Understanding the Bend Allowance and consequently the Bend Deduction of a part is a crucial first step to understanding how sheet metal parts are fabricated. When the sheet metal is put through the process of bending the metal around the bend is deformed and stretched. As this happens you gain a small amount of total length in your part. Likewise when you are trying to develop a flat pattern you will have to make a deduction from your desired part size to get the correct flat size. The Bend Allowance is defined as the material you will add to the actual leg lengths of the part in order to develop a flat pattern. The leg lengths are the part of the flange which is outside of the bend radius. In our example below a part with flange lengths of 2” and 3” with an inside radius of .250” at 90° will have leg lengths of 1.625” and 2.625” respectively. When we calculate the Bend Allowance we find that it equals .457”. In order to develop the flat pattern we add .457” to 1.625” and 2.625” to arrive at 4.707”. As you can see the Bend Allowance and Bend Deduction are closely related below.
Bend Allowance Calculator
Below is our simple Bend Allowance Calculator, it works by inputing the Material Thickness, Bend Angle, Inside Radius and K-Factor. It simply processes these variables through the equations we will discuss below. Note that the Bend Angle is the excluded angle, not the included angle.
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Bend Allowance Formula
The Bend Allowance Formula takes into account the geometries of bending and the properties of your metal to determine the Bend Allowance. You will need to know your Material Thickness (MT), the Bend Angle (B<), the Inside Radius (IR), and the K-Factor (K). The Material Thickness will be measured in decimal form, not by the gauge number. For more information on gauges and their decimal equivalents and tolerances view our Gauge Chart page. The Bend Angle will be something that you determine based on what the complimentary angle of your part is going to be. It is important to convert from the included angle to the complimentary angle before performing any calculations. The Inside Radius will be the finished radius of the included angle. For information on how the Inside Radius is determined see our post on the Air Bend Force Chart. Finally the K-Factor is a property of the material which you are bending. This property determines how the material is stretched when bending. See our post on the K-Factor for better understanding as well as charts and formulas.
The Formula itself is rather simple:
Bend Allowance Chart
Because Bend Allowance is often confused with Bend Deduction it is important to understand which value you intend on working with. Once you understand what these values represent you can use them to accurately and quickly develop flat patterns for your sheet metal parts. Creating a chart with your standard values is a key component of speeding up this process. Many software packages like Solid Works, Inventor and Solid Edge will let you incorporate a Bend Allowance Chart into its calculations when developing flat patterns.
Unfortunately sheet metal bending is not always going to be the same in every shop. The largest variations come from the materials themselves. Protective coatings, variations in the alloy and thickness as well as many other small factors all add up to give you Bend Allowances unique to your operation. This chart will get you close enough for most applications and may not require fine tuning on your part, however if you are truly dedicated to precise bending, download the excel sheet and begin plugging in your own values.
The below Microsoft Excel chart is for even numbered gauges 8 through 24 and has a default K-Factor of .33 for each. The inside radii are based off of a standard Air Bend Force Chart. You can easily modify the Material Thickness, Inside Radius and K-Factor for each thickness at the top of each column.
|8 Gauge||10 Gauge||12 Gauge||14 Gauge||16 Gauge||18 Gauge||20 Gauge||22 Gauge||24 Gauge|
|K factor||0.33||K factor||0.33||K factor||0.33||K factor||0.33||K factor||0.33||K factor||0.33||K factor||0.33||K factor||0.33||K factor||0.33|