Finding, defining, and preventing taper in press brake operations
November 8, 2005
Properly trained press brake operators understand the nuances of tapers. Armed with this knowledge and following a five-step process that includes inspecting tooling and materials, precise setup, checking the part, and making necessary adjustments, these skilled workers can complete machine setup and produce quality parts in minimal time.
"Say ye shibboleth!"...
"Do you not hear me, traveler? ... Say ye shibboleth."
A look of surprise and terror crept over the traveler's face as the third syllable finished crossing his lips. A salty, warm liquid began bubbling up in the back of his throat—a sticky, thick, and crimson ear-to-ear smile now crossed his trachea. There was no pain.
"Shib-bo-let" had been the word returned. The guard knew that this slight difference in pronunciation revealed an enemy and that it was his duty to take the traveler's life.
This story comes from a war during King Solomon's reign in which thousands of soldiers forfeited their lives, because in their native language the "th" sound did not exist, and they could not repeat the password correctly (not unlike how lollapalooza was used in World War II to discover Japanese spies).
Like sibboleth, the word lollapalooza was used in this case because the Japanese language doesn't have an "l" sound. Throughout time, whether in language or life in general, nuance has played an important role.
What does this have to do with press brakes? Plenty! Competent press brake operation also is all about nuance.
Although the metal fabricating industry struggles with the lack of skilled labor, many companies refuse to take the time to train an individual in anything more than the rudimentary skills of pushing a button or pedal. Why? For the most part, it's economics and the prevailing industry attitude that "even if I invest the time and money to train workers, they'll just move down the road for 50 cents an hour more." And yes, workers do leave for various reasons many companies fail to address.
What does all this have to do with the focus of this article—finding, defining, and applying tapers in press brake operations?
Skillful press brake operation requires more than just calling up a program and loading some tools into the brake. If it really were that simple, setups could be completed and production started in 10 minutes or less, including tearing down the previous setup.
While 10-minute setups do happen, they don't happen often. Why? Because most operators haven't been trained properly and don't grasp the nuances of forming, for example, defining tapers.
Perhaps you'll get lucky and produce a good part. Or perhaps you'll get your throat cut for mispronouncing a single syllable. So, before that red, warm, salty liquid bubbles up in the back of your throat, here are a few clues that just might get you past the guards.
New-age thinking says load the program, run a part, and then make the adjustments. This works fine for a single-bend part but falls short in more complicated bending operations. Take a simple hat-section, for example. By the time the final bend is made, finding the tapers, determining what type they are and then correcting poor or inconsistent parts is a very difficult task.
The taper may be angular, one that's off just enough to change how the next bend is gauged (Figure 1).
It also might be a dimensional taper that needs only the smallest of shims to correct (Figure 2).
A few thousandths of an inch of dimensional taper might not seem like much, but it can compound bend after bend until the overall, or float, dimension is out of tolerance.
After identifying the taper type, the question then becomes what has caused it? Is it the backstops, or is one end of the bends slightly open? And how much time are you willing to spend to track down the cause?
Using the hat-section example, you've completed the first part, and all of the flanges measure right on the money. The 90-degree bends appear to be good; however, the overall dimension has a 0.030-in. taper, and the customer-specified tolerance is ± 0.010 in. The part in the flat doesn't have a taper, and you've run a straight edge the length of the hat-section and found it parallel—proving the bends, if not square, are at least consistent.
Perhaps the tooling is worn, which can cause the inside radius to change from one end to another, or maybe only in spots. A change in radius, regardless of the forming method, can also create taper. Using a worn punch in a bottoming operation will have the same effect on the inside bend radius as using a worn V die has on the inside radius in air forming. The problem could even be as simple as the V die is not centered with the punch.
Here are the five tried-and-true steps to producing a good part:
1. Check the blank; make sure that it's square and that the edges are smooth and free of burrs and step marks.
2. Make sure your tooling isn't worn or damaged, that it's clean, and the toolholder wedges are placed back at their original settings; then install the tooling, making sure that it is centered.
3. Make your first bend, check the angle, and make adjustments if necessary. Even an angular error at this point can cause a measurable taper. If an adjustment is necessary, make another first bend and repeat the process until you get it right. A skilled operator who understands forming nuances will have it correct by the second try.
4. Check the dimensions for taper. If they are good, finish the part. If they are not, shim accordingly until the dimensions are equal and accurate.
5. Run a complete part, measure it, and fine-tune it to tolerance.
An experienced operator can define and correct forming issues quickly and accurately without overcorrecting. Teach less experienced operators the nuances of press brake operation, and don't worry about the 50 cents an hour they might get somewhere else.
Press brake operators, nuances will get you past the guards—quality control—and ensure that finished parts make it through the lines and to the customers—without getting your throat cut from ear to ear!