What is the K-factor?
K factor is the ratio of the distance from the neutral layer to the inner surface of the bend to the thickness of the sheet metal.
As shown in the figure below, k=t/T.
K factor is also called the neutral layer position factor.
According to the definition of the K factor, the K factor is a constant greater than 0 but less than 1.
But in fact, for most materials, the K factor is a number less than or equal to 0.5 in sheet metal design and processing.
K factor calculator
The following calculator can be used to calculate the K factor as well as the neutral axis’ offset for sheet metal bending.
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
Sheet metal k factor chart
GI Sheet & MS Sheet
|K Factor |
All angles, including R angle
Variation law of K factor and neutral layer
1. Even for the same material, the K factor in the actual processing is not fixed, and the specific value is affected by the processing technology.
In the elastic deformation stage of sheet metal bending deformation, the neutral layer is located in the middle of the plate thickness, but with the increase of the bending deformation of the stamping workpiece, the material mainly produces 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 when bending the plate?
We can introduce the parameter R/T, which reflects the intensity of plate deformation.
R refers to the inner radius of bending, and T refers to the plate thickness.
When the ratio of R/T is smaller, it means that the deformation degree of the plate is more intense, and the offset of the neutral layer to the inside is also larger.
The values listed in the following table are applicable to plates with a rectangular section under certain processing conditions.
It can be seen from the table below that the position factor K of the neutral layer increases with the increase of R/T.
Due to the properties of materials and different bending methods, the position of the neutral layer will be affected.
At this time, the radius of the neutral layer can be calculated according to the following formula:
ρ = R + KT
- ρ – radius of neutral layer
- R – bend inner radius
- K – neutral layer position factor
- T – material thickness
In other words, when the radius of the neutral layer is determined, the development length of the neutral layer can be calculated according to geometry, and then the development length of the sheet can be calculated.
2. Generally speaking, under the same bending process conditions, the softer the sheet metal material, the smaller its K value, and the greater the offset of the neutral layer to the inside of the bending.
There are three standard bending tables applicable to 90 ° bending in the Machinery’s Handbook:
|# 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 R bends, different bending angles will also affect the change of 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.
Frequestly asked questions about K factor
Many designers use the SolidWorks sheet metal module.
There are a lot of sheet metal parameters in the module, so how do you set them?
What is the K-factor in sheet metal parameters? How to set it up?
The K-factor is a key basic concept in SolidWorks sheet metal design and must be understood in order to learn sheet metal design and fabrication well.
What is neutral layer?
Since the K-factor is related to the location of the neutral layer, what is the neutral layer?
In the bending deformation zone, the material near the inner surface is compressed, and the closer the inner surface is, the more it is compressed.
Likewise, the material near the outer surface is stretched, and the closer it is to the outer surface, the more it is stretched.
The transition from the inner surface to the outer surface is from compression to stretching.
Assuming that a material is made up of thin layers stacked one on top of the other (most metals are actually layered), there must be a layer in the middle of the material that is neither compressed nor stretched. .
This layer we call the neutral layer.
In general, the neutral layer is invisible and inaccessible, because it is inside the metal.
Its position is related to the intrinsic properties of the material, i.e. the K-factor is related to the material.
From the definition of the neutral layer, the unfolded dimension of the sheet metal is equal to the width of the neutral layer, as shown in the figure above.
Unfolding dimension of the sheet metal = straight line A + straight line B + arc C (length of the neutral layer in the deformation zone).
Why the K-factor cannot exceed 0.5?
To figure this 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, you need to know what the neutral layer is.
You understand that bending a sheet metal part is to make it produce a very small arc, like roll bending, except that the radius of the roll bending is larger than that of sheet metal bending.
No matter how you bend, it’s impossible to get a right angle, it will always have a little 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.
But most of the time, we need to already know the dimensions of the resulting arc (bend).
But most of the time, we need to know the dimensions of the resulting arc (bend) to extrapolate 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 middle layer of the sheet will not change.
People just call the middle layer that doesn’t change the neutral layer.
With the increasing demand for dimensional accuracy of products, it has also been found that the amount that gets smaller on the inside does not necessarily equal the amount that gets larger 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.
At this point one realizes that the layer (neutral layer) that does not change size is not necessarily in the middle of the thickness of the sheet, but is more near the inside.
Divide the distance from the inside to the neutral layer by the entire thickness of the sheet, which is known as the K-factor.
Yes, you’re right, the furthest the neutral layer is from the inside is in the middle of the plate thickness.
So the distance from the inside to the middle divided by the entire plate thickness is 0.5, which means the K factor is equal to 0.5, it can’t be any greater.
These are the reasons why the sheet metal K-factor should not exceed 0.5, and I hope you understand after reading this article.
Where can we get the K factor?
It can be obtained from material suppliers, experimental data, empirical data and manuals, or by the following calculator: