This is reposted question from our Forum page, Profound Radius Bends over 90-Degree
When doing a flat development for a part with a profound radius bend over 90 degrees, is there anything special you need to do or can you still use the same Bend Allowance, Outside Setback and Bend Deduction formulas that you would use for anything else? The only reason I ask is because I notice the OSSB is quite a bit larger than the Bend Allowance, where it is usually less.
Radius Bends, Multi-breakage, BA, BD, OSSB are calculated in the usual way. The calculations are still based on the actual radius that is developed on the part when released from pressure.
A bend’s inside radius relaxes when released from pressure (Bent/Bend)*; in other words, the measurable inside radius gets slightly larger; even for bend past 90-degrees complementary.
However when you bend past 90-degrees complementary you begin to encounter “Multi-Breakage”. This is the radius that leads the punch; the greater the bend angle the greater this separation between the punch radius and the material as the actual inside radius gets smaller.
So, to answer your original question; the calculations are the same but you will need to do one of three things to establish the radius data. You can: bend a test piece and measure the radius results, you can calculate the radius and predict the radius that will appear; see my “Bending Basics”** articles for the last few months for more detail. Your last and probably best option is to use a urethane pad at the bottom of the die. This will push back against the material being formed allowing the sheet to take on the shape of the bend; less the opening of the radius when released from pressure.
* For more information about the Bent/Bend, Bend Deductions, Bend Allowance’s, etc. review the articles and columns from the media tab or consider getting a copy of TheArtofPressBrake – the digital handbook for precision sheet metal fabrication; it’s all cross referenced and indexed; find the information you need quick and easily.
**links to all Bending Basics columns can be accessed from the Menu Bar/Media/Bending Basics