I’ve been working through a backlog of reader issues – I still have a few columns to write before I catch up again. If you sent me a question and I didn’t answer it, please wait, your question might be next. With that in mind, let’s answer the question.
Q: We are trying to choose a tool that will provide 0.09 inches. radius. I threw out a bunch of parts for testing; my goal is to use the same stamp on all of our materials. Can you teach me how to use 0.09″ to predict bend radius? travel radius?
A: If you are air forming, you can predict the bend radius by multiplying the die opening by a percentage based on the type of material. Each material type has a percentage range.
To find percentages for other materials, you can compare their tensile strength to the 60,000 psi tensile strength of our reference material (low carbon cold rolled steel). For example, if your new material has a tensile strength of 120,000 psi, you can estimate that the percentage will be twice the baseline, or about 32%.
Let’s start with our reference material, low carbon cold rolled steel with a tensile strength of 60,000 psi. The internal air formation radius of this material is between 15% and 17% of the die opening, so we usually start with a working value of 16%. This range is due to their inherent variations in material, thickness, hardness, tensile strength, and yield strength. All of these material properties have a range of tolerances, so it’s impossible to find an exact percentage. No two pieces of material are the same.
With all of this in mind, you start with a median of 16% or 0.16 and multiply that by the thickness of the material. Therefore, if you are forming A36 material larger than 0.551 inches. With the die open, your inside bend radius should be approximately 0.088″ (0.551 × 0.16 = 0.088). You will then use 0.088 as the expected value for the inside bend radius that you use in bend allowance and bend subtraction calculations.
If you are always getting material from the same supplier, you will be able to find a percentage that can get you closer to the inside bend radius you are getting. If your material comes from several different suppliers, it is best to leave the calculated median value, as material properties can vary greatly.
If you want to find a die hole that will give a specific inside bend radius, you can invert the formula:
From here you can select the closest available die hole. Note that this assumes that the inside radius of the bend you want to achieve matches the thickness of the material you are airforming. For best results, try to select a die opening that has an inside bend radius that is close to or equal to the thickness of the material.
When you take all these factors into account, the die hole you choose will give you the inside radius. Also make sure that the punch radius does not exceed the bending radius of the air in the material.
Keep in mind that there is no perfect way to predict internal bend radii given all material variables. Using these chip width percentages is a more accurate rule of thumb. However, it may be necessary to exchange messages with a percentage value.
Q: Recently I received several inquiries about the possibility of magnetizing the bending tool. While we haven’t noticed this happening with our tool, I’m curious about the extent of the problem. I see that if the mold is highly magnetized, the blank can “stick” to the mold and not form consistently from one piece to the next. Besides that, are there any other concerns?
Answer: Brackets or brackets that support the die or interact with the press brake base are not normally magnetized. This does not mean that a decorative pillow cannot be magnetized. This is unlikely to happen.
However, there are thousands of small pieces of steel that can become magnetized, whether it’s a piece of wood in the stamping process or a radius gauge. How serious is this problem? quite seriously. Why? If this small piece of material is not caught in time, it can dig into the work surface of the bed, creating a weak spot. If the magnetized part is thick or large enough, it can cause the bed material to rise around the edges of the insert, further causing the base plate to sit unevenly or evenly, which in turn will affect the quality of the part being produced.
Q: In your article How Air Curves Get Sharp, you mentioned the formula: Punch Tonnage = Gasket Area x Material Thickness x 25 x Material Factor. Where does 25 come from in this equation?
A: This formula is taken from Wilson Tool and is used to calculate punch tonnage and has nothing to do with molding; I adapted it to empirically determine where the bend gets steeper. The value of 25 in the formula refers to the yield strength of the material used in developing the formula. By the way, this material is no longer produced, but is close to A36 steel.
Of course, much more is required to accurately calculate the bending point and bending line of the punch tip. The length of the bend, the interface area between the punch nose and the material, and even the width of the die play an important role. Depending on the situation, the same punch radius for the same material can produce sharp bends and perfect bends (i.e. bends with a predictable inner radius and no creases at the fold line). You’ll find an excellent sharp bend calculator on my website that takes all of these variables into account.
Question: Is there a formula for subtracting the bend from the counter back? Sometimes our press brake technicians use smaller V-holes that we didn’t account for in the floor plan. We use standard bending deductions.
Answer: yes and no. Let me explain. If it is bending or stamping the bottom, if the width of the mold matches the thickness of the molding material, the buckle should not change much.
If you are air forming, the inside radius of the bend is determined by the hole of the die and from there you take the radius obtained in the die and calculate the bend deduction. You can find many of my articles on this subject at TheFabricator.com; look for “Benson” and you will find them.
For airforming to work, your engineering staff will need to design a slab using bend subtraction based on the floating radius created by the die (as described in “Bend Inside Radius Prediction” at the beginning of this article). If your operator is using the same mold as the part it was designed to form, the final part must be worth the money.
Here’s something less common – a little workshop magic from an avid reader commenting on a column I wrote in September 2021 “Braking Strategies for T6 Aluminum”.
Reader response: First of all, you have written excellent articles on sheet metal working. I thank you for them. Regarding the annealing you described in your September 2021 column, I thought I’d share some thoughts from my experience.
When I first saw the annealing trick many years ago, I was told to use an oxy-acetylene torch, ignite only acetylene gas, and paint the mold lines with black soot from the burnt acetylene gas. All you need is a very dark brown or slightly black line.
Then turn on the oxygen and heat the wire from the other side of the part and from a reasonable distance until the colored wire you just attached starts to fade and then disappears completely. This seems to be the right temperature to anneal the aluminum enough to provide a 90 degree shape without any cracking issues. You do not need to shape the part while it is still hot. You can let it cool down and it will still be annealed. I remember doing this on 1/8″ thick 6061-T6 sheet.
I have been deeply involved in precision sheet metal manufacturing for over 47 years and have always had a knack for camouflage. But after so many years, I don’t install it anymore. I know what I’m doing! Or maybe I’m just better at disguise. In any case, I was able to get the job done in the most economical way possible with minimal frills.
I know a thing or two about sheet metal production, but I confess that I am by no means ignorant. It is a great honor for me to share with you the knowledge that I have accumulated over my life.
One more thing I know: in general, you all have a lot of experience and knowledge. Let’s say you want to share interesting tips, work habits, or just tidbits with other readers. Please write it down or draw it and send it to me at steve@theartofpressbrake.com.
There is no guarantee that I will use your email address in the next column, but you will never know. I just might. Remember, the more we share knowledge and experience, the better we become.
FABRICATOR is North America’s leading steel fabrication and forming magazine. The magazine publishes news, technical articles and success stories that enable manufacturers to do their job more efficiently. FABRICATOR has been in the industry since 1970.
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