Clamping devices have a significant influence on precision and cost-effectiveness that can be achieved in the manufacture of gearbox components.
For maximum flexibility and efficiency of production, Swiss gearbox manufacturer Rollstar AG recently invested in a large six-jaw pendulum compensation chuck with a jaw quick-change system and jaw quick adjustment. The convenient solution minimises set-up times and delivers perfect results right from the start – faster and more economically than conventional clamping solutions.
The SCHUNK ROTA NCR 1250 six-jaw pendulum compensation chuck, which Rollstar uses to manufacture gears on a Pittler PV 1600 turn machining centre, is a large-dimensioned chuck with clever features. Christian Märki, Purchasing Manager at Rollstar considers whether it was a worthwhile investment.
“Definitely,” he says. “Due to the machine design, including the clamping devices, we were able to reduce set-up times by half while decreasing the reject rate by 75%.”
In particular the investment pays off when it comes to thin-walled components and high-quality materials. The company’s aim was to achieve the necessary quality levels, reduce throughput times and lower the overall costs of producing deformation-sensitive parts in two operations while ensuring process reliability. Due to the SCHUNK ROTA NCR, parts that were previously manufactured on another machine in a three-jaw chuck with a two-pressure clamping system are now being produced faster, more precisely, and with greater process reliability.
“In the past, when we clamped a ring gear or a ring with a finished diameter of 820mm in a three-jaw chuck, the circularity or cross-sectional tolerance was higher than 0.1mm,” says Märki. “If we clamp the same component in the ROTA NCR, now we achieve cross-sectional tolerances of 0.02mm to 0.03mm. This high-precision increases the service life of our gearboxes even further because the rolling bearings are subjected to completely uniform loads.”
This means that the gearboxes can be used much longer without any maintenance works, and sudden failures are extremely unlikely. Rollstar gearboxes are valued for their extreme reliability in applications such as in tunnelling and mining, where unplanned downtimes would be very costly.
An expertly co-ordinated system
The clamping concept has been developed in close co-operation between Rollstar, Pittler and SCHUNK lathe chuck specialists. The necessary range of components, design requirements and set-up times was systematically taken into account. The decisive factor in choosing SCHUNK was the combination of the six-jaw chuck, jaw quick-change system, jaw quick adjustment, and SCHUNK’s ability to deliver quickly. Finally, the testimonials of other clients, which Rollstar visited, helped seal the deal.
The six-jaw pendulum compensation chuck SCHUNK ROTA NCR consists of a central chuck piston carrying three inner pendulums aligned at 120 degrees. Each pendulum is connected to two base jaws. This ensures workpiece centering between six contact points, adjusted in pairs. As the clamping forces are directed towards the chuck centre, optimum centering is achieved without distortion of the workpiece. The chuck perfectly adjusts to the workpiece with its oscillating jaws. With conventional jaw clamping, this configuration allows for maximum roundness of the workpieces and ensures optimum centering at the same time.
Standard jaws can be used to replace elaborate system jaws and special solutions on the ROTA NCR jaw interface. This saves time, avoids the need for cost-intensive special solutions, and reduces set-up costs. An integrated jaw quick-change system minimises set-up times on the idle machine.
“We use two sets of base jaws and set up the top jaws outside the machine, in other words, while machining is still in progress,” explains team leader Daniel Fierz. “The set-up process for the second operation then takes only 20 minutes, including cleaning.”
To do this, the base jaws are pushed open, then they automatically lock in place, and are secured with a quarter-turn of the torque wrench. Due to the jaw quick adjustment, the base jaw position can be easily adjusted in just a few simple steps.
“The quick adjustment feature allows me to set the clamping range in defined steps,” explains machine operator Willy Ummel. “The system works flawlessly and is easy to clean.”
In the past, up to two hours of machine downtime had to be calculated for a conventional set-up process of such workpiece sizes. Now it takes Ummel less than 30 minutes. Top jaw sets with hard chuck jaws and claw inserts are available for raw part clamping, and sets of soft top jaws are already bored in the different diameters for machining the part from the other side.
Perfect run-out – 75% fewer rejects
One of the main goals of the investment was for the machine to run as smoothly as possible with only minimum interruptions.
“The lathe chuck is so precise that we can use jaws again and again without re-boring,” says Fierz. “After exchange of the jaws, we achieve a run-out of 0.02mm without any additional actions.”
The vertical position of the spindle also plays a role here. Maximum freedom of movement and therefore optimum centering are ensured because the components are put onto surface-ground pillars and not on the chuck jaws, as is the case with horizontal spindles.
“The vertical machine allows for significantly better oscillation than machines with a horizontal spindle, where the friction between the component and the jaws restricts the pendulum effect,” emphasises Fierz.
When finish machining or clamping rough turned surfaces, Ummel can clamp the ROTA NCR’s pendulum in the center position so that all six jaws are moving concentrically. The use of the lathe chuck has a welcome side effect: the reject rate has fallen by around 75% since the solution was implemented. This is due to the dramatically reduced deformation, the high precision of the jaw change process and the improved flat work surface of workpieces. The reduced reject rate results in considerable savings, especially with high-quality materials such as the ferritic casts frequently used by Rollstar.
According to Märki, a decisive advantage of the clamping solution is the flexibility that Rollstar has gained in production: “Even though we typically manufacture in batches of 10, we can now squeeze in rush orders at any time without losing efficiency.”
This allowed the company to reduce stock levels without compromising its excellent service level. In the near future, Rollstar plans to machine components with diameters between 400mm and 1,600mm on the chuck. The ROTA NCR 1250 is ideal for this range of parts.
“It’s easy to clean, a breeze to adjust and extremely easy to load thanks to its horizontal position,” says Märki.