Developed especially for grinding precision spur and helical gears, worms, screws and rotors of up to 350 mm diameter and 160 mm gear face width, Holroyd GTG2 grinding centres use what is said to be a unique machining system to achieve accuracy to DIN 2 levels and beyond.
“The customer required a machine capable of grinding a variety of gears to exacting tolerances,” says Holroyd regional sales director Steven Benn. “Products to be manufactured include spur and helical gears; gears with end tip relief and root fillet radius; worm tooth forms such as ZA, ZI, ZK and ZN; worms with sunken tooth forms; and dual lead forms with both cylindrical and tapered roots. The GTG2 was selected for its versatility, reliability and accuracy – the latter achieved through high levels of on-board machine intelligence and a machining process developed to compensate for helical twist.”
Helical twist is a condition that occurs when helical gears are ‘lead crowned’ to improve meshing and 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 tool motion to deviate from a true helix. The problem is that ‘in form’ grinding has the undesirable effect of causing the profile of the tooth flank to vary across the face. Particularly in high precision and low noise applications, this variation affects gear wheel performance by concentrating loads on particular areas of teeth during meshing.
The GTG2 machine corrects this problem by calculating and controlling additional motions of the wheel during the grinding operation using dedicated software written by Holroyd engineers. In the machining process, the workpiece is rotated about its axis and the tool moved so as to vary the angle of inclination of its axis relative to the workpiece axis. As a result, generating errors are reduced along each line of instantaneous contact between the tool envelope and groove surface being machined. The outcome is better tooth contact during meshing and improvements in torque transfer efficiency.
GTG2 gear grinding centres combine rigidity with high power for either CBN or conventional deep grinding operations.
“Although the GTG2 is designed to provide DIN 2 levels of accuracy, it has generally exceeded this benchmark, achieving its ‘best ground’ performance with a tooth profile of DIN 1,” says Benn.
The machine combines high levels of intelligence with on-board features such as automatic co-ordinate adjustment, in-cycle wheel dressing, integrated profile management and co-ordinate measurement. All gear, worm and spline profiles can be verified using the integrated Renishaw probing system, enabling automatic on-machine corrections to be made if necessary.
According to Holroyd, the GTG2 is easy to program, set up and use. Features include an integrated profile management system and an advanced touchscreen Interface which allows the operator to take a typical drawing and enter co-ordinates directly into the machine. By extracting data from the on-board Renishaw probing system, the GTG2 is able to predict tiny alterations in the profile of the grinding wheel to achieve a precise result. Where a workpiece fails to meet tolerance requirements after a first grinding pass, the machine calculates the adjustments needed to grinding wheel form or axis position, transmitting the data to a 2-axis CNC wheel dressing system.
