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Optimising Turbo-Spindle Efficiency for Machining at Ultra-High Speed

Final Report Summary - OPTEMUS (Optimising Turbo-Spindle Efficiency for Machining at Ultra-High Speed)

Reducing energy demand in industry is a critical aspect of meeting global CO2 emission reduction targets. For the production of micro-sized parts and features, the miniaturisation of machine tools that produce such parts offer significant energy savings. In the OpTEMUS project, the optimisation of the efficiency of ultra-high-speed spindles was investigated, which is a key consideration of precision machine tools. Current commercial turbo-spindles suffer from very low energy efficiency, which leads to increased operating costs and environmental impact. The challenge is to increase spindle efficiency while maintaining small size, rotational accuracy and economic viability. To investigate and address these issues, this project aimed to develop and demonstrate a high efficiency turbine spindle. To accomplish this goal, a new 90° radial inflow type turbine stage (consisting of a rotor, nozzle guide vanes and volute) was proposed for micromachining spindles.
In the initial (outgoing) phase of the project, an experimental method to measure spindle torque and power output was developed to validate the efficiency estimations derived from manufacturer datasheets. This method involved mounting flywheels with known moments of inertia on the spindle shaft and measuring the acceleration and deceleration to different speeds. The very high rotational speeds attained by micro-machining spindles could result in flywheel burst and pieces of debris with a very high kinetic energy. For safety reasons a blast enclosure was therefore fabricated to provide containment of high velocity fragments. The experimental measurements demonstrated that the overall energy efficiency of a turbine grinder (spindle) was less than 20%, which was in line with earlier estimations.
The design and simulation of a new energy efficient turbine and spindle also started in the first phase of the project. The turbine types used to date for turbo-spindle applications include Pelton wheel (tangential flow), axial flow, radial outflow, and radial inflow. The radial turbines mentioned utilise rotor blades with a constant height from inlet to outlet. By contrast, in this project a 90° radial inflow turbine with an axial outflow, as typically used in automotive turbochargers, has been utilised. The turbine was specified to produce a nominal power output of 120W at 100,000 rev/min at design conditions (supply pressure and temperature of 1.7 bar and 300K respectively). The required operating conditions for a micro-machining spindle lead to a relatively low specific speed turbine design by comparison to typical turbocharger radial turbines. The design for a turbine is a compromise between efficiency, weight and size objectives. For a micro-machining turbo-spindle and machine tool, productivity and cycle time are primary objectives. It was therefore decided to employ a low reaction and highly loaded turbine stage design. This design approach allowed for reduced inertia, weight and size of both the turbine and the overall spindle, whilst still achieving a high turbine efficiency. In addition, the low reaction design reduced the turbine thrust forces and the subsequent load on the spindle bearings. Furthermore, the small diameter of the final turbine rotor reduced the blade speed to a level whereby the rotor could be manufactured using a magnesium alloy material, if further inertia or weight savings are desirable.
The aerodynamic performance of the turbine was further improved through an iterative design process whereby a number of parameters (e.g. blade profile, number of blades, radial gap between rotor and stator, rotor outlet blade angle) have been varied and their impact on performance assessed through CFD simulations. The aerodynamic performance of the turbine was constrained by both structural considerations regarding blade stress and deformation, and manufacturing considerations concerning cutting tool sizing and machining time. A spiral flow path volute was also integrated into the spindle to provide an even distribution of flow angle and pressure around the inlet to the turbine stage. A rotordynamic analysis was conducted using FEM simulations to ascertain the natural frequencies of the prototype spindle. Computational Fluid Dynamic (CFD) simulations of the final OpTEMUS turbine design indicated that a peak total-total isentropic efficiency of over 70% was achievable, using a radial inflow turbine with fully 3D geometric blade shape. The overall spindle efficiency also depends on additional losses due to windage, bearing frictional torque and minimum achievable turbine tip gap.
The final (return) phase of this project focused on the physical hardware development and testing of the prototype turbo-spindle. The attached pictures illustrate the prototype turbine, assembled spindle and spindle test stand. The final turbine rotor has an outer diameter of 25 mm and a blade span at inlet of 1.9 mm (see attached figure of turbine rotor). The prototype rotor was manufactured using an aluminium alloy material to reduce costs. The final spindle had a volume of less than 100 cm3 and weighed approximately 350 g (see attached Figures 1 – 3 showing the turbine rotor, the assembled spindle and the instrumented test rig).
To date, the prototype turbo-spindle has been successfully tested at speeds up to 90,000 rev/min and has accumulated over 14 hours running time. The distribution of the grease lubrication in the spindle bearings is a time intensive process and is still ongoing. To provide a torque load on the turbine, the prototype turbo-spindle will be coupled to a high speed electric motor/generator which will in turn dissipate the electric power via electrical resistors. Full turbine and spindle performance validation will continue over the summer 2017 under the supervision of the Fellow.
To facilitate knowledge transfer and industrial adoption of high efficiency radial turbines in manufacturing spindle technology, two funding proposals for further research and development projects have been submitted in conjunction with an industrial partner in the UK. In addition, the Fellow is currently taking part in a commercialisation programme (ICURe, SetSquared partnership) to further explore the commercial exploitation opportunities of the developed micro-turbine technology in different market sectors e.g. power tools, paint-spraying, medical and dental devices etc.

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