Troubleshooting sheet metal forming: Determining the root causes of bending failure

Question: Our company has a quality issue related to forming a bracket. We have had trouble determining the root cause and the best way to go forward. It seems that as a result of bending, a crack formed on the underside of the bracket, rust built up in the crack, and eventually the bracket broke. We’re working with 10-mm-thick steel.

We have a homemade V die with a 7-mm radius at the bottom of the die, which didn’t work out well. We then tried some larger radii in the bottom of the V die, including 9 and 12 mm.

In our analysis, we found the material contained fractures with numerous voids that run longitudinally across the fracture, with these features mainly evident closer to the outside radius. The microstructure has numerous inclusions, likely manganese sulfide inclusions. However, we examined a fracture from one sample that did not have sulfide inclusions, and we can find no features that could have led to a fracture. The formed part also has die marks on the outside surface.

We have investigated the problem, but we are not confident as to which of the factors could be affecting the failure, including material quality, mechanical strength, radius, and production method. If quality bending isn’t possible, would forging be an option for us? I appreciate any feedback you can offer.

Answer: I cannot count the number of times I have discovered the material as a root cause of forming, fracture, and failure problems. The material you have chosen may meet with chemical and technical specifications, and it may not necessarily be the root cause of the fracturing. However, the quality and consistency of your material are substandard, so it’s no surprise that it exhibits fractures when bent.

However, this is probably not the only cause. Analyzing your forming method and tooling choice, in combination with this low-quality material, may get to the root of your problem.

Cheap Material Isn’t Cheap

Poor, inexpensive material has no place in making quality, error-free parts, and using it ultimately can get very expensive, considering the cost of failure and part replacement. On top of this, you drive up labor costs because employees in the forming department (and everywhere else) are busy reworking already completed parts and fighting setups. Low or inconsistent material quality may be fine for flat parts, at least in some cases. But even here, bad material can make fabricating and assembling the simplest components more challenging—and much more expensive.

The bottom line: It’s usually not the best idea to purchase substandard material. The few dollars that you save per pound are immediately lost to labor costs.

Forming Method Problems

The die marks on your part are caused by coining and, in this case, using a too small die opening. It appears that you are applying excessive pressure to the bend, hence the die marks. This excessive pressure is required when the die angle and punch angle are both 90 degrees; using this pressure compensates for springback.

The excessive pressure of coining causes the material’s molecular structure to realign. This eliminates the springback. But when you apply so much pressure to such a small area, you destroy the integrity of the material being formed, making it prone to failure.

Even increasing the radius at the bottom of your die set—you tried 9 and 12 mm (0.354 and 0.472 in.)—suffers from the same issues. The radius at the bottom of the V die isn’t the radius you should be looking at. Remember, in air forming, bottom bending, or coining, the radius value is found on the inside radius of the bend, not the outside of the bend, and certainly not the radius in the bottom of the V die.

Substandard material no doubt contributes to your inside bend radius variation, but the tight radius might also be a contributing factor. The minimum inside radius for this 10-mm- (0.394-in.-) thick steel is 6 mm (0.236 in.), and your outside radius of 7 mm (0.280 in.) is very close to that minimum inside radius, which further stresses the material. (For more on the minimum radius, see “How an air bend turns sharp” and “Minimum versus recommended inside bend radius,” archived at www.thefabricator.com.)

You seem to be coining, which deliberately places a very small radius firmly into the steel at less than material thickness. If the radius is sharp enough, you can produce an acceptable part, but often at the expense of the press brake. For proof of this, just look at the condition of many used mechanical brakes.

The Solutions

Besides upgrading your material, you should consider air forming. This is a fundamentally different way to form parts, compared to bottoming or coining, as the radius of the bend is floated rather than stamped. This reduces the tonnage required to form a given piece of sheet metal and will not diminish the material’s integrity. Also know that homemade tooling is not always the best or cheapest way to go.

A properly done air form will produce an inside radius approximately equal to the material thickness. Air forming in and of itself creates the most stable bending situation and causes the least amount of damage to the part.

Air forming will require a complete rethinking of the bending process. There are many ways to choose a proper die. In your case, a standard rule of thumb for die selection—eight times the material thickness as the perfect die opening—should work fine, as long as you maintain a one-to-one relationship between the material thickness and inside radius.

For example, if your material thickness is 0.250 in., the inside radius should be 0.250 in. To achieve this, you would choose a 2-in. die opening (0.25 × 8 = 2.000). You would also use a punch nose radius as close to—but not exceeding—the naturally floated air-formed radius on the part. This is known as a perfect bend. For more on this, check out my 2015 series, “Grand unifying theory of bending,” archived at thefabricator.com.

Conclusion

In your initial question you asked if forging the bracket would be appropriate. No doubt a forged part would function fine for your project, but I’m not sure how cost-effective it would be.

Forging is one of the oldest metalworking processes. It produces a metal part that is stronger than the equivalent cast, machined, or formed part. The metal shaping that occurs during forging causes the internal material grains to deform and follow the general shape of the part being produced. In essence, the grains align to the shape of the final workpiece. This will improve the part’s strength and its material characteristics.

However, forging may require a large capital investment, so it may be too expensive for short part runs. This brings us back to forming, which, if done correctly and with the best material available, can produce a strong and a relatively inexpensive part by comparison.

Ultimately, you need to choose a method that meets the standard of modern manufacturing. Regardless of which you choose, for everyone’s sake, invest in quality material. Whatever metal trade you work in, poor-quality materials will cost you in the long run. This includes not only additional costs in labor, repairs, and scrap, but the cost of parts failing in the field, repairs, replacements, and, most important of all, your company’s reputation.

Steve Benson is a member and former chair of the Precision Sheet Metal Technology Council of the Fabricators & Manufacturers Association International®. He is the president of ASMA LLC, steve@theartofpressbrake.com. Benson also conducts FMA’s Precision Press Brake Certificate Program, which is held at locations across the country. For more information, visit www.fmanet.org/training, or call 888-394-4362. For more information on bending, check out Benson’s book, The Art of Press Brake: the Digital Handbook for Precision Sheet Metal Fabrication, © 2014, available at www.theartofpressbrake.com.