18-gauge 304 box for a packaging line needed #4 brushed finish on the outside and fully welded seams inside to prevent bacteria traps. Stainless fights the brake more than mild steel, so we increased the die opening and slowed the ram speed to avoid cracking on tight radii.
Aluminum
When weight matters, 5052-H32 aluminum is the standard. Drone builders use 0.063 in 5052 for battery compartments because it forms well and anodizes to a tough surface. A robotics team built twenty sensor housings from 0.080 in 5052. The finished boxes weighed under two pounds each yet passed drop tests from four feet.
Design Principles
A good box starts on paper or in CAD. The flat pattern must account for bend allowances, hem returns, and corner reliefs.
Flat Pattern Layout
Draw the base, add the four sides, and attach any flanges or lids. For a basic open-top box 10 × 10 × 4 inches, the flat looks like a cross. Each bend line needs a deduction so the finished height hits 4.000 inches exactly. Most CAD programs calculate this automatically, but the formula is straightforward: Bend deduction = 2 × (radius + thickness) × tan(angle/2) – actual bend allowance.
Corner Treatments
Leave small relief notches at inside corners so material does not bunch up during bending. A 0.030 in gap works for most gauges. Without reliefs, the metal tears or the corner bulges outward.
Fastener Locations
Plan hole patterns early. Mounting ears need extra material beyond the bend line. A common mistake is placing a hole too close to a bend; the deformation distorts the hole oval. Keep holes at least 2× material thickness away from bend lines.
Cutting Methods
Clean, square blanks make every following step easier.
Shearing
A 10-foot power shear cuts 16-gauge steel all day long with edges straight to 0.005 in. For fifty pieces, stack three sheets, clamp them, and shear the stack. Time drops from twenty minutes to five.
CNC Punching
Turret presses punch holes, louvers, and embosses in seconds. A control-panel box with thirty 0.250 in diameter holes and four rectangular knockouts took ninety seconds per blank on a 30-ton Amada.
Laser Cutting
Fiber lasers handle reflective materials like aluminum and stainless without trouble. A 4 kW fiber cut 0.125 in aluminum at 300 inches per minute. The narrow 0.006 in kerf wastes almost no material, and the heat-affected zone stays tiny.
Bending and Forming
Press brakes turn flats into three-dimensional shapes. Sequence matters: bend short flanges first, then long ones, and save the closing bends for last.
Air Bending Basics
Most shops use air bending because one set of tooling covers a range of thicknesses. For 90-degree bends in 18-gauge steel, an 0.472 in V-die gives a 0.062 in inside radius. Ram depth controls the angle.
Springback Control
High-strength steels spring back more than mild steel. Over-bend by 2–3 degrees and let the material relax to 90. Bottom bending with urethane pads eliminates springback entirely but costs more in tooling.
Box Forming Example
A 12 × 12 × 6 inch stainless electrical box required four 90-degree bends. We bent the two short sides first, then the long sides. A back gauge kept locations within 0.010 in. Final corner gaps measured 0.015 in—perfect for TIG welding.
Joining Techniques
Seams must stay closed under vibration and temperature cycles.
Spot Welding
Resistance spot welding joins 20-gauge steel in 0.8 seconds per nugget. A pedal-operated 30 kVA spot welder put eight spots along each corner of a mild-steel tool tray. Pull tests exceeded 600 lb before the metal tore.
TIG Welding
For stainless and aluminum, TIG gives clean seams. A 200-amp inverter with pulse settings welded 0.048 in 304 corners without burn-through. Slow travel speed and argon purge on the backside kept the inside bright and sanitary.
Riveting and Clinching
When welding is not allowed, blind rivets or Tog-L-Loc clinch joints work. A battery enclosure for an electric forklift used 3/16 in aluminum rivets every 2 inches around the lid. The joint passed IP65 splash tests.
Finishing and Assembly
Raw edges cut hands and rust quickly. Deburr everything.
Deburring
A Timesaver belt sander removed shear burrs in one pass. For small runs, a handheld deburring wheel works fine.
Powder Coating
After cleaning and phosphating, powder coat adds 2–3 mils of protection. A batch of fifty gray control boxes went through a 400 °F oven for twenty minutes and came out chip resistant.
Silk Screening
Permanent labels beat stickers. White ink on blue powder coat identified terminals on a motor-drive enclosure. The ink survived years of oil and solvent wipes.
Simulation and Optimization
Modern software predicts problems before the first blank is cut.
Finite-element models show thinning in deep draws and stress concentrations around holes. One study ran 120 simulations on a battery tray and reduced blank size by 8 % while keeping strength. Another paper compared springback predictions to physical parts and achieved 0.3-degree accuracy.
Running a quick simulation takes minutes and often saves hours of trial-and-error on the brake.
Conclusion
Building a sheet metal box ties together material science, geometry, and shop floor experience. Start with the end use in mind: indoor or outdoor, light or heavy, painted or natural. Lay out the pattern carefully, cut clean blanks, bend in the right order, join securely, and finish properly. Do those steps consistently and the box will fit the first time, seal tight, and last for years.
The process never gets old. Every new job brings slightly different constraints—tighter tolerances, thinner material, or a new coating spec. Meeting those challenges keeps the work interesting and the shop profitable. Grab a sheet, fire up the brake, and make something solid.
Frequently Asked Questions
Q: What gauge should I use for a toolbox that will hold 150 lb of tools?
A: 16-gauge mild steel for the bottom and 18-gauge for the sides gives plenty of strength without excess weight.
Q: How do I stop corners from tearing when bending 0.090 in aluminum?
A: Increase the bend radius to 0.125 in and add 0.060 in radius relief notches at each corner.
Q: Can I bend a box on a manual finger brake instead of a press brake?
A: Yes for shallow boxes up to 20-gauge. Clamp a straightedge as a backstop and bend one flange at a time.
Q: What is the cheapest way to make ten waterproof electrical boxes?
A: Shear galvanized blanks, form on a press brake, seal seams with polyurethane caulk, and rivet the lid with a gasket.
Q: How accurate are free CAD programs for flat-pattern development?
A: LibreCAD and Sheet Lightning give bend allowances within 0.015 in for common materials—good enough for most shop work.