September 2019

44 n PRECISION MOTION September 2019 www.drivesncontrols.com Giving the best shot S hot peening is a well-established technology for extending the lives of heavy-duty metal components, especially those used in aircraft where fatigue resistance is essential. It consists of firing thousands of tiny steel balls at the surface of the component. In an effect similar to a blacksmith’s hammer, this process produces precise compression stresses on the surface of the component. The shots affect the atomic structure of the metal surface.“Fissures in the material occur where tensile stress is prevalent,”explains Martin Hunziker, CEO of Freymatic, a Swiss machine-builder that specialises in shot- peening machines.“The connection between tensile forces and the formation of fissures is particularly critical in aircraft since lightweight construction is required. Despite their size, the blades in an engine need to be very light. This is why they are made of a titanium alloy.” The shot needs to hit the target material accurately, at the right velocity and in the correct quantities. It does not damage the surface; in fact, each hit makes the target more solid and tougher. Precision is essential in the peening process because it is not possible to carry out inline quality monitoring. Operators must therefore rely on the correct process parameters being achieved. For maximum precision and repeat accuracy, Freymatic uses shot dosage systems driven by synchronous servomotors. The motors drive a speed-controlled worm gear which, in turn, powers a conveying screw. The shot is directed by a robot arm into a hermetically sealed chamber. Its wrist is equipped with up to six different spray nozzles which can be exchanged during the machining operation. The components are blasted inside a soundproof chamber, with the high-pressure supply and dosage systems remaining outside the chamber. Freymatic’s peening system uses two pressure containers that act as a sluice between the unpressurised blasting shot store, and the servomotor-powered dosage system which is under a pressure of 1–7 bar. The system fires 0.1–12kg of shot per minute with an accuracy of ±5%. The shot is accelerated up to its exit velocity in a mixing tube that leads to the spray nozzle. This velocity, which affects key process parameters, is regulated indirectly via the spray pressure and has a tight window of tolerance. The machine’s servodrive controls were designed by a Swiss motion control specialist, TBM Automation. The challenge it faced was to develop a system that was both delicate and fast. To achieve this, the drive axes regulate the speed of the dosing screw in response to signals from throughput sensors. “If there is too much material, the speed drops, and vice versa,”explains TBM’s managing director, Silvester Tribus. Dosage drives To ensure that adjustments are carried out swiftly and precisely, the motion controls are equipped with some special functions.“We want the system to start up quickly and need a correspondingly short amount of time until the quantity and pressure have precisely engaged,”explains TBM’s Urs Tobler, who has developed an expertise in controls for shot- blasting systems. He recalls that earlier dosage drives were driven by DC motors that were difficult to regulate, especially when handling small quantities of shot, and only delivered reliable values at higher speeds. The latest servodrives and motors support linearisation of the control performance with analogue values, making precision control possible“from zero speed upwards,”according to Tobler. This controllability is said to make the process reliable across the entire speed range. This is particularly important for shot-peening because it is not possible to carry out a quality analyses after peening without damaging the components. Shots fired with velocities that are too low can also result in poor values. To determine where the optimum values lie, operators use standardised metal measurement strips called“Almen strips” which measure the effect of the shot impact by distorting the strip. Freymatic and TBM are using Kollmorgen AKD servodrives and AKM synchronous servomotors for their shot-peening machines. One attraction of these drives are that they use a single cable to link the drives to the motors, speeding up the wiring process, and saving space. The servo system also avoids the need for costly multiturn encoders which can be time-consuming to install. Instead an SD3 digital resolver is used, and an interface translates the resolver’s analogue output into a digital signal which is transmitted via the single cable. n A Swiss manufacturer of shot-peening machines used to strengthen metal components, has turned to an accurate servodrive technology to help control the demanding process. Freymatic is using servodrives to enhance the precision of its shot-peening machines.