Key takeaways
- A bend relief must clear the whole bend zone, which is one bend allowance long: 4.52 mm on a 2 mm CRCA part bent at R2, not the 4 mm that R + t suggests.
- The gap between R + t and the bend allowance grows with the radius ratio — 13% at R = t, 28% at R = 2t and 35% at R = 3t — so thick, generously radiused parts are the ones that tear.
- Relief width should be at least one material thickness in mild steel and 1.5t in austenitic stainless or aluminium 5052, with a 1.0 mm practical floor for a laser-cut slot.
- No relief is needed where the bend line runs out to a free edge; relief is needed where it stops at an internal corner, a shoulder or a tab.
- The root of the relief should carry a radius of at least 0.5t in mild steel and 1t in stainless, because a square root is a crack starter sitting in the highest-strain region of the part.
A bend relief is a notch at the point where a bend line stops. Its job is to let the bend end in free air instead of tearing into the material beyond the corner. Most drawings show one, and a large share of them are too small, because the depth is copied from a rule of thumb that uses the bend radius when it should use the bend allowance.
The error is not academic. On a 2 mm CRCA part bent 90° at an inside radius of 2 mm, the common rule gives a relief depth of R + t = 4 mm. The bend consumes 4.52 mm of flat material at that radius, so the notch ends 0.52 mm inside the deformed zone and the corner still tears. Increase the radius to 6 mm on the same 2 mm thickness and the gap between the rule and the bend allowance grows to 2.8 mm.
This is written for the design engineer who is placing the notch and the toolmaker who has to make the tool clear it. It gives the width, the depth, the root radius and the drawing callout, and it says where a relief is unnecessary. The design and CAD hub holds the rest of the rules that decide whether a part forms cleanly.
What bend relief does, and why the depth is a bend allowance
The relief converts the end of a bend line into a free edge. When the bend line runs into an internal corner, the material beyond that corner is continuous with the flange, so it resists the bend, stretches, and concentrates strain exactly where the two features meet. The notch removes that connection and lets the bend terminate.
The length of material that actually deforms is the bend allowance, not the radius. For a bend through angle θ with inside radius R and thickness t:
BA = (π/180) × θ × (R + K·t)
At 90°, with K = 0.44 for CRCA at R ≈ t, this reduces to 1.5708 × (R + 0.44t). At R2 on 2 mm sheet: 1.5708 × 2.88 = 4.52 mm. The deformed strip is 4.52 mm wide in the flat, and a relief that stops short of that strip still has material in the bend.
That is the whole correction. Depth is measured past the bend line and must exceed the bend allowance, not the radius plus thickness.
The gap grows with the radius ratio
At R = t, the bend allowance is 2.26t against the rule's 2t — 13% short. At R = 2t it is 3.83t against 3t, 28% short. At R = 3t it is 5.40t against 4t, 35% short. The parts that tear are therefore not the thin, tight ones that everyone worries about; they are the thicker parts bent over a generous radius, where the rule of thumb drifts furthest from the geometry. A 3 mm bracket at R6 is 2.5 mm under-relieved by the R + t rule.
Bend relief width and depth by thickness and material
The table gives working values for a 90° bend at R = t. Depth includes 1 mm of clearance beyond the bend allowance. The Ganesh column records what the route can cut and form.
| Material and thickness | Bend allowance at R = t | Relief width (minimum) | Relief depth past bend line | Root radius | Ganesh limit or note |
|---|---|---|---|---|---|
| CRCA 1.0 mm | 2.26 mm | 1.0 mm | 3.5 mm | 0.5 mm | 12 kW laser; 1.0 mm is also the practical slot floor |
| CRCA 1.5 mm | 3.39 mm | 1.5 mm | 4.5 mm | 0.8 mm | Kerf 0.2–0.3 mm leaves 1.2 mm clear |
| CRCA 2.0 mm | 4.52 mm | 2.0 mm | 5.5 mm | 1.0 mm | 200 T brake, 4,450 mm bed, to 10 mm |
| CRCA 3.0 mm | 6.79 mm | 3.0 mm | 8.0 mm | 1.5 mm | Root radius matters more than width here |
| Stainless 304 2.0 mm | 4.71 mm | 3.0 mm (1.5t) | 5.7 mm | 2.0 mm | Work-hardening grade; 1t root minimum |
| Aluminium 5052 2.0 mm | 4.55 mm | 3.0 mm (1.5t) | 5.6 mm | 2.0 mm | Lower elongation than CRCA; 1.5t width |
| CRCA 6.0 mm | 13.57 mm | 6.0 mm | 14.6 mm | 3.0 mm | Plate work to 40 mm on the iron worker |
Width has a lower bound set by the cut, not the forming. A laser kerf of 0.2–0.3 mm in 2 mm mild steel means a 2.0 mm slot leaves about 1.7 mm of clear width, which is adequate. Below 1.0 mm the slot becomes a cut that the machine can make but the shop cannot keep clean, and a slug that stays in the nest will mark the next sheet.
