Sheet Metal Bending Guide: Hole Clearance and Bend Relief

To keep holes from deforming during a bend, place them far enough from the bend zone or add a relief cut that lets the material move without tearing. A useful preliminary rule is to keep the nearest edge of a hole at least the inside bend radius plus two material thicknesses from the bend tangent line. For bend relief, a starting point is a relief width at least equal to the sheet thickness, with the cut extending beyond the bend area. These are design guidelines, not guarantees: material, tooling, bend direction, and production method can change the required dimensions.

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Key Takeaways

• Measure hole clearance from the hole edge to the bend tangent line, not automatically from the bend centerline.
• Use the inside radius and material thickness to establish a preliminary clearance, then confirm it against the actual forming setup.
• Add bend relief where a bend ends at an edge or intersects a flange, especially when the material could stretch or tear at the corner.
• Show material, thickness, bend direction, inside radius, and critical hole locations on the drawing so your supplier can assess manufacturability.

What Hole Clearance and Bend Relief Mean

Hole clearance near a bend

Hole clearance is the distance between a feature, such as a round hole or slot, and the region that will be plastically deformed during bending. If a hole is too close, forming can pull it into an oval shape, shift its position, or distort the surrounding surface. The risk depends on the feature’s size and shape as well as the material and bend geometry. Clearance is therefore best treated as a manufacturing design check rather than a single universal number.

Bend relief

A bend relief is a notch, slot, or cutout placed at the end of a bend to separate the deforming bend zone from adjacent material. It helps reduce tearing, buckling, and unwanted distortion where a bend terminates or meets another flange. Reliefs can be rectangular, rounded, or shaped to suit the part and cutting process. Their exact geometry should preserve the required strength and appearance while providing enough room for the material to form.

How to Estimate Hole Clearance

Start with the bend tangent

For a first-pass layout, measure from the closest edge of the hole to the bend tangent line—the point where the flat section transitions into the bend. A commonly used preliminary rule is: minimum edge clearance ≈ inside bend radius + 2 × material thickness. Because this is a rule of thumb rather than a universal manufacturing standard, confirm it with the fabricator before releasing a production drawing.

For example, with 1.5 mm sheet and a 1.5 mm inside radius, the preliminary edge clearance is 4.5 mm: 1.5 mm + (2 × 1.5 mm). This example illustrates the calculation only; it does not establish a guaranteed minimum for every alloy, tool, or bending process. If the drawing dimensions the hole center instead of its edge, add the hole radius to the required edge clearance. Make the measurement reference explicit to avoid different interpretations during quoting and inspection.

Check the actual geometry and process

A hole near a bend may behave differently depending on whether it is round, elongated, or close to an edge. A slot oriented parallel to the bend line can create a different stress and distortion pattern from a round hole, so do not apply the same clearance without review. Material thickness, temper, grain direction, bend radius, and forming sequence can also influence results. When dimensions are tight, a forming simulation, prototype, or supplier review can help identify risk before production.

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The values in this table are preliminary design guidance, not acceptance criteria. A supplier should evaluate them against the part’s material, tooling, tolerances, and inspection requirements. If a feature is function-critical, specify its allowed position or shape after forming rather than relying on a generic clearance rule alone.

How to Design Bend Relief

Choose a relief shape for the bend end

A rectangular relief is straightforward to define, while a rounded end can reduce sharp internal corners that may concentrate stress. The best choice depends on the surrounding geometry and how the part will be cut and formed. A relief that is too narrow may not separate the bend zones effectively, while an excessively large cutout can weaken the part or interfere with an assembly surface. Balance formability with the structural and visual requirements of the finished component.

Set width and depth deliberately

A preliminary relief width of at least 1.0 times the sheet thickness is often used as a starting point. For example, a part made from 2 mm sheet could begin with a relief width of 2 mm for a design review, but the final dimension should be confirmed for the specific bend and process. The relief depth should pass far enough through the bend-affected area to prevent material from remaining connected where the bend terminates. Ask the supplier to check the relief against the inside radius, bend length, and adjacent flange rather than specifying depth by habit.

Common Design Mistakes

• Measuring from the wrong reference: Hole-to-bend distance can be interpreted from the centerline, tangent line, or bend edge. State the intended reference in the drawing or CAD notes.
• Using one clearance for every material: A guideline that works for one thickness or material condition may not work for another. Reassess the design when the material specification changes.
• Ignoring the bend sequence: A feature may be clear of the bend itself but still create a tooling or access issue during forming. Review the complete forming sequence for complex parts.
• Leaving sharp relief corners without review: Sharp corners can act as stress concentrators. Consider a rounded termination where it suits the part and cutting process.
• Applying tight tolerances without a process check: Forming can affect feature position and shape. Identify critical dimensions early and discuss inspection expectations with the manufacturer.

Buyer and Supplier Checklist

Before requesting a quote, provide a clear drawing or 3D model together with the material grade, thickness, quantity, finish, and critical tolerances. Mark bend lines, bend directions, inside radii, and any holes or slots close to a bend. If the design permits alternatives, identify which dimensions or relief details can be adjusted to improve manufacturability. This information helps the supplier evaluate the part and flag open questions before production planning.

At Jinhui, we recommend reviewing hole clearance and relief geometry as part of the broader sheet metal forming discussion, not as isolated drawing details. Our team can review the information supplied for a project and discuss potential design adjustments, subject to the actual material and production requirements. For a more useful assessment, include the intended application and identify any features that must remain within a specific tolerance after bending. Final dimensions should be agreed before production begins.

Conclusion

To reduce hole distortion and bend-end tearing, start by measuring hole-edge clearance from the bend tangent and use inside radius plus two material thicknesses as a preliminary check. Add a suitably sized bend relief where a bend terminates or meets adjoining geometry, then verify the design against the actual material, tooling, and forming sequence. These starting rules support early design decisions but cannot replace project-specific manufacturability review. Share your drawing, material, thickness, and critical requirements with Jinhui to discuss a practical bending approach and the next steps for your part.

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