Small Machine Tools for Small Workpieces

In this paper two new small and lightweight spindles for the implementation in desktop sized machine tools are introduced. In both spindles, the tool shaft with a micro tool on the tip (e.g. micro end mill, micro pencil grinding tool or Electrical Discharge Machining (EDM) electrode) works directly as a rotor and is supported by aerostatic bearings. The spindles are driven via air turbines and reaches high rotational speeds in combination with high rotational accuracies. One spindle was implemented in a desktop-sized machine tool and used for micro machining of titanium and brass. With the high rotational speed of the spindles very high material removal rates can be achieved. Thus, the productivity of the micro machining process is enhanced.

This chapter describes the development of a module for micro electrical discharge machining with gaseous dielectrics, which was integrated into the machine frame as described in chapter “Introduction to the Priority Programme SPP1476—Small Machine Tools for Small Workpieces”. The module has implemented a high frequency gap width control, in which a solid-state joint-based unit was driven by piezo stack actuators. For this the development of a special control architecture that allowed a superimposed movement of the main axes and the piezo unit was necessary. The module has been tested on tool steel and fine grain hard metal as workpiece material. The process parameters for rough and fine machining were identified on an existing machining system and then transferred to the newly developed module. After the transmission of the parameters the processing results were compared with regard to the erosion duration t_ero and relative electrode wear ϑ.

In the course of miniaturisation the demand of novel manufacturing methods and tailored machine tools is ever-increasing. For manufacturing metallic micro-components the laser chemical machining presents a promising approach for a localised and smooth material processing. Its applied low laser power densities (<100 kW/cm²) open up the opportunity of using optical diffractive elements for a maskless beam shaping. In this work, Digital Micromirror Device technology (DMD) and laser processing are combined for the first time in a laser chemical machining procedure. A novel compact machining module is presented, which enables large angle illumination of DMD with diffraction efficiencies up to 9.6%. It is shown that the direct amplitude modulation is affected by optical distortions and inhomogeneous intensity profiles. As result, the material removal cannot be allocated to the related shaping. To overcome these limitations, an optical modelling of the DMD-pattern was performed taking into account the device tilting. Thus a highly accurate beam shaping has been achieved.

Small machine tools and their components are persistent topics of research. Effective miniaturisation usually requires a complex integration of various functions into a reduced number of components. The concept presented here constitutes a highly miniaturised grinding module named GrindBall. This concept combines a three-dimensional magnetic bearing with a fluidic drive system to embody a micro grinding module with contactless tool positioning and drive. The grinding tool consists of a sphere with an abrasive surface that is able to perform a shaftless micro machining, allowing novel grinding kinematics on hard and brittle materials, such as glass and ceramics. Due to its size and working principles, this concept is specifically developed for micro grinding applications in small machine tools.

In this paper the design and experimental validation of a novel hybrid actuator combining the advantages of shape memory alloys and piezoceramics in one setup is presented. The SMA-part of the actuator enables the required stroke superimposed by the piezo-part which ensures precision and the aimed dynamics. The two actuator parts are developed regarding geometry, applied load and control aspects. Beyond that control concepts for positioning applications are investigated. At the beginning the actuator parts piezo and SMA are considered as single and independent control loops to develop the base of the actuator control. Further, these control loops are interconnected to realise an overlaid actuator control. There are two concepts to be considered. Firstly, an open loop concept based on dynamic splitting using signal filters and secondly a concept basing on a cascaded arrangement to exploit the whole potential of this combination. Furthermore a hybrid actuator is built to investigate its properties. Experimental results further show its potential to serve as miniaturised positioning device for small machines.

Current machine tools for the machining of small workpieces show a remarkable disproportion of the machines’ size and the size of the components they produce. When trying to miniaturise such machine tools, feed axes play an important role. They position the workpiece and tool relative to each other and are, therefore, crucial for the accuracy and productivity of the machining process. The research and development of a highly accurate feed axis with a compact build space was the task of this project in the priority program SPP1476. Within the two phases of the priority program, different demonstrators for compact feed axis systems have been developed and evaluated. The final system uses a hydraulic fluid in order to realise a feed axis with a combined guidance system in a compact body. The use of piezoelectric valves enables a precise and highly dynamic control of the oil flows in the axis. In contrast to conventional hydrostatic guidance systems, the axis can not only absorb forces of the machining process and component movements but also provide functionalities for an active compensation of geometric errors. This compensation can be achieved by actively controlling the oil flow in the guidance system with piezoelectric orifices. The oil flow affects the gap widths between fixed guidance prisms and the moving housing of the axis. It can, therefore, compensate geometric errors by making small correctional movements in five degrees of freedom. An evaluation of the developed axis system shows that it meets the specified requirements and can provide highly accurate and dynamic movements within the desired build space.

