Textbook of Production Technology PDF

702 A Textbook of Production Technology 25. What are: Continuous and aligned, discontinuous and aligned and continuous and randomly oriented fibres? 26. Write a short note on the material ‘‘Kevlar’’. Kevlar :– Kevlar is the trade...

702 A Textbook of Production Technology 25. What are: Continuous and aligned, discontinuous and aligned and continuous and randomly oriented fibres? 26. Write a short note on the material ‘‘Kevlar’’. Kevlar :– Kevlar is the trade name of ‘‘Aramids’’, Aromatic polyamides (thermosetting plastics). It is 5 times stronger than the same weight of steel, does not rust, does not corrode and is extremely light weight. Applications : As fibres for reinforced plastics, in bullet proof vests, in underwater cables, brake linings, radial tyres, space vehicles, boats, parachutes, and skis. Kevlar Protera is a new material from Du Pont (1996). It is a high performance fabric that allows lighter weight, more flexibility and greater ballistic protection in a vest design due to the molecular structure of the fibre. Its tensile strength and energy absorbing capabilities have been increased by the development of a new spinning process. (Also see Art. P 692). 27. Classify 'Composites'. See P 688. With respect to the matrix constituent, the composites are : Organic Matrix Composites (OMCs) or polymer Matrix Composites (PMCs). These also include Carbon Matrix Composites, known as Carbon -Carbon Composites), Metal Matrix Composites (MMCs) and Ceramic Matrix Composites (CMCs). On the basis of reinforcements used, composites are : Fibre Reinforced, Laminar and Particulate Composite (See P 689). 28. Write the Constituents of Composites : These are : Matrix, Rein forcements and Fillers. For matrix material, see Q 27. Its roles are : (See P 688 and 690 also). The properties of matrix material are, in addition to given on PP 688 and 690: Reduced Moisture absorption, Low shrinkage, low co-efficient of thermal expansion, excellent chemical resistance, dimensional stability, both elevated temperature and Low temperature capacity, etc. For Reinforcements, See P 689 and P 690 (Art. 14.2.2). The common reinforcing materials are : Thermosetting Polymer, Thermoplastic polymer, Metals, Carbon and glass (GFRC). Fillers are generally present to improve properties of composites other than strength, e.g., decorative finish, handling, fire retardancy, light stabilization, air inhibition reduction, air release promotion, improved modulus, very low densities, radar transparency and electrical and magnetic properties etc. Fillers can be : minerals, metal powder ceramics, polymers or metal oxides etc. Some common filters are : particles of alumina, silica, hollow or solid glass particles, wood chips, fly ash and carbon black. 29. Write the advantages and limitations of composites. Advantages : In addition to given on P 688 : Improved dent resistance, improved torsional stiffness, are dimensionally stable, improved marine ability, simplified manufacture and assembly, close tolerances, greater reliability, Directional tailoring properties and fibre to fibre redundant load path etc. Limitations : High cost of raw materials and fabrication, are more brittle than wrought metals, Reuse and disposal difficult, environmental degradation, New problems for repair etc. 30. Define Composites : Also see P 688. They are a combination of two or more materials, see P 687, and exhibit the best properties of the individual materials and include other properties that none of the individual materials possess. They are widely used in electronic industry for producing PCBs' A recent addition to composites is: Syntactic foam, which is gas filled material consisting of hollow spherical inclusions in a polymer/metallic matrix. It has good thermal and water insulation, vibration damping, low dielectric constant etc.  Chapter 15 Tracer Controlled Machine Tools 15.1 GENERAL In tracer controlled machine tools (also known as duplicators, contour producing or copying tools), an attachment is provided that enables the machine tool to machine all types of shaped components from a master form or model of the desired shape. The master form may consist of a previously machined part or a specially made template. The cutting tool is made to follow a path that duplicates the path of a stylus or tracer finger that scans the model either automatically or by hand. The machine tool cutter is made to mechanical, hydraulic, electrical, electro-hydraulic or optical contact. A taper turning attachment is an example of a simple straight line copying device. ‘Duplicators’ are capable of reproducing external and internal profiles from templates in three dimensions, whereas ‘Profilers’ do so in two dimensions. A tracer controlled lathe will machine irregular contours, including steps, tapers, right-angle or tapered shoulders, recesses, grinding necks, radii formed surfaces, bore contours and so on. A tracer lathe is generally faster than a manually operated lathe for two or more diameters, shoulders or faces on a workpiece because the man or handling time to adjust the tool from one surface to another is eliminated. The advantages of a tracer lathe over the manually operated lathe increase as the part becomes more complex. To meet the initial cost of making and setting the template, a