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1. 1. msharizanJJ204SCREW THREAD General Objective: To understand the methods of testing and measuring elements of ISO and BSW screw threads. Specific Objectives: At the end of the unit you will be able to :  Identify the methods of measuring major diameter, minor diameter and mean diameter.  Measure and calculate major diameter, minor diameter and mean diameter of a screw thread.  To check the thread form by using the optical comparator. 1
2. 2. msharizanJJ2041.0 INTRODUCTIONAll elements of the thread influence the strength and interchange ability ofscrew thread, but the pitch, angle and effective diameter are much moreimportant than the other elements1.1 ELEMENTS OF A THREAD To understand and calculate the thread elements, the following definition relating to screw threads should be known (Fig. 1.1). root pitch major diameter mean diameter minor diameter thread angle Figure 1.1 Screw thread terminology 1.1.1. Major Diameter It is the largest diameter of the thread. This is the distance between the crests of the thread measured perpendicular to the thread axis. 2
4. 4. msharizanJJ2041.2. MEASURING THE MAJOR DIAMETER To measure major diameter of the screw, a micrometer, with anvils ofa diameter sufficient to span two threads, may be used,( Fig. 1.2). Toeliminate the effect of errors in the micrometer screw and measuring faces,it is advisable first to check the instrument to a cylindrical standard of aboutthe same diameter as the screw. For such purposes a plug gauge or a set of„Hoffman‟ rollers is useful. anvil Figure 1.2 Checking the major diameter with a micrometer1.3. MEASURING THE MINOR/CORE DIAMETER The diameter over the roots of a thread may be checked by means of aspecial micrometer adapted with a shaped anvils, (Fig. 1.3) or a micrometermay be used in conjunction with a pair of vee pieces ( steel prisms ). Thesecond method is recommended ( Fig.1.5). The steel prisms on themicrometer are pressed into the thread groove. The ends of the prisms areslightly curved and parallel to the root thread. It is important , whenmaking the test, to ensure that the micrometer is positioned at right anglesto the axis of the screw being measured, and when a large amount of suchwork is to be done, a special „floating bench micrometer‟ ( Fig. 1.4 ) is used.It is because, it supports the screw and incorporates the micrometer 4
5. 5. msharizanJJ204elements correctly located, as well as providing means for suspending thevee prisms. Fig. 1.3 Checking the core diameter of a thread with an shaped anvil micrometer Fig. 1.4. A Floating Micrometer 5
6. 6. msharizanJJ204 The prism values are stated as, Dm = W – 2TNote: Dm - mean diameter W - distance between two prism T - prism height (known) T prism W Figure 1.5 Checking minor diameter by using a micrometer and prisms1.4. MEASURING THE MEAN/PITCH/EFFECTIVE DIAMETER The three-wire method is recognized as one of the best methods ofchecking the pitch diameter because the results are least affected by anyerror which may be present in the included thread angle. For threads whichrequire an accuracy of 0.001 in. or 0.02 mm, a micrometer can be used tomeasure the distance over the wires. For threads requiring greater accuracyan electronic comparator should be used to measure the distance over thewires. In the three-wire method, three wires of equal diameter are placed inthe thread; two on one side and one on the other side (Fig. 1.6). The wires 6
7. 7. msharizanJJ204used should be hardened and lapped to three times the accuracy of thethread to beinspected. A standard micrometer may then be used to measure thedistance over the wires. For greatest accuracy, the best size wire should beused. Figure 1.6 Three wire method The hard round bars (wire) with the same size are positioned oppositeto the screw thread groove shown in the diagram above. The distance ismeasured between the outside of the round bars. The most suitable wiresize is 0.57735p. In Fig. 1.7 P is the pitch of the screw thread. The suitablewire size is quite hard to get, usually a size bigger than 0.57735p wire sizewill be used. 7
