Measured length of unfolded sheet does not match the calculated circumference

[lamprecht.mark]

Hello,

I would like to create a dxf file of a square tube to perform a 4 axis plasma cut operation with sheetcam.
For this I have created a tube with the sheet metal tool that has a parting line at one corner to create the unwinding.
When the 80 mm tube is unfolded, the dimensions do not fit. The calculated dimension should be 309.7 mm, but when I measure the sketch, I only have 297.7 mm, so I am missing a whole 10 mm. Can anyone tell me where the error is?

[jhackney1972 · Accepted solution]

Sheet metal uses a calculation to determine the amount of material consumed in the bending process which uses the "neutral" axis of the material.  This is a line, across the thickness, where the metal does not change dimension during bending.  This calculated line used a sheet metal material property called K-Factor.  This is a number that is the ratio of the full thickness to the neutral axis.  In your model, it is 0.44.  If you change this number to 1.0, your unfolded value will grow.  I am not sure of your measurements, as I did not calculate them myself, but this is one reason for the difference in material length to calculated length.