tracer lathe will be economical as compared to ordinary centre lathe) only if a number of pieces of one type are to be made on it. The components made on a tracer lathe, can also be produced on turret and automatic lathes and at faster rate because in these machinnes, several tools can be made to machine a component at the same time. However, more expensive tooling and set up and more powerful machines are required. These machines will prove economical only for relatively large quantities of components. The use of a copy lathe substantially reduces the setting costs and can enable small batches to be economically produced. Tracer machines are designed to operate on an automatic cycle and will produce components to within the limits associated with automatic lathes. Also, turret lathes and some designs of automatic lathes cannot be used for parts that must be turned whilst held between centres. Tracer control is equally fast and easy to control as numerical control. But it is more expensive and more lead time is needed to make a template for many parts (particularly the more complex one) than preparing a programme for a NC machine. However, tracer control is much cheaper than a numerical control. Its initial cost may be of the order of 1/10 or less of the cost of numerical control. 703 704 A Textbook of Production Technology Tracer cotrol can be used with all the machine tools, but major development has been done for milling machines. Parts of complex shapes, such as blanking and bending dies, metal foundry patterns, permanent moulds, plastic moulds, propellar blades and turbine blades, are milled on tracer controlled milling machines. 15.2. MECHANICAL COPYING MACHINES In these machines, the movement of the cutter is controlled through mechanical means, from a tracer or follower, following a master or a model. Mechanical tracing system is used in small machines which do engraving work and also for light milling operations such as the cutting of lettering or other fine details in dies and similar work. They are also used for milling complex surfaces of small parts of templates, in cases where the required machining accuracy is within 0.1or 0.2 mm. Many designs and forms of mechanical copying machines are available, but, here, we shall discuss only three of the common machines. Cutter and follower head X X Y Head or 2 saddle 1 Follower Cutter roller 3 4 Work Former Y Fig. 15.1. Mechanical Copying Machine. 1. The first design is shown in Fig. 15.1. Both the work and the former or master are secured to the work-table. A provision may be made to adjust the relative positions of the two. The work table can be traversed in the direction YY on the bed ways of the machine. A cutter and a follower are carried by a head or saddle that can slide on ways on the column of the machine in the direction XX. Usually, either the work-table or the cutter head is left free to slide and is acted on by a spring or other force which keeps the follower and master in contact while a steady feed motion is given to the other member. Thus, when machining the portions 1-4 and 2-3, the feed motion will be given to the work-table and the cutter head would be left free so as to move towards the left or right. Similarly, when machining the portions 1-2 and 3-4, the feed motion would be given to the follower and cutter head, keeping the work-table free to move forwards or backwards. The force to keep the follower and the master in contact with each other can be supplied by a spring, a compressed air or hydraulic cylinder or by a dead-weight acting through a cable. if the cutter and follower head is carried on a slide that can move up and down vertically, the machine could be made to copy a three dimensional contour. For exact copying of the master, the shape and size of the follower and cutter should be exactly similar. 2. Another design of a machanical copying machine is shown in Fig. 15.2. It consist of two rotary tables which are carried by a saddle which can move along the bed ways. The two rotary tables are rotated in synchronism by a mechanical feed motion. One of the tables known as work- table carries the work and the cutter. The other table carries the master and the follower which are kept in contact by a force acting on the saddle. The master thus imparts the longitudinal motion of the saddle that enables the cutter to copy the shape of the master. The axes of the cutter and the follower are fixed during cutting, but their relative position can be adjusted relative to each other. Tracer Controlled Machine Tools 705 In an alternative arrangement, the rotary tables are carried direct on the bed of the machine and the longitudinal motion is given to the saddle which carries the follower and cutter and which can slide on the column of the machine. By making the cutter and follower capable of moving vertically, the machine can be adapted for three dimensional copying. Work table Cutter Master Follower Saddle Rotary feed Motion Fig. 15.2. Mechanical Copying Machine. Disadvantages of the Above Machines (a) Frequently, large forces may have to act between the follower and the master. This will necessitate the use of a large diameter follower and hence a large diameter cutter, which imposes restriction on the forms that can be reproduced. This difficulty can, however, be overcome by modifying