Press brake basics: Bending questions from around the globe

The FABRICATOR is a U.S. publication, but thanks to the Internet, it’s a global one too. Over the years I’ve received many reader questions, and a lot of them come from elsewhere in the world. This month I thought I would share a few of them, with a final question from the U.S. thrown in for good measure.

Thanks to all for the continued feedback, and keep the questions coming!

Bending Thick in Australia

Question: I read your great article, Calculating bend allowance, outside setback, and bend deductions, from November 2012. Can these calculations be applied to the bending of mild steel plates 10 to 20 mm (0.394 to 0.787 in.) thick, or does it apply only to sheet metal? I’m bending steel plates for pump bases and have to work out the flat pattern. Typically, the bends are 90 degrees.

Answer: Mathematically and practically, the formulas apply. The real trick is knowing what the inside radius is going to be; get that correct and the math works.

Specifically, you need to correctly estimate what the inside radius is going to be and use that value in the calculations. If you are air forming, the radius is produced as a percentage of the die opening. The results you attain are affected greatly by the punch nose radius.

A perfect bend is one in which the radius and material thickness are one and the same. If the punch nose radius is significantly less than the material thickness, the force the punch exerts may be so great that it creates a crease along the bend line, creating a sharp bend. You will need to have a good understanding of what makes a bend sharp.

To ensure you always work within the tonnage limits of your press brake and tooling, first calculate how much tonnage you need. Second, identify your tooling load limits. Third, calculate the sinking tonnage limit, which, if exceeded, can embed tooling into the ram or bed. Fourth, determine your press brake’s centerline load limits.

For complete information on this topic, check out several articles archived on thefabricator.com: The four pillars of press brake tonnage limits (April 2015); How an air bend turns sharp (May 2015); and Forming aluminum on a press brake: Bending soft, not sharp (June 2015).

I also invite you to review the four-part series, “A grand unifying theory of bending,” which ran in September, October, November, and December 2015. It’s a new theory of bending, altering some long-held labels and definitions and introducing new formulas. The traditional definitions worked well, but these new definitions and formulas may help a press brake technician be even more accurate when predicting how a part will form.

For this new theory, we keep our definition of a sharp bend, which on average has a radius that’s 63 percent or less of the material thickness. We then add two new terms. A perfect bend starts at an inside radius larger than 63 percent of the material thickness and extends up to 125 percent. At the median, the perfect bend has the ideal 1-to-1 inside bend radius-to-material thickness ratio. That is, the material thickness and inside bend radius are the same.

A radius bend is defined as any bend with an inside radius greater than 125 percent of the material thickness. It requires careful calculation, not only because of significant springback, but also because of the tooling it requires. This includes the use of relieved dies.

Tonnage Considerations in India

Question: While searching on the net for information on press brakes, I came across your article on bending radius and force calculations.

We are considering the purchase of a 300-ton press brake, and I am not clear about the tonnage requirement for a bending job that involves ST46 grade, 10-mm-thick HR material with a bend that’s 2,760 mm (109 in.) long. We need to form 90-degree bends with a V-die width of 80 mm (3.15 in.).

As per the bending force calculations given by the machine supplier, the pressure requirement works out to be 277 tons, which gets us close to the machine’s capacity. For most jobs, however, this machine will be processing material that’s only 1.5 mm (0.059 in.) thick.

Will the machine’s 300-ton capacity suffice for our application, and how will such a machine perform over the long term? If we go for a 400-ton machine, will it affect the bending accuracy of thinner components, such as the 1.5-mm-thick material? Also, do you recommend hydraulic dynamic crowning of the lower beam, and is it really effective compared to motorized crowning?

Answer: First, I recommend you consider a 400-ton machine. The 277 tons of required force would seriously approach the limits of the 300-ton machine. Plus, having the extra tonnage gives you the ability to increase the tonnage load if needed. You never want to run any machine near, at, or over the rated tonnage.

I would also seriously consider the dynamic crowning feature if the parts you are producing are greater than 60 percent of the distance between the side frames.

Make sure whichever brand of machine you purchase that the tonnage requirements do not exceed the rated centerline load limit of the ram. If you exceed that limit, you will permanently bend the ram and bed; this ram upset will ruin your new press brake.

In summary, I would recommend the 400-ton machine with dynamic crowning.

Forming Offsets in Mexico

Question: I’m from Torreon City in Mexico, and I just finished reading the article “Strategies for forming offsets.” We manufacture a part with a forming offset, but we don’t know how to calculate the force. We have a 350-ton press brake. Are the force calculations the same as they are for air bending, or are there different calculations to consider?

Answer: Required tonnages for offsets vary greatly when using offset tools, mainly because they can involve either air forming or bottoming. Bottoming produces the best results but requires a lot of tonnage. Air forming requires much less tonnage, but the final bend will take on more of a Z shape rather than a true 90-degree offset. Especially when bottoming with offset tools, your tonnage calculations need to take extra force requirements into account.

Start by calculating for a single bend in mild cold-rolled steel like A36, with a 60-KSI tensile strength. Factor that value by material type—for example, 1.4 for 304 stainless. Determine the material factor by comparing your material’s tensile strength with the 60,000-PSI tensile strength of the baseline material. If the material has a 120,000-PSI tensile strength, your material factor would be 2.

Next comes the method factor. If you’re air bending, you don’t need to incorporate a method factor. If you are bottoming with your offset tool, you need to multiply by a factor of 5.

Next comes the multiple-bend-tooling factor. If your material is less than 0.250 in., multiply the tonnage calculations by 5; if thicker, multiply by a factor of 10.

You need the die opening value to calculate the tonnage; this is not the size of the offset—that is, its depth. For the tonnage calculations, it’s the width of the opening that matters, and you measure it the same as you would any other die opening, from one top corner to the other.

All this gives us the variables we need to plug into the following formula (all dimension measurements are in inches):

A point of note, in the formula above, the material thickness must be squared before multiplying it by the 575 constant if you want the answer to be correct. Also note that this tonnage calculation produces only an estimate of the tonnage that may be required.

With and Against the Grain in the U.S.

Question: I have been reading some of your articles on metal bending, and I was wondering about your expert recommendation on bending relative to the grain direction. From what I read, it is recommended to bend perpendicular to the grain for material 0.25 in. and thicker. I just need a good technical explanation of when to bend against the grain direction, when it’s not necessary, and why.

Answer: The answer to your question is … there is no absolute answer. It’s more of a general rule. When bending plate 0.25 in. or thicker, grain direction is an issue particularly in low-quality steels in which you need to form a small inside bend radius. The tighter the radius, the greater the stress on the outside surface, which in turn pulls the grains in the material apart and causes cracking on the surface.

The grain direction issue is more of a poor-quality material issue. Generally, you will not have such problems when bending materials like stainless and high-quality steels. Still, even with high-quality materials, grain direction can be an issue at the press brake if it varies from part to part—that is, if you form the same flanges but each has a different grain direction, which in turn changes your bend angle from part to part.

On thin material and aluminums, grain splitting on the outside of the bend can be the result of the finish grain. If it’s deep, the grains tend to pull apart.

My advice: Do not bend with the grain in low-quality steels. Instead, try to bend across or diagonal to the grain, and try to keep the grain direction consistent across the run of parts.