Bending Die Design: V-Die, Wiping Die and U-Die — Comparison and Selection

Sheet metal bending is deceptively simple — bend metal around a radius and call it done. In practice, the choice of bending die geometry determines whether you hit dimensional tolerance, manage springback, achieve the required surface finish, and stay within press tonnage. Three configurations dominate production bending: the V-die (air bending and bottoming), the wiping die (wipe bending or edge bending), and the U-die (channel bending). Each has a different force signature, a different springback response, and a different set of tooling cost and maintenance implications.

This article covers the engineering mechanics behind each die type, gives you the formulas and worked calculations you need at the design stage, walks through the selection decision with a practical framework, and highlights the failure modes that kill die life and dimensional repeatability. The target reader is a tooling engineer or die designer selecting or specifying a bending die for a new part program or troubleshooting an existing line.

What Bending Dies Do — Mechanics Behind the Bend

All three die types impose the same fundamental deformation: a sheet is forced past its yield stress in bending, the outer fibers stretch, the inner fibers compress, and the neutral axis shifts inward. What distinguishes the configurations is how the contact geometry controls material flow, the bending moment arm, and the normal force distribution along the sheet.

Key material parameters that govern all bending die designs:

Yield strength (σ_y): sets the minimum bending force
Ultimate tensile strength (σ_UTS): bounds the overbend compensation needed
Strain hardening exponent (n): governs how much extra force is needed past yield
Anisotropy ratio (r-value): affects springback asymmetry between rolling and transverse directions
Sheet thickness (t): appears squared or cubed in most force and moment equations

The neutral axis does not sit at mid-thickness during bending. For tight radii (R/t < 4), the neutral axis shifts inward by a factor depending on the R/t ratio. This shift is captured by the K-factor — typically 0.33 for sharp bends (R/t < 1), rising to 0.50 for gentle bends (R/t > 5).

Bend allowance (BA):

BA = (π/180) × α × (R + K × t)

Where:

α = bend angle (degrees)
R = inside bend radius (mm)
K = K-factor (dimensionless)
t = sheet thickness (mm)

Worked example: 90° bend, R = 3 mm, t = 2 mm, K = 0.42 (SPCC mild steel)

BA = (π/180) × 90 × (3 + 0.42 × 2)
BA = 1.5708 × (3 + 0.84)
BA = 1.5708 × 3.84 = 6.03 mm

This blank allowance feeds directly into strip layout and part flat pattern calculations — get it wrong and every part is dimensionally out before the die is even cut.

V-Die Bending: Air Bending and Bottoming

Die Geometry

The V-die consists of a punch with a tip radius and a die with a V-shaped groove. The sheet spans the die opening (W) and is loaded by the punch. Two operational modes exist:

Air bending: Punch stops before sheet contacts the V-groove sidewalls. Contact is at three points — the punch tip and the two die edges. Bending angle is controlled by punch travel, not by die geometry.
Bottoming (coining): Punch drives the sheet until it is fully seated against the V-groove sidewalls. Bending angle is controlled by die geometry.

V-Die Opening (W) Selection

The die opening drives bending force, minimum bend radius achievable, and springback magnitude. Standard rule:

W = 6t to 12t (general range)
W = 8t (most common starting point for mild steel)

For high-strength steel (σ_y > 500 MPa), use W = 10t to 12t to reduce tonnage and prevent cracking. For stainless steel, use W ≥ 10t.

Minimum punch tip radius (R_punch):

R_punch ≥ t (absolute minimum)
R_punch = 0.5t to 1.0t (typical for mild steel, t < 3 mm)
R_punch = 1.0t to 2.0t (AHSS grades)

Punching below minimum radius causes outer fiber cracking in AHSS and stainless.

