Bottoming and Air Forming
From the advent of the powered press brake in the 1920s to the present day, operators have formed parts with bottom bending, or bottoming. Even though bottoming has been losing favor over the last 20 to 30 years, the bending method still permeates our thinking when we bend sheet metal.

Precision-ground tooling entered the market in the late 1970s and changed the entire paradigm. So, let’s look at how precision tools differ from planer tooling, and how the shift to precision tooling changed the industry, and what it all has to do with your question.

In the 1920s, forming changed from folding on a leaf brake to forming into a V die with a matching punch. A 90-degree punch would be coupled with a 90-degree V die. The change from folding to forming was a major leap forward for sheet metal. It was faster partly because the newly developed press brake was powered—no more manual folding of every bend by hand. Plus, press brakes could bottom bend, which boosted accuracy. Backgauging aside, the increase in accuracy can be credited to the punch nose stamping its radius into the inside bend radius of the material. This was accomplished by forcing the nose of the tool to a less-than-material-thickness position. And as we all know, if we can achieve a consistent inside bend radius, we can calculate the correct values for the bend deduction, bend allowance, outside setback, and k-factors, regardless of the type of bend we’re making.

It was quite common to bottom a very sharp inside bend radius in parts. Manufacturers, designers, and craftsmen knew that the part would still hold up because it seemed that everything was overbuilt—and in truth, everything was, at least compared to today.

This was all fine and good until something better came along. The next leap forward took place in the late 1970s with the introduction of precision-ground tooling, computer numerical controllers, and improved control of hydraulic systems. You now had complete control of the press brake and its systems. But the game changer was the precision-ground tooling, which fundamentally changed everything. All of the rules for producing good parts had changed.

Read more: Getting back to air forming and bending basics on the press brake