Quick Answer
Bending tolerances for sheet metal are looser than machining tolerances because bending is a forming process with inherent springback. Typical bend angle tolerance is ±0.5–2° and bend-to-feature position tolerance is ±0.15–0.50 mm. Tight bending tolerances require compensated die design, tighter material thickness control, and supplier process validation. Specifying tolerances you do not need increases cost.
How Bending Works
Sheet metal bending uses a press brake: a punch pushes the sheet into a V-shaped die, forcing it to deform plastically. The material springs back slightly after the punch releases, so the die must overbend to achieve the target angle. Springback varies by:
- Material strength and grade
- Thickness (thicker = less springback)
- Bend angle (sharper bend = less springback)
- Bend radius (larger radius = more springback)
- Direction of bend relative to grain
Typical Bend Tolerances
| Parameter | Standard Tolerance | Precision Tolerance | Notes |
|---|---|---|---|
| Bend angle | ±1.0–2.0° | ±0.5° | Varies by angle and material |
| Bend radius | ±0.2–0.5 mm | ±0.1 mm | Depends on die radius |
| Bend-to-hole | ±0.20–0.50 mm | ±0.10 mm | Most critical dimension |
| Bend-to-edge | ±0.30–0.60 mm | ±0.15 mm | Feature to part edge |
| Overall length (after bend) | ±0.5–1.0 mm | ±0.2 mm | Accumulation of all bends |
Springback Compensation
Springback is the most misunderstood aspect of bending. The formula for compensation:
- For mild steel: overbend by approximately 1–3° per 90° bend (varies by thickness and radius)
- For aluminum: overbend by 2–5° per 90° bend (aluminum springs back more than steel)
- For stainless steel: overbend by 1–2° per 90° bend
- For high-strength steel (AHSS): overbend by 3–8° per 90° bend (springback is significant)
Buyer warning: if you specify a tight bend angle tolerance on aluminum without compensating, the supplier may need multiple trials to get it right — and charge for tryouts.
Bend Radius Guidelines
| Material | Minimum Bend Radius (× thickness) | Recommended Radius |
|---|---|---|
| Mild steel (CRS) | 0.8–1.0×t | 1.5–2.0×t |
| Aluminum (5052) | 0.5–1.0×t | 1.0–1.5×t |
| Aluminum (6061-T6) | 2.0–3.0×t | 3.0–4.0×t |
| Stainless steel (304) | 1.0–1.5×t | 2.0×t |
| High-strength steel | 2.0–4.0×t | 3.0–5.0×t |
Bending below minimum radius causes cracking at the bend outer surface. Always specify bend radius or minimum radius requirement in your drawing.
Bend Direction and Grain
Bend direction relative to rolling grain affects cracking risk and springback:
- Bend across grain (perpendicular) — lower cracking risk, more springback
- Bend with grain (parallel) — higher cracking risk, less springback
- 45° to grain — intermediate properties
Buyer action: mark bend lines relative to grain direction if it matters for your part function. If not marked, supplier will choose the most convenient direction.
GD&T for Bending
Key GD&T symbols for sheet metal drawings:
- Position (position tolerance) — controls hole-to-bend relationship; most critical for assembled parts
- Flatness — controls part flatness after bending; especially important for large panels
- Perpendicularity — controls bend perpendicularity to flange
- Profile of a line — can control bend angle if angular tolerance is critical
Common Drawing Mistakes
- Specifying both a tight bend radius AND a tight bend angle — these interact; tighter radius = harder to control angle
- Not indicating bend direction — supplier chooses; may cause cracking or aesthetic issues
- Requiring bend flatness tighter than ±0.5 mm on welded assemblies — welding distortion will exceed this; specify after-weld flatness
- Calling out machined tolerances on bent features — bending cannot hold CNC machining tolerances (±0.01 mm); if machined tolerance needed, machine after bend
- Forgetting clearance for bends — features near bends must account for bend relief; specify minimum distance
Bending Sequence and Part Distortion
Complex parts have multiple bends. Bend sequence matters:
- Supplier should plan bend sequence to avoid interference and accumulated error
- Each bend can affect previously bent features; tolerance accumulation must be managed
- For parts with many bends, tolerance stack-up analysis is needed
- Welded assemblies: always bend before welding; welding after bending causes distortion
Supplier Process Validation
For tight bending tolerances, buyers should request:
- First article inspection (FAI) with actual measured angles and positions
- Bend compensation data: supplier should provide springback data for their process and material lot
- Measurement method: CMM, optical comparator, or protractor — method affects results
- Material certificate: thickness and yield strength; these affect springback
Before finalizing the sourcing decision, many OEM buyers also compare Sheet Metal Fabrication, DFM Review, CNC Machining, and Casting Tolerances to clarify process fit, cost trade-offs, tolerance expectations, and supplier risk.
If you need application-specific guidance, drawing review, or a quotation, you can Contact YCUMETAL.
FAQ
Why does my aluminum part spring back more than steel?
Aluminum has a lower elastic modulus (stiffness) than steel. When the punch releases, aluminum springs back more for the same deformation. The supplier must compensate by overbending more, which requires process data or trial-and-error.
Can bending hold CNC machining tolerances?
No. Bending tolerance is typically ±0.1–0.5 mm and ±0.5–2° — far looser than CNC (±0.01 mm). If you need machined tolerance on a bent feature, machine it after bending.
What is bend allowance?
Bend allowance is the length of the neutral axis in the bend region. It is used to calculate the flat blank size before bending. Suppliers use bend allowance formulas (or software) to determine the correct blank size.
What if my part cracks at the bend?
Cracking is usually caused by: (1) bend radius too small for material hardness, (2) grain direction perpendicular to bend, (3) surface defects from laser or punch. Ask supplier for root cause analysis and die correction.