Bending on Press Brake
There are 3 basic ways of performing a bend on a PB:
Air bending: During the complete process there is always a gap ("air) between the sheet and the bottom of the lower die.
Bottom bending: The sheet is pushed until it touches the bottom of the die, but there's not a great force applied on the sheet at this point.
Coining: The sheet is pushed to the bottom of the die and a great pressure is applied in order to "coin" a shape with no further springback
Called air bending because of the air gap always present between sheet and bottom die. This bending method is the most common in today's fabrication.
Using Air Bend method means that while we achieve plastic deformation during the process, We do not apply such a stress (force) that there's no further movements on the steel. There's actually part of the fibers in the structure of the sheet that still tend to move back to their original shape and direction. This is what we call spring-back
Being able to estimate that return on the elastic part of the deformation (called spring-back) is what saves us a lot of machine force and headaches. It takes only 25 ton/mtr (9ton/ft) to brake .125" (3,175mm) mild steel in a V=1" while it would take about 100ton/mtr (33 T/ft) if we were to try for a complete a complete plastic deformation (aka Coining).
As just explained above. Coining is a total plastic deformation of the sheet. This means that we apply such a strong pressure into our material that we completely brake the fiber "memory" or spring-back.
This is the deformation concept of a stamping press and of course the method applied to make coins (therefore it's name).
Coining is OK when we are working on thin gauge and low UTS material, like mild steel. Things start to get complicated when we start going thicker (even over 2,5mm - 0.1").
Going back in time, first PB worked in a coning method concept. They had no ability to move the ram to a certain point. It was all a full cycle. Nowadays coining is still used, even with CNC PB, when a sharp radius and a tight tolerance in thin gauge material is needed. Or when, always on thin material, there are slots in or near the bending line or and tapered cut ends. Using coning in these operations helps diminishing deformation on the part.
Coining is the method used by many custom tools that perform single stroke shapes like hat, omega or deep U on thin gauge.
Large radius parts needing coining operations are definitely the most difficult ones to fabricate.
A large radius helps the material keep its strength making it difficult to brake into it. The force really needed to brake the material resistance to deformation will be higher on this operations than on a normal 90° bend with the same UTS material
Definitely a coning operation here on this large radius
.. mind the operator's expression!!
A kind of in between method that consists in putting the sheet against the bottom of the die in order to correct deformations in slots and tapered end cuts.
This method does require as much force (tons) as coining, but requires great expertise from the operator. The die and punch must both have the same angle so that the sheet is kept parallel between them. This method also demands a lot of trial and error. It's actually becoming less used everyday thanks to new tools like Rolla-V dies that obtain the same result with other benefits attached and with less headaches.
Other Fab Operations
Whenever a radius in the blue print is too large to be fabricated in a single stroke, we use some kind of bumping operation.
The more jits we apply on a radius, the more we get a radius style shape.
Notice the importance of the R axis for correct placement of the sheet.
One of bumping variations consists in using radius elements to coin one radius inside the previous. This creates a conical shape.
This fabrication process is used in fabrication of light poles and structural poles.
Just what's being fabricated in this video!!
There's many ways to finish a hemming detail in a part. Old style method was to use a pre bend acute set of dies and a flat upper and lower to finish the process.
This requires 2 tooling setups and can be a real headache.
Modern hemming tools can perform both processes in a single setup.