Automotive component makers rely on forging presses and shearing systems to transform metal bars into parts. The presses work efficiently, but when the metal bars are sheared to length, visual quality inspection and additional machining are needed. The shearing process directly affects the quality of the automotive part, amount of machining required, consumption of material and, ultimately, production cost. Hatebur did some research and teamed up with Moog to integrate a servo-hydraulic valve to regulate shearing forces and control their effects on the final product.
“Shearing has always been a challenge; an accepted one,” said Dr. Mihai Vulcan, who oversees strategic projects for Hatebur Metalforming Equipment Ltd., a Swiss manufacturer of high-end horizontal multi-stage presses that has been in business for 90 years. “Solving that problem is like building a Swiss clock because every movement of a press is dependent on another. For example, the transfer system, punch ejector and die ejector are all interrelated and interlocking.”
For decades, automotive component makers have relied on hot forging presses and shearing systems to transform metal bars into high-quality parts such as drive shafts, gear wheels and bearing rings, to name but a few. Component makers turn out millions of parts annually. The presses undoubtedly work very fast and efficiently, but, says Vulcan, after metal bars are cut, visual quality inspection and additional machining are needed. So, the goal for Vulcan was to improve shearing quality.
On the largest hot forging presses available today, automotive component makers produce parts using an automated multi-stage process forming metal bars with a direct pressing force of up to 2,000 tons. These presses heat, through induction, steel bars up to 90 mm (3.5 inches) in diameter to temperatures reaching 1250˚C (2282˚F) and automatically move the bars to the shearing process.
At a rate between 50-80 parts per minute, the presses will (in approximately 60 milliseconds) shear the white-hot to glowing-white-hot rod. According to Vulcan, the shearing process directly affects the quality of the auto part, amount of machining required, consumption of material and, ultimately, production cost. The decisive factor shaping the quality of each part is the surface created with the cut. The surface condition of that sheared-off section depends heavily on how well the press holds the metal bar during the cut.
“The mechanical bar-stop systems in presses used today seem like state-of-the-art technology,” Vulcan said. “But an operator cannot adjust the shear gap or the angle of the bar-stop head during the production process. And mechanical bar stops can’t react to deviations in position or changing conditions such as deteriorating shearing blades.”
According to Vulcan, nobody complains about the process. Instead, they accept (as the cost of doing business) the slight tilting of the shears and the consequent tearing that causes damage on up to 20% of the shearing surface in the form of breakouts, shingles and wrinkles leading to uneven surfaces.
Uneven Shearing Increases Cost
The result of this is that a finished forged surface requires further machining, and the
machine operator accepts waste on the blank. This increases manufacturing time and costs. One reason why a machine owner would endure a less-than-perfect shear quality and the consequent breakouts is that addressing the problem requires changing the shearing blade or adjusting the shear angle. To accomplish either of those things, an operator would have to stop the press, clear the rod and make adjustments that could take 30 minutes or more. That, of course, racks up costs in the form of lost production time.
“I had a colleague who worked 40 years with this company and fought a lot to improve cut-off quality,” Vulcan said. “Even 13 years ago, I was performing experiments and looking at every factor, like shearing gap, and we were successful in improving cut-off quality in isolated experiments. But we couldn’t find the recipe from any one successful experiment on a machine and transfer it across all machines.”
Finding an Answer by Focusing on Force
The breakthrough for Hatebur and Vulcan came when his team decided to take another look at the shearing system. They focused on not only shearing many types of components but also blade geometry, cut-off blades and clamping forces. With a hot forge press, the material and temperature can’t be modified significantly. So, the team instead focused on applying changes to the forces acting on the bar (i.e., bar stop) and during shearing. To rapidly change those forces, Vulcan realized that a servo-hydraulic valve was the key.
“Servo-hydraulic valves are dynamic, and I believe the best manufacturer for these valves is Moog,” Vulcan said.
With Moog, Vulcan set about developing a servo-hydraulic bar stop for a hot forge press. The bar stop adjusts the shearing process for hot presses. The thinking by the team was that the servo-hydraulic-powered bar stop would increase the quality of cut-offs to the extent that the reworking process could be rendered largely unnecessary.
