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How to Calculate the Bending Allowance of Press Brake

Use DA-53T CNC press brake for bending and correction 03

Unlock the key to precision bending with this comprehensive press brake guide. Learn how to calculate bend allowance using both basic and advanced formulas. Featuring bend allowance charts for materials like cold rolled steel sheet (SPCC), aluminum plates, and copper plates. Ideal for professionals seeking efficient and accurate metal forming.

  1. What is Bend Allowance?
  2. How Bend Allowance Affects the Overall Dimensions
  3. Importance of Accurate Bend Allowance Calculation
  4. The Anatomy of a Press Brake
  5. Types of Metal Commonly Used
  6. Factors that Affect Bend Allowance
  7. What is the bend allowance formula, and why is it important in sheet metal fabrication?
  8. Basic Formula for Bend Allowance
  9. How to calculate the “K” factor
  10. Real-World Example: Calculating Bend Allowance Step-by-step
  11. Bending allowance chart for cold rolled steel sheet SPCC (electro-galvanized sheet SECC)
  12. Bending allowance chart for aluminum plate

What is Bend Allowance?

To put it simply, bend allowance is the extra length of sheet metal needed for a bend. When you bend a piece of metal, it stretches. Yes, you read that right, it actually stretches like pizza dough (well, not quite like pizza dough, but you get the idea). And no, you can’t just eyeball it; you’ve got to calculate it. Why? Because a miscalculated bend allowance can lead to all sorts of problems like material wastage or, even worse, an unusable product. Think of it as the GPS for your press brake; it helps you navigate through the intricacies of bending steel plates, aluminum sheets, and other metals.

For those of you who are curious about the bend allowance formula, it’s often represented as BA = [(π/180) x (Bend Angle) x (Inside Radius + Material Thickness)]. We’ll dive deeper into how to calculate bend allowance and other related topics like bend radius formula and sheet metal bending force calculation in the upcoming sections, so hang tight!

Press Brake Bending Allowance

How Bend Allowance Affects the Overall Dimensions

Let’s get real for a moment—bending metal isn’t a casual stroll in the park. One slight mistake and your project might turn into something that resembles modern art rather than a functional piece. That’s why understanding how bend allowance affects the overall dimensions is crucial. It’s like baking a cake; you can’t just toss ingredients into a bowl and expect a Michelin-star dessert. You have to measure and calculate. But don’t worry, it’s not as complex as it sounds!

You see, when you bend a metal sheet, the metal along the inner radius compresses while the metal along the outer radius stretches. This deformation isn’t just happening in the ‘bending zone’; it extends a bit beyond it. And this is where bend allowance comes into play. This little number tells you how much extra material you need for that bend to occur without any hiccups.

Ever heard of the phrase, “Measure twice, cut once?” The same logic applies here, especially when dealing with steel plate bending radius or sheet metal bending force calculation. Miscalculating bend allowance can throw off your dimensions drastically, leading to increased material costs and time delays. Essentially, bend allowance is that extra bit you add to make sure you don’t end up short. Imagine tailoring a suit; you always leave a bit of room for that final stitch, right? The same concept applies here.

KRRASS press brake with gooseneck punch and multi-V die

Importance of Accurate Bend Allowance Calculation

If you’ve been following along, you’re probably starting to realize that bend allowance isn’t just some arbitrary number you can ignore. But how critical is it, really? Well, let’s just say it’s as essential as the yeast in a loaf of bread. Without it, everything falls flat.

Inaccurate bend allowance calculations can have far-reaching impacts. First off, there’s the obvious issue of wasting material. If your calculations are off, you’ll end up with either too much or too little material, and in the world of manufacturing, every inch and ounce counts. Not to mention, material waste leads to increased project costs, something no one wants to deal with.

But wait, there’s more. Incorrect calculations aren’t just costly; they can also be downright dangerous. Imagine a scenario where you’re working on a load-bearing structure and miscalculate the bend allowance. What you’re left with is a potentially unstable structure that poses a safety risk. In such cases, the steel plate bending radius and sheet metal bending force calculation also become paramount.

