Sheetmetal tube flat changes length?

[Rich05]

Simple tube flattened out the measurments change. using measuring tool when its bent the distance of the rim is 64.996 after flat 64.882. Its it possible sheetmetal feature will change the length of the part? This happens when unbend the tube from outside or inside edge.
Ty

[diycnc]

yes when metal bends it stretches. if your actually trying to fabricate the part you should probably leave the k factor alone. if for some strange reason you want your flat pattern to be the same size as the od of your part set k factor to 1 this will be in the sheetmetal feature at the top of the tree.

[Rich05]

great tip, thank you

so k factor will stretch the part?

this is my problem. I have a 60" x 9" piece of .120 wall 316 stainless that needs to be bent around something and bolted in. The bolt pattern has to be there when its flat for water jet, and has to be there when its bent, for visual / verification purposes it would be nice if the holes aligned when its bent over the part.


so your saying keep the k factor at 1 and the parts dimensions will not be alterted when bent?

you solved something that was torturing me thank you again. worked. I guess I should tell who ever planned this that the part will stretch most likely and that a 1 k factor is not accurate, actually the bend is fairly gentle so most likely it wont strech too much.

[diycnc]

k factor is the the neutral line in the part. if set to 1 if you want to use the outside of the radius for your flat length, set to 0 and you will use the inside, .5 will use the middle.

http://en.wikipedia.org/wiki/K-factor_(sheet_metal)

[CarbonKevin]

As already stated, K-factor is part of the determination of how much stretch a metal part develops over it's bend.

BUT, with tubes, sometimes, you aren't actually stretching the metal, per se.

A simple test - if you roll the flat into the desired tube size, then let go of it and it completely springs back, then you're staying within the elastic range of the metal - thus, no stretch, and no K-factor required. If it keeps some degree of curve, then you've moved into the plastic range of the metal, and it's stretched somewhat, meaning you'll need K-factor calculations to happen.

Over gentle curves, though, the degree of stretch is very small, and depending on your design, you may be fine avoiding K-factor entirely.

[diycnc]

when in doubt set k factor to .5

you can not create a circle out of a flat piece of metal without it stretching or compressing. it is simple mathematics. the outside of the part will have a larger circumference then the inside of the part.

this is why k factor always comes into play.

i will agree that on a large radius and thinner material the difference is negligible.

also its kind of hard to avoid k factor in sheet metal modeling software, you need to enter some value between 0 and 1 for the software to figure out the flattened length.

[CarbonKevin]

Forgive me, clearly I didn't state my point well enough. My bad.

K-Factor always exists.

BUT, depending on the sort of bend (elastic or plastic), the K-factor will either be outside the material, or inside the material...some number less than 1, or 1.

The test I already mentioned is an easy way to figure out if you should be using traditional K-factors, or a K-factor of 1.