Sheet metal parts are easy to model incorrectly because the finished 3D shape is only one part of the manufacturing problem. The flat pattern, material thickness, bend radius, tooling and bend allowance all affect whether the physical part will match the CAD design.
What Is the Inside Bend Radius?
The inside bend radius is the radius formed on the inside surface of a sheet metal bend. It depends on material, thickness, tooling and the selected manufacturing process. A radius that is too small can cause cracking, deformation or an impractical bending operation.
What Is K-Factor?
K-factor describes the position of the neutral axis through the sheet thickness during bending. It is used by CAD systems to calculate the developed flat length. The value is not universal; it changes with material, thickness, tooling and process conditions.
Bend Allowance and Flat Pattern
The bend allowance represents the length of material consumed around the bend region. In simplified form, bend allowance can be related to the bend angle, inside radius, thickness and K-factor. Your CAD system normally performs the calculation once the sheet metal parameters are defined correctly.
Five Checks Before Releasing a Sheet Metal Part
- Thickness: Confirm the material thickness in the model matches the intended stock.
- Inside radius: Make sure the selected radius is realistic for the machine and material.
- Hole-to-bend distance: Keep holes and cutouts far enough from bends to reduce distortion and tearing.
- Corner relief: Add suitable relief where intersecting bends could otherwise create unwanted deformation.
- Flat pattern: Unfold and inspect the developed profile before sending the design for laser cutting or punching.
Why K-Factor Should Not Be Blindly Copied
Using a single K-factor for every material and thickness can produce inaccurate flat patterns. If your manufacturing supplier has validated bend data, use that information. Otherwise, start with the CAD system’s recommended values and validate them with actual parts or process data.
Design for Manufacturing Tip
A good sheet metal model considers manufacturing from the beginning. Check bend sequence, tool access, flange length, minimum features near bends and whether the chosen geometry can actually be formed on the available equipment.
For related learning, see the Design Guide and the Shaft Torque Calculator for another example of practical engineering calculation.
Image: Sheet metal bending and manufacturing. Source: Unsplash.