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Press brake bending basics: Do we really need more cowbell?

Moving from bottoming to air bending—Part II

Figure 1
These split dies have seen better days.

Last month a reader submitted a question about some of the basics behind punch and die selection. He sent an image of the split die shown in Figure 1. His operation had been bottoming for years, and perhaps coining in some cases.

The reader was hoping to introduce air bending to the press brake department—a welcome move, considering how abusive bottoming and coining can be to tools and equipment.

The situation reminds me of that “Saturday Night Live” skit with Christopher Walken telling Will Ferrell in his band, “I need more cowbell!” The band members stare, dumbfounded. “Guess what?” Walken continues, “I’ve got a fever, and the only prescription … is more cowbell!” Ferrell bangs the cowbell harder and harder, drowning out everything else.

Banging away at workpieces and abusing tools until they look like the one pictured in Figure 1 is like banging away at a cowbell. People are so familiar with their operation, they ignore the fact that they’re pushing machines and tools harder and harder to make it all work. (“I need more cowbell!”) Pretty soon it doesn’t work at all.

Making the move to air bending calls for three steps. First, determine the radii you need to form in your material types and thicknesses. Second, choose the dies and punches to achieve those radii. And third, ensure your press brakes haven’t been irreversibly damaged after years of banging away and, well, giving it more cowbell.

Method Basics

Bottoming and coining are two separate bending methods. Bottoming requires clearance between your punch and die angle. Bottoming occurs when the punch radius is stamped into the part’s inside radius, and as the ram continues to apply pressure, the bend angle is forced to conform to the 90-degree V-die angle.

When bottoming, the punch nose radius should match the radius you ultimately achieve (though springback does still play a role; for more on this, see “The hows and whys of springback and springforward, archived on thefabricator.com), and the die angle determines the bend angle, usually 90 degrees.

Coining forces the punch nose into the material, penetrating the neutral axis. Technically, any radii may be coined, but traditionally coining has been used to establish a dead-sharp bend.

Coining uses a matched tool and die set. The punch nose radius determines your inside bend radius (which, again, is usually very small); you calculate the outside bend radius by adding the inside bend radius to the material thickness. The radius at the bottom of the die should match the outside radius of the part.

The reader from last month’s column wished to introduce air bending for good reason. Air bending is the method of choice these days. The punch descends into the die opening, and a naturally floated radius is formed as a percentage of the die width (see Figure 2).

Figure 2
The bend radius in air forming is “floated” as a percentage of the die width.

While the die width is critical, the included die angle has no direct effect on the bend angle. The die angle matters only as it relates to the angle of the punch, which should be the same or narrower (that is, have a smaller included angle) than the die.

Die Considerations

The tonnage required to bottom-bend was the root cause for most of the damage seen in Figure 1. Over time, continually bottoming tools will itself be the root cause of excessive wear and tear to tooling. And it’s highly likely that the press brake itself was damaged. Of course, considering the situation, operators had no choice but to bottom-bend. When tools are as old and worn as those in Figure 1, bottoming is really the only viable option to maintain consistency.

Press brake dies have three radii: two at the top corners and one at the bottom of the V. The top radii are either sharp edges, with a very small radius like 0.015 in., or a compound radius that helps reduce drag (the scraping of the material) as the workpiece is pulled into the die space. The radius at the bottom of the V is usually quite sharp, like 0.015 in. But again, the radius at the bottom of the V has no effect in air forming.

Calculating the Air-Formed Radius

You calculate an air-formed inside bend radius using the 20 percent rule. Named after the percentage used for certain grades of stainless steel, the actual baseline we use is 15 to 17 percent for 60-KSI cold-rolled steel. Start with the median (16 percent) until you find the value that best matches the material characteristics you receive from your metal supplier.

To calculate the percentage to use for other materials, compare the tensile strength with the baseline. For example, 120-KSI material would air-form its radius twice the value of 60-KSI cold-rolled steel, or between 30 and 34 percent of the die width.