the shape of the master making it possible to use a cutter of smaller diameter than the follower. (b) The master should be strong enough to withstand the forces imposed on it by the follower. This will make the master somewhat expensive. (c) Such machines tend to become single-purpose machines with a limited range of work. 3. Pantograph Machine. The Pantograph machine is extensively used for engraving and for diesinking operations. The machine utilizes the pantograph mechanism. The cutter is carried by one point of the pantograph and a tracer by the corresponding point. The ratio of the cutter movement to tracer movement is obtained by adjusting the links of the pantograph. The reproduced part can Pivots E D Base or frame of the machine P A B M C Cutter Cutter Slide T Tracer Fig. 15.3. Pantograph Linkage. 706 A Textbook of Production Technology either be an enlargement or reduction of the size of the master. For very small work, the model or master used is larger than the work. The tracer or stylus is moved around the master by hand and thus imparts the necessary motion to the cutter. A typical two dimensional pantograph linkage for an engraving machine is shown in Fig. 15.3. The arm CA is always kept parallel to arm TB by the parallelogram ABDE. Also points P and C must be adjusted so that the three points P, C and T will always lie on a straight line regardless of the positions of the linkage. The slide M helps in positioning the entire linkage system with respect to the frame of the machine. For this, it can be moved along line PA. It is fastened at the desired position with a clamping screw. P is the pivot for the entire linkage system with respect to the frame of the machine. The cutter slide helps to position the cutter on the link CA. It is then fastened with a clamping screw. It is clear that the triangles PAC and PBT are always similar. The ratio of the sides of the triangles PAC and PBT will depend upon the reduction or enlargement desired. In a three dimensional pantograph system, additional arrangement is done to impart vertical movement to the cutter. This vertical movement will be at the same enlarged or reduced scale as that at which the pantograph linkage is set. On a lathe, the simplest type of mechanical tracer-controlled contouring device is similar to taper-turning attachment in which the straight guide bar has been replaced by a template of the required shape and the guiding block by a roller or stylus. In mechanically controlled tracer machines, it is necessary to ensure reliable contact between the stylus and the template. A typical pantograph die-sinking and engraving machine (also known as pantograph mill) consists of two tables, copy table and the work table. The template or the model is held on the copy table. The copy table is held on horizontal slideway, with vertical and rotary movement. The worktable has movements in three directions controlled by screws and indicating sleeves. The patograph mechanism is carried above the two tables ad incorporates the tracer pointer and the cutter head, the spindle being driven by round or V-belt from a vertically mounted motor. The correct depth of cut is obtained by the cutter spindle feed mechaism and the form of the cutter imparts the profile to the mould or cavity. 15.3. HYDRAULIC TRACING DEVICES In hydraulic tracing system, the stylus which follows the form of the master, gives a continuous signal to the hydraulic unit, in order to cause the tool to follow a path which is a copy of the form on the master. These hydraulic units are basically `Servomechanism'', that is, a control system which magnifies a relatively small input force or signal in order to provide a large output force or signal for operating the mechanism. The chief advatage of nonmechanical tracing systems (hydraulic and electrical) is that the stylus slidingn over the template (hydraulic and electrical) is that the stylus sliding over the template profile does not carry the cutting force and the cutting force has no influence on the force of contact between the stylus and the template. This enables the contact pressure on the template to be reduced to 1 to 6 N. Because of the low pressure of the stylus on the template and the comparatively small size of the stylus, it proves feasible to tur steep transition surfaces of the contour at higher speeds ad feeds and to use templates made of iexpensive materials. A simple hydraulic circuit for the servomechanism of a tracer controlled milling machine is shown in Fig. 15.4. Its principle of operation is explaied below : The blank and the template are clamped on the table of the machine. As the table moves longitudinally, the stylus of the hydraulic tracing device follows the form of the template and will move up and down in accordance with the profile of the template, always remaining in contact with it by spring pressure on the top end of the hydraulic piston. If the stylus, in following the template form, moves downward, ports b and d of the tracer valve are opened. As a result, oil delivered by Tracer Controlled Machine Tools 707 the oil pump is admitted through port b of the valve, to the headend of the hydraulic actuating cylinder. The pressure of oil in this end of the cylinder forces the piston together with its rod and spindle head to move downward, reproducing the template