Vulcan and a team of experts designed and tested the servo-hydraulic bar stop under simulated manufacturing conditions at Hatebur. Moog’s D636 servo valve was the central motion control element of the bar stop. With a typical automotive component-maker’s manufacturing requirement of up to 80 strokes (or shearing operations) per minute, a highly dynamic servo valve would have to operate the central actuator of the bar stop.
Hatebur opted for a Moog D636 Series Digital Control Valve (DCV) with a powerful short-stroke linear motor that could handle the dynamic characteristics of the shearing process. For example, the shock and vibration from the shearing process could be mitigated by the valve’s integrated vibration decoupling feature to protect the DCV’s electronics. This allows a short-stroke linear motor to continue driving a wear-resistant spool in the valve’s bushing unit, which ensured that the press could consistently produce a superior cut and repeatedly create a quality part.
Illustrating the Procedure
The servo-hydraulic-powered bar stop and shearing procedure take 0.1 milliseconds for the change to force control. The duration of the force control depends on bar diameter and stroke rate. For the machine Vulcan and his team worked with, the force control varied from approximately 75-150 milliseconds. After the shearing process, the system changed back to position control within another 0.1 milliseconds.
During the shearing process, the servo-hydraulic valve’s integrated digital electronics ensure that any deviation from the target position is corrected within a few milliseconds by force control with position monitoring. This keeps the distance between the press’s blade and bar stop constant and precise throughout the entire shearing process, and tensile stresses are virtually eliminated. Measurement technology integrated in the servo-hydraulically operated bar stop delivered the necessary data to keep the compressive stress at a predefined level. To achieve that desired compressive stress consistently, the Moog valve can cycle between open and closed at a rate of less than 8 milliseconds if the shearing process demands it.
The machine operator can also easily control the displacement-force time profile within certain limits. With the prototype press’s servo-hydraulic valve applying forces, Vulcan and his team were able to give the machine operator a way to adjust the shearing process from a control panel without interrupting production. As the machine’s shearing blades wore or there were deviations in material, temperature or tension in the cutting zone, the operator was able to compensate for these conditions with push-button adjustments.
With the prototype complete and the approach proven to work, Hatebur and Vulcan tested the servo-hydraulic bar stop extensively on its HOTmatic HM 75 XL application in partnership with Germany-based Hirschvogel Automotive Group, a leading automotive parts supplier, using a wide variety of operating conditions and materials.
What had always eluded Vulcan in his prior experiments at perfecting cut-off had been achieved: The cut-off quality due to the drive behavior of the servo-hydraulic bar stop in a production setting mirrored what he and his team did with the servo-hydraulic bar stop in the lab. Even after the HOTmatic HM 75 XL press had manufactured several million parts for Hirschvogel, the part quality and shear were consistent.
With the servo-hydraulically equipped hot press, Hatebur also achieves a significant improvement in the quality of shearing surfaces. The servo-hydraulic bar stop ensures the shearing surfaces are now almost parallel, and the breakouts and shingles are frequently reduced from 20% (Figure 1) to an average of 1% (Figure 2). The approach is also significantly reducing the consumption of materials by Hirschvogel.
“When you’re making millions of parts per year, saving a few cents and seconds each time causes an impact,” Vulcan said. “We know that Hirschvogel has practically eliminated the need for visual quality control because of the reduction in breakouts. They’re able to reduce machining costs too.”
Since fitting its HOTmatic HM 75 XL with a servo-hydraulic bar stop, Hatebur has also developed the servo-hydraulic bar stop for three additional press models, which the company hopes to release to the market. Due to the success at Hirschvogel, Hatebur has decided that the new bar-stop system should not only be applied to new machines but also to retrofit existing applications.
Since Hatebur’s presses are normally in use for decades (e.g., the first AMP 70 went into production in 1964 and is still in service), the new technology could boost quality and profit for any number of parts makers.
For questions or additional information, please contact Mihai Vulcan, Dr.-Ing., Hatebur Metalforming Equipment Ltd. at firstname.lastname@example.org.