Bending is a manufacturing process by which metal can be deformed by plastically deforming the material and changing its shape. The material is stressed beyond its yield strength but below its ultimate tensile strength. There is little change to the materials surface area. Bending generally refers to deformation about one axis only.
Bending is a flexible process by which a variety of different shapes can be produced though the use of standard die sets or bend brakes. The material is placed on the die, and positioned in place with stops and/or gages. It is held in place with hold- downs. The upper part of the press, the ram with the appropriately shaped punch descends and forms the v-shaped bend.
Bending is done using Press Brakes. Press Brakes can normally have a capacity of 20 to 200 tons to accommodate stock from 1m to 4.5m (3 feet to 15 feet). Larger and smaller presses are used for diverse specialized applications. Programmable back gages, and multiple die sets currently available can make bending a very economical process.
BENDING ALLOWANCESWhen sheet metal is bent, the inside surface of the bend is compressed and the outer surface of the bend is stretched. Somewhere within the thickness of the metallies its Neutral Axis, which is a line in the metal that is neither compressed nor stretched.
What this means in practical terms is that if we want a work piece with a 90 degree bend in which one leg measures A, and the other measures B, then the total length of the flat piece is NOT A + B as one might first assume. To work out what the length of the flat piece of metal needs to be, we need to calculate the
Bend Allowance or
Bend Deduction that tells us how much we need to add or subtract to our leg lengths to get exactly what we want.
The location of the neutral line varies depending on the material itself, the radius of the bend, the ambient temperature, direction of material grain, and the method by which it is being bent, etc. The location of this line is often referred to as the K factor.
K-factor is a ratio that represents the location of the neutral sheet with respect to the thickness of the sheet metal part.
REVERSE ENGINEERING THE K-FACTORFirst, cut a strip of material and measure its length and thickness as accurately as possible. The width of the strip is not that critical but generally somewhere around100mm (4 inches) or so usually does the trick.
Then, bend the strip to 90 degrees, and measure its Length X and Length Y as shown in the diagram below.
NOTE: that it is very important to bend the sample piece in exactly the same manner as you plan to bend your real pieces, so that whatever you measure now becomes reproducible later.The bend radius can be extremely difficult to measure accurately but, in this case, is not critical (within reasonable limits, of course!). The reason it is not critical is that what we are interested in is a number to use in our CAD program that, with the bend radius used in our CAD program, will produce the results you are measuring in real life.
In other words the K-factor you calculate now will take into account any small inaccuracies in the bend radius measurement and compensate for it. If, for example,we are using a Bend radius of 0.5 in our CAD program, it does not matter if our real tooling radius is actually 0.4, as the K-factor, which was worked out from our real tooling, corrects for this. The only implication of this is that we may occasionally get a K-factor that seems odd (higher than 0.5, for example) if our real radius is very different from our CAD program radius. Remember though that most CAD programs such as Solidworks only accept K-factor values from 0 to 1, so if the calculated K-factor is outside these limits, then you may need to double-check your numbers.
The correct K-factor to use in your CAD program can now be calculated as follows:
BendDeduction = Length X + Length Y - Total Flat Length
OutSideSetBback = (Tan(BendAngle / 2)) * (thickness + BendRadius)
BendAllowance = (2 * OutSideSetBback) BendDeduction
K-Factor = (-BendRadius + (BendAllowance / (ลก * BendAngle / 180))) / thickness
Using this method will produce the most acceptable results other than by using a bend table.There are however also some general rules of thumb that can be used for K-factors that will generally give results that are within acceptable tolerances for non-precision sheet metal work. Some of these sample K-factors are given in the methods of bending section below.
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