Below, is a portion of “Dissecting Bend Deductions” a chapter in The Art of Press Brake textbookand is available through either a basic or premium membership in TheArtofPressBrake.com This chapter was also the basis of a similarly titled article in the Fabricator Magazine.
At initial publication this textbook has over 60 chapters with new ones being added as time goes on plus it contains 58 movies, videos and films with more coming in the future, (a sample of which can be viewed here).
Also, each chapter includes a printer friendly button at the end of each chapter allowing you to print the chapter.
The Modules are listed in bold along with some of the individual chapter topics, which include:
- The Basics (math, blueprints, measurement tools and Quality Control)
- The Cut (the cut, shear, notching, laser)
- The Holes (punching theory, the punch press, punch tooling, G-codes)
- The Bends (variables, bend functions, springback, 20%rule)
- The Brake (leaf/box brakes, brake types, controllers, punches, dies and more)
- The Roller (slip rolling, plate rolling and leveling)
- Safety (general, laser, brake, OSHA)
A reference library and training manual in one. While many of the links are disabled here they are still noted by highlighting in the standard blue text; on the actual live page these links would work link directly and along with [glossary] mouseover [/glossary] popup definitions and the search option on the side bar makes this text is easy to navigate and/or search for specific topics or information.
Dissecting Bend Deductions
[glossary]Bottom bending[/glossary] and [glossary]coining[/glossary] with the press brake had its heyday. But over the past several decades, [glossary]air forming[/glossary] has become the industry standard. So when air forming, how do you select your bottom die opening? Do you choose a die opening that’s 6 times the material thickness, [glossary]8 times rule[/glossary], 10 times, or even 12 times?
The narrower the die opening, the more tonnage it will take to bend a part. If you’re a new operator, or if you’re worried about exceeding tonnage limits, you might choose an opening that’s 10 to 12 times the material thickness; if you’re not worried about tonnage, you may reach for a die opening that’s only 6 times the thickness.
Almost every press brake comes with a tonnage chart, and many toolmakers publish information on maximum tonnage for every tool they make. You use a formula to calculate tonnage requirements for a specific job to ensure you don’t push your machine or [glossary] tooling load limit [/glossary] beyond what the press brake manufacturer’s rams [glossary]center line load limit[/glossary] specifies. Tool placement on the bed, type of bending operation, and other factors come into play.
But in press brake air forming, the die opening does far more than affect available tonnage per foot. So which is the perfect die opening—6, 8, 10, or 12 times material thickness? A widely held rule of thumb is that 8 times material thickness is a perfect die opening. This is correct, but only when there’s a one-to-one relationship between the material thickness and the desired inside radius. But if you want to put a 3-in. radius into 0.036-in.-thick material, that 8-times-material-thickness rule of thumb just won’t work.
Here’s why. In bottoming, the punch nose radius effectively stamps the material at slightly more than material thickness. In coining, the punch presses with so much force the material thins and actually realigns the [glossary] molecular structure [/glossary]of the metal, which is why coining is rarely performed these days, at least on purpose. In air forming, though, the [glossary] die opening [/glossary] sets the radius of the bend.
This becomes obvious when you see air forming in action. To air form, the punch descends to a certain point; but unlike bottoming, the workpiece doesn’t conform tightly around the punch radius. Rather, the inside radius is produced as a percentage of the die opening. If you change the die opening, you change the inside bend radius you produce—sometimes significantly.
If you change the [glossary] inside bend radius [/glossary], you change the inside radius and therefore the amount of material elongation within each bend; the [glossary] bend deduction [/glossary] (BD) in turn changes your part, and you will then be unable to form the part to the desired dimensions.
The 20 Percent Rule
[Glossary}The 20% rule [/glossary]shows just how dramatically small changes in the die opening affect the resulting inside bend radius……..