Better Peening Shouldn’t Be a Shot in the Dark

Almost any pilot will tell you that when it comes to airplane parts like wings or engine blades, failure is unacceptable.

That’s why more aerospace companies have sought to reduce the possibility of metal fatigue in such critical parts, and why some manufacturers of those pieces use a technique called shot peening, in which a hail of 0.014-in. steel balls create microscopic dents in surfaces of metal parts to strengthen the material and extending its lifetime.

It may appear that surface deformation – which extends into the material roughly three times the depth of the dents ‑ might reduce material strength than enhance it, the dents actually convert tensile stresses into compressive stresses that act to hold the material together.

But that’s if the process is done right, and with manual shot peening the risk of damaging the material rises significantly.

Innovative Peening Systems (Norcross, Ga.) is becoming fully automated by combining their proprietary software with motion control technology from Siemens Industry (Alpharetta, Ga.) to create what they say is a more reliable, high-performance machine at an economical price.

“Most automated peening machines leverage computer numerical controlled technology, often at a high cost,” says IPS president Dan Dickey. “But we worked with Siemens to use alternative intelligent motion to produce a shot peening machine that would perform just as effectively, and far more economically.”

In a shot peening machine, a reservoir introduces steel shot into a compressed-air stream that passes through a nozzle to strike the part. The process takes place inside an enclosure that protects both operator and machine, but only to a point. A typical machine contains about 2,000 pounds of shot, which amounts to hundreds of millions of particles flying around at high velocity. The potential for damage to components exposed to the shot was extremely high.

“The 0.014-inch shot is the perfect size to make its way inside the seal of any kind of moving part and lock it up,” says Nick Hart, electrical engineer at IPS. “Metal shot will get inside anything it can and short it out electrically.”

The IPS team realized early in the process that the solution was to pull the motion control elements outside of the enclosure. The resultant six-axis nozzle manipulator consists of a three-axis gantry that rides above the machine and drives a rigid arm tipped with a nozzle. The arm extends into the enclosure through a single hole with a carefully designed seal; everything else is protected.

Rather than redesigning each machine from scratch, IPS started with a standard motion platform, adjusting the enclosure for a particular project. In the case of large parts like 100-ft-long airplane wings, for example, it might gang together multiple nozzle manipulators.

“We used standardized components that have been proven in the industry, but we customize every enclosure to fit the customer’s product,” says Dickey. “They get a machine that fits their part, at a good price, but it’s not a new invention for us, every time we build a machine."

The specifications called for the nozzle manipulator to move at a maximum speed of 500 in/min. On the face of it, such a requirement is more demanding than challenging. The problem is inertia; fully charged with shot, the assembly weighs around 2000 lbs. It requires a lot of muscle to accurately position that much mass at 500 in/min. For a solution, the team turned to Siemens.

The system combines Sinamics S120 drives in Booksize format and servomotors from the 1FK7 family. Although the S120 drives feature onboard memory and processing power, the IPS team chose to use them in a centralized control architecture, in part because the components were already grouped together on the nozzle manipulator carriage and in part to provide additional protection from contamination.

While Sinamics supplied the muscle, Simotion provided the brains in the form of the D435 drive-based controller.

“It allows us the flexibility to create custom software for the shot peening application, whereas some of the alternatives in the industry do not give you a lot of freedom,” says Hart.

The Simotion system stores parameters on a removable memory card that can be swapped out to instantly update software. The controller also offers trapezoidal motion profiles, compensating for the inertia of the nozzle manipulator at high speed by providing controlled acceleration and deceleration. This approach improves performance while reducing motor wear and, consequently, points of failure.

In addition to meeting the speed spec, the system positions to 0.001 in, repeatable to within 0.005 in. The company has multiple machines deployed, including one machine operating 16 hours a day, five days per week. The others are running around the clock, five days per week, and Hart has heard of no problems, thus far

“The first machine has 18 months in the field,” says Hart. “I just talked to the customer and they say the machine is repeatable, accurate and reliable. They have no complaints.”

For more information on Innovative Peening Systems, please call 770-246-9883 or visit innovativepeening.com; for more information on Siemens Industry, call 847-640-1595 or visit usa.siemens.com/motioncontrol.

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