| Micro parts are widely used in various devices, includingminiaturized medical devices. These micro parts, made of difficult-to-cutmaterials, contain complicated structures. With the increasing demand onminiaturized medical devices, the way on how to machine these microcomponents effectively and efficiently is becoming increasingly important.Micro-wire electrical discharge machining (Micro-WEDM), which useselectrical discharges instead of mechanical forces to remove materials froma workpiece, has been proved to be an feasible approach for making3Dmicro parts that are difficult, if not impossible, to manufacture by otherprocesses. Low surface roughness and high dimensional accuracy can beobtained in micro-WEDM. In order to machine complicated micro medicaldevices, especially with a spiral structure feature, a novel micro-WEDMmachine with a horizontal wire-cutting mechanism and a rotary axis isdeveloped. The main features of this machine are as follows:Firstly, for integration of the micro-WEDM machine, key componentsare designed, which include a granite basement, a precise servo platform, ahorizontal wire-cutting mechanism, a micro-energy pulse generator, adischarge status detection system, a CNC control system.Secondly, a novel desktop horizontal wire-cutting mechanism isdeveloped, which conveys the micro-wire smoothly, and is able to controlthe wire tension steadily by adjusting a magnetic powder brake. Amathematic method is then employed to analyze the state of wire vibration,which has been found to have a great impact on the surface roughness ofmachined workpieces. Based on the vibration analysis on the wire, threemeasures are adopted to attenuate the vibration of the wire. Experimentalresults show that the vibration of the wire can be significantly suppressed by the application of these measures.Thirdly, a computer numerical control (CNC) system is developedunder the Ubuntu10.04Linux platform packed with the RTAI (Real-TimeApplication Interface) kernel. The CNC software is developed based onLinuxCNC which is an open source machine controller. Qt, across-platform user interface (UI) development tool, is employed to build aCNC UI. The UI and background modules are connected via the neutralmessage language (NML). Hardware abstraction layer (HAL) is anotherflexible platform on which various real-time as well as non-real-timemodules are connected. The motion control module, the driver of thePCI-8136interface card, the trajectory interpolation module, are alldeveloped as HAL modules and interconnected by a configuration file. Thedeveloped CNC is capable of achieving a coordinated motion along aspacious curve among three axes (the Y, Z and C axes) in a WEDMprocess.The fourth feature is that a transistor type pulse generator and anaverage voltage detection circuit are proposed to supply the electricalpower to the machining process. The detected gap voltage can be used todetermine the forward or backward motion for the servo platform. Theservo platform consists of three high precision ultrasonic ceramic linearmotors. The full-closed loop control can be achieved with the positionalfeedback from an optical scale with a resolution of50nm. The controlleremployed is a digital PID controller. By appropriately adjusting thecontroller parameters, a positional accuracy of1μm is achieved. Micromachining can be made possible with accurate positioning,.Finally, experiments are conducted on the developed Micro-WEDMmachine. With a partial-open-loop servo feed strategy and a set of properlytuned parameters, a micro horizontal kerf and a vertical kerf are machinedby using a micro-wire with a diameter of30μm. Two tiny spiral structureswith different thread pitch are then achieved in a copper pipe and stainlesssteel pipe with a diameter of1000μm, and the minimum kerf width is less than70μm. Also a micro sine structure and a micro corner structure aremachined by the developed MWEDM. Experimental results show that thedeveloped Micro-Wire-EDM machine has the capability of manufacturingmicro-mechanism parts. |
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