If you’re looking to master the art of sheet metal fabrication, then understanding the K-factor is an absolute must.
The K-factor is a fundamental concept in SolidWorks sheet metal design that determines the bending angle, shape, and size of sheet metal. But what exactly is the K-factor, and how do you calculate it?
Look no further than this comprehensive guide from MachineMfg.
In this article, you’ll learn everything you need to know about the K-factor, from its definition and calculation to the factors that affect it.
You’ll discover how the K-factor relates to the neutral layer, a crucial component in the bending deformation zone, and how it impacts the elastic control ratio of sheet metal during the bending process.
With this knowledge, you’ll be able to achieve precise processing and take your sheet metal fabrication skills to the next level.
Plus, with handy charts and calculators provided, you’ll have all the tools you need to start mastering the K-factor today.
So, whether you’re a seasoned sheet metal fabricator or just starting out, this guide is a must-read for anyone looking to take their skills to the next level.
Let’s dive right into it.
What Is the K-Factor?
The K-factor is a fundamental concept in SolidWorks sheet metal design that is crucial to mastering sheet metal fabrication.
One must first understand the K-factor.
It is the ratio of the distance between the neutral layer and the bend’s inner surface to the sheet metal’s thickness.
As shown in the diagram below, K = t / T. From the definition of the K-factor, it is clear that it is a constant greater than 0 and less than 1.
Since the K-factor is related to the position of the neutral layer, what is the neutral layer?
In the bending deformation zone, the material near the inner surface is compressed and the compression is more severe the closer it is to the inner surface.
Similarly, the material near the outer surface is stretched, and the stretching is more severe the closer it is to the outer surface.
Transitioning from compression to stretching as going from the inner surface to the outer surface, assuming that the material is stacked in thin layers (most metal materials are layered), there must be a layer in the middle of the material that is neither compressed nor stretched.
We call this layer the neutral layer.
In general, the neutral layer cannot be seen or touched because it is inside the metal.
Its position is related to the material’s inherent properties, which means that the K-factor is related to the material.
From the definition of the neutral layer, the unfolded size of the sheet metal is equal to the width of the neutral layer, as shown in the figure above.
The unfolded size of the sheet metal = straight line A + straight line B + arc C (length of the neutral layer in the deformation zone).
The K-factor is also referred to as the neutral layer position factor.
For most materials, the K-factor is a number less than or equal to 0.5 in sheet metal design and processing.
Importance of K Factor in Sheet Metal Bending
The K-factor is a crucial concept in sheet metal fabrication, as it determines the bending angle of the bending machine and also affects the shape and size of the sheet metal formed.
In sheet metal fabrication, the K-factor represents the elastic control ratio of the sheet metal during the bending process.
In other words, by controlling the size of the K-factor, one can control the degree of sheet metal deformation and achieve precise processing.
Factors Affecting K Factor Values
Before calculating the K-factor, we need to understand the factors that affect it. Here are several main factors:
- Material and hardness of sheet metal forming
The larger the material and hardness, the greater the pressure during the bending process, and thus the K-factor will also increase.
- Thickness of sheet metal
The thickness of the sheet metal directly affects the size of the K-factor. The K-factor of a thicker sheet metal is usually smaller than that of a thinner sheet metal.
- Bending angle
The greater the bending angle, the larger the K-factor. Therefore, the bending angle should be considered in the design of sheet metal parts for its impact on the shape.
- Bending tool
Different bending tools can have different impacts on the K-factor. Correctly selecting a bending tool can reduce the size of the K-factor.
How to Calculate K Factor?
The k-factor is an independent value that describes how sheet metal bending occurs in a wide range of geometric parameter situations and how it unfolds. It is also an independent value used to calculate the bending allowance (BA) under various situations, such as material thickness, bending radius/angle.
The figures below can help us better understand the detailed definition of the k-factor.
In the thickness of sheet metal parts, there is a neutral layer or axis. The sheet metal material in the neutral layer of the bending area neither stretches nor compresses, which is the only place in the bending area where it remains undeformed. It is represented as the intersection of the pink and blue areas in the diagram.
During the bending process, the pink area is compressed, while the blue area is stretched. If the neutral sheet metal layer remains undeformed, then the length of the neutral layer arc in the bending area is the same in its bent and flattened states.
