Get bent, or how I came to love K factor

It came to my attention tonight that a certain facet of bending things has not been discussed here so I thought we’d discuss it very briefly and I’d drop a few URLs for further reading.

Whenever anything is bent two things happen. One, the outer radius is stretched, and two, the inner radius is compressed. If the inner and outer stretched the same amount as the inner compressed, the maths would be super simple and for things that tend to be forgiving the simple will usually work. For other things, you might cut and then bend it is a bit more complicated.

Living hinges should be forgiving enough most of the time to ignore this, leather would be the same.

Get into plastics such as acrylic, now you are going to have t consider K factor.

What is K factor? Well, all it is is a multiplier that compensates for the fact that materials don’t always bend right down their center, in fact, they rarely do.

I had to learn to calculate bend allowances by hand but great news, Autodesk just added a sheet-metal module to Fusion 360 so learn that and all the maths will be taken care of for you. I would be beneficial to read through some though so you will understand what all is being compensated for.

For most metals, there is a published K factor but for the things we work with it will be hit and miss. Try google for finding a starting point but like kerf, you may well have to find it through experimenting with a material.

Go here for general understanding:

Go here for plastic:

These are just starters. Read as much as you want/need. Knowing that it exists is the larger part of getting where you want to be.


Super! Thanks for linking that, I’m going to do some reading up and will probably have a few hundred questions (after the holidays)! :smile:

Oh, and i Matrixed it.


I’m probably going to sit down in front of Fusions sheet metal tools most of next week, that and I’m trying to learn EAGLE.

1 Like

Did you see that AutoDESK is integrating Fusion360 and EAGLE, so designs can move from one to the other seamlessly?


Yes I did. Exciting.

1 Like

I used the feature the other day, it’s really cool!!

I am trying to cut a living hinge cornered box in fusion 360 using the sheet metal feature. The length of the bent radius when taken through the “undfold” command isn’t coming out to the correct number. For instance, I have a 90 degree corner, radius of 1.27324" which should generate a line length 1/4 of the circumference = (2PIr)/4= ((2* PI *1.27324" )/4) = 2" long line. My plan was to then cut evenly spaced 1/4" tabs along this radius so that I have a consistent tab all the way around the box. Unfortunately, it comes out to some other random shorter dimension. I finally had to change the k value to “1” to get the value to come out correctly when flattened out.

Bend I’m adding tabs to:

Initial wrong value in flat pattern:

K value changed to “1” gives correct length: