# Bend deduction and k factor questions and suggestion

• posted

Hi, I keep getting this message after trying to unfold a tube with a cut down it's side.

Sketch Arc length 1108.78mm chord Length 0.2mm Radius 176.5 Angle 359.94

Base Flange direction 1 = 420 sheet metal parameters T1 =2mm reverse direction radius 174.5

sheet thickness 2mm bend radius 174.5

Error Unfold1 The specified bend deduction value for the given radius and thickness is invalid. Try using a different bend deduction value, or use a different bend allowance type.

Now here is my theory, if Solidworks knows that the bend deduction value to be invalid, then it must know what the valid range is!

So could it not be helpful and show what the valid range is instead?

k factor 0.9525 works and the tube unfolds.

Is there a way to work out the bend deduction from the k factor?

• posted

Do you understand what k-factor is?

• posted

K-factor is a ratio that represents the location of the neutral sheet (t), with respect to the thickness (T) of the sheet metal part.

In other words K-factor = t/T.

But, our sheet metal department does not use the k-factor they use, bend deduction.

So it would make sense to use bend deduction.

On a 90 degree bend this is easy to work-out, But I am having trouble with a rolled tube in solidworks.

I know what the blank size is, I know what the bend deduction is, but I can not get Solidworks to make the part, with that information. Hence my posting here.

• posted

Generally when the inside radius is greater than or equal to twice the material thickness, the neutral axis is half way through the material, therefore k=.5.

I just tried it and it worked fine if using K factor. Flat length =1102.50. And by creating the sheet metal part this way, you will find that it recognizes the large radius and uses it as the bend radius. Do a RMB on BaseBend1 and select Edit Feature to see that.

But if I then say that the bend deduction should be 6.282 because that's the difference between the neutral axis length and the outside arc length, then I end up with the same error you do. My guess is that BD will not work properly with such a large radius relative to the material thickness. Interesting, indeed.

WT

• posted

Good. Using K-factor, you can then calculate the unbent length of the bend region. I believe this is what most call "bend deduction".

• posted

Never mind that. I was wrong. Turns out there is a good explanation of bend deduction in the help.

Always used K-factor in the past.

• posted

As Wayne has found out, BD does not work as expected, in this case.

If nothing else it has proved that I was not having a bad day, Solidworks was,lol

• posted

The K-factor is the placement of the centerline of the material in the bends. (i.e. 0,30 means its 0,3 x material thickness meassured from the inside of the bend in SolidWorks) That factor is very dependent on the bending tools used.

You should have that in mind if you use different sub-suppliers (different set of tools) for manufacturing

• posted

Thank you Ronni,

That's a useful bit of info to bear in mind.

:o)

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