When a hole enters the plastic bend zone, its edge moves with tensile and compressive strain, causing ovality, flaring, cracking or positional shift. A useful first screen is hole-edge-to-bend-tangent ≥ R+2t, where R is inside radius and t is sheet thickness. It is a starting rule, not a guarantee for every material or tool set.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
MECHANISM AND DECISION
For 2.0 mm sheet with R=2.0 mm, start with a 6.0 mm edge clearance. If only 3–4 mm is available, test the actual temper, V-die width, hole-diameter/thickness ratio and bend direction. Hard-temper aluminum, high-strength steel, low R/t and a slot normal to the bend are more sensitive. Measure the nearest feature edge, not merely its center.
PRACTICAL METHOD
1. Mark bend tangents, edge clearance and hole function class during DFM. 2. A normal clearance hole may use validated compensation; dowel, bearing and close-fit holes should preferably be drilled or reamed after bending. 3. Measure at least five trial parts before and after bending for diameter, ovality and location to datums; inspect CTQs in final formed condition. 4. If the feature cannot move, consider a relief, larger R or a slot, while checking residual ligament and load direction.
BASIS
Use a measured K-factor or bend deduction for flat development; express geometric requirements using ISO 1101:2017 and ISO 5459:2024. Do not present a supplier rule of thumb as an unconditional material law.
PROJECT CHECKLIST
Grade and temper; t, R and V-die; feature clearance; hole function; burr direction; final inspection state; post-machining permission.