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Metal Bend Allowance & Springback

Sheet metal bending is a vital and common manufacturing process used in many American industries. Sheet metal forming, as this metal-working task is also known, is invaluable for making diverse products like original equipment in large construction machinery to small but specialized components in material handling logistics systems. A great part of the nation’s economy would be severely curtailed without effective sheet metal bending techniques, equipment and expertise.

Sheet metal bending entails taking flat metal stock from sheets and applying force or pressure to change its shape into the desired configuration. Manufacturing presses can be powered pneumatically, hydraulically or electrically. Also, they range in capacity from light-duty punch and die sets to massive presses capable of applying tons of force to bend and permanently alter thick metal.

Regardless of size or power, all sheet metal bending techniques take standard variables into account when planning a job. Sheet metal forming technicians need to calculate metal springback reaction and compensate for metal bend allowance. Calculating sheet metal bend allowance and knowing how to reduce springback takes considerable expertise and requires professional knowledge to consistently produce top-quality end products

What Is Bend Allowance?

When technicians bend sheet metal from its original flat and straight shape to a bent configuration, they also change its physical dimensions. The metal’s length and width elongate because of molecular action and the laws of physics and mathematics. The force of bending the material causes material at the bend point to compress on the inside of the bend radius and stretch on the outside. This causes the metal stock’s external measurement to grow.

This physical deformation increases the material area in a finished product compared to the length of the original stock. This increase might be slight for simple end products requiring only one bend, but they can drastically alter the amount of sheet metal needed to produce a product with multiple bends. Product designers and professional fabricators compensate for the change in material dimensions by building in bend allowance into their starting measurements.

To understand what occurs to cause dimension changes in formed sheet metal objects, it’s necessary to know the physical and mathematical forces that occur when a metal sheet experiences tremendous pressure. Bending metal requires altering the molecular state of an inert object into a newly formed product. Those two states are:

  • Elastic: This is where the metal is bending or stretching. It’s much like how a rubber or elastic band reacts under force. The elastic state is only momentary while the metal goes through the forming press or brake. Elasticity causes metal surfaces on the outside of the bend to experience tension forces, while the inside of the bend’s radius goes through compression. Compression increases molecular density, while tension decreases density. Therefore, the metal on the outside surface expands its length while the inside surface shortens. The tension force must always be greater than compression or the metal would exceed its yield capacity and break.
  • Plastic: This is the inert or stable condition an object is in before and after it experiences forces putting it in an elastic state. Although a metal product may be in a physical plastic state, it can still be under tension and compression. However, a plastic object will have stable dimensions even if it grows or stretches during a bending operation. The difference in length between before and after the bending process is an important allowance that sheet metal fabricators need to calculate when preparing materials.

While calculating bend allowance, or bend deduction, is somewhat predictable, it isn’t an exact science — too many variables exist. Some of the factors influencing bend allowance include:

  • Material Composition: Metals with soft properties are more elastic than hard materials.
  • Material Thickness: Thicker materials will stretch more than thin products when bent.
  • Material Grain: Cross-grain bending reacts differently to straight-grain bends.
  • Temperature: Warmer materials are more elastic than cold metals.
  • Die and Punch Sizing: Bending tool design affects metal stretch.
  • Pressure: Force inside the press or brake also affects plasticity.

Designers, engineers and fabricators use mathematical calculations to determine sheet metal bend allowance. One of the core math elements is called the K-factor, which is a neutral axis line inside the sheet that runs horizontally with the metal between the inner compressed surface and the exterior under tension. While the inner surface within the bend area contracts and the outer dimension expands, the neutral — or K-factor — dimension remains constant regardless of material thickness and severity of the bend.

K-factors are the ratio of compression to tension forces occurring within the sheet metal bend. Their values normally range in the 0.25 area but can never exceed 0.5. This is because it’s physically impossible for compression at the bend’s inside to be greater than tension forces on the outside.

The K-factor axis is a control figure used to determine the bend allowance and forecast precise material requirements before altering the metal’s configuration. This can be a complex figure to calculate. Fabricators often refer to charts which table K-factors. This is a historic approach within the sheet metal fabrication industry, but today, advanced software programs are available for computer-assisted design.

Another significant factor contributing to bend allowance is the type of bending process a fabricator selects. Each one results in different plastic and elastic forces being exerted on the sheet metal material. These are the three common processes used in fabrication brakes:

  • Air Bending: This is the most popular brake style and the easiest to use. Air bending incorporates an open brake design where the outside radius of the bend does not contact the die face. Air bending doesn’t require as much force as the other two brake designs. However, the downside to air bending is a phenomenon called springback, where the elasticity remains in the metal after the brake releases and tries to spring back to its original shape. Springback is another important factor that fabricators need to calculate and allow for in the bending process.
  • Bottom Bending: In this brake design, the sheet metal makes full contact with the die. Bottom bending requires more force than air bending and is used for thicker and harder sheet metal materials that need full compressive power to move between the plastic and elastic states. A drawback to bottom bending is the time and energy required to complete bends. However, because bottom bending crushes the sheet metal, it removes most of the residual elasticity that causes springback.
  • Coining: The earliest sheet metal brakes and presses utilized the coining process, which is similar to stamping out coins. Coining is a bottom bending approach that forces the punch into the die seat. It requires considerable force and can weaken the material within the bend radius. Coining can prevent practically all springback, but the weakening effect is often a disadvantage.

