Corner relief for sheet metal: why a box corner tears

A box corner tears because two bends are trying to use the same piece of material. Each bend needs a strip one bend allowance wide to deform. Where two bend lines meet at 90°, those strips overlap in a square at the corner, and that square is asked to stretch in two directions at once. It cannot, so it tears — usually at the second bend, and usually a few millimetres from the corner where the relief was supposed to be.

Most drawings carry a corner relief of some kind. The common one is a 3 mm hole or a 2 mm radius, drawn because it looks right on the flat pattern. On 1.5 mm CRCA at R1.5 the bend zone is 3.39 mm wide, so a 3 mm hole leaves material inside both zones. The relief exists on the drawing and not in the forming.

This is written for the design engineer laying out a folded enclosure and for the toolmaker who has to bend it in sequence. It gives the geometry, the sizes by thickness, the choice between a hole and a notch, and the sealing consequence. The design and CAD hub holds the rest of the design rules.

Why a box corner tears without relief

At a corner, three things happen at once. Both flanges are bent from the same sheet. The material at the corner is shared, so it has to accommodate two bend allowances rather than one. And the outer fibre at the corner is in tension in two perpendicular directions, which suppresses the transverse contraction that a simple bend relies on.

That last point is the mechanism. In a straight bend, the outer fibre stretches along the bend and contracts across it, and the contraction is what keeps the strain near the material's elongation. At a corner, the second bend prevents the contraction, so the material behaves closer to plane strain. CRCA drawing grade typically shows 28–32% elongation in a tensile test; at R = t the outer fibre strain in the bend is t/(2R + t) = 33%, which is already at the limit before the second direction is added.

So the corner is not a slightly worse bend. It is a different strain state at the point where the material has the least freedom to flow.

The relief has to clear both bend zones

The geometry is simple once the bend allowance is on the table. Two bend lines meet at a corner point. The material within one bend allowance of each line deforms when that line is bent. The square where both conditions are true belongs to both bends, and it has to go.

BA = (π/180) × θ × (R + K·t)

At 90° with K = 0.44 and R = 1.5t, this is 1.5708 × (1.5 + 0.66)t = 3.39t. For 1.5 mm CRCA that is 3.39 mm. A square notch with a 3.4 mm side removes exactly the shared square.

A round hole is less efficient. Centred on the corner point, it has to reach the far corner of that square, which is BA away in both directions, so its radius must be BA × √2 = 4.79 mm — a 9.6 mm hole. The area removed is π × 2 BA², about 6.3 times the square notch, and on a 25 mm flange that is a large bite out of the corner.

The 45° cut deserves a note. It removes the corner entirely rather than notching beside it, so the two flanges end in a mitre that can be welded with a single pass. It is the usual choice on a frame or a plinth where the corner is visible and has to be closed.

The notch grows with thickness faster than the radius does

The square notch scales with the bend allowance, so it grows roughly linearly with thickness at a fixed radius ratio. At 2 mm CRCA and R2, the allowance is a third larger than at 1.5 mm — 4.52 mm against 3.39 mm — and a round hole that clears it becomes Ø12.8 mm. At 3 mm and R3 it is 6.79 mm, and the clearing hole is Ø19.2 mm. On a 40 mm flange that hole removes almost a quarter of the flange width, which is the point where the designer should stop drawing a circle and draw the notch or the mitre instead.

The bend allowance also depends on the K-factor the flat pattern was calculated with, and the relief has to use the same one. Take K as 0.33 from a CAD default instead of 0.44 from a proved bend table and the allowance at 1.5 mm falls from 3.39 mm to 3.13 mm, so the notch is cut 0.26 mm small. That is inside the laser tolerance and outside the forming tolerance: the part that tears will be the one where the flat pattern and the notch were generated from different assumptions.

The flange height and the bend sequence decide the rest

A relief that clears both zones can still be impossible to form, because the press brake needs somewhere to put the tooling.

Bend the two short flanges first, then the two long ones. By the time the long flanges go up, the short ones are standing in the way of the punch, and the inside height of the box has to exceed the tooling envelope or the bend cannot be made. That is why box height and relief size are chosen together: a deeper notch lets the tooling sit closer to the corner, and a taller box gives the punch more clearance. The box bending article works through the sequence and the gooseneck tooling that makes it possible.

