【正文】 is lets the programmer mand axis motion in a very logical manner. Refer to , 3. All discussions to this point assume that the absolute mode of programming is used6. The most mon CNC word used to designate the absolute mode is G90. In the absolute mode, the end points for all motions will be specified from the program zero point. For beginners, this is usually the best and easiest method of specifying end points for motion mands. However, there is another way of specifying end points for axis motion. 畢 業(yè)設計（外文翻譯 ） 4 In the incremental mode (monly specified by G91), end points for motions are specified from the tool39。s current position, not from program zero. With this method of manding motion, the programmer must always be asking How far should I move the tool? While there are times when the incremental mode can be very helpful, generally speaking, this is the more cumbersome and difficult 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 regar