Choose Press Brake Tooling Forming Aluminium
How To Choose Press Brake Tooling Forming Aluminium
Choosing a Die Width
First, to form the aluminum workpieces you mentioned, you will be air forming. This means the radius generated in the bend is a ratio or percentage of the die opening, known as the 20 percent rule. This is only a label as the percentages vary by material type. The softer the material, the smaller the percentage. And for very soft material, you need to factor in where the bend turns sharp and the punch nose radius. To get deeper into the weeds on this topic, check out “What makes an air bend sharp on the press brake?” from the November 2016 issue, archived at www.thefabricator.com.
That being said, you have two options: (1) Buy quite a few lower dies for every application and desired inside radius or (2) purchase a die width that produces the largest inside radius you need, test-bend the different materials with the die, and record the resulting inside bend radii. The radii values are then used in your calculations or CAD system to generate the bend functions: outside setback, bend allowance, and bend deduction.
Choosing a Die Angle and Die Width
Because you are air forming, and because some of the bends have a reasonably tight bend angle, a 30-degree-included die angle would be an excellent choice, giving you the widest possible range of forming options. Unless you are bottom bending, the included angle of the die will not affect the resulting inside radius.
Again, in air forming, the inside radius forms as a percentage of the die width, or die opening. So you’ll need to have tools capable of producing the smallest and largest inside bend radius required. Depending on the specifications of the parts you are building and the required inside radius, you might need to purchase another die that’s halfway between your smallest opening (which produces your smallest inside bend radius) and largest opening (which produces your largest radius).
Bend angles that exceed 90 degrees complementary will produce an inside radius in the part smaller than the nose radius of the punch. This effect is known as multibreakage. This manifests itself as the inside radius of the material separating from or leading the punch nose through the forming process. It is very difficult to predict, and a test bend for your application may be the best course of action.
If you have a urethane block of midrange durometer—40 to 60 on the Shore A scale—you can use it as a “solid hydraulic” that pushes back against the bend’s leading radius, forcing the material against the radius of the tool and allowing it to take on the radius of the tool. You could also use an old piece of high-pressure water hose laid in the bottom of the die.
Without the urethane backup, the inside radius will continue to collapse until the required bend angle is reached; this is true when forming in relieved dies. Small-radius bends also collapse until reaching a zero radius upon reaching a hem. For bends being formed using standard or aircraft channel dies, the radius will follow the tool radius.
Punch Angle and Radius
Because you are air forming, the radius you achieve is a function of the die opening; the punch itself is only the “pushing unit.” If you follow my recommendation on die selection using a 30-degree-acute die, you will also need an acute punch. In this case, a 28-degree-included angle would be a good choice, because it gives you 2 degrees of clearance between the punch and die faces. The clearance keeps someone from bottoming or side-loading the die and causing it to break.
To achieve consistent, high-quality parts, you need to choose a punch nose radius that’s as close as possible to the naturally floated inside radius of an air formed bend, but without exceeding the floated radius. If your punch nose radius exceeds the naturally floated radius, the material will tend to take on the new, larger radius. For example, if the floated inside radius of your air bend is 2.25 mm, the nearest punch nose radius that is available commercially off-the-shelf is 2.00 mm. This tool will give you the most stable part-to-part bend possible.