V-Die Bending Force Calculation

For air bending, the bending force F is:

F = (1.33 × σ_UTS × b × t²) / W

Where:

F = bending force (N)
σ_UTS = ultimate tensile strength (N/mm²)
b = bend length (mm, perpendicular to bending direction)
t = sheet thickness (mm)
W = die opening (mm)

For bottoming, multiply air bending force by 3–5:

F_bottoming = (3 to 5) × F_air

Worked example: SPCC (σ_UTS = 340 MPa), t = 2 mm, b = 500 mm, W = 16 mm

F_air = (1.33 × 340 × 500 × 4) / 16
F_air = (1.33 × 340 × 500 × 4) / 16
F_air = 906,400 / 16 = 56,650 N ≈ 57 kN ≈ 5.8 tonf

For the same setup bottoming at 4× air bending force: F_bottoming ≈ 228 kN ≈ 23 tonf. That is a 4× jump in press tonnage requirement — a critical selection factor.

Springback in V-Die Bending

Springback angle (Δα) after air bending is approximately:

Δα ≈ 3 × (σ_y / E) × (R/t) × α_bent

Where:

σ_y = yield strength (MPa)
E = elastic modulus (MPa), 210,000 for steel
R/t = ratio of inside radius to thickness
α_bent = bend angle imposed by punch (degrees)

Worked example: σ_y = 280 MPa, E = 210,000 MPa, R = 4 mm, t = 2 mm, α_bent = 90°

Δα ≈ 3 × (280/210,000) × (4/2) × 90
Δα ≈ 3 × 0.001333 × 2 × 90
Δα ≈ 0.72°

For AHSS with σ_y = 600 MPa:

Δα ≈ 3 × (600/210,000) × 2 × 90 ≈ 1.54°

Air bending springback is substantial and strongly material-dependent. Bottoming nearly eliminates springback by setting residual compressive stress — at the cost of much higher tonnage and faster punch wear.

Wiping Die (Edge Bending): Mechanics and Design

How a Wiping Die Works

In a wiping die (also called a wipe die or edge bending die), one end of the sheet is clamped by a pad, and the punch wipes the free end over the die radius to form a flange. Contact is along the die radius — not at two points like a V-die. The punch moves vertically or at an angle, rotating the sheet around the die radius.

The wiping die is the natural choice for:

Single-flange bending at the sheet edge
Hemming pre-forms
Narrow flanges where V-die geometry cannot clamp the part
Parts with return flanges or stepped geometry

Wiping Die Force Calculation

The bending force for a wiping die is:

F_wipe = (0.5 × σ_UTS × b × t²) / R_die

Where R_die is the die edge radius. Note the denominator uses die radius rather than die opening — wiping die forces are generally lower than V-die bottoming but higher than V-die air bending for the same material and thickness, because the moment arm is shorter.

Worked example: σ_UTS = 340 MPa, b = 300 mm, t = 2 mm, R_die = 2 mm

F_wipe = (0.5 × 340 × 300 × 4) / 2
F_wipe = 204,000 / 2 = 102,000 N ≈ 10.4 tonf

In addition to bending force, the clamping pad must exert sufficient force to prevent sheet sliding. Pad force is typically:

F_pad ≥ 0.3 × F_wipe (minimum)
F_pad = 0.5 × F_wipe (recommended for AHSS)

Insufficient pad force causes the sheet to pull inward during bending — this shows up as a shortened flange and inconsistent angle.

Die Radius Selection for Wiping Dies

R_die = 1.0t to 2.0t (mild steel)
R_die = 2.0t to 3.0t (stainless 304/316)
R_die = 2.0t to 4.0t (AHSS DP600–DP980)

The die radius becomes the inside bend radius of the part (approximately, ignoring springback). Too small and the outer fiber cracks; too large and the angle is hard to hold.

Springback in Wiping Dies

Springback in wiping dies is larger than in V-die bottoming but comparable to V-die air bending. Because the bend is formed around a single radius rather than a three-point span, the springback is slightly more predictable. Standard compensation: design the punch angle 1–3° past final part angle for mild steel, 3–5° past for AHSS.

The wiping die punch must clear the flanged part on the return stroke — interference between punch face and formed flange is a common geometry error. Check the clearance angle:

θ_clearance = 90° - α_final + 2° minimum clearance

For a 90° flange: θ_clearance = 90° - 90° + 2° = 2° minimum — this is tight and often leads to designs with negative punch taper angles to ensure clearance.

