Shrinking Volumes Focus Cost-Cutting on Press Brakes

With the rapid advancements in laser cutting technologies in the last few years, it has been easy for fabricators to focus their attention on how these advancements can help them be more competitive in tight markets. Higher-power lasers are able to cut faster and process thicker materials. Linear-drive motors transverse sheets quicker and accelerate faster in detailed profiles. Rapid piercing and power-gating techniques have improved. New resonators, the source of laser beams, have allowed faster cutting in thin material with less electricity consumption.

In many cases, the appeal of cutting faster for less has been so alluring that it has distracted fabricators from greater opportunities to lower costs in other areas of their operations.

Take, for example, the often overlooked press brake. 

Press brakes are commonly viewed as simple machines that do not capture the imagination in the way lasers do. Lasers have the appeal of futuristic technology. They excite us. Press brakes seem boring by comparison; but to ignore the advancements in forming technology can be quite costly to shops that have no spare margin to waste.

Stollco Industries in Port Coquitlam, B.C., is a custom tool and die shop, as well as a fabricating shop.  Floyd Stolle, a partner in the company, runs the fabricating side of the business. With most of the company’s growth occurring on the fabricating side, its highest capital priority is keeping that end of the business technologically current.  Their lasers are current models, for example, but Stolle likes to say, “I own lasers to get work.  I own press brakes to make money.”

Staying current with forming technology allows Stolle to make that statement.

The advancements in forming technology are quite different from the advancements in cutting technology; forming is focused on precision and setup time, rather than power and speed. That is to say, it is not that a new precision press brake can bend a part faster, it is that it can be set up to bend a part faster, and that it will bend it correctly the first time.

Let’s have a look at the entire setup cycle for a press brake from the very beginning, that is, from 3-D design, to the first part. 

Unfolding Your Part

The first step in the cycle is to unfold the 3-D design into a flat profile to be punched or laser-cut. This unfolding step requires that the flat pattern, or blank, be shrunk to allow for the stretching of the material during bending.  

The amount of shrinkage per bend, called a bend deduction, is calculated using a value called the k-factor that is dependent on the material type and thickness, the bending technique, the inner radius and angle of bend, and the punch and die set used to make the bend.  If the bend angles are accurate and k-factor is correct, the part will bend into the correct shape and size. 

A precision press brake can bend a part faster because it can be set up faster and will bend it correctly the first time.

Standard k-factors work well, but the complexity and number of the variables that affect it can lead to tolerances that are unacceptable in some applications.

This can lead to the costly cycle of having to go back to the flat layout to adjust the bend deduction and then cut another part to test on the press brake.  When the final part is correct, the layout can be saved and production volumes can be cut, but both material and time have been wasted in the process.

Some 3-D CAD software packages have unfolding capabilities that use standard k-factors to correct blank sizes. Others have modules that are specific to tool brands that can capture subtleties of tool design geometry that affect bend deductions. 

Some builders of press brakes have become vertically integrated and manufacture their own tooling and develop their own software. To achieve the most accurate results, use an unfolding program that was developed by a company that uses empirical data from its own punch and die sets to calculate accurate bend deductions the first time.  Great flexibility and cost savings can be gained by shortening the development time to first bend.

Bend Deduction

Validation of the bend deductions in the final part requires that the bend angles also be correct.  This is another step in the setup cycle. Today most fabricators use a forming technique called air bending. 

Air bending involves stopping the punch before it reaches the bottom of the die.  The position of the punch in the die determines the bend angle.  The technique requires highly accurate and repeatable positioning of the punch and die, but requires less tonnage than the older bottoming technique and can produce a range of angles from a single punch and die set.

Accurate positioning of the punch and die starts with precision punches and dies, as well as precision tool clamps, and accurate and repeatable positioning of the press brake ram and backgauges.  Without these, the setup process often involves the tedious process of trial-and-error placement of shims, combined with clamp adjustments. Again, both material and time are wasted.

Current precision press brakes use intelligent tool systems that maintain the required tool tolerance in any position in the clamps.  The tools are self-centering, interchangeable, and reversible.  They are permanently inscribed with all key tool information, and segments short enough to be handled safely with one hand are equipped with clips that can allow them to be removed vertically for fast and flexible reconfiguration of the tool arrangement.