8. 8. msharizanJJ204 Fig. 1.7. Conditions when measuring with wires1.4.1. Best Size Wires. Wires which touch the thread at the pitch diameter are knownas "Best Size" Wires. Such wires are used because the measurementsof pitch diameter are least affected by errors that may be present inthe angle of the thread. The above analysis for the distance over wires holds goodprovided the wire touches the thread somewhere on its right side, andprovided the thread angle is correct. The extremes of wire sizes whichtouch on the straight sides and which can be measured are shown at(a) and (c), Fig.1.9. For ISO metric, unified and Whitworth threadsthese limiting sizes are given in Table 1.1 8
9. 9. msharizanJJ204 Table 1.1. Wire sizes for thread measurement ( p = pitch of thread)Thread Max. Min. „Best Size range forForm Wire Wire Wire‟ Best wireISO metric and 1.01p 0.505p 0.557p 0.534pUnified 0.620pWhitworth 0.853p 0.506p 0.564p 0.535p 0.593p Pitch (P) A h r W C H B DE 60o 2 D E P/2 Figure 1.8. Three-wire measurement Note: W = Distance over wires DE = Pitch/ Effective Diameter Dw = Wire diameter = 600 From the Fig. 1.8, mean/pitch diameter can be calculated by applying the following formula; 9
10. 10. msharizanJJ204 AD = AB cosec = r cosec 2 2 P H = DE cot = cot 2 2 2 P CD = 0.5H = cot 4 2 P h = AD – CD = r cosec – cot 2 4 2 and distance over wires (W) = DE + 2h + 2r P = DE + 2 {r cosec – cot } + 2r 2 4 2 P = DE + 2r cosec - cot + 2r 2 2 2 P = DE +2r ( 1 + cosec ) – cot 2 2 2 and, since 2r = d (the diameter of the wire), P W = DE + d ( 1 + cosec ) – cot (1) 2 2 2 From this general formula we may apply the special adaptation forcommon threads. 10
11. 11. msharizanJJ204 Figure 1.9. a) ISO metric and unified b) Whitworth(a) ISO metric and unified Fig. 1.9 (a)The effective diameter lies 0.3248p inside the crest of the thread, Hence DE = D – 0.6496p = 60 and cosec =2 2 cot = 1.732 2 P W (over wires) = DE + d (1 + cosec ) – cot 2 2 2 P =D – 0.6496p + d(3) – (1.732) 2 = D +3d- 1.5156p (2) 11
12. 12. msharizanJJ204(b) Whitworth Fig. 1.9(b)Depth of thread = 0.64p, so that DE = D – 0.64p = 55 and cosec = 2.1657 cot = 1.921 2 2 P Hence W ( over wires) = DE + d { 1 + cosec }- cot 2 2 2 P = D -0.64p + d 3.1657) - (1.921) 2 = D + 3.165d - 1.6 p (3)1.5. OPTICAL COMPARATOR An optical comparator or shadowgraph (Fig. 1.10a and 1.10b) projectsan enlarge shadow onto a screen where it may be compared to lines or to amaster from which indicates the limits of the dimensions or the contour ofthe part being checked. The optical comparator is a fast, accurate means ofmeasuring or comparing the work piece with a master. It is often used whenthe work piece is difficult to check by other method. Optical comparators areparticularly suited for checking extremely small or odd-shaped parts, whichwould be difficult to inspect without the use of expensive gauges. Optical comparators are available in bench and floor models, whichare identical in principle and operation. Light from a lamp passes through acondenser lens and is projected against the work piece. The shadow causedby the work piece is transmitted through a projecting lens system, whichmagnifies the image and casts it onto a mirror. The image is then reflectedto the viewing screen and is further magnified in this process. The extent of the image magnification depends on the lens used.Interchangeable lenses for optical comparators are available in the followingmagnifications: 5 x, 10 x, 31.25 x, 50 x, 62.5 x, 90 x, 100 x, and 125 x. 12
13. 13. msharizanJJ204 A comparator chart or master form mounted on the viewing screen isused to compare the accuracy of the enlarged image of the work piece beinginspected. Charts are usually made of translucent material, such as celluloseacetate or frosted glass. Many different charts are available for special jobs,but the most commonly used are linear-measuring, radius, and angularcharts. A vernier protractor screen is also available for checking angles .Since charts are available in several magnifications, care must be taken touse the chart of the same magnification as the lens mounted on thecomparator. Many accessories are available for the comparator, increasing theversatility of the machine. Some of the most common ones are tilting workcentres, which permit the work piece to be tilted to the required helix anglefor checking threads; a micrometer work stage, with permit quick andaccurate measuring of dimensions in both direction; and gauge blocks,measuring rods, and dial indicators used on comparators for checkingmeasurement. The surface of the work piece may be checked by a surfaceilluminator, which lights up the face of work piece adjacent to the projectinglens system and permits this image to be projected onto the screen. 1.5.1. To check the angle of a 60o thread using an optical comparator 1. Mount the correct lens in the comparator. 2. Mount the tilting work centres on the micrometer cross- slide stage. 3. Set the tilting work centres to the helix angle of the thread. 4. Set the work piece between centres. 13