U-Die (Channel Bending): Double-Bend Geometry

Configuration and Applications

A U-die simultaneously forms two parallel bends in a single stroke, producing a channel or U-shaped cross-section. The punch has two shoulders that contact the sheet at the two bend locations; the die has a matching channel cavity. A knockout or ejector is required to release the part from the die cavity.

U-die applications:

Structural channels and hat-sections
Electrical enclosure frames
Automotive structural members
Bracket and mounting profiles

U-Die Force Calculation

Total force is approximately twice the force for a single V-die bend of equivalent geometry, adjusted for simultaneous action:

F_U = 2 × (1.33 × σ_UTS × b × t²) / W

Where W is the effective die opening at each bend location. For a symmetric U-section with flange spacing D and wall depth h, the punch shoulder width equals D and the die cavity depth must be at least h + 5 mm for ejection clearance.

Worked example: SECC (zinc-coated, σ_UTS = 360 MPa), b = 400 mm, t = 1.5 mm, W = 12 mm

F_U = 2 × (1.33 × 360 × 400 × 2.25) / 12
F_U = 2 × (430,560 / 12)
F_U = 2 × 35,880 = 71,760 N ≈ 7.3 tonf

U-Die Springback — The Web Bowing Problem

U-die bending introduces a secondary complication absent in single-bend configurations: web bowing. After forming, the web (the flat bottom of the U) bows upward — concave when viewed from above — because the outer surface of the web is under tension during bending at both corners and springs back asymmetrically.

Web bow magnitude:

δ_bow ≈ (σ_y × D²) / (8 × E × t)

Where D = web width (flange-to-flange inside distance).

Worked example: σ_y = 280 MPa, D = 80 mm, E = 210,000 MPa, t = 1.5 mm

δ_bow ≈ (280 × 6400) / (8 × 210,000 × 1.5)
δ_bow ≈ 1,792,000 / 2,520,000 ≈ 0.71 mm

For a web 80 mm wide in 1.5 mm SPCC, expect ~0.7 mm bow without compensation. Correction options:

Add a crown to the punch web (0.3–0.5× δ_bow depth)
Add a rubber insert in the die cavity base to pre-load the web
Use a triple-action tool with a web-pressing pad

Comparison Table: V-Die vs Wiping Die vs U-Die

Parameter	V-Die Air Bending	V-Die Bottoming	Wiping Die	U-Die
Bending force (relative)	Low (1×)	High (3–5×)	Medium (1.5–2×)	2× V-air
Springback	High (requires angle compensation)	Minimal	Medium	Medium–High
Angle repeatability	±0.5–1.5°	±0.1–0.3°	±0.5–1.0°	±0.5–1.5°
Inside radius control	Poor (air bending)	Good	Good	Good
Minimum flange length	3.5t minimum	3.5t minimum	1.5t possible	2.0t minimum
Part accessibility	Excellent	Excellent	Limited to edge	Limited to channel
Tooling cost	Low	Low–Medium	Medium	Medium–High
Setup flexibility	High (angle = f(stroke))	Low	Medium	Low
AHSS suitability	High (low force)	Poor (tonnage)	Good (if R adequate)	Good
Coating damage risk	Low	Medium (contact pressure)	Medium	Medium
Typical tolerance ISO 2768	Medium (m)	Fine (f)	Medium–Fine	Medium (m)

Step-by-Step Bending Die Selection Framework

Step 1: Define the part geometry

Identify all bend locations, angles, and inside radii
Classify each bend: single angle, double angle, edge flange, channel
Note minimum flange lengths — this immediately eliminates some configurations

Step 2: Identify material constraints

For AHSS (σ_y > 500 MPa): avoid bottoming; favor air bending or wiping with generous radii
For stainless 304/316: use wiping or air bending with R ≥ 2t; avoid bottoming
For galvanized/coated: prefer air bending to minimize contact pressure and coating damage
For aluminum 5052-H32: air bending with R ≥ 1.5t; bottoming increases cracking risk

Step 3: Calculate minimum flange length requirement

V-die: minimum flange = (W/2) + punch travel clearance ≈ 4t–6t
Wiping die: minimum flange = 1.5t + die radius overhang
U-die: minimum web depth = die cavity depth + knockout clearance

If the part’s flanges are shorter than these minimums, the die type is disqualified.