shape on the blank. The oil from the rod end of the cylinder drains through port d back to the tank. The spindle head will cotinue to move downward until the ports b and d are covered by the body of the tracer valve. Hydraulic actuating Cylinder Spring Tracer valve Oil Pump Relief Valve a Spindle b head c d Stylus Blank Template Table Fig. 15.4. Hydraulic Tracer-controlled Milling Machine. When the stylus moves upward, ports a and c of the tracer valve are uncovered. Oil from the pump is admitted through the port a to the rod end of the actuating cylinder, while the oil from the head end drains through port c back to the tank. The piston, together with piston rod and the spindle head travels upward until the ports a and c are again closed by the valve body. In this manner, the power cylinder which is rigidly linked to the spidle head, reproduces the motions of the tracer system. The tracing efficiency may be very high, of the order of 0.01 or 0.02 mm. The various tracing functions employed in tracer controlled milling machines are discussed below. One or more of these functions may be used simultaneously to generate the desired complex surfaces. 1. Depth tracing. Here the depth of the entire shape to be duplicated in the workpiece may be varied relative to the master or model. Depth tracing positions only one machie slide or axis which is controlled from the tracing follower, while one or more of the machine slides or axes are fed by hand or automatically, as discussed above (Fig. 15.4). The principle of a copy millig machine, three dimensional system (under depth tracing) is illustrated in Fig. 15.5 (a). The vertical movement of the knee is controlled by a hydraulic tracer valve. The stylus scans the master in the vertical plane, while the machine table moves in the logitudinal direction. The milling cutter moves vertically in unison with the vertical movement of the tracer. Straight line feeding path across the three dimensional master are employed as shown, by the movement of the machine table in the longitudinal direction. At the ned of each longitudinal traverse, the cross feed is advanced through a pick feed mechanism. 2. 360° or 2-axis tracing. This system is illustrated in Fig. 15.5 (b). The tracer and the milling cutter remain at a constant depth during a complete tracing. In order to machine a three dimensional shape, several tracings are made as shown. These machines use flat templates and irregular contours, radial cam profile and constant depth grooves of any contour on a flat face can be milled on such machines. Other applications include the cases where form cutters are used to great advantage. 708 A Textbook of Production Technology Substituting one of the two machine slide axes with a rotating axis makes it possible to machine aerofoil blades for gas turbines or grooves in drum cams. Tracer Tracer Pick feed Master Master Cross Cross Longitudinal Longitudinal Fig. 15.5. Copy Milling. 3. Combination tracing or 3-dimensional tracing. Three axes of the machine are controlled simultaneously from a single follower. This function is available either with automatic or hand feed. For the automatic combination tracer, there exists a limitation on the ability of the tracer to follow the model when the angle for the depth functions exceeds 35°. No such limitations exist when hand feed is used. This system has the flexibility that permits it to be used as only a depth tracer or as a 360° tracer. Machine equipped with the combination tracer, together with servocontrol to the three axes are very versatile. 4. Depth Control and 360° tracing. In this system two separate models are used. The making of the models is greatly simplified compared with the complexity of the three dimensional models required in combination tracing. Die-Sinkers. Milling machines with depth tracing and contouring functions are known as die-sinkers. In “hand die-sinkers” power and hand feed is employed for machine slides, without feed rate modifications to the feeding slides. The feed rate is varied manually according to the contour of the master or model. “Automatic die-sinkers” are equipped with power and hand feed to the machine slides with feed rate modifications, automatic feed reversal and pick feed or progression. Automatic feed rate modifications will produce a constant feed rate between the cutter and the surface being generated. In such machines, provision for 360° profiling is also made. Copying Lathes. As discussed under Article 15.1, copying lathes are used for profile turning. These machines are used for turning of parts such as shafts, axles, piston rods, internal and external stepped and form surfaces etc., in batch production. The latest version of copying lathes have a slant bed to facilitate heavy duty copy turning. One and two dimensional systems are employed on turning equipment for engine lathes. 