There’s also the issue of time. Corrections take time, especially if you’re dealing with large-scale projects. You’ll need to re-measure, re-cut, and re-bend, setting you back significantly in terms of project timelines. Remember, time is money in the manufacturing world.

So, getting your bend allowance calculation right the first time is not just advisable—it’s imperative. Think of it as the thread that holds the fabric of your project together. Neglect it, and you’re opening up a Pandora’s box of complications.

The Anatomy of a Press Brake

Before you fully grasp the concepts like bend allowance and steel plate bending radius, it’s essential to understand the machine you’re working with—a press brake. Just as you wouldn’t drive a car without understanding its basic components, you shouldn’t operate a press brake without knowing its anatomy.

Main Components and Their Functions

A press brake is more than just a hunk of metal; it’s a symphony of parts working in harmony. Let’s break down its main components:

  • Bed and Ram: The bed is the stationary lower part where you place the sheet metal. The ram is the upper moving part that delivers the force required to bend the metal.
  • Back Gauge: Think of this as your ruler; it’s what you use to accurately position the metal before the bend.
  • Die and Punch: These are the actual tools that contact your metal sheet to make the bend. The punch pushes the metal into the die to create the bend.
  • Control Panel: This is the ‘brain’ of the operation, where you input all your settings like bend allowance, angle, and force.

How Press Brakes Differ from Other Bending Equipment

You might wonder, “Aren’t all bending machines the same?” The answer is a resounding no. While all bending machines serve the purpose of bending metal, the way they achieve it can differ widely.

Press brakes, for instance, offer precision like no other. They are often programmable, allowing you to input complex bend sequences, unlike simpler machines like folding machines or roll benders which offer less control. This is why a press brake is your go-to machine for projects that require high levels of accuracy, like intricate metal assemblies.

And let’s not forget that with press brakes, you have the flexibility to change out the die and punch to adapt to different bending needs, something many other machines don’t offer.

Types of Metal Commonly Used

The recipe for a successful bending project doesn’t end at knowing your press brake inside and out, or even mastering the art of bend allowance calculations. The type of metal you’re working with is the cherry on top. It’s like choosing the right type of wood for a piece of furniture. Each metal has its own properties, advantages, and challenges.

Steel and its Varieties

Steel is often the go-to choice for many manufacturers. But did you know that not all steel is created equal? We’ve got carbon steel, stainless steel, and high-strength low-alloy steel, among others.

  • Carbon Steel: Known for its versatility and affordability. It’s commonly used but watch out for its susceptibility to corrosion.
  • Stainless Steel: Your choice when you need corrosion resistance. However, it often requires higher bend force calculations.
  • High-Strength Low-Alloy Steel: If you’re looking for strength without the weight, this is your go-to. Ideal for automotive applications.

Aluminum and its Alloys

Aluminum is the featherweight champion in the world of metals. It’s lighter than steel but still offers decent strength. Its alloys, such as 6061 and 7075, offer increased strength and other desirable properties.

  • 6061 Aluminum: Known for its versatility and weldability. Often used in frames and structures.
  • 7075 Aluminum: If you’re looking for strength, 7075 trumps most aluminum alloys and is ideal for aerospace applications.

Other Metals and Special Considerations

Of course, the world of metals isn’t limited to just steel and aluminum. You’ve got other options like copper, brass, and titanium. Each comes with its own set of special considerations, such as conductivity in copper or corrosion-resistance in brass. These metals might require specialized bend radius formulas and bend allowance calculations.

Press brake schematic

Factors that Affect Bend Allowance

Understanding how to calculate bend allowance is like cooking a gourmet meal; you need to know how each ingredient—or in this case, factor—affects the final product. Skipping over these crucial elements could lead to results that are far from appetizing. So, what are these key ingredients you ask?

Material Thickness

Thickness is not just a dimension; it’s a critical factor in determining how your material will behave during the bending process. Thicker materials generally require more force, and they can significantly impact your bend allowance formula.