Step 4: Calculate bending forces and select press

Calculate F for each die type using formulas above
Add 25% safety factor for press selection
Verify tonnage does not exceed 80% of press rated capacity at BDC

Step 5: Establish springback compensation strategy

Air bending: angle-correct by punch CNC travel or use springback correction tables
Bottoming: minimal — verify by bend trial
Wiping: overbend punch by calculated Δα
U-die: account for web bowing; add crown or rubber pad

Step 6: Verify die life requirements

High-volume production (>500,000 hits/year): use D2 or equivalent for punch, with TiCN coating
Medium volume: use SKD11 or equivalent; no coating required for mild steel
Low volume: use 1.2083 (AISI P20) pre-hardened; cost-effective for short runs

Common Mistakes and Failure Modes in Bending Die Design

1. Die opening too narrow for the material Result: excessive bending force, punch tip cracking, workpiece cracking at outer fiber. For AHSS, this is the single most common tooling failure at tryout. Always verify W ≥ 8t for mild steel and W ≥ 10t for AHSS before cutting steel.

2. Punch radius below material minimum Result: outer fiber fracture, especially at the 3–9 o’clock position on curved flanges. For DP600 and above, punch radius below 1.5t will cause micro-cracking even if the part looks acceptable initially — cracking propagates in service.

3. Insufficient pad force in wiping dies Result: sheet slides under the pad during bending, flange length is short and inconsistent. Toolmakers often underspec the spring pack. Minimum pad pressure = 5–8 N/mm² of pad contact area for mild steel; 10–15 N/mm² for AHSS.

4. No ejector stroke calculation in U-dies Result: the formed channel locks onto the punch and cannot be ejected without deformation. Die cavity depth must account for minimum ejector travel: typically h_part + 8–10 mm.

5. Ignoring web bowing in U-dies Result: assembled parts with gaps at fastener locations, failed GD&T flatness calls. Web bowing is predictable — calculate it before tryout and compensate proactively, not reactively.

6. Wrong K-factor for bend allowance Result: parts are short or long in the developed flat. Using K = 0.50 for a tight R/t = 1 bend is a 15–20% error in bend allowance. Match K-factor to the actual R/t and material combination using published tables (e.g., DIN 6935 Table 1, or Machinery’s Handbook bend allowance tables).

7. Hardness mismatch between punch and die Result: the softer component galls. In V-die sets, punch and die should be within 2 HRC of each other; the die should be 58–62 HRC, punch 60–64 HRC for production tooling.

8. Die corner radius (R_die_edge) at V-die mouth not deburred or radiused Result: sheet scratching, galling, and zinc coating pickup in galvanized steel. V-die mouth edges must be polished to Ra ≤ 0.4 µm and radiused to 0.5–1.0 mm minimum.

Industry Applications by Die Type

Automotive Body Panels — Wiping and Progressive Wipe Dies

Door inners and B-pillar reinforcements use multi-stage wiping sequences. A pre-bend wipe at 30°, followed by a finish-wipe at 90°, distributes strain across two strokes and reduces cracking risk in DP780 and DP980 grades. The finish wipe tool has negative back taper of 3° to ensure springback brings the angle to 90° nominal.

HVAC and Ductwork — V-Die Air Bending (CNC Press Brake Analogue)

Galvanized sheet for duct flanges (DX51D+Z275) is air-bent with standard V-die tooling at W = 10t. Angles are program-controlled; the same tool set handles 90°, 135°, and 45° bends by adjusting punch stroke. K-factor library entries for each gauge and grade are maintained in the CNC controller.

Electrical Enclosures — U-Die with Knockout

2 mm mild steel (SPCC) enclosure frames are formed in a single U-die stroke. Web width 100–150 mm requires 0.8–1.2 mm crown compensation on the punch base. Production rate: 600–900 strokes/hour on a 63-ton mechanical press.

