Using benchmarking for bend deductions
Benchmarking is a very good idea for your operation ... just make sure your benchmarks are your own.
Benchmarking is using measurement or evaluation to judge similar processes, parts, charts, and methods.
The term benchmark is believed to come from medieval times—more specifically, form the stonemasons who built the great cathedrals of Europe. Before that time local craftsmen built every piece of furniture and cut every piece of stone to size by judging length, width, and height by eye. This meant there were no standards by which to judge components for length, width, and height relative to any other part. In other words, everything was eyeballed.
At some point someone came up with the idea of placing a mark or gouge into a workbench to indicate the length or width of a part. By comparing each leg of furniture or block of stone against the benchmark, the builder could be assured that each subsequent piece was pretty much the same size as the previous one.
Modern Version—Bend Deduction Charts
A benchmark, then, is a measurement that can be used as a reference for other measurements or data sets. In the modern sheet metal shop, bend deduction charts are a good example of benchmarking. A bend deduction chart created in a shop where bending methods, bend types, and tooling are agreed upon by all becomes a valid benchmark.
Ideally, modern benchmarks would serve the same universal purpose as the notch in the bench did for our ancient brothers. A test run of the material being formed would validate the bend deduction values at any time.
However, when inappropriate benchmarks are used, problems arise. For instance, benchmarks can be rendered invalid when they're used in a different shop where tooling or forming methods differ from the benchmarks' source.
Figure 1 and Figure 2 show two different bend deduction (BD) charts for the same material thickness and bend radii. Notice how different the bend deduction values are, even though both are correct for the shop in which they were created. One shows a bend deduction of 0.106 for a 1/32-in. inside radius for 16-gauge cold-rolled steel; the other chart shows it is 0.136. These are perfect examples of how another shop's benchmark may or may not be correct for you. If you try to apply someone else's benchmark to your process, chances are it won't match the marks on your workbench, leaving you with one leg that is too long.
Mathematics: Establishing a Common Benchmark
As long as you use the same forming methods and tooling that were used to create the original data set, validating the original numbers should be easy. If everyone used the following formulas and processes to create of their bend deduction charts, they could establish a common benchmark.
To set a standard benchmark for your bend deduction charts, you must take an accurate measurement of the inside radius, either by meeting part specifications or by planning for what is going to be done. This radius measurement can be taken on the tip of the punch radius in bottom bending or coining. It also can be taken as a percentage of the V-die opening in air forming using these percentages—20 percent for 304 stainless steel, 15 percent for cold-rolled steel, 15 percent for H32 5052 aluminum, and 12 percent for hot-rolled pickled in oil. Simply multiply the V-die opening by 0.20, 0.15, or 0.12 to find the value for the inside radius.
Once a value for the inside radius is set, simply plug it into the following formulas to establish a proper benchmark in your bend deduction charts for a given material thickness and inside radius.
Bend Allowance (BA)
BA = [(.017453 x Rp) + (.0078 x Mt)] x ^
Outside Setback (OSSB)
OSSB = [tangent (^/2)] x (Mt + Rp)
Bend Deduction (BD)
BD = (2 x OSSB) - BA
^ = Complementary angle of bend
Mt = Material thickness
Rp = Radius of the punch (coining and bottom bending) or inside radius (air forming)
As long as the inside radius of the bend can be measured with a radius gauge, also a benchmark, the benchmark supplied by mathematics can be yours to apply in your shop and in any other shop that measures and calculates these values.
The key to valid benchmarking is understanding how the benchmark relates to the tasks, tools, and methods employed in your particular environment. Benchmarking the tasks that you generally perform rather than using a benchmark someone else has created will steer you in the right direction, unless they also are using mathematically derived benchmarks. Then you're OK.