Understanding the Bend Deduction and consequently, the Bend Allowance 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 Deduction is defined as the material you will have to remove from the total length of your flanges in order to arrive at the flat pattern. The flange lengths are always measured to the apex of the bend. In our example below a part with flange lengths of 2” and 3” with an inside radius of .250” at 90° will have a total length of 5”. When we calculate the Bend Deduction we find that it equals .293”. In order to develop the flat pattern, we will subtract .293” from 5” to arrive at 4.707”. As you can see the Bend Deduction and Bend Allowance are closely related below.

Bend Deduction in sheet metal 1

Bend Deduction Calculator

Below is our simple Bend Deduction Calculator, it works by inputting 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.

Bend Deduction Formula

The Bend Deduction Formula takes into account the geometries of bending and the properties of your metal to determine the Bend Deduction. 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 to 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 For Bend Deduction is rather long because it incorporates the Out Side Set Back as well as the Bend Allowance.

Bend Deduction in sheet metal 2

If you have your Out Side Set Back (OSSB) and Bend Allowance (BA) then the Bend Deduction Formula is shortened considerably.

Bend Deduction in sheet metal 3

Bend Deduction Chart

Because Bend Deduction is often confused with Bend Allowance 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 Deduction 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 Deductions 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 on 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.


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