What is bend allowance: K Factor, Y Factor, Bend Table

If you are working in the sheet metal fabrication industry, you might have noticed that the sheet metal part’s unbend size is different from the bent version of the same part. Right?

Have you ever wondered why it is so? Well, that is due to the bend allowance or bend deduction. Let me have the privilege to explain the bend allowance in this article.

What is a bend allowance? Bend Allowance Vs Bend Deduction

Bend allowance is the amount of material that you need to add when you bend or unbend a sheet metal part

Bend deduction is the amount of material that you need to deduct when you bend or unbend a sheet metal part.

Logically both bend allowance and bend deduction are the same things. The terminology is different just because of how we dimension it. Here is an example.

Now let us take an example to understand how bend allowance or bend geometry can affect flat part dimension.

In this example, this is an L-shaped bracket having a dimension of 5.00 and 3.00 inches. So you may think that if we Unbend this part, the flat sheet size will be 8.000 inches. But you are wrong.

The size will be less than 8.000 inches. If you see the flat pattern size, it is 7.61 inches. So there is a difference of (8.000-7.610 )=.390, which is called bend deduction. How we arrived at this number depends on many factors that we will discuss later in this article.

Why does Bend Allowance exist? What is K factor, Y Factor, and Neutral Axis

Well, to understand bend allowance, we need to understand how sheet metal behaves when it comes under the tool. Below is a snapshot of a bend and the flat pattern of the bend

In this example
T= Thickness of the sheet
R= Internal Bend Radius
L= Flat Length of the bend
δ= Distance between the inside radius of the bend and the neutral axis
N= Neutral Axis

When a sheet metal forms a bend, the inner portion ( Covered by δ) is always in compression which reduces the part length. On the other hand, the outer portion ( Covered by T-δ) is under expansion that always tries to elongate the part.

To compress this compression and expansion, bend allowance exists so that we get the correct length of the flat part.

Bend allowance depends on the following factors.

• Material composition in the sheet metal
• Percentage of carbon content in the sheet metal
• Bend tool radius
• The angle of the bend
• The thickness of the sheet metal

What is a neutral axis in sheet metal

The neutral axis is the line that remains constant irrespective of compression or expansion. In FIGURE-1, the red line is the neutral axis.

The position of the neutral bend line or neutral axis decides the length of the flat part. Typically neutral axis is at the center of the sheet metal. But it can be anywhere in the sheet metal.

What is the K factor in sheet metal?

The k factor is the ratio of the neutral axis location to the sheet metal thickness. In this Figure 1, the K factor can be calculated as (δ/T). The value of the K factor varies from .33 to .50. K Factor depends on material composition and percentage of carbon contents.

K Factor= (Distance between the inside radius of the bend and neutral axis)/ Thickness of the sheet

With the K factor, you can find out the location of the bend axis. If you know the K factor and sheet metal thickness, you can find out the location of the neutral axis using the below formula.

Neutral Axis location= K Factor x Sheet Thickness

K factor of different materials ( Approximate)

What is the Y factor in sheet metal?

The issue with the K factor is that it does not consider internal stress. Y factor does consider internal stress. With the Y factor, you will get a precise bend allowance. Y factor of any material can be calculated using the below formula. The default value of the Y factor is .50

Y Factor= K factor X (π/2)

Bend allowance formula: Bend Allowance Calculator

Now here we are in the fun part about how to calculate the bend allowance. Well, most of the companies already use the standard bend allowance datasheet given by the supplier.

But in case you are in a project where you need to calculate the bend allowance, you can use this formula. This formula holds good for most of the materials.

But as I said the bend allowance also depends on tool radius and numerous other factors. So this formula or any other formula will give an approximate value. The exact value needs to be calculated using the trial and error method.

Bend Allowance = (Π/180)x A x((R+(KT))

Where,

A= Bend Angle
R= Inner bend radius
K= K Factor
T= Sheet Thickness
Π=3.142

Flat-length calculator for sheet metal

To calculate the flat length of any sheet metal part, you can either calculate the bend allowance and add it to the total length or use the below formula. This formula holds good for most of the material but gives an approximate value.

Flat Length = ((Π/2 )x R) + (Y x T))x ( A/90)

Where,

A= Bend Angle
R= Inner bend radius
Y= Y Factor
T= Sheet Thickness
Π=3.142

Bend Table in sheet metal

As I said earlier in this article most of the companies use their standard bend allowances that they get from suppliers. So the bend table is a chart that has standard bend allowance data for different materials with different thicknesses and different bending operations.

No one prefers to calculate bend allowance using formulas and instead use bend tables. Here is an example of a bend table.

In this table, you can see that the table gives the bend allowance value against the sheet thickness and the inner radius. Also if anything goes out of the range you can use the formula to calculate the bend allowance.

Frequently Asked Questions (FAQ)