Bending guru Steve Benson catches up with reader emails to answer questions about hemming and bending calculations.Getty Images
I get a lot of emails every month and I wish I had time to respond to all of them.But alas, there isn’t enough time in the day to do it all.For this month’s column, I’ve put together a few emails that I’m sure my regular readers will find useful.At this point, let’s start talking about layout-related issues.
Q: I want to start by saying that you write a great article.I found them very helpful.I’ve been struggling with an issue in our CAD software and can’t seem to find a solution.I’m creating a blank length for the hem, but the software always seems to require extra bend allowance.Our brake operator told me not to leave a bend allowance for the hem, so I set the CAD software to the absolute minimum allowed (0.008″) – but I still ran out of stock.
For example, I have a 16-ga.304 stainless steel, outside dimensions are 2″ and 1.5″, 0.75″.Hem to the outside.Our brake operators have determined that the bend allowance is 0.117 inches.When we add the dimension and hem, then subtract the bend allowance (2 + 1.5 + 0.75 – 0.117), we get a stock length of 4.132 inches.However, my calculations gave me a shorter blank length (4.018 inches).With all that said, how do we calculate the flat blank for the hem?
A: First, let’s clarify a few terms.You mentioned bend allowance (BA) but you didn’t mention bend deduction (BD), I noticed you didn’t incorporate BD for bends between 2.0″ and 1.5″.aspect.
BA and BD are different and not interchangeable, but if you use them correctly, they both take you to the same place.BA is the distance around the radius measured at the neutral axis.Then add that number to your outside dimensions to give you the flat blank length.BD is subtracted from the overall dimensions of the workpiece, one bend per bend.
Figure 1 shows the difference between the two.Just make sure you are using the correct one.Note that the values ​​of BA and BD may vary from bend to bend, depending on bend angle and final inner radius.
To see your problem, you are using 0.060″ thick 304 stainless steel with one bend and 2.0 and 1.5″ outside dimensions, and 0.75″.Hem at the edge.Again, you didn’t include information about the bend angle and inside bend radius, but for simplicity I calculated the air assuming you made a 90 degree bend angle on 0.472 inches.die.This gives you a 0.099 inch.Floating bend radius, calculated using the 20% rule.(For more on the 20% rule, you can check out “How to Accurately Predict the Inner Bend Radius of Air Formation” by typing the title into thefabricator.com’s search box.)
If it is 0.062 inches.The punch radius bends the material by more than 0.472 inches.Die opening, you achieve 0.099 inches.Floating within the bend radius, your BA should be 0.141 inches, the outer setback should be 0.125 inches, and the bend deduction (BD) should be 0.107 inches.You can apply this BD for bends between 1.5 and 2.0 inches.(You can find BA and BD formulas in my previous column, including “Basics of Applying Bending Functions.”)
Next, you need to calculate what to deduct for the hem.Under perfect conditions, the deduction factor for flat or closed hems (materials less than 0.080 inches thick) is 43% of the material thickness.In this case, the value should be 0.0258 inches.Using this information, you should be able to perform a plane blank calculation:
0.017 inches.The difference between your flat blank value of 4.132 inches and mine of 4.1145 inches can easily be explained by the fact that hemming is very operator dependent.what do i mean?Well, if the operator hits the flattened part of the bending process harder, you’ll get a longer flange.If the operator does not hit the flange hard enough, the flange will eventually shorten.
Q: We have a bending application where we form various metal sheets, from 20-ga.Stainless to 10-ga.Pre-coated material.We have a press brake with automatic tool adjustment, an adjustable V-die on the bottom and a self-positioning segmented punch on the top.Unfortunately, we made a mistake and ordered a punch with a 0.063″ tip radius.
We are working on getting our flange lengths consistent in the first part.It was suggested that our CAD software was using the wrong calculation, but our software company saw the problem and said we were fine.Will it be the software of the bending machine?Or are we overthinking?Is it just a normal BA adjustment or can we get a new punch with 0.032″ stock.radius help?Any information or advice would be greatly appreciated.
A: I will address your comment about buying the wrong punch radius first.Given the type of machine you have, I’m assuming you’re air forming.This leads me to ask several questions.First, when you send the job to the shop, do you tell the operator on which mold the opening design for the part is formed?It makes a big difference.
When you airform a part, the final inner radius is formed as a percentage of the mold opening.This is the 20% rule (see the first question for more information).The die opening affects the bend radius, which in turn affects BA and BD.So if your calculation includes a different achievable radius for the die opening than the one the operator uses on the machine, you have a problem.
Suppose the machine uses a different die width than planned.In this case, the machine will achieve a different inner bend radius than planned, changing BA and BD, and ultimately the part’s formed dimensions.
This brings me to your comment about the wrong punch radius.0.063″ unless you’re trying to get a different or smaller inner bend radius.The radius should work fine, that’s why.
Measure the obtained inner bend radius and make sure it matches the calculated inner bend radius.Is your punch radius really wrong?It depends on what you want to achieve.The punch radius should be equal to or less than the floating inner bend radius.If the punch radius is greater than the natural floating bend radius on a given die opening, the part will take the punch radius.This will again change the inner bend radius and the values ​​you calculated for BA and BD.
On the other hand, you don’t want to use a punch radius that is too small, which can sharpen the bend and cause many other problems.(For more on this, see “How to Avoid Sharp Turns.”)
Apart from these two extremes, the punch in air form is nothing but a push unit and does not affect BD and BA.Again, the bend radius is expressed as a percentage of the die opening, calculated using the 20% rule.Also, be sure to apply the terms and values ​​of BA and BD correctly, as shown in Figure 1.
Question: I am trying to calculate the maximum lateral force for a custom hemming tool to ensure our operators are safe during the hemming process.Do you have any tips to help me find this?
Answer: Lateral force or lateral thrust is difficult to measure and calculate for flattening a hem on a press brake and in most cases is unnecessary.The real danger is overloading the press brake and destroying the punch and bed of the machine.Ram and bed overturned causing each to bend permanently.
Figure 2. Thrust plates on a set of flattening dies ensure that the top and bottom tools do not move in opposite directions.
The press brake typically deflects under load and returns to its original flat position when the load is removed.But exceeding the load limit of the brakes can bend machine parts to the point where they no longer return to a flat position.This can permanently damage the press brake.Therefore, be sure to consider your hemming operations in tonnage calculations.(For more on this, you can check out “The 4 pillars of a press brake tonnage.”)
If the flange to be flattened is long enough to flatten, the side thrust should be minimal.However, if you find that the side thrust seems excessive and you want to limit the movement and twisting of the mod, you can add thrust plates to the mod.The thrust plate is nothing more than a thick piece of steel added to the bottom tool, extending up beyond the top tool.The thrust plate mitigates the effects of side thrust and ensures that the top and bottom tools do not move in opposite directions to each other (see Figure 2).
As I pointed out at the beginning of this column, there are too many questions and too little time to answer them all.Thank you for your patience if you’ve recently sent me questions.
In any case, let the questions keep popping up.I will reply to them as soon as possible.Until then, I hope the answers here help those who asked the question and others facing similar issues.
Uncover the secrets of using a press brake in this intensive two-day workshop August 8-9 with instructor Steve Benson to teach you the theory and mathematical fundamentals behind your machine.You’ll learn the principles behind high-quality sheet metal bending through interactive instruction and sample work problems throughout the course.Through easy-to-understand exercises, you’ll learn the skills needed to calculate accurate bend deductions, choose the best tool for the job, and determine the correct V-die opening to avoid part distortion.Visit the event page to learn more.
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