【正文】 ult method of specifying motion and beginners should concentrate on using the absolute mode. Be careful when making motion mands. Beginners have the tendency to think incrementally. If working in the absolute mode (as beginners should), the programmer should always be asking To what position should the tool be moved? This position is relative to program zero, NOT from the tools current position. Aside from making it very easy to determine the current position for any mand, another benefit of working in the absolute mode has to do with mistakes made during motion mands. In the absolute mode, if a motion mistake is made in one mand of the program, only one movement will be incorrect. On the other hand, if a mistake is made during incremental movements, all motions from the point of the mistake will also be incorrect. Assigning program zero Keep in mind that the CNC control must be told the location of the program zero point by one means or another. How this is done varies dramatically from one CNC machine and control to another8. One (older) method is to assign program zero in the program. With this method, the programmer tells the control how far it is from the program zero point to the starting position of the machine. This is monly done with a G92 (or G50) mand at least at the beginning of the program and possibly at the beginning of each tool. Another, newer and better way to assign program zero is through some form of offset. Refer to . Commonly machining center control manufacturers call offsets used to assign program zero fixture offsets. Turning center manufacturers monly call offsets used to assign program zero for each tool geometry offsets. 畢 業(yè)設(shè)計（外文翻譯 ） 5 Fig. 4 Flexible manufacturing cells A flexible manufacturing cell (FMC) can be considered as a flexible manufacturing subsystem. The following differences exist between the FMC and the FMS: 1. An FMC is not under the direct control of the central puter. Instead, instructions from the central puter are passed to the cell controller. 2. The cell is limited in the number of part families it can manufacture. The following elements are normally found in an FMC: ? Cell controller ? Programmable logic controller (PLC) ? More than one machine tool ? A materials handling device (robot or pallet) The FMC executes fixed machining operations with parts flowing sequentially between operations. High speed machining 畢 業(yè)設(shè)計（外文翻譯 ） 6 The term High Speed Machining (HSM) monly refers to end milling at high rotational speeds and high surface feeds. For instance, the routing of pockets in aluminum airframe sections with a very high material removal rate1. Over the past 60 years, HSM has been applied to a wide range of metallic and nonmetallic workpiece materials, including the production of ponents with specific surface topography requirements and machining of materials with hardness of 50 HRC and above. With most steel ponents hardened to approximately 3242 HRC, machining options currently include: Rough machining and semifinishing of the material in its soft (annealed) condition heat treatment to achieve the final required hardness = 63 HRC machining of electrodes and Electrical Discharge Machining (EDM) of specific parts of dies and moulds (specifically small radii and deep cavities with limited accessibility for metal cutting tools) finishing and superfinishing of cylindrical/flat/cavity surfaces with appropriate cemented carbide, cermet, solid carbide, mixed ceramic or polycrystalline cubic boron nitride (PCBN) For many ponents, the production process involves a bination of these options and in the case of dies and moulds it also includes time consuming hand finishing. Consequently, production costs can be high and lead times excessive. It is typical in the die and mould industry to produce one or just a few tools of the same design. The process involves constant changes to the design, and because of these changes there is also a corresponding need for measuring and reverse engineering . The main criteria is the quality level of the die or mould regarding dimensional, geometric and surface accuracy. If the quality level after machining is poor and if it cannot meet the requirements, there will be a varying need of manual finishing work. This work produces satisfactory surface accuracy, but it always has a negative impact on the dimensional and geometric accuracy. One of the main aims for the die and mould industry has been, and still is, to reduce or eliminate the need for manual polishing and thus improve the quality and shorten the production costs and lead times. Main economical and technical factors for the development of HSM Survival The ever increasing petition in the marketplace is continually setting new standards. The demands on time and cost efficiency is getting higher and higher. This has forced the development of new processes and production techniques to take place. HSM 畢 業(yè)設(shè)計（外文翻譯 ） 7 provides hope and solutions... Materials