Appliance Outer Panels — V-Die Bottoming for Class-A Appearance

Washing machine outer panels in 0.8 mm SECC require ±0.15° angle tolerance and Ra ≤ 0.8 µm at the bend line (visible surface). Bottoming with mirror-polished punch (Ra ≤ 0.1 µm) is mandatory. Air bending cannot hold angle tolerance at this thickness.

Structural Brackets — U-Die with AHSS

DP600 (1.5 mm) hat-section brackets for automotive chassis: R_punch = 3 mm (2× t), W at corners = 16 mm, pad force = 80 kN over 200 mm pad. Springback at both flanges = 2.8° measured at tryout; punch corrected by 3° overform. Web bow measured at 0.4 mm over 60 mm span — within 0.5 mm GD&T flatness call.

FAQ

Q1: How do I choose between air bending and bottoming for a production part? Air bending is the default for AHSS, coated materials, and any material over 4 mm thick. Bottoming is justified when you need ±0.1° angle repeatability and the material is mild steel or low-carbon at thickness ≤ 3 mm. Bottoming on AHSS causes premature punch tip cracking and requires 4–5× the tonnage — it is rarely cost-effective.

Q2: What is the minimum flange length a wiping die can form reliably? A wiping die can form flanges as short as 1.5t under ideal conditions — clamped edge, small die radius, mild steel. In practice, 2.5t is a reliable minimum for production. Below 2.5t, the pad loses grip and the flange geometry becomes inconsistent. If the flange is shorter than this, consider a combination die or restrike.

Q3: How much does die opening affect springback in air bending? Springback scales approximately linearly with R/t, and the effective R/t in air bending is determined by the die opening. Wider opening = larger effective bending radius = more springback. Specifically: doubling the die opening approximately doubles the springback angle. This is why tight-tolerance applications use bottoming or active angle correction systems.

Q4: Can V-die and wiping die operations be combined in a progressive die? Yes — this is standard practice in progressive dies for complex bracket profiles. The V-die station performs the main angle bend; a downstream wiping station forms the edge flange. Critical: the wiping station must be after all major V-die stations to avoid part instability on the strip carrier.

Q5: What punch tip radius should I use for DP980 steel? DP980 (σ_y ≈ 700 MPa, elongation ≈ 6%) is among the most crack-sensitive grades. Minimum punch radius: 3t for transverse bends (perpendicular to rolling direction), 4t for longitudinal bends. Use air bending exclusively; never bottom DP980. Die opening: 12t minimum.

Q6: How do I calculate the required tonnage for a U-die on a press with 100 tons capacity? Calculate F_U using the formula above, add 25% safety margin, and verify F_U × 1.25 ≤ 80 tons (80% of press capacity). The 80% rule prevents off-center loading damage to the press frame. For U-dies, also check the press table size — U-die toolsets are wider than equivalent V-die toolsets.

Q7: What causes cracking at the inside bend radius in stainless 304? Three common causes: punch radius below 2t, bending in the transverse direction (across rolling direction — always bend stainless 304 with the bend line parallel to rolling direction when possible), or r-value anisotropy causing uneven strain distribution. Increase punch radius to 2.5t and verify strip orientation before committing to production tooling.

Q8: How often should V-die and punch sets be reground? For mild steel production (SPCC, SECC), regrind punch tip when tip radius increases by more than 0.2 mm from nominal, or when burr height on the formed part exceeds 0.1 mm. A typical regrind interval is 150,000–300,000 strokes for mild steel. For AHSS (DP600+), inspect every 50,000 strokes and regrind at 0.1 mm radius growth.

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Conclusion

Bending die selection is not a preference — it is a calculated decision driven by material grade, part geometry, angle tolerance, and production volume. V-die air bending handles the widest material range at the lowest tonnage and tooling cost, at the price of springback variability that must be managed through stroke control or material characterization. Bottoming delivers tight angle repeatability but is limited to mild steel and lower-strength materials where tonnage multiplication remains within press capacity. Wiping dies excel at edge flanges and short-flange geometry, with moderate springback and well-understood pad force requirements. U-dies form channels in a single stroke but introduce web bowing that must be calculated and compensated at the design stage.

