Bending Plate

October 2012

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In the plate fabrication industry, I’ve noticed a shift towards bending thicker material, and at the same time, there has been a definite increase in the use of high tensile-strength materials that have high strength but are thinner, but still pose issues for bending.

How do we manage the large variety in material thicknesses and tensile strengths?

Is it possible to get good bend results without endless trial bends?

We can differentiate between the following thickness groups: thin sheet is greater than 0.5 mm to 3.0 mm, thick sheet is larger than 8.0 mm to 15.0 mm and plate is greater than 15 mm up to 120 mm. Everything over 0.250 in. is called plate in the U.S.

Then there are the different material types: cold-rolled steel, coated materials, hot-rolled steel and high-tensile-strength steel.

I’ll discuss the bending techniques and problems of thick sheet or plate in hot-rolled and high-tensile-strength form.

Parts made from plate or high-tensile-strength materials are mostly expensive parts. They have already cost considerable amounts of money to cut and basically are used in high-end equipment. Most of the time, re-working bent parts, re-bending, flattening, etc. are not an option. The scrap bin is the end destination for thick parts that were not correctly bent. Therefore, it’s more important to make good parts from the start.

So what are the basic problems bending plate material?

Material concerns

Problem:There are different tensile-strength zones within a piece part. Typically an operator recognizes this when his parts come in all colors of the rainbow.

Effect: Bend angles depend on where the part is bent. If the bend is across the different zones, you’ll see varying angles on the same bend. Along the zones, the result per bend will vary.

Problem: Springback is a concern. Although all press brake controllers allow corrections today, these corrections are always made after the part has been bent. There’s no guarantee that the next part behaves the same way. However, springback doesn’t greatly vary throughout a material batch.

Effect: Good material has a springback that doesn’t vary that much. Usually an angle correction will suffice. However, predicting springback isn’t easy. Most of the time corrections are one bend behind.

Problem: Sheet thickness has variations that causes the same problems as springback.

Effect: Differences in sheet thicknesses directly results in bend-angle variation. This also results in more or less bending force, which results again in different deformation of the machine, the crowning for example.

Problem: Heat stress builds up along the material’s edges, the result of flame, plasma or laser cutting.

Effect: Right at the beginning of the bend, the stress brought into the material by the cutting procedure releases, resulting in strong longitudinal bows.

Problem: Different surface hardness on the top and the bottom of the parts is the result of the cooling procedure during the steel manufacturing.

Effect: The effect is different bend results depending on the bend direction.

Problem: Bending along or against the grain direction gives different end results.

Effect: The direct effect is a change in bend radius, resulting in different bend angles at the same punch position. Also the springback varies.

Problem: Edge bulging happens when the bend on the outside of the sheet is stretched and the inside compressed. The bend length over the bend part gets shorter on the outside and longer on the inside.

Effect: Your part is no longer flat. It can’t be flush mounted. Because of the outside bend line being stretched and the inside compressed, your parts start to bow.

Part size

Problem: Handling: Parts are heavy and sometimes dangerous to manipulate

Effect:“Hanging down” causes extreme stress on machines and tools. It also influences the bend result. In the worst case a secondary bend is formed just after the die radius.

Machine and tool

Problem: The punch radius is smaller than the material thickness for mild- and structural-steel plate.

Effect:Punch carving: The tip of the punch makes a groove in the sheet. The depth of this groove is difficult to predict and can hardly be compensated for. In the worst case, the sheet might form micro cracks and the part is structurally compromised.

Problem:The punch radius is smaller than the specified minimum. Especially for high-tensile-strength steels.

Effect: Parts crack or split in two.

Problem:The punch radius is too big. Especially a problem with high-tensile-strength steels. At the end of the bend there is a gap between the top of the sheet and the tip of the punch.

Effect: Punch lifting: A space is forming between the tip of the punch and the top of the material during the bend. Parts are slightly over bent. It also increases the necessary bend force.

Problem: The V-opening is too small. For high-tensile-strength steels, it’s important to keep to the manufacture’s recommendations. Where you would normally use six to eight times the material thickness for a die opening, you might need a die opening that is 20 times the thickness.

Effect:Extreme load on the die radius and possible part cracking. Increases the “punch lifting” effect.

Problem: The bigger the part, the more your machine deforms. You need to automatically compensate for these deformations to concentrate on the real material problems.

Effect: Parts with different lengths or bending through holes alter the bend force. This results in machine deformation that needs to be compensated for to keep the bend results stable.

Problem:The deformation of the upper beam needs to be compensated for by your crowning system, ideally in real time and automatically.

Effect: Often the material properties change from one blank to the next. The brake’s crowning system needs to directly react to these changes to avoid bend angles varying over the length of the part.

If the machine needs a lot of crowning, it might result in part bowing or permanent deformation of the upper or lower beam.

These are the main problems in bending plate and high-tensile-strength materials, which are quite a lot for an operator to handle. Some can be worked around or solved. Some you’ll just have to live with.

Let’s have a look at the problems that can be solved by not creating them in the first place. This starts in your production office. You can solve some of them easily. For instance, design your parts with the bending operation in the foreground. Develop your blanks using the right tools. Check out the material constructor’s recommendation. Ask your press-brake-manufacturer’s application department for help.

