Creating unfolded sheet blanks, with K factor compensation

There is theoretical line, neutral axis, based on observation that runs through any material having a thickness when bent that does not deform. The outer surface stretches and the inner surface compresses on the corners. In Rhino therefore you would term it the neutral surface. Deriving this neutral surface and then flattening it gives you the size of the piece of material you need to cut out pre-bending. It is always longer than the upper surface when flattened and always shorter than the lower surface when flattened.

For those who want to understand the theory, which I wont go into here, just to name the key components and their definitions.

  1. Internal radius, the bend radius on the internal surfaces. Critical variable.
  2. The angle of the internal radius.
  3. Thickness of material.
  4. K factor this is material, machine and bending machine type dependent. Bending shops have tables for material types, material thickness and their various machines. There are some material manufacturers who actually specify a global K factor, shops would adjust these slightly to match their machines.
  5. Bend Allowance The theoretical length of the arc of the neutral axis at the bend point.
  6. Bend Deduction The Amount of material to be removed from the blank outside surface dimension for each bend. Half on each side.

Now you have the theory, and you can dig deeper if you need to, here is the fun part, Rhino solves your headache of sitting and calculating all these things and can do it all automatically with a couple of clicks, could be fully automated into a command if need is great enough.

1.Procedure Select all inner surfaces using Ctrl Shift Click, or using Pascal’s Python Script “ExtractTangentFaces” copy this surface to new location.

2 From the drop down use Offset Surface (_OffsetSrf) Flip direction to point to outside Surface. Choose Solid “No” from menu bar. Enter thickness of material Multiplied by K factor ( a decimal fraction between 0 and 0.5) In the example shown 1.2192 X 0.38 ( K factor for this material at this bend shop is stated at 0.38)

  1. Unroll this newly created Surface using drop down menu Surface>Surface Flattening>Unroll Developable Surface. Make sure the world origin is clear of objects as the result is going to be delivered at 0,0,0. Explode “No” Labels “No” Keep Properties “No” Enter

You have your blank ready to dimension to send to bending shop for bending.


Have a beautiful day



Things become more complex if you have up and down bends. You would then have to offset in both directions and extract the right surfaces and stitch these back together, then unroll.

Or just send the shop the final result you want to achieve and they rebuild it in SolidWorks Sheetmetal or whatever they use because this stuff is a bit of a black art, the K factor can vary by the phase of the moon, so just leave it up to the experts.

The “middle surface” is probably close enough for a crude DIY prototype, though.

In some cases even the inner surface is the correct one to use. We used this method a lot for a student project involving 1mm aluminum.

That’s probably the best advise.

Yeah I haven’t done much of that stuff but what I’ve seen the guys who do this all day can achieve amazing accuracy. You know, except when they make a mistake in SolidEdge Sheetmetal and make a $20K prototype assembly 2" too big.