Therefore, the bending allowance (BA) should be equal to the length of the neutral layer arc in the bending area of the sheet metal part. This arc is represented as green in the Figure.
The position of the neutral layer in sheet metal depends on specific material properties, such as ductility.
Assuming the distance between the neutral sheet metal layer and the surface is “t,” that is, the depth from the surface of the sheet metal part to the sheet metal material in the thickness direction is t.
Therefore, the radius of the neutral sheet metal layer arc can be expressed as (R+t).
Using this expression and the bending angle, the length of the neutral layer arc (BA) can be expressed as:
BA = Pi*(R+T)*A/180
To simplify the definition of the neutral layer in sheet metal and considering the applicability to all material thicknesses, the concept of the k-factor is introduced. Specifically, the k-factor is the ratio of the thickness of the neutral layer position to the overall thickness of the sheet metal part, that is:
K = t/T
Therefore, the value of K is always between 0 and 1. If a k-factor is 0.25, it means that the neutral layer is located 25% of the thickness of the sheet metal material, and if it is 0.5, it means that the neutral layer is located at the halfway point of the entire thickness, and so on.
Combining the above two equations, we can get the following equation (8):
BA = Pi*(R+K*T)*A/180 (8)
Where some values such as A, R, and T are determined by the actual geometric shape.
K Factor Calculation Formula
According to the above calculation, we can easily derive the formula for calculating the k-factor:
- BA=Bending allowance
- R=Inside bending radius
- K=k-factor, which is t/T
- T=Material thickness
- t=Distance from the inner surface to the neutral axis
- A=Bending angle (angle through which the material is bent)
Based on the given information:
Sheet metal thickness T = 1mm Bend angle A = 90° Bend radius R = 1mm Bend allowance factor BA = 2.1mm
The formula to calculate the K factor is:
Substituting the given values in the formula, we get:
K = (2.1 × 180/(3.14 × 90) – 1)/1
Simplifying this equation, we get:
K ≈ 0.337
Therefore, the K factor for the given parameters is approximately 0.337.
K Factor Calculator
We provide two different calculators to calculate the value of the k-factor. The final results may have slight differences, but they will definitely meet your needs.
If the bending allowance and inside bending radius are known, you can use the following calculator to calculate the k-factor as well as the distance from the inner surface to the neutral axis.
If only the inside bending radius and material thickness are known, you can use the following calculator to calculate the k-factor.
K Factor Chart
The following are K-factors for common metal materials.
- Soft copper or soft brass: K=0.35
- Semi-hard copper or brass, mild steel, aluminium etc.: K=0.41
- Bronze, hard bronze, cold rolled steel, spring steel, etc.: K=0.45
K factor chart
(All angles, including R angle)
|Bend deduction |
(only applicable to 90 corners)
|Material thickness |
Note: The bend allowance for copper is the coefficient when the inner angle of the bend is R3. If an acute punch is used for bending, the bend allowance value should refer to the one for aluminum alloy or be determined through trial bending.
Variation Law of K Factor and Neutral Layer
1. Even for the same material, the K-factor in actual processing is not constant and its specific value is impacted by the processing technology.
In the elastic deformation stage of sheet metal bending, the neutral axis is situated in the middle of the plate thickness.
However, as the bending deformation of the stamped workpiece increases, the material undergoes mainly plastic deformation.
At this time, the plastic deformation is unrecoverable, and the neutral layer will shift to the inner side of the bending with the change of the deformation state.
The more severe the plastic deformation of the material, the greater the offset of the neutral layer to the inside of the bending.
So how can we reflect the intensity of plastic deformation during plate bending?
We can use the parameter R/T to reflect the intensity of plate deformation. R represents the inner radius of bending, and T stands for plate thickness.
A smaller R/T ratio indicates a higher level of plate deformation and a greater shift of the neutral layer inward.
The data in the table below apply to plates with a rectangular cross-section under specific processing conditions.
As shown in the table, the neutral layer’s position factor K increases as R/T increases.
Material properties and bending techniques can affect the position of the neutral layer.
At this time, the radius of the neutral layer can be calculated according to the following formula:
ρ = R + KT
- ρ – radius of the neutral layer
- R – bend inner radius
- K – neutral layer position factor
- T – material thickness
In simpler terms, once the radius of the neutral layer is determined, its development length can be calculated based on geometry, and then the sheet’s development length can be calculated.
