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Chapter: Automation, Production Systems, and Computer Integrated Manufacturing : Numerical Control

Applications of Numerical Control(NC)

Applications of NC a. Machine Tool Applications b. Other NC Applications c. Advantages and Disadvantages of Numerical Control



The operating principle of NC has many applications. There are many industrial operations in which the position of a workhead must be controlled relative to a part or product being processed. The applications divide into two categories: (1) machine tool applications and (2) nonmachine tool applications. Machine tool applications are those usually associated with the metalworking industry. Nonmachine tool applications comprise a diverse group of operations in other industries. It should he noted that the applications are not always identified bv the name "numerical control"; this term is used principally in the rnach(ne tool industry


Machine Tool Applications


The most common applications of NC are in machine tool control. Machining was the first application of NC and it is still one of the most important commercially. In this section, we discuss NC machine tool applications with emphasis on metal machining processes.


Machining Operations and NC Machine Tools. Machining is a manufacturing process in which the geometry of the work is produced by removing excess material (Section 2.2.1), By controlling the relative motion between a cutting tool and the workpiece, the desired geometry is created. Machining is considered one of the most versatile processes because it can be used to create a wide variety of shapes and surface finishes.It can be performed ut relatively high production rates to yield highly accurate parts at relatively low cost.


There       are four  common    types of machining        operations: (a)  turning, (b)  drilling,


(c) mil1ing,and (d) grinding. The four operations arc shown in Figure 6.10. Each of the machining operations is carried out at a certain combination of speed, feed, and depth of cut, collectively called the cutting canduions for the operation. The terminology varies somewhat for grinding, These cutting conditions are illustrated in Figure 6.10 for (a) turning,


(b) drilling. and (c) milling. Consider milling. The cutting speed is the velocity of the tool (milling cutter) relative to the work, measured in meters per minute (feet per minute). This is usually programmed into the machine as a spindle rotation speed (revolutions per minute). Cutting speed can be converted into spindle rotation speed by means of the following equation:

where fr = feed rate (mm/min , in/ min)N = rotational speed {rev/min),n, = number of teeth on the milling cutter. and f = feed (mmj'tooth. in/tooth]. For a turning operation, feed is defined as the lateral movement.of the cutting tool per revolution of the workpiece, so the unns are millimeters per revolution (inches per revolution). Depth of cut is the distance the tool penetrates below the original surface of the work (mm, in). These are the paramet~rs that must be controlled during the operation of an NC machine through motion or position commands in the part program


Each of the four machining processes is traditionally carried out on a machine tool designed to perform that process. Turning is performed on a lathe, drilling is done on a drill

press, milling on a milling machine, and so on. The common NC machine tools are listed in the tollcw.ng along with their typical features:


  NC lathe, either horizontal or vertical axis. Turning requires twoaxis, continuous path control, either to produce a straight cylindrical geometry [called straight turning) or to create a profile (contour turning).


   NC boring mill, horizontal and vertical spindle. Boring is similar to turning. except that an internal cylinder is created instead of an external cylinder. The operation requires continuous path, twoaxis control.


  NC drill press. These machines use pointtopoint control oftbe workhead (spindle containing the drill bit) and two axis (xy) control of the worktable. Some NC drill presses have turrets containing six or eight drill bits. The turret position is programmed under NC control. thus allowing different drill bits to be applied to the same workpert during the machine cycle without requiring the machine operator to manually change the tool.


NC milling machine. Milling machines require continuous path control to perform straight cut or contouring operations. Figure 6.11 illustrates the features of a fouraxis milling machine.

Numerical control has had a profound influence on the design and operation of machine tool>. One of the effects has been that the proportion of time spent by the machine cutting metal is significantly greater than with manually operated machines. This causes certain components such as the spindle, drive gears, and feed screws to wear more rapidly. These components must be designed to last longer on NC machines. Second, the addition of the electronic control unit has increased the cost of the machine, therefore requiring higher equipment utilization. Instead of running the machine during only one shift, which is usually the convention with manually operated machines, NC machines are often operated during two or even three shifts to obtain the required economic payback. Third, the increasing cost of labor has altered the relative roles of the human operator and the machine tool. Consider the role of the operator. Instead of being the highly skilled worker who controlled every aspect of part production, the tasks of the NC machine operator have been reduced to part loading and unloading, toolchanging, chip clearing, and the like Owing to these reduced responsibilities, one operator can often run two or three automatic rnachines,


The functions of the machine tool have also changed. NC machines are designed to be highly automatic and capable of combining several operations in one setup that formerly required several different machines. They are also designed to reduce the time can" sumed by the noncutting elements in the operation cycle, such as changing tools and loading and unloading the workpart. These changes are best exemplified by a new type of machine

   Batch production, NC is most appropriate for parts produced in small or medium 101 sizes (batch sizes ranging from as low as one unit up to several hundred units). Dedrented automation would be uneconomical for these quantities because of the high fixed cost. Manual production would require many separate machine setups and would result in higher labor cost longer lead time. and higher scrap rate.