The bending force formulas, springback equations, and K-factor selection covered in this article give you the numbers needed to specify tooling before cutting steel. In every case, verify with bend trials on production-representative material before final tool approval — mechanical properties within the same grade and supplier can vary ±10%, which shifts bending force and springback meaningfully.

Get the die geometry right at design stage and bending operations become stable, predictable, and repeatable across millions of strokes.

Demirezen Engineering — Bending Die Design and Consulting

Demirezen Engineering provides bending die design, die tryout support, and springback analysis for production stamping operations. Whether you are specifying a new U-die for a structural bracket or troubleshooting cracking in an existing AHSS bending line, contact us with your part drawing and material certificate.

WhatsApp: +90 543 341 6183
Website: demirezenengineering.com

Internal Links

External References

DIN 6935:2011 — Cold bending of flat products made of steel; standard bend radii, bend allowances and deduction values for tool design
Schuler GmbH, “Metal Forming Handbook” (Springer, 1998) — Chapters 4–6 cover die bending mechanics, force calculations, and springback in production stamping
AIDA Engineering Ltd. Technical Library — Application notes on press selection for air bending, bottoming, and U-die forming: www.aida-global.com/en/technical

Suggested Images and Diagrams

Three-configuration comparison diagram — Side-by-side cross-sections of V-die air bending, V-die bottoming, wiping die, and U-die, with labeled punch radius, die opening, and contact geometry. ALT: “Cross-section comparison of V-die, wiping die and U-die bending configurations showing punch contact points and material deformation zones.”
Force vs. stroke curve — Overlaid F-s curves for air bending, bottoming, and wiping die at the same material and thickness. ALT: “Bending force vs. punch stroke curves comparing air bending, bottoming and wiping die for 2 mm mild steel.”
Springback angle diagram — Before/after overlay showing overbend angle and final part angle for air-bent SPCC vs. DP600. ALT: “Springback angle illustration showing overbend compensation requirement for mild steel vs. dual-phase steel in air bending.”
Web bowing schematic — U-channel cross-section with exaggerated web bow, dimensional callouts for δ_bow and web width D. ALT: “U-die web bowing defect schematic with bow magnitude calculation variables labeled.”
Selection flowchart — Decision tree: material grade → flange geometry → tolerance requirement → die type recommendation. ALT: “Bending die type selection flowchart for sheet metal stamping: V-die vs wiping die vs U-die decision logic.”

Bending Die Design: V-Die, Wiping Die and U-Die — Comparison and Selection

Bending Die Design: V-Die, Wiping Die and U-Die — Comparison and Selection

What Bending Dies Do — Mechanics Behind the Bend

V-Die Bending: Air Bending and Bottoming

Die Geometry

V-Die Opening (W) Selection

V-Die Bending Force Calculation

Springback in V-Die Bending

Wiping Die (Edge Bending): Mechanics and Design

How a Wiping Die Works

Wiping Die Force Calculation

Die Radius Selection for Wiping Dies

Springback in Wiping Dies

U-Die (Channel Bending): Double-Bend Geometry

Configuration and Applications

U-Die Force Calculation

U-Die Springback — The Web Bowing Problem

Comparison Table: V-Die vs Wiping Die vs U-Die

Step-by-Step Bending Die Selection Framework

Common Mistakes and Failure Modes in Bending Die Design

Industry Applications by Die Type

Automotive Body Panels — Wiping and Progressive Wipe Dies

HVAC and Ductwork — V-Die Air Bending (CNC Press Brake Analogue)

Electrical Enclosures — U-Die with Knockout

Appliance Outer Panels — V-Die Bottoming for Class-A Appearance

Structural Brackets — U-Die with AHSS

FAQ

JSON-LD FAQ Schema

Conclusion

Demirezen Engineering — Bending Die Design and Consulting

Internal Links

External References

Suggested Images and Diagrams

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