Sheet Metal Bending – Bend Allowance and K-Factor
>> What is Sheet Metal Bending?
Sheet metal bending is the most common process in sheet metal enclosure manufacturing. It is a process where sheet metal is made to desired shape by plastically deforming the metal about one axis with a press brake.
The critical information here is – in the bending process, the metal is stressed beyond its yield strength but below its ultimate tensile strength. There is almost no change to metal’s surface area.
The basic metal bending process goes like this.
1) The sheet metal is placed on the Vee die, held in place with hold-downs, positioned in place with stops and gages.
2) The upper part of the press, the appropriately shaped punch, presses down and forms the desired v-shaped bend.
>> What Machines Are Used in Sheet Metal Bending?
Sheet metal bending is done with press brakes. Standard press brakes have a capacity of 20 to 200 tons to accommodate sheet stock from 3 feet to 15 feet. Bending is a very flexible process when combined with programmable back gages and multiple die sets, since a large variety of different shapes can be made in bending process.
Operating a press brake could be the most difficult task in a metal-fabricating shop. It requires a highly trained operator with hundreds of hours of training and experience to tackle the ever-increasing amount of precision required. Mistakes are costly and usually lead to the metal being scrapped.
>> What Are Bend Allowances?
Bend allowance comes from the fact that when sheet metal is bent, the inside surface of the bend is compressed and the outer surface of the bend is stretched.
So, when we want a 90 degree bend in which one panel side has a length of A, and the other panel side has a length of B, the total length of the flat piece will NOT be A+B, but rather equals A+B plus a bend allowance or A+B minus a bend deduction(depending on how we measure A and B).
Let’s look at the illustrations below.
So bend allowance is a term which describes how much material is needed between two panels to accommodate a given bend. Determining bend allowance is commonly referred to as “bend development” or simply “development”.
>> Get the Correct Bend Allowance with a Test Piece
Bend allowance is fairly easy to predict and calculate for most standard circumstances, but it can turn out to be tricky for some cases. So it is always a good idea to make a test piece for your bend allowance test before investing into costly tooling or production parts.
One of the easiest way to make a test piece is to shear a piece to an exact length, and then form it using the exact process that will be used to create the part. After the part is formed, the part is measured and compared to the expected lengths and the bend allowance is adjusted as needed.
While most bend allowances can be predicted with ease and will develop correctly, there is no perfectly scientific method for predicting bend allowance due to the many factors like tooling conditions, actual vs. planned thickness, forming method and the given part tolerance. Most companies will develop their bend allowances based on standard formulas, standard forming practices and historical trial and error.
These bend allowances can be measured for many materials and scenarios and then tabulated so that the table can be used by CAD programs (such as Solidworks, Pro-Engineer, Autodesk Inventor, etc) to produce accurate sheet metal work.
>> What is K-factor?
As we explained in bend allowance, when sheet metal is bent, the inside surface of the bend is compressed and the outer surface is stretched, but somewhere within the thickness of the metal lies its Neutral Axis, which is a line in the metal that is neither compressed nor stretched.
The location of the neutral line varies depending on the material itself, the radius of the bend, the ambient temperature, direction of the material grain, and the method by which it is being bent, etc. The location of this neutral line is referred to as the K-factor.
Many CAD programs also work out bend allowances automatically by using K-factor calculations.
K-factor is a ratio that represents the location of the neutral line with respect to the thickness of the sheet metal. Let’s take a look at the following illustration.
Bend allowances are calculated using a K-factor as follows. This formula is pretty accurate and straight forward. However, the problem is that we need to know the correct K-factor first. We will check out how to get the K-factor by reverse engineering.
>> Reverse Engineering K-Factor
As we discussed in “Get the Correct Bend Allowance with a Test Piece” above, the most accurate way of finding the correct K-factor to use in your CAD program is by using the reverse engineering method with a test piece.
First, cut a piece of sheet metal and measure its length and thickness as accurately as possible. The width of the piece is not very critical but 4 inches will be OK.
Then, bend the piece to 90 degree, and measure its Length A and Length B as shown below. It is critical that you bend the sample piece in exactly the same manner as you plan to bend your real pieces.
The correct K-factor to use in your CAD program can be calculated as follows.
Bend Deduction = A +B – Total Flat Length
Outside Setback = (Tan(Bend Angle/2)) * (Thickness + Bend Radius)
Bend Allowance = (2 * Outside Setback) – Bend Deduction
K-factor = (-Bend Radius + (Bend Allowance / (π * Bend Angle / 180))) / Thickness
This method produces the most accurate results other than by using a bend table.