2, Repeat enters. Batches of the same parts are produced at random or periodic inter" vah. Once I],e NC part program hus been prcpareo, parts can he economically produced in subsequent batches using the same part program,


.1. Complex parr geometry. The part geometry includes complex curved surfaces such as those found on airfoils and turbine blades. Mathematically defined surfaces such as circles and helixes can also be accomplished with NC. someor these geometries would he difficult if not irnpossrhl c to achieve accurately using conventional machine tools.


    Much fIINa! needs to he removed [eomthe workpan, This condition is often associated with a complex part geometry, The volume and weight of the final machined part is a relatively small fraction of the starting block. Such parts are common il1 the aircraft industry to fabricate large structural sections with low weights.

    Many seperate machining operation on the port, This applies to parts consisting of many machined features requiring different cuttmg tools, such as drilled anoror tapped hole" slots. flats. and so on, If these operations were machined by a series of manual operauons. many setups would be needed. The number of setups can usually be reduced significantly using NC.


    The part is expensive. This factor is often a consequence of one or more of preceding factor, l 4. and .'1. It can also result from using a highcost starting work material When the part is expensive. and mistakes in processing would be costly. the use ofNC helps to reduce rework and scrap losses


   Punch: presses for sheet metal hole punching. The twoaxis NC operation is similar to that of a drill press except that holes are produced by punching rather than by drilling.

   Presses for sheet metal bending. Instead of cutting sheet metal, these systems bend sheet metal according to programmed commands.

   Welding machines. Both spot welding and continuous arc welding machines are available with automatic controls based on NC.

    Thermal cutting machines, su<.:has oxyfuel cutting, laser cutting, and plasma arc cutting. The stock is usually flat; thus, twoaxis control is adequate. Some laser cutting machines can cut holes in preformed sheet metal stock, requiring fourorfive axis control.


   Tube bending mllchines.Automatic tube bending machines are programmed to control the location (along the length of the tube stock) and the angle of the bend. Important applications include frames for bicycles and motorcycles.


          Other  NC Applications


The operating principle of NC has a host of other applications besides machine tool control. However, the applications are not always referred to by the term "numerical control." Some of these machines with NCtype controls that position a workhead relative to an object being processed are the following:


   Electrical wire wrap machines. These machines, pioneered by Gardner Denver Corporation, have been used to wrap and string wires on the back pins of electrical wiring boards to establish connections between components on the front of the board. The program of coordinate positions that define the back panel connections is determined from design data and fed to the wire wrap machine. This type of equipment has been used by computer firms and other companies in the electronics industry.


Component insertion machines. This equipment is used to position and insert com. ponents on an xy plane, usually a flflt hoard or panel. The program specifics the .rand y~axis positions in the plane. where the c:ompone?ts are to be located. Component rnsernon machines find extensive applications for inserting electronic components  into pnntcd circuit boards. Ma~hincs ~re available for either.throughhole or sur~acemount applications as well as similar Insertiontype mechanical assembly operations.


   Drafting machine.I.Automated drafting machines serve as one of the output devices for a CAD/CAM [computer.aided design/computeraided manufacturing) system. The design of a product and its components are developed on the CAD/CAM system. Design iterations arc developed on the graphics monitor rather than on a me chanica! drafting board. When the design is sufficiently finalized for presentation, the output is plotted on the drafting machine, basically a high speed xy plotter.


    Coordinate  measuring  machine.  A coordinate   measuring  machine  (CMM)  is an in


spection machine used for measuring or checking di~ensions of a part. The C~ has a probe that can be manipulated In three axes and Identifies when contact IS made against a pan surface. The location of the probe tip is determined by the CMM control unit. thereby indicating some dimension on the part. Many coordinate measuring machines are programmed to perform automated inspections under NC. We discuss coordinate measuring machincs in Section 23.4.