If you know the die width, you can calculate the inside bend radius. If you’re bending 0.125-in.-thick A36 mild steel in a 1-in. die and using the average of 16 percent (per the 20 percent rule), you will achieve an inside bend radius of about 0.160 in.

You may be using the 8x rule; that is, eight times the material thickness will give you your perfect die width. (For more on this, see “6 steps to successful die selection for press brakes,” archived at thefabri cator.com.) The 8x rule can work well in the reader’s application, whether he jumps to air forming or continues bottoming.

Just know that in an air form, the 1-in. die would float a 0.160-in. inside radius—perfectly fine, as long as it meets customer requirements (or if the customer doesn’t care about the inside radius). Just use the 0.160-in. inside bend radius for your calculations for bend allowance, outside setback, K-factor, and bend deduction.

Achieving Air Bending Perfection

If your inside radius is as close to perfect as possible—that is, the inside bend radius equals the material thickness—you will achieve the most consistent bends and the most consistent results. If your customers’ prints allow for it, this perfect bend relationship is your best option.

To achieve this, you can determine your optimal die width for that perfect inside radius simply by looking at available die widths, dividing them by 16 percent, and choosing one that gets you as close to perfect as possible, where the inside bend radius and material thickness are the same.

In the current application, with 0.125-in.-thick material, the closest to perfection for metric dies would be 18 mm (0.707 in.), a very common die width in the precision-ground families of tooling. Sixteen percent of 0.707 equals 0.113 in. This value is as close as you can get to the perfect inside radius of 0.125 in. The closest imperial die width would be 11/16 in. (0.687 in.) or ¾ in. (0.750 in.).

Once you determine the tools you need, try to invest in the greatest number that your budget allows. How you select a die width depends on what you have currently and your tooling budget. If your budget allows for only two die widths, you would want to pick one nominal die width value that can accommodate a range of material thicknesses, say, between 0.125 and 0.250 in. You then would pick a second nominal die width to accommodate material from 0.250 to 0.375 in.

Die angles do not affect the final bend angle in air forming. If you want to ensure that operators cannot even attempt bottoming, choose an included die angle less than 88 degrees, or even channel dies with perpendicular sides.

Once you have your dies for air forming, you now can determine the inside bend radius you will achieve. As long as you use that die over a small range of material thicknesses, that radius value should hold true. If you use a 1-in. die to bend 0.125-in.-thick material, your resulting radius should be about 0.160 in. But if you use material thicknesses outside what is appropriate for the die width, your results will vary from that 0.160-in. value.

Punch Nose Considerations

In bottoming and coining, the punch nose radius produces the inside bend radius. In air bending, the punch nose doesn’t determine the radius, but it does apply force, which increases significantly the narrower the punch tip becomes.

A bend “turns sharp” when the material can no longer withstand the force being applied to it and starts to crease along the bend line. This happens when the force (tonnage) required to form the workpiece exceeds the force required to pierce and crease it. For more on this, check out “What makes an air bend sharp on the press brake,” archived at thefabricator.com.

The crease of a sharp bend will tend to amplify the usual angular variations in the bend angle caused by the variables within the material, including grain direction and hardness. These variables cause the bend angles along with the dimensions to change from part to part.

When a punch nose radius exceeds the floated radius of the air-formed part, the part will take on that larger radius. This changes your bend allowance, outside setback, and bend deduction calculations.

Styles of Punches and Dies

While there are plenty of tooling style options, we will be discussing the four main contenders. These four can be grouped into two categories, precision-ground and planed tooling. Yes, plane tooling is cheaper to purchase upfront, but the labor cost incurred in its use will eat up that savings very quickly.

Precision-ground tools come in two styles, European and New Standard. These are commonly manufactured to an accuracy of ±0.0008 in. on all surfaces. Planed tooling comes in two styles, traditional planed and precision-planed, both of which on average are manufactured to an accuracy of ±0.005 in. over 10 feet from a single X-Y coordinate. That’s pretty good. Still, this means a total error between tooling centers could be as much as 0.010 in.