14. 14. msharizanJJ204 5. Mount the vernier protractor chart and align it horizontally on the screen. 6. Turn on the light switch. 7. Focus the lens so that a clear image appears on the screen. 8. Move the micrometer cross-slide stage until the thread image is centralized on the screen. 9. Remove the vernier protractor chart to show a reading of 30o. 10. Adjust the cross-slides until the image coincides with the protector line. 11. Check the other side of the thread in the same manner.Note: If the threaded angle is not correct or square with the centreline, adjust the vernier protractor chart to measure the angle of thethread image. Other dimensions of the threads, and width of flats,may be measured with micrometer measuring stages or devices suchas rods, gauge blocks and indicators. 14
15. 15. msharizanJJ204helix angleFigure 1.10 (a). Checking a thread form on an optical comparator Figure 1.10 (b) Principle of the optical projector 15
16. 16. msharizanJJ204 ACTIVITY1.1. Draw and label a schematic drawing of how you would check the core diameter of an external V-thread.1.2. Using „best‟ wire sizes determine the distance of the wire for M 20 x 2.5 ISO metric thread.1.3. Why is the three-wire method is one of the best method of measuring the pitch diameter of a V thread?1.4. With the aid of a labelled diagram, briefly explain how you would use an optical comparator to check the thread angle of 60o 16
17. 17. msharizanJJ204 GEARGeneral Objective : To understand the concept of gears and gearingSpecific Objectives : At the end of the unit you will be able to:  Know the types and functions of gears in engineering.  Know, sketch and label the parts of gears.  Understand the method of measuring spur gear. 17
18. 18. msharizanJJ2042.0 INTRODUCTION Gears are used to transmit power positively from one shaft to anotherby means of successively engaging teeth (in two gears). They are used inplace of belt drives and other forms of friction drive when exact speed ratiosand power transmission must be maintained. Gears may also be used toincrease or decrease the speed of the driven shaft, thus decreasing orincreasing the torque of the driven number.2.1. TYPES OF GEARS 2.1.1. Spur gear Spur gears, Fig. 2.1, are generally used to transmit power between two parallel shafts. The teeth on these gears are straight and parallel to the shafts to which they are attached. When two gears of different sizes are in mesh, the larger is called the gear while the smaller is called the pinion. Spur gears are used where slow to moderate- speed drive are required. Gear . Pinion Figure 2.1. Spur gears Figure 2.2. Internal gears 18
19. 19. msharizanJJ204 2.1.2. Internal gears Internal gears, Fig. 2.2., are used where the shafts are parallel and the centers must be closer together and that could be achieved with spur or helical gearing. This arrangement, provides a stronger drive since there is the greater area of contact than with the conventional gear drive. It also provides speed reductions with a minimum space requirement. Internal gears are used on heavy duty tractors where much torque is required. 2.1.3. Helical gears Helical gears, Fig.2.3, may be used to connect parallel shafts or shafts which are at an angle. Because of the progressive rather than intermittent action of the teeth, helical gears run more smoothly and quietly than spur gears. Since there is more than one tooth in engagement at any one time, helical gears are stronger than spur gears of the same size and pitch. However, special bearing (thrust bearings) are often required on shafts to overcome the end thrust produced by these gears as they turn.Figure 2.3. Herringbone gears 2.1.4. Helical gears Figure 2.4. Herringbone gears 19
