Airbender discrepancy against IV

[Stian]

Hi. We have a airbender in the workshop from rolleri.

I use a 40mm die, internal radius 4,5 on knife.

Thickness of test sample was 5mm, S355 carbon steel. Europeen standard.

90 deg. bend, with 50mm lenght both sides. ( see picture)

When this is the input, the airbender calculate that the flat lenght of the test-piece should be 89,9mm, to achieve 50mm lenghts on both sides.

To achieve what the machine calculate, my kfactor is down to 0,240. Quite far away from standard 0,44 etc.

kelly.young has embedded your image for clarity.

[RobSinglehurst]

How have you worked out the bend radius? I normally measure the outside radius with radii gauges and then subtract the material thickness. When bending 5mm MS with a 32mm die I use a bend radius of 4.5mm and with a 50mm die I use a bend radius of 6.5mm therefore I should think your radius for a 40mm die should be a bit bigger than 4.5m. 

My K Factor is the default 0.44

[Stian]

I use a 40 die with a 4,5 ri knife. Measuring the internal radie after bending it is 4,5 or 4,4mm. The radie is acceptable.

My problem seems to be flat lenght versus leg lenght after bending.

Input on machine is 50mm lenghs on both sides, 90 degree bend, 5mm thickness and the airbender says the flat lenght is supposed to be 89.90mm to achieve this.

My Inventor says this is achieved with 91,58mm flat lenght. Kfactor 0,447. 

[Frederick_Law]

0.44 is not a standard, its just a number.

So use whatever you found that work with your setup.

I usually use 0.3 or 0.28, I think.

Your 0.24 is pretty close to 'rule of thumb' or 'minus 2 thickness on 90 deg bend'.

[swalton]

As @Frederick_Law says, k-factor is a number that Inventor uses to make the CAD model match reality.  Adjust the k-factor until the test sample comes out right.

I used individual k-factors for each combination of material (steel, stainless, aluminum alloys), thickness, die width, and punch radius.  This requires cutting, measuring, forming and measuring test samples for each variation of those four variables.  Our drawings have a note that calls out the k-factor, die width, and punch diameter used to create the flat pattern.  The material call out in the title block defines the alloy and thickness.

Some materials may behave differently depending on the grain/rolling direction of the stock.  If so, it may be necessary to make test samples and adjust k-factors for each bend or watch the grain direction when cutting out the material blanks.

I add the k-factor information to the bend notes on the flat patterns that I send to our outside sheetmetal vendors.    I hold the vendors responsible for the formed shape, so they are free to use the flats that I produce or their own from the formed stp file that I send.

[Stian]

kf= 0,233 is supposed to be corecct, if you lay it out on the airbender machine. 

I am watercutting some samples to test now. 

From my reading on the subject, the kf on mild steel is between 0,3 and 0,447.

>0,447 is on harder steel types.

Any information where this rule of thumb is from?