Feature sponsored by Test & measurement T
he machining accuracy of a CNC machine tool is affected by many factors, including machine mechanics, control electronics, machine rigidity, thermal stability and the geometric accuracy of the cutting tool.
Small cutting tools require precision grinding to ensure accurate tool geometries. Precision tool grinding is a machining-based production process that uses abrasives to obtain the required geometry and dimensional tolerances for a cutting tool. Demand in India for precision machining has hugely increased, such that India’s machine tool market is now one of the fastest growing in the world. To seize this opportunity and meet the increasing demand, India-based company, Tool Grinding Technologies (TGT), has developed a range of precision tool grinding machines equipped with linear motors and Renishaw’s FORTiS enclosed absolute position encoders, which help to improve machining accuracy and process stability.
THE CHALLENGE
Linear motor technology has matured in recent years and has become important in industrial applications. Modern linear motor designs have achieved significant improvements in heat dissipation, thrust and cost. Automation systems and CNC machines, such as precision grinding machines, increasingly use linear motors in place of traditional ball screws. The precision and accuracy of a linear motor servo is heavily influenced by the quality of the linear position feedback from the encoder. Sub-Divisional Error (the measurement error within one signal period) is a critical specification for smooth velocity control, which, as tool grinding machine manufacturers know, is important for surface finish quality. Low positional jitter allows optimisation of servo loop gain. CNC profile grinding employs small, incremental linear movements of the tool grinder’s axes to produce the interpolated trajectory; therefore, accurate motion control is vital. The working environment of a tool grinding machine is harsh and includes machining
vibrations, and contaminants such as cooling fluid and grinding swarf. Vibrations in the machine can coincide with the natural resonant frequencies of position encoders. Therefore, positioning accuracy is improved if the encoder has the ability to damp resonant vibrations to prevent high-amplitude disturbances from entering the position control loop. In these cases, encoders with tuned-mass damping can significantly enhance CNC machine performance. Mr Ranganatha, one of TGT’s Directors, explains: “In the past, TGT’s grinding machines used ball screws. To further improve the accuracy and to sustain this accuracy long-term, we decided to employ linear motors in our newest models. There are many technical aspects to the design. Issues that needed to be addressed included environmental protection of the linear motors, encoder installation, shock resistance, maintenance, and servicing requirements.”
THE SOLUTION The linear motors in TGT’s latest models of tool grinder require linear position feedback from a suitable enclosed linear encoder designed for harsh environments. As a long-time customer of Renishaw, TGT collaborated with Renishaw engineers in testing and evaluating the FORTiS encoder. The FORTiS enclosed encoder design is built upon industry proven RESOLUTE absolute encoder technology and provides high resistance to the ingress of liquids and solid debris contaminants. FORTiS encoders also feature integrated, specially-designed tuned mass dampers that not only deliver class-leading 30 g vibration resistance, but also ensure that vibrations at the encoder’s natural frequency are strongly damped; thereby, considerably reducing the risk of disturbances to the servo control loop. “We have had good experience of using
Renishaw products over many years, such as the LP2 probe system and the RESOLUTE absolute encoder for direct-drive rotary tables, and the performance has been very good.” says Ranganatha. “When Renishaw recommended the
FORTiS enclosed optical encoder for our new grinding machines, the necessary testing and evaluation work was carried out without hesitation,” he continues.
As one of the leading tool grinder manufacturers in India, TGT has pioneered the introduction of precision tool grinders with linear motors in recent years.
V2 ADVANCED MAXIMA In 2021, TGT launched the V2 ADVANCED Maxima - a five-axis high precision tool grinding machine comprising three linear axes (X, Y and Z) and two rotary axes (A and C). Tubular linear motors are employed on the crosswise X-axis and the longitudinal Z-axis of this machine with strokes of 300 mm and 680 mm, respectively. Both axes are equipped with the FORTiS-S enclosed encoder system, which is the standard-sized variant of the FORTiS encoder. The vertical Y-axis is equipped with a ball screw system as standard, but customers can also choose a linear motor-driven axis with a FORTiS-S encoder if required. The rotary A- axis is mounted on the rotary C-axis and both axes control the orientation of the workpiece using direct drive motors, equipped with the RESOLUTE absolute rotary encoder, to reduce backlash. Both the RESOLUTE and FORTiS-S encoders output BISS-C serial comms for direct interface with a Beckhoff controller. During processing, the A-axis rotates the workpiece, which is clamped on the workpiece spindle, while the linear axes (X, Y and Z) control the position of the grinding wheel. This tool grinder is also equipped with
Renishaw’s LP2 series probe system for workpiece set-up prior to grinding. In the early
44 February 2023 Instrumentation Monthly
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