Bend Radius Chart for Sheet Metal: Materials, Thickness & Design Guidelines

Sheet metal bending seems easy until parts crack during bending, exhibit more springback than expected, or return with unexpected tolerance variations from the manufacturing workshop. All such issues can be traced back to one major problem: the bend radius used is not compatible with the material. With a bend radius chart, one has a reference point based on actual material properties rather than assumptions, thereby producing reliable bending results.

Introduction: What Is a Bend Radius Chart?

The bend radius is defined as the measure of the inner radius of curvature of a bend. It is measured from the center of the bend arc to the inner side of the material. A small bend radius describes a tight and sharp bend, while a large bend radius indicates a gradual curve. Minimum bend radius is described as the tightest bend one could bend a certain metal at a certain thickness without causing cracking on the outer surface or permanent deformation of the metal grains.

The bend radius chart is a table that relates material, material thickness, and temper to the minimum bend radius. It eliminates any guessing game when designing parts since you would not want to know only at first article inspection that the 3mm thick aluminum flange had cracked along the bend line.

Standard Bend Radius Chart (Inside Radius as a Multiple of Material Thickness)

Here, t = material thickness. Minimum values are at the inside bend radius of the sheet at room temperature, bending perpendicular to the grain direction.

They are for comparison purposes only. Actual results will vary depending upon the state of your tooling, press brake tonnage, bend rate, and grain direction.

Are you still not sure which radius works for your material and thickness? Send us your drawing, and our engineers at Premium Parts will check it before you go to production. 

How Grain Direction Controls Your Minimum Bend Radius

Every sheet metal has a grain direction, which is oriented parallel to the rolling direction in the factory. Depending on how you bend your parts, the position of the bend line with respect to this grain direction will greatly affect the extent to which the material stretches without failing.

Bending against the grain:

The bend line is perpendicular to the rolling direction, so it’s definitely safer. Material fibers are oriented parallel to the bend line, thus stretching uniformly and preventing failure. This is the position reflected in most bend radius tables, such as the one above.

Bending with the grain:

The bend line is parallel to the rolling direction. This makes the material fibers stretch longitudinally, and they will most probably fail due to tensile force at this position. Practically, you must increase your minimum inside bend radius by 50% to 100% if you want to bend with the grain.

The rule that governs practical bending is easy: keep your bend lines perpendicular to the rolling direction whenever possible. If your part has many bends in various directions, which occurs often with brackets and enclosures, then you have at least one bend in the rolling direction. Mark this bend on your drawing, make the radius larger, and talk to the shop before finalizing your design.

Working with a part that has bends in multiple directions? Talk to our team at Premium Parts before finalizing the drawing. Grain direction issues are much cheaper to fix at this design stage. 

Design for Manufacturability (DFM) Rules

Observing the minimum bend radius is critical, but not enough. Here are other considerations that often cause problems in production.

• It is mandatory to have a bend relief for notched corners. In case a bend passes near a cut edge, such as a tab bending up from a notched corner, material can get torn out along the notch line during forming. The bend relief opening is usually made to be the same width as the material thickness, with a little extra extending into the bend line past its tangent. If no bend relief is shown on your drawings for notched corners, manufacturing could provide it without consulting you, or it would return cracked.
• The distance between the bend and the edge should be observed. All holes, slots, or cutouts that are too close to bend lines will deform during the forming operation. In general, the minimum required distance from the edge of a hole to a bend tangent line is 2.5 times the thickness of the material. For slots parallel to the bend line, this number is greater. If a hole must be close to a bend, place it on the outside of the bend rather than the inside.
• A minimum length for the flange also exists. There must be sufficient length of the material for the press brake die to clamp and form the flange. A short flange will result in the material slipping, non-uniform bend angles, and thus rejection of the part. The minimum flange length required is normally four times the material thickness, although this is dependent on the tooling available in that shop.
• Compensation for spring back needs to be considered from the start. Spring back occurs in all metals since there is an elastic portion in the bending process. Higher yield strength metals have a tendency to spring back further. 6061-T6 metal parts, which are bent to a 90-degree angle, will spring back a number of degrees once they are removed from the tooling. To accommodate this springback, overbending is done. The extent of it, however, varies from shop to shop depending on the type of material, lot, and speed of the press.

Need help dialing in springback compensation for a high-strength alloy? Our engineers at Premium Parts can review your bend specs and advise on overbend values before the job goes to press. 

Conclusion

The bend radius chart is the decision-making guide made for use before the design is finalized and tooling is placed into production. The minimum inside bend radius requirement isn’t just a guideline; going under it for a specific material and gauge will increase the risk of cracking that cannot be reliably solved even by changing the tooling configuration. There are many factors affecting one another, such as material selection, temper, direction of the grain, bend relief, and flanges, and a mistake in any of them will cause the need for part rework.

It’s crucial to have the chart available while designing the parts if the team works with sheet metal daily, not when the part has to be inspected. If your part has tight bend radii, specialty materials, or complex forming sequences, contact our team at Premium Parts. We will review the design and flag any issues before production starts. 

FAQs

What happens below the minimum bend radius? 

The outer surface of the bend exceeds the tensile strength of the material. There will be visual cracks on the surface or even an outright fracture of the bend. Even if there are no visible cracks on the surface of the bend, the material will still be weakened and eventually fracture under fatigue conditions.

Can 6061-T6 aluminum be bent to a tight radius? 

This is not reliable at room temperature. For 6061-T6, the minimum inside bend radius is about four times the thickness of the material. This will also require proper grain orientation. In case you have tight bends on your part, try using 5052-H32 or annealing and then heat treating the material.

Does material thickness change the minimum bend radius? 

Yes, greater thickness means greater area that is resisting distortion and thus increased tensile stress on the outer fiber. The minimum bending radius needs to increase accordingly with increasing thickness. Always refer to the chart based on thickness, not just alloy.

Should the bend always be perpendicular to the grain direction? 

Yes, where your design permits. Perpendicular bends will give the smallest radius that is safe and has a minimum chance of crack formation. Where bends need to be parallel to the grain, make the radius 50% larger than the normal value, and preferably consult with experts beforehand. Reach out to our team at Premium Parts, and let our engineers help you.