The root radius is the part of the relief that is most often drawn square. A square root sits in the highest-strain region of the part and acts as a crack starter under fatigue or during the bend itself. Half the thickness is the floor for mild steel; one thickness for austenitic stainless, which work-hardens at the cut edge and has less reserve.
The failure it prevents, and what it looks like
Two failures, and they are distinguishable on the bench.
The tear. The corner separates along a line running from the internal corner into the flange, usually at 30–60° to the bend line. It starts at the point where the bend zone meets the unrelieved corner. The mechanism is strain concentration: the material at the corner is asked to accommodate the bend while the surrounding sheet suppresses the transverse flow, so it behaves closer to plane strain than to uniaxial tension. A CRCA grade with 28% elongation in a tensile test does not have 28% available at that corner.
The open end. The flange end bows away from the base and the included angle at the end of the bend differs from the angle in the middle. On a 600 mm bend with a 40 mm flange, a 1° difference between the middle and the end moves the flange tip by 40 × tan(1°) = 0.70 mm. That is enough to fail a ±0.5 mm flange position callout while the middle of the part measures perfectly. The bending tolerance article covers how to measure the difference; the relief is what prevents it.
A third symptom is a small "ear" or lip standing proud at the end of the flange. It comes from material that flowed sideways instead of along the bend, and it is the same mechanism as the tear, caught before it became a crack.
Where a relief is needed, and where it is not
The rule is about the end condition of the bend line, not about the bend itself.
Needed. Where the bend line terminates at an internal corner, a shoulder, a step in the outline, a tab, or another flange, so that material continues past the end of the bend. This is the case for a bracket with a raised tab at each end, and for any part where the flange stops short of the edge it runs to.
Not needed. Where the bend line runs out to a free edge of the blank. The end of a plain channel is the standard example: the flange ends in air, the bend can terminate, and there is nothing to tear. Adding a notch there removes section for no benefit and adds a cut.
Not a substitute. A hole close to the end of a bend line does a poor impression of a relief, because it does not extend through the bend zone. The rule for a hole near a bend is different and is owned by the hole-to-bend distance article: R + V/2 + t. Do not use a hole as a relief unless it happens to satisfy both rules.
There is a flange-length consequence too. The notch consumes material at the end of the flange, so the usable flange is shorter by the relief length, and the minimum flange that the press brake tooling can form still applies. The minimum flange length guide gives the V-die relationship; the practical check is that the flange remaining after the notch still sits on the die shoulders.
Relief as an open slot, not an enclosed cutout
Draw the relief open to the edge. It is one contour instead of a closed cut plus a pierce, it leaves no slug in the nest, and it removes the risk of a drop-out being carried under the sheet.
On a 12 kW fibre laser, an open slot at the end of a bend line runs as part of the outer contour and costs almost nothing in cut length. An enclosed slot needs a pierce, a lead-in, and a slug that has to fall clear; at four corners per part that is four extra pierces and four chances to mark the next sheet. The cut edge itself is classified under ISO 9013, which covers the geometrical specification and quality tolerances of thermal cuts — a relief cut to a normal quality class is fine, because the edge is not a functional surface.
If the relief has to be enclosed — because the notch would break into a sealing face or a mating edge — treat it as a cut feature with a real tolerance and dimensions, not as a design afterthought.
What to put on the drawing
- The relief depth past the bend line, given as a minimum of one bend allowance at the drawing's radius and thickness.
- The relief width along the bend line, at least one material thickness.
- Both dimensions referenced to the bend line, not to the flange edge, so they survive a flange-length change.
- The root radius, with a note that it must not be sharp.
- Whether the notch is open to the edge or enclosed.
