How to Avoid an Out of Spec Angle Bend in Sheet Metal

If you're working with sheet metal, you are very likely to add a bend to your part. Bending is one of the most common sheet metal fabrication processes, and it is very easy to have an out-of-spec angle bend.

A specification of some sort will exist for the part you’re making. It may be as informal as a visualization of the part in your mind, or as formal as a manufacturing drawing or CAD model published to multiple manufacturing sites. Understanding the variables that affect an angle bend and how they can influence the quality of a bend is critical to ensuring you create a product without an out-of-spec angle bend.

Grain Direction

Sheet metals have grains. These have a direct impact on the bending process and the quality of a bend. The grain direction results from the mill’s rolling process, and grains are parallel to the rolling direction. Rolling sheet metal results in the directional alignment and stretching of metallurgical structures, forming a grain.

Bending along the grain will be easier and requires less bending force, but it is also very likely to induce cracks on the outside radius of the bend. The cracking can be reduced by increasing the bend radius. Bending transverse to the grain will require more bending force, but a tighter bend radius can be achieved before cracking appears on the outside bend surface.

Minimum Bend Radius

The minimum bend radius is a very complex variable that directly affects the quality of a bend. Not only does it vary by material type (e.g. steel, aluminum, stainless steel, etc.) but also by sheet thickness and heat treatment. A material specification sheet, available from the raw material supplier, should contain minimum bend radius advice for bends parallel and transverse to the grain.

Note that the recommended minimum bend radius will likely be different whether you’re bending parallel or transverse to the grain direction. Each material will have a minimum radius-to-thickness ratio for each grain-to-bend orientation. The fundamental principle is that thicker, harder materials will have a larger bend radius.

Springback

Anyone who’s ever bent sheet metal knows that the bend opens up, resulting in an out of spec angle bend. This can be frustrating, and understanding why this happens can help you compensate for it.

Springback is caused by two main factors:

1. The displacement of molecules in the material.
2. Stress and strain differences between the inner and outer bend surfaces.

When bending sheet metal, the material is divided into two layers separated by a neutral line. As the material is bent the neutral line moves and is no longer in the middle of the material thickness.

Figure 2 shows a detailed section of the stress in a bend. The inside of the bend will experience compression and the outside tension. Because a material’s compressive strength is greater than its tensile strength, the bend will tend to open after bending. The neutral line move is compensated for with a correction factor that will be discussed in the next section.

Various material properties affect springback. Springback will be higher:

• For higher yield strength materials
• Where a larger minimum bend radius is required
• The greater the bend radius to material thickness
• For a larger die opening when air forming a bend

It is possible to compensate for springback. A range of equations incorporating material thickness, bend radius, and material springback factor can be used to accurately predict the amount of springback. The equations are complex and will require additional reading to understand how and when to use them.

Correction Factor (k-factor)

Bending stretches the material. This means that the neutral line in the bend is no longer in the middle of the part. The problem with this is that the bent part will be a different size compared to the part’s flat pattern. To compensate for the movement of the neutral line a correction factor (known as a k-factor) is used.

The k-factor is an empirical constant and is specific to a material, its thickness, the bend radius, and the bending method used. The k-factor is a bending allowance that results in a flat pattern that better reflects reality.

Table 1 shows a “rule-of-thumb” guideline for different materials, their hardness, bending method, and bend radius. A quick internet search will find k-factors for other material types.

If you have access to CAD software that has a dedicated sheet metal module, use it to design your bent part. It will already have k-factors loaded and will automatically compensate for bend stretching.

Bend Radius to Material Thickness Ratio

The closer the bend radius value is to the material thickness, the closer the springback experienced will be to the material’s natural springback. But when the bend radius becomes very large – eight times the material thickness or more – springback increases drastically. Generally speaking, at a 1:1 ratio the expected springback is as follows:

• 304 stainless steel: 2–3°
• Cold rolled steel: 0.75–1°
• Hot rolled steel: 0.5–1°
• Copper and brass: 0–0.5°

As an example, a 0.8 mm mild steel sheet with a 0.8 mm bend radius has a springback of 0.5–1° whereas the same thickness sheet with a ±68 mm bend radius can have springback as much as 30°.

Localized Stress

Localized stress along or inside the bend’s radius area can result in an out of spec angle bend or a limit to the size of the bend radius. High-heat cutting processes, like laser cutting and flame cutting, produce a hardened edge that can influence the bend. Remove these hardened edges and sharp corners, and dress sheared edges to reduce or remove microfractures in critical areas.

Keeping a close eye on these variables and fully understanding their impact on the quality of a bend will give you the best chance of keeping your bends within their design specification. Xometry’s team of specialists can provide you with the best advice on part design, fabrication, and quality control.