Understanding Bend Deductions
Published: May 26, 2020
Sheet metal fabrication, and specifically, precision sheet metal fabrication is an excellent way to create and built robust products by means of cutting, forming, and assembling processes. Projects are generally designed or modeled and then sent to fabrication shops to bid on, and if won, fabricated to print. Often times, parts are not designed with best practices around sheet metal fabrication in mind requiring sheet metal shops to do a lot of work behind the scenes, getting prints ready for production using standard fabrication processes. One topic rarely covered at length and seems to create the most confusion is around Bend Deductions and consequently Bend Allowances.
Considering the Bend Deduction and Bend Allowances is a critical first step in designing sheet metal parts as it affects nearly every following step in the fabrication process. More so, it will allow you to achieve the correct size and dimensions needed in the flat pattern. The flat pattern is what the part looks like before any bends have happened. The lengths in the flat pattern will be different from in the bent state. This is because metal material when formed in a bending process is both stretched and compressed depending on the thickness and the type of material.
The Bend Deduction BD is defined as the difference between the sum of the flange lengths (from edge to the apex) and the initial flat length. In other words, the material you will have to remove from the total length of the flanges in order to arrive at the proper length in the flat pattern. In the example below, the part has flange lengths of 2” and 3” with an inside radius of .250” at 90° will have a length of 5”. When the Bend Deduction is calculated we find that it equals .293” in length. In order to develop the flat pattern, we will subtract .293” from 5” to arrive at 4.707”. The image below shows the close relation between Bend Deduction and Bend Allowance.
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. Values you will need to know will include your material thickness (MT), the Bend Angle (B<), the Inside Radius (IR), and the K-Factor (K). When entering the Material Thickness, use the decimal form in place of the gauge number. For your convenience, we have provided a material thickness chart, here: Material Thickness Guide. To determine the Bend Angle properly you must use the complementary angle of the part feature. It is important to convert the included angle to the complimentary angle before you perform the calculation. The Inside Radius will be the finished radius of the included angle. Lastly, the K-Factor is a property of the material substrate you are bending. This property reflects how the material stretches when formed.
Do not be intimidated by the length of the formula for Bend Deduction. It is long because it solves for the Out Side Set Back (OSSB) as well as the Bend Allowance.
If the Out Side Set Back (OSSB) and Bend Allowance (BA) are available then the Bend Deduction Formula is considerably shorter.
BEND DEDUCTION CHART
Bend deduction can easily be confused with bend allowance so it is important to understand the difference and when each value is needed. Creating flat patterns in sheet metal part drawings becomes easy once familiarized with each of these concepts. One key to success in speeding up the process of finding these values id to use a chart with standard values based on material substrates and thickness. We use Solid Works at Meta Fab but other software packages are available that easily help figure the bend deduction or bend allowances.
Sheet metal bending and forming is not always going to be the same in each fabrication shop. The biggest variation in fabrication comes from the material thickness itself. Protective coating, variation in the alloy and thickness as well as many other factors all add up to give you bend deductions which are unique to your processing methods. The chart below is a good starting point for most sheet metal processing applications and may not require many changes. However, if you are dedicated to precision fabrication then use the excel file below and plug in your own data to tailor it to your operations.
The below Excel chart is for even-numbered material gauges 8 through 22 and has a default K-Factor of .448 for each. You can easily modify the Material Thickness, Inside Radius, and K-Factor for each thickness at the top of each column. This chart is for reference only – you should always verify against your process for better results.
Bend Deductions are the first and more critical aspects when working with fabricated sheet metal. If missed, it will wreak havoc on your formed parts, done well and you’ll have a precise measurement you and your clients will be happy with. We hope you’ve enjoyed this write up and found it useful. Check back for more articles around press brakes and sheet metal forming.