The neutral axis is a theoretical area lying at 50 percent of the material thickness while unstressed and flat. To understand the k-factor, you need a firm grasp of a few basic terms, the first being the neutral axis. The k-factor allows you to calculate the bend allowance, the outside setback, the bend deduction, and the flat layout of the precision part you’re forming. Once developed, the value of the k-factor will enable you to predict the total amount of elongation that will occur within a given bend. It’s a mathematical multiplier that allows you to locate the repositioned neutral axis of the bend after forming. It’s the base value needed to calculate bend allowances and ultimately the bend deduction. Of all the mathematical constants used in precision sheet metal fabrication, the k-factor stands out as one of the most important. One thing led to another, and I eventually found that to give a complete answer, my journey would take me not only to k-factor calculations, but the y-factor, minimum radii, kinetic friction, and grain directions-all key ingredients that make the sweet, subtle, complicated gumbo that is the science of bending. Where do these k-factor values come from, and how do you calculate them without a chart? The reader thanked me for the response, but then said he wanted to know more. I explained how the k-factor was used and referred him back to the usual k-factor charts. A reader wrote me asking me about the k-factor and calculating bend allowances. This is the first part of a two-part series. Describing that shift is what the k-factor is all about. If you know those for a given piece of steel tube, you can calculate pretty accurately what the deflection's going to be without ever touching it, And if you know the specification of the steel, you can also determine the yield point by calculation.The thinning sheet forces the neutral axis to shift inward toward the inside bend radius. The determining factors for the deflection you're inducing in your "test" are 1) the diameter of the tube and 2) the thickness of the tube. There's no way you will come close to that level of force using your arm unless the tube is very thin or very small diameter.
However, when you bend the tube to form the curve you're talking about in the rest of the article, you've gone into the plastic region where the steel won't spring back straight. When you remove the force, the steel tube resumes its original shape because you're still in the elastic region. What you're doing in this step is seeing how much deflection results from a given force - in this case from your arm. It's a material property unrelated to the geometry of the steel piece you're looking at. "Yield strength" is a property of the steel, the stress at which the steel will begin to deform plasticly - that is, it won't spring back into shape when you remove the force. What you're talking about in Step One is NOT "determining the yield strength of the metal." Rather, you are determining the STIFFNESS of the tube section.
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