SolidWorks 2011 Parts Bible - Matt Lombard [276]
Bend Allowance
You can control the Bend Allowance by using one of four options:
• Bend Table
• K-Factor
• Bend Allowance
• Bend Deduction
Bend Table
Two general types of Bend Tables are available, text-based and Excel-based. The first few rows of each type of table are shown in Figure 21.5. Each table can use K-Factor, Bend Allowance, or Bend Deduction.
FIGURE 21.5
Sample text- and Excel-based Bend Tables
Sample Bend Tables can be found in the lang\english\Sheetmetal Bend Tables subdirectory of the SolidWorks installation directory. While the values may not be what you need, the syntax and organization are correct. You may want to contact your sheet metal fabrication shop to see what they are using for a table or equations.
Note
Data from Gauge Tables and Bend Tables have been consolidated, but both legacy types can still be read.
K-Factor
When sheet metal is formed from a flat sheet, bending the metal causes it to stretch slightly on the outside part of the bend and to compress slightly on the inside part of the bend. Somewhere across the thickness of the sheet is the Neutral Plane, where there is no stretching or compression. This Neutral Plane can be at various places across the thickness, depending on the material, tooling, and process. The ratio of the distance from the inside bend surface to the Neutral Plane to the thickness is identified as the K-Factor, where .5 means halfway, 0 means on the inside face, and 1 means on the outside face. Typically, you can expect values between .5 and .3.
Bend Allowance and Bend Deduction
Bend Allowance and Bend Deduction are specific length values, not a ratio like the K-Factor. The Bend Allowance is essentially the arclength of the Neutral Plane through the bend region. The Bend Deduction is the length difference between a sharp corner and the radius corner, as expressed by the formula in Figure 21.6.
The three values are related, as shown in Figure 21.6. The dark rectangle represents the bend area. Material outside of the bend area really does not matter, although it is usually shown and used in the generally accepted formulas about bend calculations for sheet metal.
FIGURE 21.6
Calculating the Bend Deduction from the Bend Allowance and K-Factor
You usually use a ratio t/T (the K-Factor) from a published table or by asking your sheet metal vendor what values they typically use. The values from the tables have been developed experimentally by bending a piece of metal of known length and then measuring the arclength of the inside of the bend and the arclength of the outside of the bend. By comparing these numbers to the original linear length of the bent area, you can find the t value and thus the K value. From the K value, the BA (Bend Allowance) value can be calculated, and from that, the BD (Bend Deduction) value is easy to find.
The specific formulas for finding these numbers are not as important as an intuitive grasp of what the numbers mean and how they are used, at least in relation to using SolidWorks to model sheet metal parts. The numbers used to fill out Bend Tables using K, BA, or BD values are typically taken from experimentally developed tables.
Bend Calculation Tables
Bend Calculation Tables are Excel spreadsheets that enable you to divide bend angles into ranges and assign flat (also referred to as developed) length equations for each range. You can also assign an equation for the K-Factor.
The Excel table must be set up like the example shown in Figure 21.7 in several respects.
FIGURE 21.7
Setting up the Bend Calculation Table
The declarations in the first several rows must include the following items in the first column (capitalization is not important, but the colon must be as shown). Although the first column may also contain other declarations,