What is metal sheet fabrication?

The short answer

What the process actually involves

A fabricated part is built in stages, and each stage changes what the next stage can hold. Cutting sets the blank outline; every hole, notch and slot is located from the same datum. Forming then introduces the third dimension, and this is where most tolerance is lost or won. Joining adds material or creates a metallurgical bond. Finishing is the last chance to fix a cosmetic defect, because after coating the surface is sealed.

How thick can the metal be, and on which process?

Thickness is the first constraint to check, because it selects the process rather than the other way round. Laser cutting, plasma cutting and waterjet all cut sheet, but their useful ranges are very different, and bending has its own limit set by the press brake and the material's ductility.

Two practical consequences follow. First, above roughly 12 mm, forming becomes a press-forging or heavy-plate operation rather than ordinary sheet work. Second, the thinner the material, the more the part behaves like foil during handling - so fixtures and nesting matter more than the machine's rated capacity.

How sheet metal fabrication compares with other processes

The common assumption is that sheet metal is the "cheap" process. It is more accurate to say it is the process with the lowest tooling barrier, which is a different claim. Where a progressive die needs USD 10,000 to 250,000 before the first part exists, a fabricated assembly needs fixtures and press brake tooling that cost a fraction of that.

Where sheet metal fabrication stops working

• Minimum bend radius. As a rule of thumb the inside radius should be at least one material thickness for mild steel, and larger for high-strength or T6 aluminium. Tighter than that and the outer surface cracks.
• Holes too close to a bend. Keep hole edges at least 2.5 times the thickness plus the bend radius from the bend line, or the hole distorts when the material stretches.
• Springback. Metal relaxes after the punch lifts, so the die has to over-bend. High-strength and austenitic stainless spring back the most and may need a second strike.
• No undercuts or internal cavities. If a feature cannot be reached by a punch or a tool from one of two directions, fabrication is the wrong route.
• Tolerance ceiling. ±0.1 to ±0.2 mm is realistic for fabricated features. If the print calls for ±0.01 mm, that feature belongs on a machining operation.

Design rules that decide cost

1. Use standard sheet thicknesses. Stepping to the next stock size instead of a nominal 3.0 mm can cut material cost and lead time immediately.
2. Standardise bend radii. One or two radii across the part means one or two press brake setups instead of five.
3. Keep bends in one direction where possible. Re-orienting the part between bends adds handling and setup time on every unit.
4. Add the bend allowance to the flat pattern. The flat blank equals the sum of the legs plus the bend allowance, where BA = angle × (r + K × t) × π/180 and K typically falls between 0.33 and 0.50.
5. Design for the finish. Anodising and plating add thickness, so threads and press-fit bores need allowance or post-finish machining.

Sheet metal fabrication at SOMI Custom Parts

We run laser cutting, press brake forming, stamping, welding and surface finishing under one roof, which means the sequence problem above gets solved at quoting rather than after the first article. Send a drawing and an annual quantity and we will come back with the process route, the flat pattern, and the tolerance each feature can actually hold: see sheet metal fabrication, metal stamping, or start a project through our inquiry form.