Bend Radius

Ah, the bend radius, the very curvature of your bend! This is where your bend radius formula comes into play. A smaller radius will result in a sharper bend, requiring more force and affecting the bend allowance. The larger the radius, the softer the bend, meaning a different calculation.

Bend Angle

How far do you want to go? 90 degrees? 45? This angle plays a significant role in determining the bend allowance. Higher angles generally mean more material and thus, require precise bend calculations.

Grain Direction

Last but not least, the grain direction. Think of it as the natural flow of the metal. Bending against the grain could lead to cracking or weakening of the material. Therefore, grain direction should be considered in your sheet metal bending force calculations.

What is the Bend Allowance Formula, and Why is it Important in Sheet Metal Fabrication?

You’ve probably heard the phrase “Measure twice, cut once.” In sheet metal fabrication, it’s more like “Calculate twice, bend once.” Making accurate bends requires a deep understanding of the bend allowance formula. So, what is this magical formula, and why is it so crucial?

The Bend Allowance Formula Unveiled

In its simplest form, the bend allowance formula can be expressed as:

BA = [(π/180) × (R + (K × T)) × A]
  • BA: Bend Allowance
  • π: Pi, approximately 3.14159
  • R: Bend Radius
  • K: K-Factor, a value that represents the location of the neutral axis
  • T: Material Thickness
  • A: Bend Angle

Mastering this formula is akin to holding the cheat sheet to a challenging exam. It helps you anticipate how the metal will behave, guiding you to a bend that’s as accurate as possible.

The Significance in Sheet Metal Fabrication

Why should you care about bend allowance formulas when bending sheet metal? The answer is simple: precision and cost-effectiveness. Getting your bends right the first time eliminates waste of materials and time—a win-win in the world of manufacturing.

A miscalculation could mean the difference between a successful project and a pile of unusable metal. By understanding and effectively using the bend allowance formula, you’re not just bending metal; you’re bending it in the most efficient way possible.

Press Brake Bending Allowance2

Basic Formula for Bend Allowance

When diving into the world of sheet metal fabrication, one term you cannot afford to ignore is bend allowance. But what is the formula behind it, and why does it matter?

Origins of the Bend Allowance Formula

The bend allowance formula emerged to precisely estimate the stretch length needed for bending operations. This formula is intrinsically tied to the Inside Radius (I.R.) dictated by the lower V die opening. For materials like mild steel, the inside radius is calculated as (5/32x lower V die opening (W), when the punch radius is smaller than (5/32 x W).

Crucial Equations for Bend Allowance

The formula for bend allowance can be categorized into two fundamental equations based on the inside radius (I.R.) and material thickness (t):

If I.R.< Material Thickness (t)
Bend Allowance = (3.1416/180) x (I.R. + t/3) x A

If I.R.> 2 x Material Thickness (t)
Bend Allowance = (3.1416/180) x (I.R. + t/2) x A

Where A = 180 – Bend Included Angle.

Interpolating Between Formulas

If the inside radius happens to equal t or 2t, or falls between t and 2t, you would interpolate the bend allowance from the two primary formulas.

An Alternative Formula for Bend Allowance

Another robust formula for calculating bend allowance is:


Bending Allowance = A*π/180*(R+K*T)

In this formula, A is the bend angle in degrees, R is the inside bend radius, K is a constant, and T is the material thickness.

Simplifying the Formula for 90-degree Bends


Bending Allowance = π/2(R+K*T)

In sum, mastering these bend allowance formulas is invaluable for any sheet metal fabrication professional. It ensures not only the correct calculation of material length but also promotes efficient and precise bending operations.

FIGURE 1. The bend allowance, shown here, differs from the bend deduction (BD), which is subtracted ("deducted") from the total outside dimensions to develop the flat blank. The outside setback, also shown here, is measured from the tangent point to the apex of the bend.

How to Calculate the “K” Factor

The “K” factor is a critical variable in bending calculations, representing the location of the neutral axis in the material being bent. It’s essentially the ratio of the neutral axis’s position to the material thickness.

