Sheet Metal: Understanding K-Factor

understanding-k-factor
Posted in: Mechanical Design

SOLIDWORKS sheet metal tools are relatively straightforward, but may not be clearly understood. In sheet metal, there is a powerful bend constant known as the K-Factor. It ultimately allows you to estimate the amount of stretch without knowing what type of material you are bending. In practical terms, it’s just a generic bend allowance to use when you don’t know the process or machine that is going to be used to bend the sheet. I’d like to review K-Factor and how K-Factor applies to your sheet metal designs. Here is a book definition of K-factor:

K-Factor- A constant determined by dividing the thickness of the sheet by the location of the neutral axis, which is the part of sheet metal that does not change length. So if the thickness of the sheet was a distance of T = 1 mm and the location of the neutral axis was a distance of t = 0.5 mm measured from the inside bend, then you would have a K-Factor of t/T = 0.5/1 = 0.5.

Now let’s take a look at an example of K-Factor. Let’s presume you are creating a sheet metal part in SOLIDWORKS. Let’s start out with a sketch of 10 mm by 10 mm. Once you activate the base flange tool, you have an available list of parameters you need to specify to determine how sheet metal is going to stretch.

In the picture above, I specified the parameters as follows: Thickness is T = 1.5 mm, Bend Radius is R = 1.25 mm and K-Factor is set to 0.3. Remember, if we needed to know the placement of the neutral axis we would use the formula, K = (t/T) -> t = K × T = 0.3 × 1.5 = 0.45 mm. We can now apply the Bend Allowance Formula using the above information. The Bend Allowance Formula will determine the length of the arc at the neutral axis from one bend line to the other. Let’s take a look at how the Bend Allowance Formula can be applied, using the parameters we specified.

BA = 2 × π × A (R + K×T) ÷360

BA = 2 × 3.14 × 90(1.25+ 0.3×1.5) ÷360

BA = 2.67 mm

WHERE:

BA = Bend Allowance

π = 3.14

A = Bend Angle

R = Bend Radius (inside bend radius)

K = K-Factor

We can see the bend allowance in the flat patterned state evaluates to 2.67 mm. (Note: In the flat patterned state I have the merge solids option selected off. This allows me to take the measurement of the length of the bend.)

You can also determine the size you would need to cut your sheet. L1 and L2 have sketched dimensions specified. You can verify this by flattening out the sheet and measuring the length from the edge to the center.

LF = L1 + L2 – R ×2 +BA

LF = 10 + 10 – 1.25 × 2 + 2.67

LF = 20.17 mm

So you would need a size cut to 20.17 mm. If necessary, you can determine the amount of stretch. The total length when bent is Total Length = 8.75 + (π×2.75÷2) +8.75 = 21.82 mm resulting in the stretch = 21.82 – 20.17 = 1.65 mm.

Practically, if you are wondering how to determine K-Factor for your industry, it is simply determined by material type. In other words, calculations have already been done for you. In SOLIDWORKS there is a list of sample tables for materials such as steel and aluminum. These sample tables yield acceptable results, but should always be verified by your company first. A sample steel gauge table looks like below. Tables can be found at Program Files\SolidWorks Corp\SolidWorks\lang\english\Sheet Metal Gauge Tables folder.

Notice the above K-Factor has already been set for you. So when you set your parameters, you can be confident that a K-Factor of 0.5 will lend your acceptable results. Also, notice below K-Factor is pre-set in the property manager, as well as thickness (gauge) and bend radius.

I hope you enjoyed this tutorial on K-factor. Please check back on this page for more updates on SOLIDWORKS, other products, and our ever growing list of products we specialize in. If you have any questions about SOLIDWORKS or Sheet Metal design, please CONTACT US.

September 28, 2015
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Comments
Matthew BUchholtz
February 14, 2017
Good morning. I just read through your tutorial which I found very informative. I have a question regarding the formula when it comes bends that are either greater or less than 90 degrees. I have come across several formulas that all get me to the same if not close answer but I'm seeing different answers in regards to bends that are over or under 90 degrees. Can you shed any light on this for me at all? Any information is greatly appreciated. Regards, Matthew Buchholtz
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gaurav thaper
March 3, 2017
Good morning. I have a question in our mind regarding the calculation of band allowance. How i calculate banding allowance practically. Plz provide sutable help.
Reply
Nathan Martin
September 27, 2018
This is so interesting and useful blog about sheet metal understanding k factor. This is really a informative blog for all peoples who wants to know about this. Thanks for sharing with us.
Reply
Nicole Drake
September 27, 2018
Hi Nathan, Thanks for your feedback! We appreciate it.
Vineyard machinery
March 19, 2019
This is very informative and it came to me at the right time when I am, planning to creating a sheet metal part and metal design process working. These simple tips are really simple.
Reply
Phil McIntosh
May 11, 2019
With the majority of sheet metal jobs, if the K-factor were to be a little out you probably wouldn't even notice. Getting into heavy gauge bending (4mm / 3/16" and up) is another matter. Get your K-factor wrong here and something probably won't fit together with something else. Thanks for the very informative video.
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Daphne Gilpin
August 12, 2019
Thanks for explaining that the K-factor is determined by the relationship between the thickness of the sheet and its neutral axis. My husband needs to find a supplier where he can buy the plate metal he needs for his project. I'm glad I read your article because discussing the K-factor with the supplier he chooses should make sure he ends up with the right thickness of plate metal for the project!
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Franklin White
June 29, 2020
Thanks for explaining that the K-factor for your industry is determined by the material type that you choose. My brother wants to work with sheet metal. I'll tell him about the K-factor and how he'll need to choose his materials wisely.
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