Bend line– The straight line on the surface of the sheet, on either side of the bend, that defines he end of the level flange and the start of the bend.
Bend radius – The distance from the bend axis to the inside surface of the material, between the bend lines.
Bend angle – The angle of the bend, measured between the bent flange and its original position, or as the included angle between perpendicular lines drawn from the bend lines.Sometimes specified as the inside bend radius. The outside bend radius is equal to the inside bend radius plus the sheet thickness.
Neutral axis – The location in the sheet that is neither stretched nor compressed, and therefore remains at a constant length.
K-factor – The location of the neutral axis in the material, calculated as the ratio of the distance of the neutral axis T, to the material thickness t. The K-factor is dependent upon several factors (material, bending operation, bend angle, etc.) and is greater than 0.25, but cannot exceed 0.50. K factor = T/t
Bend allowance – The length of the neutral axis between the bend lines or the arc length of the bend. The bend allowance added to the flange lengths is equal to the total flat length.
The K-factor is the ratio between the the neutral axis to the thickness of the material.
Importance of the K-factor in sheet metal design
The K-factor is used to calculate flat patterns because it is related to how much material is stretched during bending. Therefore it is important to have the value correct in CAD software. The value of the K-factor should range between 0 – 0,5. To be more exact the K-factor can be calculated taking the average of 3 samples from bent parts and plugging the measurements of bend allowance, bend angle, material thickness and inner radius into the following formula:
Some basic K-factor values are shown here. Use these as a guideline.
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