1. One Dimensional System. In this system, the longitudinal movement of the tool is provided by the carriage driven by the standard feed mechanisms and does not depend on the tracer system. The working of the hydraulic tracer system is identical to that for milling machine explained above (Fig. 15.4). Upon longitudinal feed of the carriage, the stylus follows the profile of the template and operates a hydraulic valve that admits or exhausts oil under pressure on the opposite ends of a piston which transmits its motion to the tool slide. The neutral position of the control valve corresponds to tracing on a section of the template parallel to the spindle axis, i.e., when the cross feed should be disengaged. In one dimensional system, three classifications are found : (i) Tool slide at 45° to the centre line of the machine. The compound rest of the engine lathe is removed and the tracer controlled tool slide is mounted in its place at an angle. When Tracer Controlled Machine Tools 709 this angle is to the line of centres, it is not necessary to stop the longitudinal feed of the carriage to produce shoulders at right angles to the centre line of the job. The carriage feeds continuously and the cut is uninterrupted, Fig.15.6(a). Feed Feed 30° 80° Contour Contour Tool slide Tool slide 45° (a) (b) Feed Feed 80° 60° 60° 70° Contour Contour Tool slide Tool slide (c) (d) Feed Tool Slide angle 45° 30° Contouring with a variable (e) angle tool slide Fig. 15.6. Tool Slide Angle Settings. (ii) Tool Slide angle 90°, Fig. 15.6 (b, c). The standard carriage provides longitudinal feed. The stylus controls the in-and-out motion of the cross-slide and also actuates a hydraulic clutch and brake that stops the carriage feed for right angle facing cuts. When the stylus contacts a shoulder, it disengages the clutch and applies the brake while lesser deflection produce in-and-out motion of the tool. (iii) Tool-slide angle variable, Fig. 15.6 (d, e): This feature permits setting the tool slide at the optimum angle for the most difficult cutting conditions in the contour. The greatest angle of the contour is bisected by the angle of the tool slide. For step shafts, the tool slide normally is set at when set at the stepest practical contour that can be traced continuously is one with a included angle in the downward direction and a included angle outward. Stylus Template Stylus Tracer Work switch Bed Cross slide A B Saddle Electromagnetic Continuously Spring strip clutches running motor (a) (b) Fig. 15.7. Electrical Tracer System. 2. Two Dimensional Control. Two co-ordinate hydraulic-tracer-controlled systems, in which longitudinal feed is dependent on cross feed, are usually used in automatic lathes at the present 710 A Textbook of Production Technology time. This system is similar to one dimensional system with the addition of carriage-feed control. Feed rates for both motions are arranged in the hydraulic control valve to give a resultant feed rate tangential to the point of contact between the stylus and the template, i.e., uniform across the entire template. The feed rates are independent of the normal feed control of the machine. This type of control can contour more complex shapes than the one dimensional system and provides a uniform feed rate and thus a uniform finish. If the form of the contour being turned requires an increase in the depth of cut at any particular point, i.e., increased infeed of the cross slide, the longitudinal feed will be automatically reduced. When, on the other hand, the depth of cut is reduced, the longitudinal feed is automatically increased. Template Location. The template may be located at the front or near of the lathe. It may be fixed in position or a floating template carrier may be employed. In the latter case, the template carrier fits in longitudinal ways in a bracket attached to the base of the tracer unit. The Bracket moves in and out with the cross slide. The template-carrier ends are mounted so they can slide in the transverse direction. The tool slide is advanced until the stylus contacts the template, then the cross slide is advancted manually to the required dial reading for the cut and the longitudinal feed is engaged. Once in contact, the stylus, the template, tool and tracing head remain in correct relationship to each other while the cross slide is advanced or withdrawn. It is not necessary to align the template carrier exactly, because the carriage, as it feeds straight along the bed, automatically positions the template carrier parallel to the centre line of the job. Tracer Controlled Turret Lathes 1. Slide-tool Type Tracer. A hydraulic slide tool type contouring attachment that can be mounted to a face of the hexagon turret is used for contour boring and turning. These tools are single directional. The tool is mounted vertically into the tracer slide tilted back to a 60° angle to the machine centre line to allow climbing a 90° shoulder. The longitudinal turret apron feed is used to traverse the template and the tracer slide advances or retreats on the 60° angle to follow the contour. Short to medium length contour bores are ideal jobs for this tracer. The bores are roughed out using other hexagon turret tools and the tracer used for finish operations. 