2. Generally speaking, under the same bending conditions, the softer the sheet metal material, the lower its K value and the larger the offset of the neutral layer to the inside of the bend.
There are three standard bending tables applicable to 90-degree bending in Machinery’s Handbook.
K Factor Table for Different Materials
|# 1||Soft brass, copper||0.35|
|# 2||Hard brass, copper, mild steel, aluminum||0.41|
|# 3||Hard brass, bronze, cold rolled steel, spring steel||0.45|
3. For smaller inner radius bends, the bending angle can also impact the change in the K factor.
The larger the bending deformation angle is, the greater the offset of the neutral layer to the inner side of the bend is.
Frequently Asked Questions About the K Factor
Why Do We Need to Calibrate the K-Factor?
In the process of sheet metal bending calculation, we often need to calibrate the k-factor. So why do we need to calibrate the k-factor?
In SolidWorks, the deduction value for non-90-degree angle bending is only calculated by input. This can be very troublesome.
To avoid having to calculate the deduction value for non-90-degree angle bending, the k-factor is used instead.
However, how do we accurately determine the k-factor for different sheet metal thicknesses? This requires calibration.
Here is an analysis of how to calibrate:
- The first step is to determine the deduction value required for different sheet metal thicknesses in practice.
- The second step is to calibrate the k-factor in SolidWorks. When drawing sheet metal, set the inner radius to 0.1 for calibration because different inner radii have different k-factor unfoldings. Note that the inner radius must be set to 0.1 for calibration. Some may ask, what if the inner radius is not 0.1 after calibration? In this case, simply change it to 0.1 for unfolding.
- The third step is the calibration stage. In SolidWorks, bend a 10x10mm sheet metal with a thickness of 1.5mm at a 90-degree angle with an inner radius of 0.1 and a deduction value of 2.5mm to obtain an unfolding length of 17.5mm.
- The fourth step is to change the deduction value to the k-factor. Start by setting an approximate value, for example, 0.3. The unfolding length will not be 17.5mm. Then try adjusting the k-factor until the unfolding length is 17.5mm. In this way, the k-factor can be calibrated to 0.23, which will result in an unfolding length of 17.5mm.
- Repeat this process to calibrate different values and record them in a table.
How to Optimize the K-Factor in Sheet Metal Fabrication?
After understanding how to calculate the k-factor and the factors that affect it, we need to know how to optimize it. Here are some important optimization methods:
Choose appropriate materials and hardness
Choosing suitable materials and hardness is very important. This can reduce the pressure during the bending process, thus reducing the k-factor.
Design the thickness of sheet metal reasonably
When designing sheet metal, it is necessary to consider the thickness of the sheet metal reasonably. The smaller the thickness of the sheet metal, the smaller the pressure during the bending process, thereby reducing the k-factor.
Control the bending angle
When designing sheet metal, it is necessary to pay attention to controlling the bending angle. The larger the bending angle, the larger the k-factor, which can cause deformation. Correctly selecting the bending angle can reduce the size of the k-factor.
Choose the appropriate bending tool
The selection of sheet metal bending tools is also very important. Choosing the appropriate bending tool can reduce the k-factor and thus reduce deformation.
Why Can the K-factor Not Exceed 0.5?
To figure out the reason why the K-factor can’t exceed 0.5, you first need to know what the K-factor is.
To figure out the K-factor, it’s necessary to understand what the neutral layer is.
You understand that bending a sheet metal part involves creating a small arc, similar to roll bending, but with a smaller radius than that of sheet metal bending.
Regardless of the method used, it is impossible to achieve a perfect right angle in bending and there will always be a slight arc.
If the lower die radius is small, the workpiece radius is small; if the lower die radius is large, the workpiece radius is large.
Then we come to the neutral layer.
As you know, sheet metal parts have a thickness.
When bending it into an arc, you will notice that the length dimensions of its inner surface are reduced, while the length dimensions of its outer surface are enlarged.
This is where the bending allowance comes from.
For example, if you bend an angle-like part with an outside diameter of 20 x 20, it will always unfold to less than 40, no matter how thick the plate is.
This is because the dimensions of its outer surface become larger after bending.
So if you design the unfolded size to be 40, the bent size is 20 on one side and over 20 on the other.