Material problems are a different matter. We’ll look at each problem and propose a solution.

1. Cut a narrow band of material just after the part. Make your test bends in this narrow band and use the corrections on the long part. Not all machines have this capability however.

Another option is to use an angle measuring system.

2. Springback can be calculated. For mild steel and a 90-degree bend, you can use the following formula: SP =((V/th)*0.2262)+0.4452 where “SB” is springback in degrees, “V” is V-opening and “th” is the material thickness, both in mm. If your part shows a different value, there is another machine or material deviation causing this. You would need to correct accordingly.

There are many angle measuring systems on the market. Some of them automatically compensate for the measured springback. For plate, laser-based systems are easier to use then mechanical ones.

3. Sheet thickness variations result in angle variations one-to-one. Today most controllers can be connected to digital calipers, either by wire or via bluetooth. High-end machines can measure the thickness in-process and compensate in real time during the bend.

4. If there is a lot of heat placed into the material during cutting, any stress is released at the beginning of the bend. The result is a bow. The amount of bowing depends on the thickness, tensile strength, amount of heat brought into the part and the distance of the first bend from the edge. There’s not a lot that can be done about it apart from re-thinking your parts.

If it’s possible to add a bend close to the edge in the other direction, you can avoid these bows. This anti-bow bend does not need to be 90 degrees. And even in the same direction, it might give you less bow.

5. During the sheet-manufacturing process, cooling on the top and bottom of the sheet is not always even. This leads to faster cooling on the top (mostly), resulting in a slightly harder surface compared to the other side. Coiling plate doesn’t help either. The plate is still very hot during the coiling process. During de-coiling, the top surface is pushed together and the bottom surface is stretched. This results in different bend angles at the same penetration depth as shown in this drawing.

Another option is to use an angle measuring system.

6. Grain direction doesn’t change the tensile strength, so springback doesn’t vary a lot just because of the grain direction. What does change is the force needed to bend the part, up to 15 percent. Because of this, you need to adapt your crowning and other force compensation depending on where the bend is applied.

Another not very well understood effect is the change in the inside radius. With the same penetration depth (the machine doesn’t t know the grain direction) a different inside radius results directly in another angle.

Avoid 80 percent of this problem by using punches with one and a half to two times the sheet thickness as a radius.

7. Edge bulging is a purely mechanical phenomenon. When you push material on one side, you stretch it on the other, and something has to give.

Bulging doesn’t influence the bend results, but it sometimes does make assembly difficult. It’s easily avoided by cutting a bend relief radius.

8. “Hanging down” can deform your parts. Heavy, long parts that are bent very close to the edge might deform just after the die radius. Reducing the bend speed doesn’t always give the desired results. A robot or sheet supports might be your solution.

9. If you’re bending plate, your radius must be at least the material thickness. Anything smaller might result in cracking or denting. Your parts might lose their mechanical strength without you noticing it.

The positive side of using appropriate punch radii is that your bend results are more stable. Grain direction and tensile strength variations cause less of a radius difference if your radius is bigger than the material’s natural radius. One to two times the thickness is a good rule.

10. For high-tensile-strength steel, the radius must be even bigger. Some materials, like Hardox 500 or Armox, prescribe up to nine times thickness as a punch radius and a V-opening of 18 times the thickness.

11. Using a 2 in. radius on all your parts is not going to solve all your problems. For the hard materials especially, you might experience “punch lifting.” Of course the punch itself doesn’t lift, but there is a gap between the tip of the punch and the top of the sheet. This gap can’t be calculated, and it directly influences the bend angle. In extreme cases the material might even collapse.

12. Using a V-opening that’s too small increases the force you need to bend your parts, and it has very similar effects as point number 10. For mild and structural steel, use eight to 10 times the thickness as a V-opening. For high-tensile-strength materials, check the manufacturer’s recommendation. This might be up to 18-times sheet thickness.

Another problem is the radius wear. Bending high-tensile-strength sheet ruins your dies pretty quickly.

Use dies with hardened radius bar inserts. These are easy to change and a lot less expensive as the complete die. Also use as much fractionized tools as possible. It is easier on the operators and the wear is more evenly spread throughout the too’sl life cycle.

13. If you bend a small part, you’ve wasted a couple of dollars. Over-bend a trailer beam and you’ve wasted hundreds of dollars.

14. Apart from all the material inconsistencies, your machine is bending, twisting, opening, bowing and yawing too.

If your press-brake controller checks and compensates for all the machine deformations, you can concentrate on the material. We call this a “neutral” machine. The machine is not adding any errors to the part. One particular manufacturer invented and patented a method of compensating all machine deformations — side-frame deflection, upper and lower beam crowning, material position and thickness, even side-frame elongation caused by temperature differences.

There are a number of instances where angle-measuring systems are mentioned as a solution. Is it the ultimate solution to all problems? No, certainly not. They are a high investment, it slows your processes down, they’re fragile, and they’re only useful if the machine itself adapts for the differences in material properties, side-frame deflection, crowning etc. This means that you must carefully consider if such a system is the solution for you and your production.

Another point is tool changing. Just changing to the right tools makes your bend results more stable. This means a multitude of upper tools. Or at least radius inserts. There are some clever systems on the market that make tool changing easy and keep the investment low.

Bystronic

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