Hello Oliver,
here is a little bit basic knowledge that may help you.
Every material and alloy has it's own minimum bending radius.
A thumb rule for soft steel and VA-stainless steel is the minimum inside radius = thickness of the sheet.
At the other side you may have aircraft grade aluminium alloy with a radius 5 times or more
of the thickness to avoid cracks on the outside of the bending line.
If you google the technical data of your material you will find two numbers named "Rm" and "Rp0,2".
The first is the absolute strength of the material in Newton or Megapascal per mm^2.
The second and smaller number is the border between elasticity und plasticity of the material.
It is clear- the load of the material shouldt stay inside the elasic range.
If the load always change from one side to the other it shouldt be not higher then 50% of Rp0,2 if you are
using black steel und not more then 30% with aluminium.
The next thumb rule say that Rp0,2 is about 80% of Rm with the most black steel alloys.
Not so with VA. Here you have not more then 20 - 25 %. That means it has a much higher range of plasticity and hold less
forces than a black steel with the same Rm-number.
What has this to do with bending of sheet metal?
Most sheet metal shops are working with laser cutter and bending press devices.
The bigger part of working is with stainless steel sheets. Typical material number 1.4301.
There are different toolsets for the press. Because time is money they dont like to change the tools to much.
Sometimes you can find an inside radius of 3 mm with a 5 mm thick sheet on a stainless steel construction.
Here it is not a big problem. Because of its plasticity you find no cracks on the outside of the bending.
The old style bending mashines, where you pull the sheet around a bar with the right radius on the edge are the only
way you can get the radius you want.
With the modern bending presses they have a script with a lot of numbers to every tool.
The radius change with every degree of the bending angle in every different toolset and material!
Other influences are: the type of material (alloy), the charge they got delivered, bending with or across the grain.
In all cases where safety of the part is importend, please ask the people of the shop first wich size of radius they can
establish with there tools, the material and the angle of your bending. If the radius is a little bit bigger than the minimum
radius you need it's ok. Then you can change your drawing radius.
If the radius is below the safety number they have to change the toolset.
As long as you do the bending works not for yourself you don't have to make the drawing for the flated part.
You are not responsable for the right length of the cutting and all the mathematics. A good shop need the drawing
of the ready construction, the minimum bending radius and the tolerance of the lengths.
Only the shop is responsable for rigth sizes of the cutting.
Good shops around my hometown and everywhere works with tolerances of +/- 0,1 mm.
In the case you have a flat sheet without any bending and with some curves on it you give this as a .dxf data
with the scale 1:1 and without any frames and script fields to the shop.
Write the thickness on it - thats all.
Hope this help you a little bit.......good luck......have fun
Hans
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