How to lay out a sheet metal box or tray by hand

A tray or open box is one blank with four flanges folded up. Each way, the blank is the outside size less one bend deduction per bend, and the corners are cut out so that no metal has to bend two ways at once. Here is the whole layout for a 300 × 200 × 50 mm tray in 2 mm sheet, with the checks the FAA handbook gives.

Published 2026-10-31. Rather have the numbers worked out? Box and Tray Blank Calculator does it in your browser.

1. Work out one bend

All four bends are 90° with the same inside radius R, thickness T and K-factor K. Here R = 2 mm, T = 2 mm, K = 0.33:

Outside setback = R + T = 4.00 mm (FAA, p. 4-59).
Bend allowance = π/2 × (R + K × T) = π/2 × (2 + 0.33 × 2) = 4.18 mm.
Bend deduction = 2 × setback − allowance = 3.82 mm.

The K-factor is best taken from a test bend on your own brake — see how to find your K-factor from a test bend. The same relations for a single bend are in working out the flat length of a bent part.

2. Size the blank, each way

Along the length: 300 + 50 + 50 − 2 × 3.82 = 392.36 mm. Along the width: 200 + 50 + 50 − 2 × 3.82 = 292.36 mm. The FAA’s developed-width rule — flats plus the bend allowance for each bend (p. 4-66) — gives the same: 46.00 + 292.00 + 46.00 + 2 × 4.18 = 392.36 mm. The FAA forms a box “the same way as the U-channel” (p. 4-71); using the rule in both directions is our reading of that.

Check it the FAA’s way: the developed width must come out smaller than the sum of the outside dimensions; if it is larger, the arithmetic is wrong (p. 4-66). Here 392.36 is less than 400 and 292.36 less than 300.

3. Mark the bend lines

The straight part of each flange is the height less the setback: 50 − 4.00 = 46.00 mm. Measure from each of the four edges of the blank:

outer tangent line (flange side) at 46.00 mm,
bend centreline at 46.00 + 4.18/2 = 48.09 mm,
inner tangent line (base side) at 46.00 + 4.18 = 50.18 mm.

To line the sheet up in a hand brake with a radius nose bar, the FAA draws a sight line inside the bend area, one bend radius from the tangent line that goes under the nose bar, and sets it directly below the edge of the radius bar (p. 4-66). For a tray the base is the part under the clamp — the sides already folded ride between the fingers (p. 4-72) — so, on our reading, that is the inner tangent line, and the sight line is at 50.18 − 2.00 = 48.18 mm from the edge.

4. Cut out the corners

Where two bends meet at a corner, material has to be removed to make room for the flanges (FAA, p. 4-71): if the two bend areas overlapped, compressive stress would build up in the corner and could crack the part (p. 4-72). The FAA asks relief holes to reach 1/32 to 1/16 in past the inside bend tangent lines, to allow for error in bending (p. 4-72). This page applies the same margin to a square notch: from each corner, 50.18 + 0.79 (1/32 in) = 50.97 mm each way.

After folding, the corner is open: each flange stops R + T + 1/32 in = 4.79 mm short of the outside corner, which leaves a gap of R + 1/32 in = 2.79 mm between the end of one flange and the inside face of the next.

The FAA’s own layout method finds where the inside tangent lines intersect, moves out along the bisector by the radius of the hole and drills there — so the edge of the hole touches the intersection — then trims off the rest of the corner, which also leaves an open corner (p. 4-72). On the same page it also calls centring the hole on the intersection good practice. For the diameter it gives about twice the bend radius (p. 4-71), or — the “most common method” — the bend radius, with a minimum that depends on the thickness: 1/8 in for aluminium alloy sheet up to 0.064 in (p. 4-72). A square notch avoids having to choose.

5. Fold

In a box and pan brake (finger brake), bend two opposite sides first. Then set the fingers so the sides already folded ride up in the gaps between them as the leaf is raised for the other two sides (FAA, pp. 4-71–4-72).

A welded, closed corner

For a corner that will be welded, the FAA wants touching flanges: “The length of the flange should be one material thickness shorter than the finished length of the part so only the insides of the flanges touch” (p. 4-72). That needs a different corner cut from the open one above; neither this page nor the tool lays it out.

Common mistakes

Adding the flanges without the bend deductions: 400 × 300 mm instead of 392.36 × 292.36 mm, 7.64 mm too long each way.
Notching only to the bend centrelines (48.09 mm): three quarters of the overlap of the two bend areas is left in the corner, where the FAA warns it can crack (p. 4-72).
Drawing the sight line in the middle of the bend instead of one radius from the tangent line under the nose bar (FAA, p. 4-66). In this example the two are only 0.09 mm apart; with a 10 mm inside radius in the same sheet they are 1.63 mm apart.
Using inside dimensions as outside ones. From an inside size, add two thicknesses to the length and width, and one thickness to a height measured from the inside face of the floor.

How this matches the tool

The box and tray blank calculator takes the outside size, thickness, inside radius and K-factor and draws this blank — outline, square corner notches and the four bend centrelines — as a DXF. The worked numbers on this page were produced by its code when the page was built.

Sources

Skip the arithmetic

Box and Tray Blank Calculator — The flat blank for a sheet metal tray or open box: blank size from the outside dimensions and bend deductions, bend lines, corner notches and a DXF.

Free, in your browser; nothing you type is sent anywhere. How the tools are checked is on the Verification page. Rather have the part drawn for you? Drafting for small shops, quoted per job.