The neutral factor for a sheet metal bend defines the position of the neutral string inside the material. The value can be 0 up to 1 (outer side = thickness of sheet). If you take a look to a cross section of a sheet metal bend the neutral string is the material section, which will neithet be strengthen or compressed through the bending procedure. The neutral factor depends on the raw material, the bend angle and the bending machine. It used to calculate the bend allowance to determine the flatten length of the sheet. In our case we are using experimental neutral factors.
Neutral factor is most important when you are using thick materials.
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To start with, the use of the so-called 'Neutral Factor' (AKA the 'K-Factor'), as already explained, is used to compute the 'bend allowance' so as to adjust the size of the flat stock to account for the deformation of the metal caused by the forming of flanges and such. Generally speaking, the smaller the inside radius of the bend, as compared to the thickness of the metal, the more critical it is that we compensate for the deformation of the material. Also, softer metals, such as Aluminum and Brass, will have a greater deformation while harder metals, like steel and bronze, will have less.
When working with Sheet Metal design software, such as NX Sheet Metal, the 'Neutral Factor' can either be tied to a material library or by simply entering an explicit value. Note that there are material specifications where you can find recommended 'Neutral Factors' for specific materials. Of course you could also perform your own tests if it's critical for your application. For information about how to go about this, go to:
However, there ARE certain 'rule-of-thumb' practices that can often be used for basic metal forming applications.
For many applications where it may not be critical you can often assume that for bend radii close to the material thickness, that for 'softer' materials you can use a factor of 0.33, and for 'harder' materials, a factor of 0.44 will often be acceptable. But keep in mind that there is a certain point, as the bend radius used is larger and larger as compared to the metal thickness, that one can often simply assume that there will be NO appreciable difference in the length of the formed versus unformed or flattened states. Unless tests indicate otherwise for the particular material and forming method used, you can often assume a 'Neutral' or 'K-Factor' of 0.50 when the inside bend radius is equal to or greater then 3 times the material thickness. But no matter what sort of material or forming methods are used, when dealing with homogeneous, isotropic materials, the 'Neutral Factor' can NEVER exceed 0.50.
Anyway, that should help you better understand how the 'Neutral Factor' is used.
John R. Baker, P.E.
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Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
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