sheet metal fabrication design guide

Sheet Metal Fabrication Design Guide

Basic Principles

Sheet Metal Fabrication is the technique of shaping parts from a single sheet of metal through processes like cutting, punching, stamping, and bending.

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3D CAD files are transformed into machine code, which directs machinery to accurately shape metal sheets into the desired parts. Sheet metal components are renowned for their robustness, making them perfect for applications such as chassis. When producing low-volume prototypes or large-scale production runs, these parts can become economically viable due to the significant setup and material costs involved initially.

Since parts are derived from a single sheet, consistency in thickness is crucial. Adhering to design specifications and tolerances is essential to achieving parts that align closely with the intended design and effective sheet metal cutting.

Bend Line: This is the straight line on the metal sheet that marks the limit of the flat flange and the beginning of the bend.

Bend Radius: This refers to the distance from the center of the bend to the inner surface of the material, located between the bend lines.

Bend Angle: This angle is measured between the bent flange and its original position, or as the angle formed by perpendicular lines drawn from the bend lines. It can sometimes be referred to as the inside bend radius, while the outside bend radius equals the inside radius plus the material thickness.

Neutral Axis: The point within the sheet metal that remains neither stretched nor compressed, maintaining its original length.

K-factor: This value indicates the location of the neutral axis in the material, determined as the ratio of the neutral axis distance (T) to the material thickness (t). The K-factor could vary based on several factors, including material, bending operations, and bend angle, typically ranging from 0.25 to 0.50. It is computed as K-factor = T/t.

Bend Allowance: The length of the neutral axis spanning between the bend lines, or the arc length of the bend, is integrated into flange lengths to equal the total flat length.

K-Factor

The K-factor is a pivotal ratio in sheet metal design as it correlates to the amount of material stretched during bending.

Importance of the K-factor in sheet metal design

The K-factor is crucial in calculating flat patterns since it relates to the degree of material stretch during bending. Correct input of this value in CAD software is necessary, typically falling within a range of 0 to 0.5. For precision, the K-factor can be derived by averaging measurements from three bent parts and applying the values of bend allowance, bend angle, material thickness, and inner radius into a specified formula.

Some basic K-factor values may be used as guidelines.

Springback

During bending, internal material stresses can result in a slight springback post-operation. Because of this elastic recovery, a precise over-bend is required to achieve the target bend radius and angle. The ultimate bend radius often becomes larger than initially intended, and the final bend angle diminishes. The ratio of the final bend angle to the initial one is termed the springback factor (KS). This effect can depend on various factors, including material type, bending operation, and initial parameters.

Dimensions:

To avoid fracture or distortion, maintain the inner bend radius to be at least equal to the material thickness.

Bend Angles:

A tolerance of +/- 1 degree for all bend angles is considered acceptable within the industry. The flange length should be a minimum of four times the material thickness.

Rule of Thumb:
It’s advisable to maintain uniform radii across all bends; additionally, flange lengths need to be at least four times the material thickness.

Material Thickness, t:

Material thickness does not correlate proportionately to the tonnage required during bending, as doubling the thickness does not lead to a doubling in tonnage. Instead, it is linked to the square of the thickening, meaning that a two-fold increase in thickness results in a fourfold increase in tonnage.

Work Piece Length, L:

Just as with material thickness, tonnage requirements are directly proportional to the workpiece length; doubling the work length doubles the necessary tonnage. However, when dealing with shorter pieces (under 3 inches), the required tonnage may be significantly less than what proportionality would suggest, safeguarding the die from potential damage.

Air Force Bending Chart:

This chart indicates the tonnage necessary for bending various thicknesses of sheet metal. It serves as a valuable reference for sheet metal designers to establish minimum flange lengths based on tooling for different V-blocks, as well as the appropriate bend radius. The data presented is sourced from the Armada Air Force bend guide.

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