Cnc machining nm09/2
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- 2.5 Work handling
- 2.6 Tool changers
- 2.7 Work holding
- 2.8 Flexible manufacturing systems
- Transfer lines
- NC machines
2.4 Buffer storagePrimarily, buffer storage s used in N.C. and CNC controls to permit smooth continuous motion from block to block. 
Figure 2.7 Most modem NC and CNC controls are equipped with buffer storage. As shown above, this feature allows the control to accept information into a buffer register while an operation is being performed from the active machine registers. When that operation is completed, the information is transferred from buffer storage to the machine actuation registers. This transfer of information is instantaneous, thereby reducing the time between tape reading and machine performance. Buffer storage reduces the amount of dwell time between machine operations because the next block of tape is read and stored while the machine is executing the previous block. Part finish is also better because the cutter does not come to a halt to process the next block of information in the middle of curves, angles, or other part configurations. Buffer storage may differ from one control to another, some CNC controls being capable of decoding and buffering 200 blocks of program information ahead of machine motion. In this situation program errors can be detected and corrected during program trialing without necessarily having to restart the program from the beginning. 2.5 Work handlingRaw materials and components must be handled at various stages of manufacture, also tooling and fixturing will require handling at some time. The more automatic any of these processes can be made, the more efficient a system will become. The trade-off against the shortest time it takes to convert raw materials into finished assemblies will be the cost of building or installing automated handling equipment. Obviously, mass production lends itself to automation at all levels, but efficiencies can be improved in jobbing shops by reducing manual handling as much as possible. Work handling methods that can be used singly or jointly commonly in use are:
2.6 Tool changersMost modem CNC Machining Centres have some form of automated tool change system which can select and load tools as they are required. Both the process of selection and loading are controlled by the Machine Control Unit which responds to program instructions by activating the tool change routine. Turret type tool changers are often 6, 8 or 12 station turrets designed to hold stub arbor or collet mounted milling cutters. 
Figure 2.8 Magazine type tool changers have the program align the required tool with the tool change arm. When instructed the grip arm rotates and grips both the new and old tool in the spindle. Both tools are then withdrawn and rotated 180 °. The grip arm then retracts and loads the new tool and replaces the old tool in the magazine. 
Figure 2.9 2.7 Work holdingCNC machining centres come equipped with a ‘tee slot’ table which can be used as a means of supporting the work or a holding device. In practice, clamping the work directly to the machine table is not a preferred option because of the problems of ensuring each successive workpiece is clamped in exactly the same position. The more preferred option is to use some from of holding device which has the ability to not only hold the work but also accurately locating the job. Such devices are normally set up and fixed to the table. Examples of work holding devices include:
Work held in a machine vice is normally located to a fix reference. For example: Each workpiece is held with the left hand edge set 25 mm in from the fixed jaw of the vice. 
Figure 2.11 Fixtures Fixtures are used to not only hold the workpiece but just as importantly they are used to locate each successive part in exactly the same position. Setting blocks or ‘dummy tables’ are normally and accessory which is clamped to the machine table. These blocks may vary in size and design however, they do generally include:
Figure 2.12 Setting blocks 2.8 Flexible manufacturing systems2.8.1 Computer integrated manufacturing systemsComputer integrated manufacturing systems (CIMS) are designed to fill the gap between high volume production transfer lines and low production NC machines. The relative position of the CIMS concept is illustrated in figure 2.13. 
Figure 2.13 Relative position of the CIMS concept Transfer linesTransfer lines Transfer lines are very efficient when producing parts in large volumes at high output rates. The limitation on this mode of production is that the parts must be identical. These highly mechanised lines are inflexible and cannot tolerate variations in part design. A change over in part design requires the line to be shut down and re-tooled. If the design changes are extensive, the line may be rendered obsolete. NC machinesStand-alone numerically controlled machines are ideally suited for variations in work part configuration. Numerically controlled machine tools are appropriate for jobbing shop and small batch manufacturing because they can be conveniently reprogrammed to deal with product changeovers and part design changes. CIMSIn terms of manufacturing efficiency and productivity, a gap exists between the high production rate transfer lines and the highly flexible NC machines. This gap includes parts produced in mid range volumes. These parts are of fairly complex geometry. The production equipment must be flexible enough to handle a variety of part designs. Transfer lines are not suited to this application because they are inflexible. NC machines are not suited to this application because their production rates are too slow. The solution to this mid volume production problem is the Computer Integrated Manufacturing System. Directory: file -> view view -> Western Europe after wwii view -> Author study: paul fleischman view -> Qpa1 7ela study Guide view -> Arctic Oil/Gas Neg view -> November 1970 Approximately 150,000-500,000 deaths Bhola Cyclone Bangladesh view -> Grade Level: 6 Unit of Study: Caribbean glce view -> Citation: Duffield, Katy view -> Table of Contents Lesson # view -> Chinese Wind Energy Disad Download 0.53 Mb. Share with your friends:
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