2. Single Directional cross-sliding Hexagon Turret Tracer. This type of control is adapted to the cross motion of the cross-sliding hexagon turret to provide a means of controur machining of deep grooves (bores). The standard longitudinal saddle feeds are used to move the unit along the ways as the cross-sliding turret moves laterally under control of the template and tracing unit to produce the desired contour. Contour cutting can be done with any one or all six stations on the hexagon turret, and an infinite number of cuts can be taken from one station, an adjustment for size is made through the hand wheel controlling cross motion of the turret. On cuts that do not require contour control, the unit is operated as a standard cross-sliding hexagonal turret and the mechanical feeds are employed. The contouring slide (the cross-sliding hexagonal turret slide) operates at 90° to the centre line of the machine, thereby limiting the angles cut to a maximum of between 45° and 60°, depending on the cut and finish requirements of the job. 3. Two Dimensional Cross-sliding Hexagon Turret Traces. Two directional contouring is also adapted to cross-sliding hexagon turrets to allow tracing complex shapes. This unit has no angle limitations and the shape that it can produce is limited only by the geometry of the cutter. This arrangement produces the contour by combining control of both longitudinal and cross feeds of the cross-sliding hexagon turret. These feeds are controlled and powered either electrically or hydraulically. Primarily, the unit is for contour boring and facing. Contour turning is limited in length by the overhang of the cutter holder from the face of the hexagon turret. As with single directional unit above, contouring can be done from any one or all six hexagon turret stations and the tracing unit can be disengaged to allow use of carriage feed for standard operation. Tracer Controlled Machine Tools 711 15.4. ELECTRIC TRACING SYSTEMS An electric tracing device has an electrically coupled tracer which follows a master profile of the shape to be produced. Through an electric control, it imparts an identical motion to the spindle of the machine tool. In a three dimensional, copy milling machine, the tracer and the cutter are identical in size and shape. Under the automatic electric control the tracer slowly scans the area covered by the master profile. The depth of cut is automatically controlled by the tracer. At the end of the each stroke of the tracer, the operator reverses the direction of motion and puts on an increment of feed to cause the action to advance a small distance from the previous passage. Thus, the entire surface of the master is covered by the tracer in a series of narrow parallel lines with the cutter faithfully following the path of the tracer and reproducing the shape of the master. The action of the tracer is very light and the master can be made of any material. Also, the sensitivity is very fine and it is possible to reproduce the most delicate gradations of surface. For a two dimensional work, the profile tracer guides the vertical and horizontal movements of the machine after the cutter has been set into the depth required. This operates against four contacts at corresponding to the four directions of travel and changes in direction of travel are obtained by energising or deenergising magnetic clutches which operate the movements of the machine. The sensitivity is so high that a movement less than 0.025 mm on its point is enough to cause a change in direction of machine travel. Parts such as moulds for plastic lavatory seats and drop forging dies for a crankshaft can be made on such machines. Fig. 15.7 shows an ‘on off’ electrical system as applied to a lathe. In the figure, the tool is shown to be operating on a parallel portion of the work. The tracer switch, Fig. 15.7 (b), will be in its neutral position with both the contacts A and B open. Because of this, neither of the electro- magnetic clutches will be engaged and the cross-feed screw will be stationary. When the tracer or Column Tracer unit Stylus Master Column Head Work Cutter Bed Spindle Saddle Fig. 15.8. Keller Die-Sinking Machine. stylus reaches the inclined portion of the template (due to the longitudinal movement of the saddle), it will get deflected inwards. When this deflection is sufficient to close the contacts A, the electromagnetic clutch giving outward motion of the cross-slide will be engaged. The cross-slide feed screw will make the cross-slide to move outwards a constant rate. This will continue until the deflection of the tracer is reduced. The contacts A open again and the cross-feed will stop. With the continued longitudinal feed of the saddle, the tracer will again get deflected and the complete above cycle will be repeated. Thus, the cross-feed is not continuous so as to obtain a smooth curve. Instead a series of steps will be obtained. The size of these steps will depend on the magnitude of 712 A Textbook of Production Technology the tracer movement required to make and break the contacts A. With proper design of the system, the size of the steps can be made very small—usually smaller than the totool marks left by a sharp- nosed tool in ordinary turning. The inclination of the steps will depend upon the saddle feed rate/ crossfeed rate. it can be made larger by using a low saddle feed rate and a high cross-feed rate, it cannot equal Thus, in this system, square shoulder cannot be copied. In spite of this defect, the system is popular because it is simple and reliable. Keller Die-Sinking Machine. A keller die-sinking machine based on “on-off” electrical tracing system is shown in Fig. 15.8. The cutter spindle and the tracer unit are carried on the head which moves up and down the column. This column in turn can move perpendicular to the plane of paper on ways provided in the saddle. The