Question: I’ve been struggling to understand how the bend radius (as I pointed out) in the print relates to tool selection. For example, we are currently having issues with some parts made from 0.5″ A36 steel. We use 0.5″ diameter punches for these parts. radius and 4 inches. die. Now if I use the 20% rule and multiply by 4 inches. When I increase the die opening by 15% (for steel), I get 0.6 inches. But how does the operator know to use a 0.5″ radius punch when printing requires a 0.6″ bend radius?
A: You mentioned one of the biggest challenges facing the sheet metal industry. This is a misconception that both engineers and production shops have to contend with. To fix this, we’ll start with the root cause, the two formation methods, and not understanding the differences between them.
From the advent of bending machines in the 1920s to the present day, operators have molded parts with bottom bends or grounds. Although bottom bending has gone out of fashion over the past 20 to 30 years, bending methods still permeate our thinking when we bend sheet metal.
Precision grinding tools entered the market in the late 1970s and changed the paradigm. So let’s take a look at how precision tools differ from planer tools, how the transition to precision tools has changed the industry, and how it all relates to your question.
In the 1920s, molding changed from disc brake creases to V-shaped dies with matching punches. A 90 degree punch will be used with a 90 degree die. The transition from folding to forming was a big step forward for sheet metal. It’s faster, partly because the newly developed plate brake is electrically actuated – no more manually bending each bend. In addition, the plate brake can be bent from below, which improves accuracy. In addition to the backgauges, the increased accuracy can be attributed to the fact that the punch presses its radius into the inner bending radius of the material. This is achieved by applying the tip of the tool to a material thickness less than the thickness. We all know that if we can achieve a constant inside bend radius, we can calculate the correct values for bend subtraction, bend allowance, outside reduction and K factor no matter what type of bend we are doing.
Very often parts have very sharp internal bend radii. The makers, designers and craftsmen knew the part would hold up because everything seemed to have been rebuilt – and in fact it was, at least compared to today.
It’s all good until something better comes along. The next step forward came in the late 1970s with the introduction of precision ground tools, computer numerical controllers, and advanced hydraulic controls. Now you have full control over the press brake and its systems. But the tipping point is a precision-ground tool that changes everything. All the rules for the production of quality parts have changed.
The history of formation is full of leaps and bounds. In one leap, we went from inconsistent flex radii for plate brakes to uniform flex radii created through stamping, priming and embossing. (Note: Rendering is not the same as casting; see the column archives for more information. However, in this column, I use “bottom bending” to refer to both rendering and casting methods.)
These methods require significant tonnage to form the parts. Of course, in many ways this is bad news for the press brake, tool or part. However, they remained the most common metal bending method for nearly 60 years until the industry took the next step towards airforming.
So, what is air formation (or air bending)? How does it work compared to bottom flex? This jump again changes the way radii are created. Now, instead of punching out the inside radius of the bend, the air forms a “floating” inside radius as a percentage of the die opening or the distance between the die arms (see Figure 1).
Figure 1. In air bending, the inside radius of the bend is determined by the width of the die, not the tip of the punch. The radius “floats” within the width of the form. In addition, the penetration depth (and not the die angle) determines the angle of the workpiece bend.
Our reference material is low alloy carbon steel with a tensile strength of 60,000 psi and an air forming radius of approximately 16% of the die hole. The percentage varies depending on the type of material, fluidity, condition and other characteristics. Due to differences in the sheet metal itself, the predicted percentages will never be perfect. However, they are pretty accurate.
Soft aluminum air forms a radius of 13% to 15% of the die opening. Hot rolled pickled and oiled material has an air formation radius of 14% to 16% of the die opening. Cold rolled steel (our base tensile strength is 60,000 psi) is formed by air within a radius of 15% to 17% of the die opening. 304 stainless steel airforming radius is 20% to 22% of die hole. Again, these percentages have a range of values due to differences in materials. To determine the percentage of another material, you can compare its tensile strength to the 60 KSI tensile strength of our reference material. For example, if your material has a tensile strength of 120-KSI, the percentage should be between 31% and 33%.
Let’s say our carbon steel has a tensile strength of 60,000 psi, a thickness of 0.062 inches, and what’s called an inside bend radius of 0.062 inches. Bend it over the V-hole of the 0.472 die and the resulting formula will look like this:
So your inside bend radius will be 0.075″ which you can use to calculate bend allowances, K factors, retraction and bend subtraction with some accuracy – i.e. if your press brake operator is using the right tools and designing parts around the tools that operators are used.
In the example, the operator uses 0.472 inches. Stamp opening. The operator walked up to the office and said, “Houston, we have a problem. It’s 0.075.” Impact radius? Looks like we really have a problem; where do we go to get one of them? The closest we can get is 0.078. “or 0.062 inches. 0.078 in. The punch radius is too large, 0.062 in. The punch radius is too small.”
But this is the wrong choice. Why? The punch radius does not create an inside bend radius. Remember, we’re not talking about bottom flex, yes, the tip of the striker is the deciding factor. We are talking about the formation of air. The width of the matrix creates a radius; the punch is just a pushing element. Also note that the die angle does not affect the inside radius of the bend. You can use acute, V-shaped, or channel matrices; if all three have the same die width, you will get the same inside bend radius.
The punch radius affects the result, but is not the determining factor for the bend radius. Now, if you form a punch radius larger than the floating radius, the part will take on a larger radius. This changes the bend allowance, contraction, K factor, and bend deduction. Well, that’s not the best option, is it? You understand – this is not the best option.
What if we use 0.062 inches? Impact radius? This hit will be good. Why? Because, at least when using ready-made tools, it is as close as possible to the natural “floating” inner bend radius. The use of this punch in this application should provide consistent and stable bending.
Ideally, you should select a punch radius that approaches, but does not exceed, the radius of the floating part feature. The smaller the punch radius relative to the float bend radius, the more unstable and predictable the bend will be, especially if you end up bending a lot. Punches that are too narrow will crumple the material and create sharp bends with less consistency and repeatability.
Many people ask me why the thickness of the material only matters when choosing a die hole. The percentages used to predict the air forming radius assume that the mold being used has a mold opening suitable for the thickness of the material. That is, the matrix hole will not be larger or smaller than desired.
Although you can decrease or increase the size of the mold, the radii tend to deform, changing many of the bending function values. You can also see a similar effect if you use the wrong hit radius. Thus, a good starting point is the rule of thumb to select a die opening eight times the material thickness.
At best, engineers will come to the shop and talk to the press brake operator. Make sure everyone knows the difference between molding methods. Find out what methods they use and what materials they use. Get a list of all the punches and dies they have, and then design the part based on that information. Then, in the documentation, write down the punches and dies necessary for the correct processing of the part. Of course, you may have extenuating circumstances when you have to tweak your tools, but this should be the exception rather than the rule.
Operators, I know you are all pretentious, I myself was one of them! But gone are the days when you could choose your favorite set of tools. However, being told which tool to use for part design does not reflect your skill level. It’s just a fact of life. We are now made of thin air and no longer slouch. The rules have changed.
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Post time: Aug-25-2023