Unique gear grinding process offers low noise and precision

Dean Palmer reports on a new process for manufacturing helical, spur and worm gears that enables designers to develop more cost effective power transmission products A British company has developed a unique, revolutionary gear grinding process and patent applied for machining system that achieves at least DIN 2 accuracy levels on helical, spur and worm gears. Apart from the obvious benefits to the production engineering of gears, the breakthrough process should interest design engineers who need to develop more cost effective power transmission systems for their customers. The company responsible for the technology is Holroyd based in Rochdale, part of the Renold Power Transmission group. The company's engineering director Dr Tony Bannan explained the aim of the project: "The objective was to develop the lowest cost, highest accuracy grinding machine for helical and worm gears in the world and I am sure that we have succeeded. Although the GTG2 [grinding machine] is designed to provide DIN 2 levels of accuracy, generally, it has exceeded this level under optimum operating conditions, producing its 'best ground' performance with a tooth profile of DIN 1, considered by many to be the pinnacle of grinding performance." The performance of Holroyd's GTG2 grinding machine, which won the Queen's Award for Enterprise 2004 innovation category, is the result of a design synergy that combined a patent-pending machining method with high levels of machine intelligence and on-board features such as automatic coordinate adjustment, in-cycle wheel dressing, integrated profile management and CMM. But which companies will benefit from the new technology? According to Holroyd, gear manufacturers will now be able to compensate for 'helical twist', an unwelcome condition that occurs when helical gears are lead crowned to improve meshing and to reduce noise and wear. Lead crowning varies the amount of material removed from the flank of a tooth across the face width, by causing the motion of the tool to deviate from a true helix. As Bannan explained: "In form grinding, this [helical twist] has the undesirable effect of causing the profile of the tooth flank to vary across the face. In many applications, this error is not of concern, but in high precision and low noise cases, it affects gear wheel performance by concentrating loads on certain areas of the teeth during meshing." Holroyd's machine corrects this type of problem by calculating additional motions of the grinding wheel and then controlling them during the grinding operation using specially-developed software written by its own engineers. During machining, the workpiece is rotated about its axis and translated parallel to its axis relative to the tool. The tool is also moved to vary continuously the angle of inclination of its axis relative to the axis of the workpiece. In this way, the deviation of the groove surfaces actually produced from those desired and specified are reduced along each line of instantaneous contact between the tool envelope and the groove surface being machined. The result is better tooth contact during meshing and improvements in torque transfer efficiency. The eight-axis CNC grinding machine has an on-board touch-screen interface, an integrated profile management system which help to replace the variability of operator skills in precision machining set up. The machine is able to take data from its on-board probing system (supplied by Renishaw) and predict the very small alterations required to the profile of the grinding wheel to achieve any desired profile 'on the fly'. Newcastle University's Gear Centre of Excellence was also heavily involved in the development of the prototype GTG2 machine. Already, Holroyd has secured its first customer. Micro-Precision of Hemel Hempstead, which makes precision parts for aerospace & defence, medical, motorsport and commercial markets, said it was "highly pleased" with its new GTG2 grinding machine and "had already achieved grinding accuracies to DIN 1 on some gear faces" during the machine acceptance trials. As Barry Cave, director at Micro-Precision commented: "Following our acceptance of the machine, Holroyd installed it and within four days it was up and running. All our subsequent experiences with the machine have confirmed our early conclusions that it is very accurate and flexible and the perfect tool to enable us to expand our precision gear grinding business. Some of our set up times have been reduced three days to three hours and we have the flexibility to quickly change gear forms to meet customer requests." Pointers * This unique gear grinding process was the result of a collaboration between the Gear Centre of Excellence at Newcastle University and grinding machine specialist Holroyd * The patent applied for machining process achieves accuracies to DIN 2, but regularly achieves DIN 1 accuracies under optimum operating conditions * The process compensates for helical 'twist' by accurately controlling the cutting tool via on-board probing system and custom developed software