Understanding the “K” Factor

The “K” factor varies depending on the material, bending operation, and machine used. It falls within a range of 0 to 0.5, usually. A “K” factor closer to 0 means more stretching, whereas a factor closer to 0.5 indicates more compression.

Methods for Calculating “K” Factor

The “K” factor can be calculated using empirical methods, by physically bending a sample piece and measuring it, or through specialized software for sheet metal design.

Empirical Method

The empirical method involves creating a test piece with the same material, thickness, and bend radius as your project. Perform the bending operation, measure the lengths of the bend and flat sections, and use these measurements to calculate the “K” factor.

Software Method

Modern CAD software often includes tools for estimating the “K” factor based on the input variables like material properties, bend radius, and angle. This method is generally quicker but may require adjustment for high-precision applications.

Important Considerations

The “K” factor is not a constant value for all materials or bending operations. It should be verified or calibrated for each new project or material type to ensure accurate bending calculations.

Real-World Example: Calculating Bend Allowance Step-by-step

Material and Machine Details

For this example, let’s assume we are working with mild steel of 0.125-inch thickness, a bending angle of 90 degrees, and an inside bend radius of 0.25 inches.

Calculating with Basic Formula

Using the basic formula for bend allowance:

Bend Allowance = Angle * (π / 180) * (Radius + K-factor * Thickness)

Assuming a K-factor of 0.42, we get:

Bend Allowance = 90 * (π / 180) * (0.250 + 0.42 * 0.125) = 0.475"

Verifying with Advanced Formulas

To verify, we can use advanced formulas tailored to the material’s properties. For mild steel, with a lower V die opening, the formula becomes:

If I.R.< Material Thickness (t): Bend Allowance = (3.1416/180) x (I.R. + t/3) x A

Here, A = (180 - 90) = 90 and I.R. = 0.25, t = 0.125.

So, Bend Allowance = (3.1416/180) * (0.25 + 0.125/3) * 90 ≈ 0.477"

The values are close, confirming the accuracy of our initial calculations.

Bending Allowance Chart for Cold Rolled Steel Sheet SPCC (Electro-Galvanized Sheet SECC)

TVAngle0.60.811.21.522.533.544.55Shortest size
V4900.91.4          2.8
V4120 0.7           
V4150 0.2           
V690        4.5
V6120 0.70.861         
V790       5
V7120 0.80.91         
V890       5.5
V830 0.30.340.40.5        
V1090    2.73.2      7
V10120    1.31.6       
V10150    0.50.5       
V1290    2.83.654.5     8.5
V1245    11.31.5      
V1260    1.722.4      
V12120    1.41.72      
V1490      4.3     10
V14120      2.1      
V14150      0.7      
V1690      4.55    11
V16120      2.2      
V16150      0.8      
V1890      4.6     13
V18120      2.3      
V18150      0.8      
V2090      4.85.1 6.6  14
V20120      2.3  3.3   
V20150      0.8  1.1   
V2590       5.76.47  17.5
V25150       111.2   
V3290         7.58.2 22
V32120          4  
V32150          1.4  
V4090          8.79.428
V40120          4.34.6 
V40150          1.51.6 

Bending Allowance Chart for Aluminum Plate

TVAngle0.60.811.21.522.533.544.55Shortest size
V4   1.4         2.8
V6   1.6         4.5
V7   1.61.8        5
V8      5.5
V10     2.43.2      7
V12     2.43.2      8.5
V14      3.2      10
V16      3.244.8    11
V18        4.8    13
V20        4.8    14
V25        4.85.46  17.5
V32          6.36.9 22

Conclusion: Your Path to Precision Bending

Mastering the art of bend allowance is pivotal for achieving precise, cost-effective results in metal forming. For those looking to further optimize their metal bending processes, our Press Brakes manufactured by Krrass are the perfect solution. Visit to explore a wide range of solutions for all your sheet metal forming needs.

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