In the project “Modular desktop machining centre with SMA actuation” within the SPP1476, the Chair of Production Systems has developed modular and standardised SMA actuators for use in a machine tool axis.

Aiming to reduce the size of machine tools for micro production significantly, an innovative planar drive has been developed. It generates forces and torque by fluid jets impinging on drive profiles. This work shows the evolution of the drive. Starting from theoretical considerations regarding the fluid dynamic working principle, a linear actuator was implemented. Then four linear actuators were fused in one slide to form a XY-motion stage. Modifications of the drive principle allowed to extend the movement capacity to unrestrained rotations. In addition to the air powered prototypes, a design for liquid media was tested.

Short travel ranges up to approx. 25 mm as in future small machine tools enable linear direct drives with simple single-phase design. Especially designs with moving magnet(s) and an iron core stator winding allow for large actuator constants, i.e. high forces at little losses and small volume. Different types of those compact, dynamic and cost-effective linear axes and tables as well as a novel planar direct drive have been developed, built and tested. They feature integrated ball or flexure guides, integrated incremental or absolute position sensors with resolutions from 0.16 to 1.25 \(\upmu \)m, embedded flatness-based position control, sensorless force control and various control interfaces. Selected prototypes and their features are presented.

In micro manufacturing the advantages of compliant mechanism—e.g. the high precision and the absence of friction—are particularly useful. However, the design process of appropriate compliant mechanisms is a complex task with a wide number of research areas to consider. This article addresses the challenging process of designing compliant mechanisms for feed units in small machine tools. Therefore a comprehensive design methodology for compliant mechanisms is developed and the corresponding tools are analysed. To generate an adequate topology a deeper look into the mechanical characteristics of compliant mechanism is carried out to achieve an efficient model of the mechanism in the final application. Subsequently, a control problem for optimising the trajectory of a prototype feed unit is developed and implemented. Based on these findings a comprehensive design methodology of new feed units with compliant mechanism is proposed. This methodology is applied to prototypes of feed units and the results are shown. Eventually, the manufactured feed unit is integrated in a small machine tool that is assembled of other modules which were developed within the research program.

In cooperation of the two institutes FHR and IHE a high precision radar sensor was realised, which features a flexible integration in different micro machining modules. The feasibility of high precision and robust absolute measurements near the tool centre point is one of the main advantages of the radar technology. The realised radar sensor uses an integrated silicon-germanium (SiGe) transceiver chip which allows a cost-effective, energy-efficient, compact, and mass-market-capable solution. The system is working in the frequency range from 68 to 93 GHz providing a high modulation bandwidth of 25 GHz leading to a range resolution of a few mm and a record µm-accuracy. For improving the measurement and reducing the fabrication costs of this measurement system another sensor working at 240 GHz was developed as well. With a compact design of the measurement front-end a flexible integration even in constricted available space is possible. In combination with the Field Programmable Gate Array (FPGA) back-end measurement rates up to 100 Hz can be achieved, thus a closed loop control of the micro machining tool can be realised.

A modularised and small machine tool concept developed within a six year research programme allows for a new changeability in production. For a suitable, quick, easy and precise (re)configuration of those developed modules to a complete machine tool, an interface is required. This interface shall provide mechanical positioning as well as the transfer of electrical energy and information. Additional intelligence and safety features shall be integrated. Besides the product development of this multifunctional coupling a scientific approach for the development itself as well as to cope with constantly changing external requirements inflicted by the research partners is presented within this contribution.

A concept is presented, which allows the individual combination of several machine modules into a flexible manufacturing unit by using a modular principle. Here, these are size-adjusted machine tools with a much better ratio of installation space to the workpiece size than that of conventional machines for micro manufacturing. This results not only in a smaller area in the shopfloor, but also in cost savings for acquisition and operation of the machines. A modular and (re)configurable machine concept also ensures a user-oriented machine design that can be easily and quickly adapted to special manufacturing tasks. The low dead load permits additionally a high mobility and quickly changeable installations of multiple machines within a process chain.