- The default tolerance that applies, usually ISO 2768-1 class m for a non-functional cut.
- A note that the notch must clear the bend zone, so the toolmaker can adjust it without a drawing change.
ISO 129-1 governs how those dimensions are presented, and the reason to follow it is practical: a relief dimensioned from the flange edge moves whenever the flange length changes, and the change is invisible on the shop copy.
What Ganesh actually runs
Ganesh cuts on a 12 kW fibre laser with a 40 ft × 16 ft bed, supported by 6 kW and 3 kW machines, in mild steel, CRCA, stainless, aluminium, brass and copper, up to 20 mm plate on the laser and 30–40 mm on the iron worker. Cutting a relief slot is a contour operation, so there is no tooling cost and no minimum quantity to justify it.
Forming runs on the 200 T, 5-axis CNC press brake with a 4,450 mm bed and Delem control, to 10 mm. The bend table and springback correction are stored in the programme, which is what keeps the end of a relieved bend consistent with the middle across a repeat order. 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.
The tool room builds the press tools, welding fixtures and checking fixtures in-house, so a relief that needs adjusting after the first article is a drawing change and a re-cut by the same team, not a new supplier conversation.
When bend relief is the wrong answer
An enclosure corner that has to be sealed cannot use an open notch: the notch is a leak path, and the IP rating — IEC 60529 classifies the degrees of protection an enclosure provides — is a property of the closed assembly. In that case the corner is welded and finished, or the box is drawn rather than folded, and the design conversation moves to the weld sequence rather than the notch.
The second wrong case is a part that is failing for a different reason. A tear at the corner can also come from a bend radius below the material's minimum, a burr on the cut edge, or a grain direction that puts the bend across the rolling direction. A relief will mask the first two on the first batch and fail again when the coil changes. Check the radius against the minimum bend radius by material before adding metal to the notch.
Send the flat pattern with the bend lines marked, or request a quotation with the thickness, material and radius. The review will place the relief against the actual bend allowance and say whether the corner needs a notch, a larger radius, or a different corner treatment. If the part is already in production and tearing, include the first-off and the last good part; the difference between them usually names the cause.
Frequently asked questions
How wide should a bend relief be?
At least one material thickness in mild steel or CRCA, and 1.5 times the thickness in austenitic stainless or aluminium 5052. Below 1.0 mm the laser kerf and the slug removal make the slot unreliable, so a 1.0 mm floor applies at the thin end of the range.
How deep should bend relief go?
At least one bend allowance past the bend line, plus about 1 mm of clearance. For a 2 mm CRCA part bent 90 degrees at R2 that is 4.52 mm plus clearance, so 5.5 mm. The common R + t rule gives 4 mm and leaves the notch inside the bend zone.
Do I need bend relief on a 1 mm part?
Yes, wherever the bend line stops at an internal corner. The bending moment is lower, but the strain at the corner is the same fraction of thickness, and a 1 mm part with a 3 mm wide tab will still tear the tab off at the root.
Can bend relief be an enclosed slot?
It can, but an open slot to the edge is better. It needs no pierce, leaves no slug in the nest, and the laser can run it as a contour. An enclosed slot adds a pierce per relief and a drop-out that has to be cleared before the next part.
Does bend relief change the flat pattern?
Only where the relief removes material that the bend allowance calculation assumed. If the notch is placed outside the bend zone, the flat length is unchanged. If it cuts into the zone, the bend behaves as a shorter bend line and the angle at the ends opens up.
Where does bend relief go on a drawing?
Dimension it from the bend line, not from the flange edge: the length along the bend line, the depth past the bend line, the width, and the root radius. A depth dimensioned from the flange edge moves when the flange length changes.
Sources and further reading
- ISO 2768-1:1989 — General tolerances, Part 1: linear and angular dimensions without individual tolerance indications · reviewed 3 Nov 2026
- ISO 129-1:2018 — Technical product documentation, presentation of dimensions and tolerances · reviewed 3 Nov 2026
- ISO 9013:2017 — Thermal cutting, classification of thermal cuts, geometrical product specification and quality tolerances · reviewed 3 Nov 2026
Does the bend tear at the corner?
Send the drawing and the flat pattern. We will mark where the bend zone actually ends, size the relief against the bend allowance, and quote the part with the notch in the right place.
Request a quote