The Five Orders of Bend Radius
There are five orders of the inside bend radius (Ir). At the heart of all things precision, the Ir is what we use to calculate our bend allowances (BA) and bend deductions (BD). The five are as follows:
Sharp radius bend
Minimum radius bend
Perfect radius bend
Surface or radius bend
Profound radius bend
Sharp Radius Bend
A sharp radius bend is one where the center of the bend is creased. This creasing is caused when pressure is applied to an area so small that the applied tonnage exceeds the material’s ability to resist that force, allowing the punch nose to pierce the material surface.
Creasing the center of the radius causes variations in material thickness (Mt), yield strength, tensile strength, and grain direction. These in turn lead to angle variations in the final bend and variations in the bend deduction (BD). At their worst, sharp bends produce a weak point in the sheet metal and cause the bend to fail in the final product.
Whether a bend turns sharp is a function of the material, not the sharpest punch nose in your shop. When the punch tip is too small in comparison to the tonnage required to form, the load will be concentrated on such a small area the punch will begin to pierce the surface of the material.
From here you have two choices. First is to stay with the sharp bend and calculate the BA, outside setback (OSSB), and BD using the value for the naturally floated radius. If the punch nose radius must remain the same, you will need to watch the bend angles closely during production. Again, because sharp bends pierce the surface of the material, they amplify bend angle variations from changes in the material properties, grain direction, thickness, and tensile and yield strengths.
Your second option is to still calculate the BA, OSSB, and BD using the naturally floated inside radius—only this time, you change the punch nose to a radius as close as possible to the naturally floated radius without exceeding the radius value. If your punch nose exceeds the floated radius value, the material will take on the new, larger radius, again changing all your BD values and the flat blank.
Keeping the punch nose radius as close as possible but still less than the floated Ir will give you the most stable and consistent bend angle and, by extension, stable linear dimensions.