Flange height also sets a floor on the notch. The notch consumes the end of each flange, so the remaining flange has to sit on the die shoulders. At 1.5 mm, a 12 mm flange with a 3.4 mm notch leaves 8.6 mm of usable flange, which is workable; a 10 mm flange leaves 6.6 mm and may not be. The minimum flange length guide gives the V-die relationship to check against.

There is a tolerance consequence too. Every notch is a free end, and a free end moves. The corner relief trades a tear for a small, controlled opening, and the flange position at that end is then governed by the bend, not by the notch. If the flange position is critical, dimension it in the middle of the flange and let the ends float, rather than dimensioning to the notch edge.

Sealing the corner afterwards

A relief is a hole. On an enclosure that has to keep dust or water out, that hole is a path, and the IP rating belongs to the closed assembly, not to the panel. IEC 60529 classifies the degrees of protection an enclosure provides against access to hazardous parts and against the ingress of water; the number is earned by the finished enclosure, so an open corner has to be closed before the claim can be made.

The usual route is to weld the notch and dress it. That introduces the next problem: weld shrinkage pulls the corner in, and a box that formed square can finish 1–2 mm out of square over a 600 mm side. The cure is a fixture and a sequence, not a tighter notch — the welded sheet metal enclosures article covers the distortion side, and the welding fixture design guide covers how the corner is held while it cools.

Where the enclosure is not sealed — a rack frame, a plinth, a canopy internal bracket — leave the notch open. Welding a non-sealed corner adds cost and distortion for no functional gain.

What to put on the drawing

• The corner relief size, dimensioned from the intersection of the two bend lines.
• The form: square notch, round hole, or 45° cut, stated rather than implied by the outline.
• The root radius, at least 0.5t in CRCA and 1t in austenitic stainless.
• A note that the relief must clear both bend zones, with the bend allowance or the radius and thickness given so the shop can check it.
• The flange height, and whether it is measured before or after the notch.
• The bend sequence, if the tooling clearance depends on it.
• Whether the corner is welded, and to what finish.
• The default tolerance, usually ISO 2768-1 class m for a non-functional cut, with tighter callouts only where the corner is a datum.

ISO 2768-1 does not cover relief geometry — its scope is linear and angular dimensions where the drawing gives no individual tolerance — so the relief has to be dimensioned explicitly or left to the shop with a clear note. The second option is acceptable only if the note says what the notch has to achieve.

What Ganesh actually runs

Ganesh cuts the flat pattern on a 12 kW fibre laser with a 40 ft × 16 ft bed, in CRCA, mild steel, stainless, aluminium, brass and copper, up to 20 mm on the laser and 30–40 mm on the iron worker for plate work. A corner notch is a contour cut, so the relief costs cut length rather than tooling, and the edge is classified under ISO 9013 for the quality class the drawing calls.

Forming runs on the 200 T, 5-axis CNC press brake with a 4,450 mm bed and Delem control, to 10 mm, with the bend sequence and springback correction stored per part. Welding runs on three CLOOS robotic MIG cells and more than 30 manual MIG/MAG, spot and SPM stations, with fixtures designed and built in-house — which is what allows a relieved corner to be welded and held square in the same operation.

Coating runs through seven-tank pretreatment, a 16 ft × 16 ft shot blast chamber and an 18 ft × 14 ft electrostatic powder booth, with DFT read on a gauge and adhesion checked by cross-hatch. Typical sheet metal tolerance is ±0.5 mm, tighter on critical features by agreement, across five units in Mumbai (Vasai) and Nagpur under ISO 9001:2015 and ISO 14001:2015.

When corner relief is the wrong answer

A drawn box has no corner seam to relieve, and adding a notch to a drawn part removes material the draw needs. If the corner has to be both sealed and undistorted, the draw is often the better route, and it is worth saying so before the folded design is frozen.

The second wrong case is a corner that is failing for a different reason: a bend radius below the material minimum, a burr on the cut edge, or a grain direction that puts the bend across the rolling direction. A bigger notch will hide those on the first batch and fail again when the coil changes. Check the radius against minimum bend radius by material before enlarging the relief.

Send the flat pattern with the bend lines and the box height, or request a quotation with the material, thickness and finish. The review will size the notch against both bend zones, check that the tooling can reach the second corner, and tell you whether the corner should be open, welded or drawn.