Bend allowance is sometimes referred to as bend deduction. The two terms are similar but not exactly equal. Bend deduction is a reverse calculation where the additional stretch measurement is deducted or removed from the material requirement calculation. This might be a cart-horse issue, but all sheet metal fabricators know how important bend allowance is to their finished products.

The Importance of Bend Allowance

Calculating and compensating for bend allowance is highly important for accuracy in finished sheet metal products. Anticipating bend allowance is a core principle for designing and fabricating precise products that perform with perfection. Ignoring bend allowance measurement would result in a poor fit and finish as well as possible end-product failure.

Bend allowance might not seem important for simple sheet metal work like building flashing or manufacturing material handling devices like shelving. However, calculating bend allowance has a cumulative effect that increases the margin of error when developing complex products with multiple bends required from a single stock of sheet metal.

Metal housings for electronic components are a prime example of how important bend allowance is to the accuracy of finished products. Bend allowance is also highly important to precision-driven industries such as custom sheet metal fabrications for industrial enclosures and environmental solutions. Original equipment manufacturers (OEM) also expect exact bend allowances for their important sheet metal works.

Although bend allowance is an extremely important function of sheet metal craftsmanship, it’s not the only calculation and anticipation involved in designing and making metal products. Springback reactions also occur when forming sheet metal bends and anticipating springback values is also a critical design factor. Bend allowance and springback calculations go hand-in-hand with professional sheet metal production.

What Is Springback in Sheet Metal Bending?

Springback is a physical function of sheet metal bending. When sheet metal stock transforms from a plastic state, into an elastic condition and then back to a newly configured plastic existence, the metal’s molecular properties can retain its original instructions. This is similar to the metal having a memory and trying to reestablish its past.

In other words, the metal is trying to spring back into its initial state by retaining energy and transforming it into a reactive force. This is a natural phenomenon that occurs in metal, and knowledgeable fabricators always take springback forces into account when building precise sheet metal products. They are also aware of these contributing factors that determine how much springback to expect:

  • The material’s chemical composition
  • Yield strength in the material
  • Physical properties in the metal such as grain
  • Material thickness and overall size
  • Brake or press design being used
  • The temperature of the sheet metal and tools
  • Deformation rate known about the material
  • Bend radius prescribed by the product design
  • Bend allowance factors anticipated and factored in

K-factor ratios also play a big role in calculating springback in sheet metal fabrications — so do trial and error prototypes, as well as personal experience of the fabricator. Like bend allowance, precisely determining springback isn’t completely predictable. That’s especially so when working with newly designed sheet metal products and unfamiliar materials.

Fortunately, professional sheet metal fabrication companies have experienced staff using state-of-the-art equipment to compensate for metal springback.

How to Compensate for Metal Springback

Nothing beats personal experience when it comes to knowing how to compensate for metal springback when working with sheet metal. Knowing the factors affecting springback is highly important — so is having the right type of equipment, applying either an air bending, bottom bending or coining process.

Experienced fabricators start production by building their material’s known properties into their springback calculations. They also compensate for springback by using the precise pressure required for the bend and the radius degree they’re planning to achieve. Then, professional metal fabricators use a technique called overbend.

Overbend occurs exactly how it sounds. Fabricators account for a calculated springback occurring in their material and literally bend the material over the finished radius point, so the metal will spring back into the precise angle expected in the finished product. Top-end metal fabricators use natural springback forces to their advantage, not their disadvantage.

While springback happens from strain and stress at the molecular level, custom metal fabricators see them playing out on the surface in their everyday sheet metal projects. Precision fabricators overbend their seams exactly enough so the elastic recovery stops at their desired point. Inexperienced fabricators can go too far with overbends, though. Excessive bends can result in a ruined product, while insufficient bends require inefficient re-bending. Neither of these mistakes will happen when professional sheet metal companies undertake custom metal fabrication.

Considering Bend Allowance & Springback During Custom Metal Fabrication

Experienced custom metal fabricators always consider bend allowance and springback when designing and manufacturing end-use products. It’s part of the professionalism expected from these highly-skilled craftspeople. They’re used to extreme tolerances when creating custom pieces for exacting customers.

While experience is a key component in manufacturing custom sheet metal work, these fabricators would suffer without precision bending equipment and excellent material to work with. They also need to be familiar with other sheet metal manufacturing services such as laser cutting, welding, grinding and powder coating. This is where a professional custom sheet metal fabrication company like APX York Sheet Metal excels.

Contact APX York Sheet Metal for Custom Part Design & Metal Bending

APX York Sheet Metal has more than 70 years of experience in custom part design and metal bending. Since 1946, our business steadily evolved into the leading sheet metal fabrication shop in Central Pennsylvania. We efficiently manufacture tailor-made products instead of relying on products made of stock sheet metal.

Our 65,000-square-foot facility is a single provider for all custom sheet metal work, including design, fabrication and finishing. By streamlining operations, we have increased efficiency to bring your products from a concept to a conclusion in short order. Our custom sheet metal fabrication facility serves all of Central Pennsylvania and Northern Maryland. We focus on delivering top-quality products at competitive prices with excellent customer service.

With state-of-the-art equipment and advanced business processes, our skilled staff rise to meet your sheet metal challenges no matter what they might be. Our in-house expertise, equipment and technology provide you with fast turnaround time on small and large orders. Call APX York Sheet Metal at 717-767-2704 to find out more about our product design and metal bending services. You can also reach us any time through our online contact form.

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