Bending under the bottom is stamping the radius of the punch tip in the work piece. It is different from coinage, that places the whole face of work pieces beneath a tonnage, which makes it flow. In the bottom bend the punch angle must fit the estimated angles of work pieces leaving a-few-degree adjustment for back spring, accordingly, the availability of an 88 degrees cutting tool to reach 90 degrees. First the work piece is pressed towards the punch, later the die radius gets pressed in work pieces, that gain punch angles and after getting discarded the work piece meets the die by springing backwards.
For compensation punch angles should appear less than die angles with some degrees making over-bending possible by forcing punch-tip in the work piece. Its large sizes must be excluded, otherwise the tool will be damaged. This technique gives operators fine repeatability when correctly configured. Fine tuning of the bottom bend application demands awareness on tools, materials, tonnage, as well as timing of various steps to enable operators work efficiently.
Bottom bend and air bend are sometimes mixed up. They often cause an operator to identify differences in radii while bending angles of 90 and 80 degrees. The reason is that in the bend of 80-degree angles punch does not get bottomed in the sheets in the way bottom bend does.