Precision tools and dies alone, however, do not ensure a correct bend angle every time.  The accurate positioning of the punch in the die directly depends on the precise thickness of the material, something fabricators should not take for granted.  They may have ordered 0.060-in.-thick steel, but received a lot of 0.058-in.

Imprecise material thickness typically is compensated for with a correction bend.  A correction bend involves making a single bend of, say, 90 degrees, based on calculations using, in this case, 0.060-in.-thick steel. The part is then bent and the actual resultant angle is measured.  An offset value is then entered into the control to compensate for the material thickness variation.  In this case, the variation is 0.060 - 0.058, or 0.002 in., and that value is very close to the offset that will be entered into the control.  The technique is simple and works well as long as the lot of steel remains consistent at 0.058 in., in this case.

Thickness variability is not the only material-based variation that can affect bend angles.  Another important variable, especially in thicker material like 3/16 in. or 1/4 in., is tensile yield strength. 

When a 90-degree bend is required, the math is worked out for an overbend of about 92 degrees.  This is done because the material relaxes somewhat, or springs back, due to its tensile yield strength after the tonnage on the tool is released.  For small parts, a correction bend usually works well to compensate for yield strength variations, but many fabricators struggle in longer bends because yield strength can vary over the length of the blank.

Several new precision press brakes can measure some or all of the material tolerances that affect springback.  For example, precision rams can use the tools and dies to measure the material thickness like a micrometer.  This is a good start.  However, for the most demanding applications, look for systems that measure the springback of the material directly and perform a compensating bend. 

Only these systems are capable of capturing all contributing tolerances that lead to angle variation, such as yield strength and grain direction. 

To overcome this example of yield strength variance over a long bend, you need a system that measures springback at two or three points along the bend and applies separate compensations. 

A top-quality correction system can achieve consistent results within 0.33 degree of target in the first bend, even in challenging cases like long bends with varying yield strength.   These systems can allow shops to achieve virtually zero scrap and eliminate setup parts, a potentially huge savings in some applications.

Understanding Press Brake Efficiency

The total cost of setting up a press brake, from bend deductions to tool setup to material compensation, is spread out over the total run for that part.

Over the years fabricators have watched their production volumes shrink both because of higher-customized end products and the movement of the remaining high-volume work offshore.

These reduced quantities also reduce the number of parts that setup costs can be spread over.

Consider two machines that are competing to produce 200 parts. Press brake 1 does not have intelligent tools and requires 20 minutes for its tools to be loaded into the machine and then tweaked and adjusted to achieve correct bends. Press brake 2 does have intelligent tools and can be set up in five minutes. 

Let’s use a typical part with six bends and an operator who can perform six bends per minute.  That works out to the easy value of one part per minute. This means that each press brake takes 200 minutes to form the entire run. Press brake 1 has an efficiency of 200/(200+20)=91 percent, and press brake 2 has an efficiency of 200/(200+5)=98 percent.

This example does show a significant difference in efficiency, but watch what happens to the numbers when the run is reduced to 40 parts. 

Press brake 1 now has an efficiency of only 40/(40+20)=67 percent, and press brake 2 has an efficiency of 40/(40+5)=89 percent. 

As the run quantity decreases, the efficiency difference increases.  There is more to this than the cost of time.  The first press brake requires setup-parts for the bend-adjustment cycle.  This not only increases scrap material costs, but the cost of punching or laser cutting the setup parts in the first place.   The time and material wasted by the first brake in this example are from tool setup only.  If the blank size has to be developed by trial and error, even higher costs have to be spread out over fewer parts.

One of the most attractive advantages of lasers is that they do not require tool setup time.  What is very important to recognize is that the virtual elimination of setup time with lasers is a cost advantage that fabricators captured 20 years ago.

Lasers continue to get faster and offer intriguing new technologies and techniques to reduce costs, but when it comes to removing setup time, there’s not much left to pick at.

Press brakes, on the other hand, are not getting much faster at bending because of safety regulations that limit axis speeds.  What current technology gets you in forming is faster setups and repeatable accuracy.  That is, they are more efficient and effective, and that is what matters in a market of shrinking volumes where setup costs take center stage.

In Port Coquitlam, B.C., this is summarized by the words of Stolle: “My lasers are high powered and very fast, but as smaller-volume work becomes more common, it is my press brakes that win bids and maintain my margins.”

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