Bend allowance when using Autodesk Inventor’s “Bend Part” feature.

Most users of the Sheetmetal tools in Autodesk Inventor are familiar with bend allowances (usually using k-factor), which allow them to “calibrate” their inventor models to ensure that the flat patterns it creates will produce accurate blanks. This is necessary to produce accurate finished parts.

Recently a customer contacted us to ask us if similar functionality was available for the “bend part” feature, as he was having issues getting the cut lengths correct for parts that were made by bending standard sections. This raised an interesting topic, so I have attempted to shed some light on adjustments that can be made to ensure your parts come out the way you expect them to.

To create a “Bend Part” feature in Inventor, you first click the button that is hidden under the drop-down menu on the “Modify” panel which is on the “3D Model” tab. The only input geometry that you are required to create is a sketch line on a plane that the part will be bent around. The key thing here, is that the line you create is not the axis that represents the center of the bend, it is actually a line that represents the start of the bend. The bend axis is offset from this line in a direction normal to the plane that it is sketched on, by the distance you set as “Bend Radius” in the dialogue.

“Bend Part” Dialogue

So what if you put a bend in your part, and the finished part doesn’t come out the correct dimensions? This will happen to varying degrees based on the method you are using to bend the part, and how it forms the material. Different bending processes and bend radii will vary the amount of “stretch” in the part. Have a look at the following video, and see the effect of moving the workplane that contains the bend line:

To illustrate further, here are 3 copies of the same part with the workplane at different offsets. You can see that the bend radius is only consistent at the plane where the bendline is:

So as you can see, moving the workplane, affects the final length of the part. By doing this, you are effectively changing the inside bend radius by moving the reference, so there is some compromise here. Saying that, this is an easy way to add a “fudge-factor” to get your bent parts to come out correctly. 

To take this further, there are 2 things that I think would make things easier. Firstly, you could add the iLogic control for moving the ‘bend reference plane’ to a custom part like the one shown, and then replace the Content Center family template with this file. This would mean that anytime you place a piece of section from that family, the bend reference control would already exist in it.

The second thing would be to add a “bend allowance” parameter to your content family table that get’s added to the “B_L” parameter in the calculation for “G_L” This would then give you more accurate cut lengths for standard sections that are being bent. To do this however, you’d have to do some bend testing and work out the allowance factors for your particular bending process.

Hope this helps some of you.

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