The 4T rule is a core guideline in sheet metal design to ensure bending accuracy, requiring functional features (e.g., holes, bosses) to be at least 4 times the material thickness (4T) from the bend line, preventing deformation or fracture during bending. Widely used in aerospace and automotive industries, it’s key to Design for Manufacturability (DFM).

What is the 4T rule for sheet metal?

1. Core Logic

During sheet metal bending, outer layers endure tensile stress and inner layers compressive stress, with a 2-3T zone near the bend line as the stress concentration area. Features too close cause material flow-induced deformation or cracking. For example, a 0.05-inch aluminum sheet needs a 0.2-inch (4×0.05) safety distance; otherwise, hole deviation may exceed ±0.1mm tolerance.

2. Application & Design Strategies

  • Hole Layout: Holes for post-punch bending must meet 4T. If not, “bend-first then punch” works but adds cost. In medical device enclosures, shielding holes need 4T from bend lines to avoid EMC issues.
  • Boss/Stiffener Design: Bosses (e.g., rivet nut positions) near bend lines risk root stress concentration. A case saw boss fractures until 0.3mm 工艺缺口 (process notches) and layout adjustments fixed it.
  • Complex Structures: Multi-bend parts require 4T compliance for all lines. U-groove sidewalls and bottom bend lines must both meet 4T to prevent instability.

3. Industry Practices

  • Tolerance & Process Optimization: CNC press brakes use die compensation, but insufficient distance limits effectiveness. A car parts maker embedded 4T in CAD templates via simulation, boosting design efficiency by 30%.
  • Material Impact: High-strength materials like 304 stainless steel (1.5mm thick) need 6mm (4×1.5) distance; aluminum can use the 4T minimum.
  • Space-Limited Solutions: Local slots or process holes release stress. An industrial cabinet used 1.5T-wide slots near bend lines, reducing hole distance to 2.5T while maintaining strength.

4. Violation Risks

  • Functional Failure: Deformed holes caused assembly misalignment in an appliance case, increasing rework costs by 20%.
  • Cost Rise: A medical device project saw 5% scrap due to hole deviation; adopting 4T rules cut defects to 0.3%.
The 4T rule is flexible, adjusted by material and bend radius. Small batches may use 3T, but mass production needs fatigue testing. CAD tools like SolidWorks can auto-detect violations, enhancing efficiency.

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