I will provide you with a clearer and more gradual method, which is usually used for accurate bending processes on metal parts. Through this article, we mainly aim to solve the basic calculation methods for cracks, rebound, and dimensional errors, as well as the selection of attacks and basic techniques.
01Understand the Material Before You Bend
Different metals react differently to bending. Common materials like low-carbon steel,aluminum 5052, and stainless steel 304 each have distinct minimum bend radii and springback behavior.
Low-carbon steel (e.g., AISI 1008) : Can bend sharply – minimum inside radius = 0.5× material thickness.
Aluminum 5052 : Good formability – minimum inside radius = 1× thickness. Avoid 6061-T6 (cracks easily).
Stainless steel 304 : High springback – require over-bending by 2–5°.
Actionable check: Always check the material certificate or perform a test bend on a scrap piece before production.
02Calculate the Bend Allowance Correctly
Bend allowance (BA) is the length of the neutral axis through the bend. Wrong BA leads to incorrect flat patterns and final part dimensions.
Formula for 90° bend:
BA = (0.017453 × Inside radius + 0.0078 × Material thickness) × Bend angle
(For steel, using a K-factor of 0.446)
Simplified rule of thumb (for steel, 90°):
BA ≈ (Radius + Thickness/3) × 1.57
Example: 3mm steel plate, inside radius = 3mm → BA ≈ (3 + 1) × 1.57 = 6.28mm per bend.
> Source: Common industry practice from tooling manufacturers and sheet metal handbooks (e.g., “Machinery’s Handbook”).
03Choose the Right V-Die Opening
The V-die opening determines the required tonnage and the achievable inside radius.
Standard rule: V-die width = 8× material thickness (for steel up to 6mm thick).
For 2mm steel → 16mm V-die.
For 4mm steel → 32mm V-die.
Inside radius result: ≈ 1/6 of V-die width (air bending). Example: 16mm V-die → inside radius ≈ 2.7mm.
Common mistake: Using too narrow a V-die causes excessive tonnage, part marking, and possible die damage.
04Calculate Required Tonnage to Avoid Overload
Use this formula for air bending mild steel (tensile strength ≈ 450 MPa):
Tonnage per meter = (Thickness² × 450) / (V-die width × 12)
Example: 3mm steel, 24mm V-die → (9 × 450) / (24 × 12) = 4050 / 288 ≈ 14 tons/meter.
For a 1.5m long part → 21 tons needed. Always add 20% safety margin.
> Data consistent with standard press brake tonnage charts (e.g., Cincinnati, Amada – general reference).
05Control Springback with Over-Bending or Bottoming
Springback is the elastic recovery of metal after bending. It is unavoidable but can be compensated.
Measured springback angles (air bending, 90° target):
Steel, 3mm, R=3mm: ~0.5–1°
Stainless 304, 2mm: ~3–5°
Aluminum 5052, 2mm: ~1–2°
Two reliable methods:
Over-bend: Punch to 92° for a 90° final part (adjust by test).
Bottoming (coining): Use a punch and die that match the final angle – almost zero springback but requires higher tonnage (3–5× air bending force).
06Avoid Cracking on Outside of Bend
Cracking happens when the outside fiber strain exceeds material elongation.
Minimum bend radius (R_min) to avoid cracking:
R_min = (50 / Elongation% – 1) × Thickness
Example: Steel with 20% elongation → (50/20 – 1) = 1.5 → R_min = 1.5× thickness.
Signs of imminent cracking:
Orange peel texture on aluminum
Fine lines appearing on the outside surface
Audible popping during bending
Immediate fix: Increase inside radius or use annealing (for some grades).
07Inspect and Adjust with Common Tools
After bending, check critical dimensions with these methods:
Daily checklist before production:
1. Clean V-die and punch surfaces.
2. Lubricate only if required (dry bending is common for steel).
3. Run a test piece and measure angle with a protractor or gauge.
08Repeat of Core Principle
To achieve qualified processing of workpiece bending, four core criteria are essential: precise bending compensation value, V-shaped lower die matching with plate thickness, good rebound compensation, and reasonable inner fillet matching material. As long as any one of them is ignored, bending defects will inevitably occur.
09Actionable Conclusion
Select the workpiece that is currently difficult to bend and process, measure the actual thickness of the board, and choose a V-shaped lower mold with 8 times the thickness of the board; Calculate the bending compensation value and perform three bending tests using punch angles of 90 °, 92 °, and 94 ° respectively. Record the corresponding angle parameters that can accurately achieve a 90 ° right angle after rebound, and apply this parameter setting uniformly to all subsequent workpieces of the same model.
