Bend radius is the inside arc a material can tolerate without cracking, kinking, or losing strength. Set the radius too tight and you waste parts; set it too generous and you add weight and cost. This guide explains how to pick the right bend radius for sheet-metal brackets, welded tubes, injection-molded living hinges, and flexible cables. Follow the tables, formulas, and checklists below to get a first-time-right bend on the shop floor.
Indholdsfortegnelse
- Why the correct bend radius saves money
- Key terms for bend calculation
- Minimum bend radius by material
- How thickness affects radius choice
- K-factor and bend-allowance basics
- Press-brake tooling selection
- Tube and pipe bending limits
- Plastic sheet and living-hinge design
- Bend radius in flexible cables and hoses
- Inspection methods for bend quality
- Cost impact and cycle-time chart
- Need bending & machining services?
- Quick recap before releasing drawings
Why The Correct Bend Radius Saves Money
A bend that is too tight risks cracked paint, fatigued copper conductors, or fractured stainless corners—triggering scrap or warranty claims. An overly large bend wastes material and space and forces bigger tooling. Correct radius design:
- Reduces springback rework on press brakes.
- Maintains flow area in hydraulic tubes.
- Meets cable-manufacturer warranty for bending life.
Key Terms For Bend Calculation
Before we dive into tables, know these three variables:
| Term | Symbol | Definition |
|---|---|---|
| Inside bend radius | Ri | The curve measured on the inner surface of the bend. |
| Bend angle | θ | Angle between two legs after bending (usually 90°). |
| K-factor | K | Neutral-axis location as a ratio of thickness. |
Minimum Bend Radius By Material
The next table lists conservative starting radii for common sheet metals. Values are expressed as multiples of sheet thickness (t).
| Materiale | Temper | Min Ri (×t) | Comment |
|---|---|---|---|
| Aluminum 5052-H32 | 1/2-hard | 0.8 × t | Great for tight electronics brackets |
| Aluminum 6061-T6 | Full-hard | 3 × t | Cracks below 2 × t |
| Mild steel CRS 1018 | Cold-rolled | 1 × t | Stable springback of ~1° |
| Stainless 304 2B | Annealed | 1.5 × t | Use larger punch nose to cut galling |
| Spring steel 1095 | Hardened | 4 × t | Consider laser cut + form hot |
| Kobber C110 | Annealed | 0.5 × t | Excellent ductility—watch edge ripples |
How Thickness Affects Radius Choice
The thicker the sheet, the more the inner fibers must compress. A simple rule is:
Ri minimum = Max( t , 0.8 × t + 0.2 mm )
For a 0.8 mm CRS part, min Ri ≈ 0.8 mm. For a 4 mm plate, Ri ≥ 4 mm. Beyond 6 mm thickness most shops switch to air bending with large V-dies; fixed inside radius follows the punch nose rather than this formula.
K-Factor And Bend-Allowance Basics
Unfold length in CAD needs the bend allowance (BA). This equation uses K-factor and radius:
BA = (π/180) × θ × (Ri + K × t)
For 90° (θ = 90) in mild steel 1 mm thick, punch nose 1 mm, K ≈ 0.33:
BA = 1.5708 × (1 + 0.33) ≈ 2.08 mm
Insert that BA into flat-pattern length to avoid rework.
Press-Brake Tooling Selection
Tooling choice locks in the achievable radius. Use this quick lookup for V-die width (V) and expected inside radius (Ri) for air bends.
| Sheet thickness t (mm) | Recommended V-die width V (mm) | Resulting Ri (≈ 0.16 × V) |
|---|---|---|
| 0.8 | 8 | 1.3 |
| 1.5 | 12 | 1.9 |
| 3.0 | 24 | 3.8 |
| 6.0 | 50 | 8.0 |
If you need a sharper radius than the V-die rule, switch to coining—high tonnage but near-zero springback.
Tube And Pipe Bending Limits
Tube bends are measured by the centerline radius (CLR) divided by outer diameter (OD). Most draw-bending aims for CLR ≥ 1.5 × OD to stop ovality.
| Materiale | OD (mm) | Wall (mm) | Min CLR (×OD) | Mandrel needed? |
|---|---|---|---|---|
| Stainless 316L | 25 | 1.5 | 2.0 | Yes—ball + link |
| Aluminum 6061-T4 | 19 | 1.2 | 1.5 | Yes—plug |
| Steel DOM | 32 | 2.0 | 1.6 | No—wiper only |
| Copper HVAC | 12 | 1.0 | 1.2 | No—rotary draw |
Plastic Sheet And Living-Hinge Design
Thermoplastics cold-bend when thin. For thicker plates, use heat strip-forming.
- Polycarbonate sheet ≤ 3 mm: Min Ri ≈ 100 × t at 23 °C cold; 2 × t at 150 °C heat.
- PP living hinge: Groove to 0.25 mm, radius 0.13 mm, cycles > 1 million.
- ABS: Bend warm (90 °C) to Ri ≥ 25 × t; below that stress-cracks.
Bend Radius In Flexible Cables And Hoses
Cables publish static (installed) and dynamic (moving) radii. Use larger of the two.
| Cable type | Outer Ø (mm) | Static Rmin | Dynamic Rmin | Bemærk |
|---|---|---|---|---|
| Cat-6 Ethernet | 6.2 | 4 × Ø = 25 mm | 8 × Ø = 50 mm | Avoid pairs twist change |
| Hydraulic hose 3/8″ | 19 | 75 mm | 100 mm | Check pressure derate at 90° |
| Fiber optic single-mode | 2.0 | 30 mm | 50 mm | Below 30 mm causes 0.5 dB loss |
Inspection Methods For Bend Quality
After bending, verify edge stretch, angle, and radius.
- Radius gauge: Stainless leaf set compares inside arc.
- Digital protractor: Confirms bend angle ±0.1°.
- D-capsule dye test: Finds cracks >25 µm on aluminum.
- Tube ovality caliper: OD difference/OD ≤ 8 % for fuel lines.
Cost Impact And Cycle-Time Chart
Inside radius | Press-brake cycle time | Scrap risk index 0.8 × t | ████ 8 s | ███████ 7 1.0 × t | ███ 6 s | ████ 4 2.0 × t | ██ 5 s | ██ 2 4.0 × t | █ 4 s | █ 1
Tight radius increases time and scrap. Where form height allows, boost Ri to at least t to halve risk.
Need Bending & Machining Services?
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Quick Recap Before Releasing Drawings
- Check material spec—temper changes radius rule.
- Ensure inside radius ≥ t for mild steel, ≥ 3 × t for 6061-T6.
- Add K-factor bend allowance to flat pattern; verify in CAM.
- For tubing, set CLR ≥ 1.5 × OD and add mandrel if wall/OD < 0.07.
- Label critical radii on drawings and note inspection gauge.
Follow these guidelines and your bends will close easily, hold tolerance, and keep both operators and accountants happy.