saddle can itself slide in or out along the bedways of the machine. Both the master and the work are clamped to the face of the right hand column, which is an integral part of the machine. The machine can work in two manners : 1. By giving a constant downward feed motion to the head and a step by step motion to its column (left) on its ways in the saddle, or 2. By giving a constant cross feed to the column (left) and a step-by-step motion on the head. In either case, the appropriate member is returned to its starting position between the steps and the contour of the work would be obtained by the in-and-out motion of the saddle, which is controlled by the tracer unit. The in-and-out motions of the saddle are obtained from constantly running electric motors which by means of electro-magnetic clutches can be connected to the lead- screws which move the saddle. These clutches are controlled by contacts which are made or broken by the movement of the tracer. Since only a limited number of downwards (or sideways) traverses can be made in covering the whole area of the master, it is clear that the surface produced will consist of a series of ridges and hollows which must be removed by subsequent processes, usually by hand grinding and filing. 15.5. AUTOMATIC TRACING In all the tracing systems discussed above, metal templates or masters are employed to guide the cutters. In automatic tracing, the scanner directly follows the line drawings. Such machines may operate on the pantograph principle or electrical controls. In one design of such machines, the line drawings are made with a conductive ink. A tracer disc maintains contact with the line through control of a spark gap. In another type of machine, the scanner in the form of electric eyes follows regular ink lines on clear plastic film. The scanner can follow a line as fast as 125 mm/min with 0.025 mm travelling accuracy and 0.075 mm stopping accuracy. Drawing are made to a scale of as much as 10 : 1. PROBLEMS 1. Define a tracer controlled machine tool. 2. Differentiate between “Duplicators” and “Profilers”. 3. Compare a tracer controlled lathe and a manually operated lathe. 4. Compare a tracer controlled lathe with turret and automatic lathes. 5. Compare tracer control and numerical control of machine tools. 6. Explain with the help of neat diagrams, the mechanical copying machines. Discuss their drawbacks. 7. Explain the principle of a pantograph machine. 8. With the help of a neat diagram, explain the working of a hydraulic tracer controlled milling machine. 9. With the help of neat diagrams, explain the following principles of copying : (a) Three dimensional system. (b) Two dimensional system. 10. With the help of a neat diagram, explain the tracing system. 11. With the help of a neat diagram, explain the working of Keller Die-Sinking machine. 12. Explain automatic tracing.  Chapter 16 Numerically Controlled Machine Tools 16.1. GENERAL In the modern age of business competition, the aim of manufacturing activity is to manufacture the parts of desired quality at the minimum possible cost resulting in profit to the organisation. To compete in the market, the quality of the product will have to be reliable and consistent with delivery of the product offered on schedule. For consistency in quality especially in the manufacture of complex components, human intervention has to be eliminated. This necessitates the use of special purpose or automatic machine tools. These machine tools are highly specialised and have high rates of production. However, their initial costs are high. Due to these factors, these machine tools are suitable and economical for mass production only. Moreover, these machine tools are inflexible (that is, the complete set up will have to be changed for a new product). Due to this inflexibility also, these machine tools are not suitable for job or small batch production. Thus, when the main requirement is : quantity, consistency in quality and delivery schedule, mass production or automation is the answer. Mass production machine tools include : automatic unit machine tools and automatic transfer machines etc. However, the mass production system of manufacturing accounts for only about 20% of the total of manufactured parts. The remaining demand is met by jobbing and the batch production systems. More than half of the total machine tools are engaged in small lot and piece production. These include general purpose machine tools, hydraulic tracer controlled machine tools and programmed operation cycle machine tools. The general purpose machine tools, eventhough highly flexible (can easily change from one product to another), are not suitable for mass production, because of longer set up times, machine and tool adjustments, and very low machine utilization time (maximum upto about 30% of the total time). All this results in low rates of production and delays in the delivery schedule. Also, they require too highly skilled operators. Because of the intervention of operators, (eventhough skilled) the consistency in quality can not be assured. Therefore, when consistency in quality and delivery schedule are of prime importance, these machine tools are not suitable. Similarly, hydraulic tracer controlled machine tools etc. (which are mainly used for small batch productions) require longer set up times while