K-factor In Sheet Metal Pdf
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We have one other term to discuss before talking about the k-factor, which is our bend radius. The bend radius is measured on the inside of the part, not on the outside of the part. The bend radius is measured on the inside of the part because the part goes under compression and tension. The inside of this part is in compression. This area is actually compressed and formed to create the inside of the bend. And then the outside of the bend is in tension. So when we bend, we actually end up deforming the area with the bend, and the area under tension ends up moving inward towards the neutral line.
The biggest difference between the k-factor and the y-factor in sheet metal bending is that the y-factor takes the internal stresses of the material into account more so than the k-factor does. This means that calculations involving the y-factor are slightly more accurate than those involving k-factor, but also quite a bit more complicated and uses different calculations for other values in bending, such as bend allowance.
Calculating the y-factor for a bent sheet metal part is really only necessary for highly complicated bends in unique materials, and most shops and machinists prefer to use k-factor as the industry standard.
It started innocently enough. A reader wrote me asking me about the k-factor and calculating bend allowances. I explained how the k-factor was used and referred him back to the usual k-factor charts. The reader thanked me for the response, but then said he wanted to know more. Where do these k-factor values come from, and how do you calculate them without a chart?
To understand the k-factor, you need a firm grasp of a few basic terms, the first being the neutral axis. The neutral axis is a theoretical area lying at 50 percent of the material thickness while unstressed and flat. The neutral axis is a shifty guy; that is, it shifts toward the inside of the bend. The theoretical line of the neutral axis will remain the same length both before and after the bend is complete.
During bending, while the area between the neutral axis and the inside surface comes under compressive forces, the area between the neutral axis and the outside surface is stressed by tensile forces. The neutral axis is the zone or plane that separates the tension from the compression. The neutral axis position depends on the bend angle, inside bend radius, and method of forming.
The k-factor is a constant determined by dividing the location of the shifted neutral axis by the material thickness of the sheet. The area within the sheet defined as the neutral axis does not get compressed on the inside of the neutral axis or expanded on the outside. The neutral axis does not suffer any change [of] length during a bending operation.
Say you have a 1-millimeter (mm) material thickness. In a flat state the material has a neutral axis located at 50 percent of the thickness, at 0.5 mm. Bend the material, and the neutral axis shifts to 0.446 mm, as measured from the inside surface of the bend. We define this neutral axis shift as t, as shown in Figure 2. We calculate k-factor by dividing t by the material thickness (Mt): k-factor = t/Mt,
The k-factor is nothing more than a multiplier that can give you an accurate value for the relocated neutral axis. And if you know the bend allowance, you can extract the k-factor from it. Once you know the k-factor, you can use it to predict the bend allowance for various angles.
The k-factor is fundamental to designing precise sheet metal products. It allows you to anticipate the bend deduction for a large variety of angles without having to rely on a chart. While modern bend deduction charts now are reasonably accurate, historically bend calculation charts, both for bend allowances and bend deductions, were notorious for their inaccuracies. They were usually only valid for the manufacturing environments in which they were created. And many of these charts are still floating around.
The chart in Figure 3 shows the range of k-factors you can have, from 0.50 all the way down to 0.33. And the k-factor can be even smaller. In most applications, the k-factor is given as an average value of 0.4468.
A common problem in both the sheet metal and plate industries involves parts designed with an inside bend radius much tighter than necessary. It can wreak havoc in the press brake department and cause cracking on the outside surface of the bend.
A bend made too sharp develops plastic deformity from the excessive stress caused by the bending. The problem will manifest itself as fracturing on the outside surface, altering the bend allowance. The smaller the inside bend radius, the more the neutral axis will shift toward the inside surface of the bend.
In this case, the minimum inside bend radius is two times the material thickness. Note that this is just a rule of thumb that gives you a ballpark figure. Finding the correct minimum bend radius for steel or aluminum plate requires a little research and should include data from your material supplier and another critical ingredient in your k-factor gumbo: whether you are bending with or against the grain.
The grain direction, created in the direction the sheet is rolled at the mill, runs the length of the full sheet. You can see it on a new piece of sheet metal by noticing the direction of visible lines running through it. When the sheet is made, its particles become elongated in the direction of rolling.
Grain direction is not a surface finish, which is made by sanding or other mechanical procedures. Nevertheless, finish surface scratches do make the material more susceptible to cracking, especially when the finish grain is parallel to the natural grain.
Because the grains are directional, they cause variations of the angle and, potentially, the inside radius. This dependence on orientation is what we call anisotropy, and it plays an important role if you want to make precise parts.
When the metal is bent parallel (with) the grain, it affects the angle and radius, making it anisotropic. Incorporating the metals anisotropy qualities are an essential part of making accurate predictions for k-factor and bend allowances.
Bending with the grain forces the neutral axis inward, changing the k-factor once again. And the closer the neutral axis gets to the inside surface of the bend, the more likely cracking is to occur on the outside of the radius.
The Fabricator is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The Fabricator has served the industry since 1970.
I found your theArtofPressBrake.com and realized that aside from this question, maybe there is more I could learn. I want to help our design engineers create more manufacturable parts. I would say that I have a good understanding of the basics, but there are still issues that I come across in production parts that I tuck away to keep in mind for future designs. Are you able to answer my question on K-factors with a general recommendation without going into too much theory or calculations?
Your press brake and stamping press form sheet metal in different ways. On the press brake you are air forming, while on the stamping press you are stamping or coining. These are all distinct methods of forming, and each is calculated differently because of how the radius is produced in the workpiece.
Even if you are producing a sharp bend, the smallest radius you can use for your bend calculations is the minimum bend radius, if you want your numbers to work out in practice. Note also that air forming a sharp bend usually is very detrimental to consistency. The crease in the center of the bend tends to amplify any angular variations caused by changes in material grain direction, hardness, thickness, and tensile strength. The sharper and deeper the crease, the greater the effect.
For instance, 304 stainless steel forms a radius 20 to 22 percent of the die width, while a radius in 5052-H32 aluminum forms at 13 to 15 percent of the width. The general rule here is this: The softer the material, the tighter the inside radius.
Take the customary default K-factor value of 0.446, multiply it by the material thickness, and you know where the neutral axis will relocate. What we are doing in essence is forcing the measured length from a larger radius (that is, the length of the neutral axis at 50 percent of the material thickness) onto a smaller radius. The same total measured length spread over the smaller radius means we have excess material, or elongation.
Note that the material type, method of forming, and the relationship of bend radius to material thickness all give us different K-factors. These in turn affect the total amount of elongation that occurs and the bend deductions we need to use.
The K-factor is defined mathematically as t/Mt, where t is the neutral axis location and Mt is the material thickness. Because of the specific properties of any given metal, there is no easy way to calculate that value perfectly, hence the chart in Figure 2.
To calculate the K-factor, you need to collect some information. First, you need to know the dimensions before and after forming and measure the inside radius as accurately as possible. An optical comparator is a good first choice because of its accuracy; other options include gauge pins and radius gauges.
Take the total of the formed inside dimensions, subtract the flat size, and you get the bend allowance (BA). Then measure the complementary bend angle and inside bend radius (Ir). With those data points, along with the material thickness (Mt), you can solve for the K-factor (all dimensions are in inches):
This formula uses a K-factor of 0.446. Still, if you have any change in the method of forming, type of material, or the ratio of inside bend radius to material thickness, you will have a different K-factor value. To incorporate this new value, you can use an expanded version of the same formula. You then determine the OSSB, then use the result along with the BA to calculate your bend deduction:
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