    Tap" laying machines for polymer compo5ite5. The workhead of this machine is a dispenser of uncured polymer matrix composite tape. The machine is programmed to lay the tape onto the surface of a contoured mold, following a backandforth and crisscross pattern to huild up a required thickness. The result is a multilayered panel of the same shape as the mold.


   Filament winding machines for polymer composites, This is similar to the preceding except that a filament is dipped in uncured polymer and wrapped around a rotating pattern of roughly cylindrical shape


Additional     applications       ofNC include  cloth  cutting,  knitting,        and riveting.


       Advantages  and Disadvantages  of NC


When the production application satisfies the characteristics in Tahle 6.5, NC yields many benefits and advantages over manual production methods. These benefits and advantages translate into economic savings for the user company. However, NC is a moresophisticated technology than convenuuna! production methods are.ano there are drawbacks and costs that must be considered to apply the technology effectively. In this section, we examine the advantages and disadvantages of NC.


Advantages of NCo The advantages generally attributed to NC, with emphasis on machine tool applications, are the following:


   Nonproductive time is reduced. NC cannot optimize the metal cutting process itself, hut it does increase the proportion of time the machine is cutting metal. Reduction in noncutting time is achieved through fewer setups, less setup time, reduced workpiece handling time, and automatic tool changes on some NC machines. This advantage translates into labor cost savings and lower elapsed times to produce parts.


Greater accuracy and repeatability. Compared with manual production methods, NC reduces or eljminate~ variations that arc due to operator sk.ill differences, fatigue, and other factors attributed to inherent human variabilities. Parts are made closer to nominal dimensions, and there is less dimensional variation among parts in the batch.

   Lower scrap rates. Because greater accuracy and repeatability are achieved, and because human errors are reduced during production, more parts are produced within tolerance. As a consequence, a lower scrap allowance can be planned into the production schedule. so fewer parts arc made in each batch with the result that production time IS saved.


   Impedion requlremems arC reduced. Less inspection is needed when NC is used because parts produced from the same NC part program are virtually identical. Once the program has been verified, there is no need for the high level of sampling inspection that IS required when parts are produced by conventional manual methods.


Except for tool wear and equipment malfunctions, NC produces exact replicates of the part each cycle

   Morecomplex part geometries are possible. t\C technology has extended the range of possible part geometries beyond what is practical with manual machining methods. This is an advantage in product design in several ways: (1) More functional features can he devigned into a single part. thus reducing the total number of parts in the product and the associated cost of assembly: (2) mathematically defined surfaces can be fabricated with high precision; and (3) the space is expanded within which the designer's Imagination com wander to create new part and product geometries.

  Engmeenng ,hangf's ':an be accommodated more 8racefully. Instead of making alterations in a complex fixture so that the part can be machined to the engineering change. revisions are made in the NC part program to accomplish the change

Simpler fixtures are NC' requires simpler fixtures because accurate position" thc toolis accurnplished by the NC machine tool. Tool positioning does not


   Shimer manufacturing lead times. Jobs can be set up more quickly and fewer setups arc required per part when NC is used. This results in shorter elapsed time between order release and completion.


   Reduced paris inventory. Because fewer setups are required and job changeovers are easier and faster. NC permits production of parts in smaller lot sizes. The economic lot size ISlower in NC than in conventional batch production. Average parts inventory is therefore reduced.


   Less fioor.\pm:;erequaed, This results from the fact that fewer NC machines are requircd to perform the same amount of work compared to the number of conventiona! machine tools needed. Reduced parts inventory also contributes to lower floor space requirements.


   Operator skilllevel requirements (Ire reduced. The skill requirements for operating an NC machine arc generally less than those required to operate a conventional rna. ~hine tool. Tcndin.g an NC machi~le tool usually consists only of loading and unloadmg parts and periodically changing tools. The machining cycle is carried out under program control. Performing a.comparable machining cycle on ~ conventional machine requires much more participation by the operator, and a higher level of train. ing and skill are needed.


Disadvantages of NC. On the opposing side, there are certain commitments 10 NC technology that must be made by the machine shop that installs NC equipment; and these commitments, most of which involve additional cost to the company, might be seen as disadvantages. The disadvantages of NC include the following

   Higher utilization of Ne equipment. To maximize the economic benefits of an NC machine tool. it usually must he operated multiple shifts. This might mean adding one OJ ]WO exira shifts to the plaJll'~ normal operations, with the requirement for supervision and other staff support.

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