The tip radius of the punch

Natural product radius

When air bending sheet metal on a press brake, the formed product radius is not equal to the tip radius of the punch. The product radius is formed naturally and is called the natural product radius. How large that radius becomes depends on the sheet material, the bent angle and the V-opening of the die.  

The formed internal product radius is a percentage of the V-opening. Read more about determining the V-opening of the die.

Optimal radius

But if the V-opening determines the product radius, then what about the tip radius of the punch? If you can estimate the natural radius, you can determine the tip radius.

The optimal tip radius of the punch is as close as possible to the natural radius that will be formed, but it does not exceed it. This creates a stable and predictable process.

If the tool is not available or you want to limit the number of tool changes, you can deviate from the optimal radius.

Radius too small

If the tip radius of the punch is smaller than the natural radius, then you can bend predictably. But be careful not to choose the tip radius too small! The tip will then eat into the material. This results in a less predictable bending angle, markings in the bending line and tool wear.

Example: with steel (90°, V=8xS), the tip will start to eat into the material if the tip radius is smaller than about 2/3 of the natural radius.

Radius too large

When the tip radius of the punch is larger than the natural radius, the material forms around the tip of the top tool. The material may buckle and separate from the tip. This affects the calculated bend deduction in the flat pattern and is usually not desirable.

Radius bending

Do you want to bend with a large radius relative to the sheet thickness? A special bending application is radius bending. With radius bending the radius is indeed formed by the shape of the radius part of the top tool.

Advice

If the tip radius is larger or much smaller than the natural product radius, this can affect the angular accuracy and bend deduction. Marking may also occur in the bending line and the tool may suffer excessive wear. Choose the punch tip radius as close to the natural radius as possible without exceeding it. Use bending simulation software or the WILA Tool Advisor for an appropriate tool advice.