This coordinate reference makes planed tools much more difficult to work with. No two pieces are alike. Each has a slightly different centerline, and sometimes even a different tool height. Maybe it’s only a few thousandths, but that’s enough for several degrees of angular difference from tool section to tool section.

If you cut your tools to the length you need, and then don’t remate them cut for cut and facing the same direction (front to back) when installed together on the press brake, they don’t work. Tools must be kept in the order they were cut when being installed together in the machine. This inability to mate different tools makes building parts requiring multiple tool sets (staging) a very difficult if not impossible operation.

Still, planed tools are great if you are using them as a single piece of tooling and have little if any staging. They cost less than precision-ground tools, they can deal with high tonnage loads, and a lot of machines are designed to use them (though adapter blocks are not always a practical solution).

Note that precision-planed tools are not at first what they seem. These are no more accurate and suffer all the same issues as traditional planed tooling. They just come in European tool profiles. Also, there is a difference between “precision planed” and “precision tooling” ground and traditional profiles. The biggest telltale sign that you’re looking at true precision-ground tooling is this: The tool does not come in long lengths, such as more than 3 ft.

The severe tooling damage shown in Figure 1 points to another problem: How badly hammered is the press brake that used this tool? If it is old and beat up, the benefits derived from precision-ground tooling will be lost.

Whether the brake somehow made it safely through years of bottom bending or you’ve just bought a new brake, deciding whether to invest in traditional planed or precision planed depends on your product mix.

Precision-ground tools do not suffer from the variation that planed tools introduce. This makes precision tools much easier to use. Their precision allows for some very sophisticated setups in the press brake. If you’re moving toward precision work, a precision-ground tool is more than likely the better choice. Then again, there’s the cost—substantially more than planed tooling.

As an aside, what the industry really needs to use is a true precision-ground punch and die set but in traditional planed styles and profiles. The set should fit regardless of the mounting profile, and the sets should have the same shut heights and centers as their precision cousins. That means they could be mixed and matched as needed with the same setup. It would really be the best of both worlds.

Making the Move

Regardless of whether we will ever see such a mythical tool as the one just described, or if we continue with the same tools that can be bought today, one thing is for sure: When transitioning to air bending, take your time, use the tips I have offered here and from others, and think it through.

If there’s one takeaway from all of this, it’s the following: Press brake tooling is a consumable. There is no need to fight worn tooling or antiquated methods; you’re just doing more of the same, giving it more cowbell. There’s no money in it, only headaches.

Steve Benson is a member and former chair of the Precision Sheet Metal Technology Council of the Fabricators & Manufacturers Association International®. He is the president of ASMA LLC, steve@theartofpressbrake.com. Benson also conducts FMA’s Precision Press Brake Certificate Program, which is held at locations across the country. For more information, visit www.fmanet.org/training, or call 888-394-4362. The author’s latest book, Bending Basics, is now available at the FMA bookstore, www.fmanet.org/store.

Air Forming Transition Tips

• If you want to be sure bottoming cannot be attempted, choose an included die angle of less than 88 degrees.

• Invest in the biggest selection of tools that your budget allows.

• Assign small ranges of material thicknesses for each die width, and select each die width based on the median in each thickness range.

• If customer requirements allow, use the die width that causes the floated radius in the material to be as close as possible to the material thickness.

• Whatever the measured inside radius is after the workpiece is bent and released from forming load, use it when calculating K-factors, bend allowances, outside setbacks, and bend deductions.

About the Author
ASMA LLC

Steve Benson

2952 Doaks Ferry Road N.W.

Salem, OR 97301-4468

503-399-7514

Steve Benson is a member and former chair of the Precision Sheet Metal Technology Council of the Fabricators & Manufacturers Association. He is the president of ASMA LLC and conducts FMA’s Precision Press Brake Certificate Program, which is held at locations across the country.