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Bend allowance is a measure of how much extra material is required to accommodate a bend in sheet metal. This is a function of the bend angle, bend radius, thickness of the material, and a variable called the K-factor. Bend allowances vary based on the angle of the bend, the type and thickness of the material, and the method used to create the bend. Calculating the bend allowance is necessary to create a finished product of the correct size.
When a piece of sheet metal is bent, the overall length of the piece changes because of compression on the inside of the bend and tension on the outside. A line through the thickness of the bend, called the neutral axis, does not change in length when the part is bent. Material to the inside of this line is in compression, while material on the outside of this line is in tension. The location of the neutral axis varies based on the angle of the bend and the radius of the bend.
The K-factor is a ratio of the distance of the neutral line from the inside edge of the material to the thickness of the material. Generally, the K-factor is not less than 0.25, and it cannot be greater than 0.5 because it is not physically possible for the compression on the inside of the bend to be greater than the tension on the outside. Essentially, the K-factor is an indirect measure of the ratio of compression to tension in the bend — the higher the K-factor, the greater the compression. The K-factor is dependent on the material used and the type of bend being created.
To calculate bend allowance, the K-factor is multiplied by the thickness of the material; that number is then added to the bend radius. This number is multiplied by the angle of the bend times pi over 180. The formula is bend allowance = (K-factor x thickness + radius) x angle x (pi/180). Once bend allowance is calculated, it is added to the required finished length to obtain the material length required to create the bent piece.
Machines commonly used to bend sheet metal include brake presses — also known as press brakes — and box-and-pan brakes. There are multiple methods for creating bends on each kind of brake. Materials commonly bent using brakes include aluminum and mild steels; some types of ductile plastics may also be bent using these machining methods.
I have been working with light steel for quite some time now. The company I work for makes lightweight manufacturing machinery for companies that want to find cheaper and more sustainable ways to produce their products.
Knowing the bend allowance of the steel that we use is very important. If we poorly design our machinery, it not only affects us as a company, but our customers as well. Selling them a low quality machine that will inevitably break down will halt their production cycles.
To prevent mistakes in our products, we often times use testing strips to determine the bend allowance of our steel. We take a scrap pieces of the materials and calculate and test their bend
allowance. If it passes the tests, then we go ahead and bend the steel that will be used in the finished product.
Although it costs money to test these materials before building them, the money we save in faulty products is worth it. My company takes pride in building machinery that lasts a long time. Proper testing is a way for us to maintain this quality.
Reading this article takes my mind back to my days as an undergraduate civil engineering student. I took about a half dozen classes on the properties of different building and construction materials. Steel, iron, aluminum, copper and other metals were definitely ones that I studied intensely. I did numerous lab studies to determine the bending allowance of different materials.
In one class, the professor had us make a sculptor out of steel, and the artwork had to include at least seven bends. At first I had a hard time calculating the bend allowance of the steel. But with practice, I was able to grasp the concept of the K-factor, and was even able to eyeball the bend allowance
of the steel that we used. It took a while, but after a few material failures, my teammates and I finished our sculptor.
After we were done, our professor informed us that there was such a thing as a bend allowance chart. This chart displays the bend allowance of certain sheet metals based on their characteristics. Having this chart would have made our project a whole lot easier. But, I must say I appreciate the chart a lot more now that I understand the concepts behind it.
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