However, in most cases, it is necessary to know the dimensions of the resulting bend (arc) ahead of time.
But in most cases, it is necessary to know the dimensions of the resulting bend (arc) to calculate its unfolded dimensions.
It has long been believed that no matter how thick the sheet is, no matter how much the inner dimensions get smaller and how much the outer dimensions get larger, the size of the middle layer of the sheet will not change.
The middle layer that remains constant is referred to as the neutral layer.
Due to the growing demand for product dimensional accuracy, it has been discovered that the amount of reduction on the inside does not necessarily match the amount of expansion on the outside.
Especially when the resulting arc is small (such as a bend), it tends to get 0.3 smaller on the inside, but 1.7 larger on the outside.
It becomes apparent that the layer (neutral layer) that remains constant in size is not necessarily located in the middle of the sheet’s thickness, but rather is closer to the inside.
The distance from the inside to the neutral layer divided by the entire thickness of the sheet is referred to as the K-factor.
Yes, you’re right, the furthest the neutral layer can be from the inside is in the middle of the plate thickness.
Therefore, the distance from the inside to the middle divided by the entire plate thickness is 0.5, resulting in a K-factor of 0.5, which is the maximum value it can attain.
These are the reasons why the K-factor in sheet metal should not exceed 0.5. I hope this article has helped clarify your understanding.
Where Can We Get the K Factor?
The K-factor can be obtained from material suppliers, experimental or empirical data, manuals, or the calculator provided above to calculate.
K Factor Calculation Tips and Tricks
The K-factor varies for different materials and thicknesses of sheet metal. During the actual fabrication process, external factors such as the fixtures and bending force used for bending can cause discrepancies between the dimensions of the raw material and the final product.
Therefore, the K-factor needs to be adjusted according to the actual processing conditions in order to obtain an empirical value that can accurately predict the results of sheet metal fabrication.
Applications of K Factor Calculator
After years of experience accumulation in the sheet metal fabrication industry, the bend allowances for each type of sheet metal have been summarized and perfected by predecessors.
So, what is the use of the K-factor?
Nowadays, sheet metal design personnel commonly use 3D design software, such as SolidWorks and Creo. In order to ensure the accuracy of the unfolded dimensions of sheet metal parts during the design process, the bend allowance or K-factor must be set correctly. Some software requires the use of the K-factor in certain situations.
In such cases, the K-factor can be calculated based on the known bend allowance in order to ensure the accuracy of the model.
- How do you calculate K factor?
The K-factor is calculated by dividing the material thickness by the location of the neutral axis, which is the position where the material neither stretches nor shrinks. It is typically determined by performing a test bend with a sample piece of material and then measuring the outcome.
- What is K value factor?
The K-factor is a value used in sheet metal manufacturing to account for the bending and stretching that occurs during the process. It’s a ratio of the distance from the inside bend to the neutral line over the material thickness. The K-factor is used in calculations to predict the amount of elongation or compression that will occur in the bend area of the sheet metal.
- What is the K factor of sheet metal?
The K-factor of sheet metal depends on the material and bending process, and it typically ranges between 0.3 and 0.5. This value is used to calculate the bend allowance or bend deduction in the sheet metal fabrication process.
- Is a higher or lower K factor better?
Neither higher nor lower K factor is inherently better – it simply depends on the material being used and the specific manufacturing process. The K factor is a characteristic of the material and the bending operation, and it is used to accurately predict the bend allowance or bend deduction in sheet metal work.
- K-factor for different materials?
The K-factor can vary significantly between different materials due to their individual physical properties. For example, aluminum generally has a K-factor of around 0.5, while steel might have a K-factor of around 0.4. The exact values depend on the specific alloy and the bending process used.
- Does K factor change with thickness?
Yes, the K factor can change with the thickness of the material. Thicker materials usually have lower K factors because they are less flexible and less prone to stretching or compressing during the bending process.
- What is the K factor of sheet metal?
As mentioned, the K-factor of sheet metal is a ratio that describes how much the material will stretch or compress during bending. It varies between different materials and processes, typically ranging from 0.3 to 0.5.
- What is the K factor for aluminum?
The K-factor for aluminum is generally considered to be around 0.5. However, this can vary depending on the specific alloy of the aluminum and the bending process used.