changing over to new jobs as these machines require cams, templates, stops, electrical trip dogs etc. Because of the above factors, a great need was felt for machine tools that could bridge the gap between highly flexible general purpose machine tools (which are not economical for mass production) and highly specialized, but inflexible mass production machines. Numerically controlled 713 714 A Textbook of Production Technology machine tools have taken up this role very well. These machines are highly flexible and are economical for producing a single or a large number of parts. Numerical control, NC, can be defined simply as control by numbers. Electronics Industries Association defines NUMERICAL CONTROL as “a system in which actions are controlled by the direct insertion of numerical data at some point. The system must automatically interpret at least some protion of these data.” In numerically controlled machine tools, the input information for controlling the machine tool motion is provided by means of punched paper tapes, plastic tapes(Mylar), floppy disk, hard disk or magnetic tapes in a coded language. Thus, with numerical control, the operation and motions of a machine tool are controlled electronically. NC machine tools are thus automated production machines. The key difference in this system is that of “reprogrammability”. In NC, a changeover to a new product, does not require extensive physical changes in the machine set up. Only a change in the control program itself is required. Thus, the flexibility of general purpose machine tools can be combined with the precision and accuracy of special-purpose machine tools. The human intervention, as an interface translating the design information into machine activities, is replaced by some form of information processing device, such as a computer. The principles of NC were established in 1950s. However, with the availability of low-cost programmable control, based around the microprocessor in 1970s, this technology diffused widely. This opened up the development of the concept of Computer Numerical Control (CNC), and other functions such as automatic tool change or part manipulation. All this led to highly sophisticated, multipurpose machining centres, complete with a range of support functions such as tool change, head change, transport and manipulation and so on-all under computer control. The next development was the idea of Direct (or distributed) Numerical Control (DNC), in which more than one machine tool (alongwith associated functions) could be grouped into a manufacturing cell under the overall control of a larger, supervisory computer. The early numerically controlled machine tools were milling and profiling machines that provided faster and less expensive means for producing aircraft parts. Now, NC, has been applied to other machine tools, such as lathes, drilling and boring machines, welding and flame cutting machines, punching machines and inspection devices 16.2. WORKING OF NC MACHINE TOOL To understand the working of a NC machine tool, let us refer to Fig.16.1. The first two steps, component drawing and process planning are similar in both operator and NC machine tools. In the operator controlled machine tools, the operator controls the cutter position during machining. He also makes the necessary adjustments and corrections to produce the desired component. However, in numerically controlled system, the place of the operator is taken by the data processing part of the system and the control unit. In the data processing unit, the co-ordinate information regarding the component is recorded on a tape by means of a teleprinter. Tape is fed to the control unit which sends the position command signals to slide-way transmission elements of the machine tool. At the same time, the command signal is constantly compared with the actual position achieved, with the help of position feedback signal derived from automatic monitoring of the machine tool slide position. The difference in the two signals, if any, is corrected until the desired component is produced. The conventional NC system consists of basically two parts : Machine Tool and Machine Control Unit (MCU). The main elements of a NC machine tool are shown in Fig. 16.2. These are : 1. The machine control unit (MCU), also known as N.C. console or Director. 2. The drive units, or actuators. 3. The position feed back package. Numerically Controlled machine Tools 715 Component drawing Feed back Process Operator Completed planning component Command Machine Tool (a) Operator controlled machine tool Component drawing Process planning Feed back Tape Completed Programmer Control unit preparation component Machine Positon tool Command (b) Numerically controlled machine tool Fig. 16.1. Position feed Back Package Servomotor Control Unit Magnetic box Machine Manual tool control Fig. 16.2. Main Elements of a NC Machine. 4. Magnetic Box. 5. Manual control. The hardware also includes the associated circuits. The functional elements involved in MCU are : 1. Data Input. The instructions for manufacturing the component are written in a Coded language (Paper tape is the most commonly used device) are read by a tape reader. 2. Data Processing. The instruction undergo electronic processing resulting in information in the form of electrical signals (pulsed commands).