Generative Sheet Metal Design Processes

Generative Sheet Metal Design Processes

Overview: Three basic processes compose the majority of sheet metal manufacturing: bending, forming, and blanking (also called shearing). These processes use tools that apply force to thin sheet material deforming or shearing the material to a specific shape. It's important when designing a sheet metal part to understand the physical limitations of bending, forming, and shearing. Following basic design rules when designing quality sheet metal parts can tremendously impact the processes and costs required to manufacture the part.

Bending Concepts

Bending is the uniform straining of material around a straight axis. The bend must take place in the plastic range of the material for the bend to remain permanent. When the sheet is bent, the inside radius is in compression and the outside radius is in tension. This distorts material differently on each side causing the sheet to become longer overall. Only at the neutral axis does the sheet retain its initial dimensions.
Two common ways to bend sheet metal are V-bending, and punch and die bending. The V-bend is usually accomplished by using a brake press. A brake press has a long bed and a selection of standard V-blocks that can make acute, obtuse, and 90-degree bends. The punch and die technique uses a punch that bends the sheet over a die. A pressure pad holds the material in place during the bend. This is commonly known as forming.

Blanking Concepts

Blanking uses a tool, usually a punch and die, to stamp out a peripheral shape in a single stroke. You can add other features into the tool to produce additional shapes, such as holes and internal profiles, as well as bent flanges in more advanced tools. To blank a profile, break or shear away the material from the parent material. As illustrated in the following figure, the shearing action takes place by stretching the material into a die, which puts the top of the material into compression and the bottom of the material into tension.

This causes a reduction in the cross-sectional area at the edges of the punch and die. Fractures usually occur in the reduced areas, which may eventually break as more force is applied.

Flat Patterns

The flat pattern is a two-dimensional layout of a three-dimensional part defining the shape and size of the flat sheet before it is formed. The flat pattern determines the bend radius length when it is flat and adds this length to the straight sections of the part. A mathematical formula, the Bend Allowance Formula (BAF), calculates the flat length of the bend radius

Bend Allowance Formula

The Bend Allowance is calculated near the center of the material thickness. The neutral axis, which is the location of the calculation, locates where the bending forces, the tension, and compression are zero.
The location of the neutral axis depends on many factors, such as the hardness of the material being bent, the bend radius, and the process used for the bending. Use a factor called the K Factor to calculate the neutral axis in the bend allowance formula. Common values for K Factors work well for parts with standard tolerances.

A general rule for the K Factors is to use .33 if the bend radius is less than the material thickness, .44 if the bend radius is one to two times the material thickness, and .5 if the bend radius is more than two times the material thickness. For precise bending, the actual K Factor is determined by trial and error based on the material and the process. The general formula for bend allowance is illustrated in the following figure.

The bend allowance must be calculated for each bend and added to the straight sections of the part to calculate the overall developed length of the flat pattern. The bend lines and bend tangent lines are included in the flat pattern to assist manufacturing personnel in aligning the bending equipment. The following figure illustrates a typical flat pattern with the appropriate information included.