Configuring bend tables of a CAD sheet metal module

All CAD users when designing sheet metal parts are familiar with bending and unbending operations. Usually they design sheet metal parts fully formed (or unbend) and after the sheet metal module generates a bend part based on the unbend part. Bending lines and angles also are generated. The generation of the unbending part is based on a mathematical calculation of an unbend curve for each bending geometry. Standards like the din 6935 describe this calculation methodology. The main parameter of this calculation is a constant, the neutral axis position. There are numerous references about bent allowance calculation ("The length of the neutral axis, or bend allowance,… Cold-working processes –sheet and coil stock, Tool & Manufacturing Engineers Handbook SME).

The manufacturers of bending equipment embodied the methodology inside the CNC controller. Usually a CNC user is programming the bending press, feeding it with inputs like the profile, material, sheet metal thickness and the bending tool. The CNC controller calculates the bend allowances and finally the unbend length of the profile. This procedure is simple enough for manufacturing all the bend profiles, but usually the situation is more complicated. The profile is just a side projection of a long sheet metal part with numerous cuts holes special punch features (cut, shapes, threads, etc.). All these features are made with the punching machine. The punching machine must be programmed based on data of the unbend sheet metal version of the part. The CAD software builds the bend part, and the next question is the similarity between the produced unbend part and the unbend profile length of the CNC controller.
The CAD software usually is programmed with data for a default bend table. If we want similar unbends profiles we must invest more time for the configuration of the sheet metal CAD module. The parameters are basically three: material, sheet metal thickness, and tool geometry. From statistics are known the potential combinations: materials x thickness x tool geometry. The configuration methodology is simple, the CAD designer designs a simple profile with a few bends. The CNC user programs the bending press with the data of this profile and calculates the unbend length according to press software for the potential combinations.

For each combination the CAD users makes an error-and-try procedure in order to achieve the same unbend length. The CAD user can change two useful parameters inside the bend table: the inside radius of the bend (depends on the bend angle and tool geometry) and the neutral axis position constant (depends on material and tool geometry). For the same tool the inside radius change when the bend angle changes.  The precision of the method is not affected because this change is small (about a few hundred's of a millimeter) between a 90 degrees and an oblique or acute angle. In a few words the steps are:

1. Construct a simple profile with few 90 degrees angles (2 or 3).
2. Make a table with column headers the thickness, the bend tool (inside radius of bend angle) the material and the total unbend length. In each row we will enter the data and the results for each combination.
3. Program and simulate (calculation of the resulting unbend length) the profile in the CNC software for each combination.
4. With the table data configure the CAD software bend tables (input the inside angle and the sheet metal thickness). Construct the unbend version of the part and measure the unbend length.
5. Change the neutral axis position constant until the CAD calculated unbend length is equal to CNC unbend length (error-and-try method).

With this procedure you will construct as many bent tables as the potential combinations dictated by your manufacturing department.