20. 20. msharizanJJ204 Herringbone gears, Fig. 2.4., are resembles of two helicalgears placed side by side, with one half having a left-hand helix andthe other half a right-hand helix. These gears have a smoothcontinuous action and eliminate the need for thrust bearings.2.1.5. Bevel gears When two shafts are located at an angle with their axial linesintersecting at 90o, power is generally transmitted by means of bevelgears, Fig. 2.5. Figure 2.5. Bevel gears2.1.6. Miter gears When the shafts are at right angles and the gears are of thesame size, they are called miter gears, Fig. 2.6.. Figure 2.6. Miter gears Figure 2.7. Angular bevel gears 20
21. 21. msharizanJJ2042.1.7. Angular bevel gears However, it is not necessary that the shafts be only at rightangles in order to transmit power. If the axes of the shafts intersectat any angle other 90o, the gears are known as angular bevel gears,Fig. 2.7.2.1.8. Hypoid gears Bevel gears have straight teeth very similar to spur gears.Modified bevel gears having helical teeth are known as hypoid gears.The shafts of these gears, although at right angles, are not in thesame plane and, therefore, do not intersect. Hypoid gears are used inautomobile drives, Fig. 2.8. Worm Worm gear Figure 2.8. Hypoid gears Figure 2.9. Worm and worm gears2.1.9. Worm and worm gear When shafts are at right angles and considerable reduction inspeed is required, a worm and worm gear may be used, Fig. 2.9. Theworm, which meshes with the worm gear, may be single or multiplestart thread. A worm with a double-start thread will revolve the 21
22. 22. msharizanJJ204 worm gear twice as fast as a worm with a single-start thread and the same pitch. 2.1.10. Rack and pinion When it is necessary to convert rotary motion to linear motion, a rack and pinion may be used, Fig. 2.10. The rack, which is actually a straight or flat gear, may have straight teeth to mesh with a spur gear, or angular teeth to mesh with a helical gear. Pinion Rack Figure 2.10. Rack and pinion2.2. GEAR TERMINOLOGY top land/peak face width 2 Fig. root addendum circle face circular pitch flank thooth thickness pitch addendum liner pitch circle clearance dedendum pitch diamete outside base dedendum r diamete diamete circle r r Fig. 2.11 Parts of a spur gear 22
23. 23. msharizanJJ2042.2.1. Addendum Addendum is the radial distance between the pitch circle andthe outside diameter or the height of the tooth above the pitch.2.2.1. Dedendum Dedendum is the radial distance from the pitch circle to thebottom of the tooth space.2.2.3. Pitch diameter Pitch diameter is the diameter of the pitch circle which is equalto the outside diameter minus two addendums.2.2.4. Base diameter The diameter of the circle from which the involute isgenerated; which is equals to pitch diameter times the cosine of thepressure angle.2.2.5. Pitch circle Pitch circle is the circle through the pitch point having itscentre at the axis of the gear. 23
24. 24. msharizanJJ2042.2.6. Pitch line The line formed by the intersection of the pitch surface and thetooth surface.2.2.7. Face width - The width of the pitch surface.2.2.8. Tooth thickness The thickness of the tooth measured on the pitch circle.2.2.9. Top land - The surface of the pitch cylinder.2.2.10. Base diameter - The diameter of the root circle.2.2.11. Root - The bottoms of the tooth surface. 24
25. 25. msharizanJJ2042.3. MEASUREMENT AND TESTING OF GEARS 2.3.1. Gear-tooth vernier caliper The gear-tooth vernier, Fig.2.12, is an instrument for measuring the pitch-line thickness of a tooth. It has two scales and must be set for the width (w) of the tooth, and the depth (h) from the top, at which the width occurs. AO = R Figure 2.12. The gear-tooth vernier caliper 25
26. 26. msharizanJJ204NOTE: The following considerations of gear elements, thesymbols below will be used for the quantities. T/t = No. of teeth P = Diametral pitch ( inch gear ) P = Circular pitch D/d = Diameter of pitch circle R/r = Radius of pitch circle = pressure angle M = Modul Add/A = Addendum Ded/D = Dedendum Circular pitch = x Modul MThe angle subtended by a half tooth at the centre of the gear( AOB), Fig. 2.12, is given by, 1 360 90 = x = ; T = no. of teeth 4 T T w 90 90 AB = = AO sin = R sin 2 T T D = Modul x No. of Teeth, and MT R =R 2 MT i.e. D = 2R =MT and R= 2 26
27. 27. msharizanJJ204 w 90 MT 90Hence = R sin = sin 2 T 2 T 90and w = MT sin (1) TTo find h we have that h = CB = OC – OB MTBut OC = R + Add = +M 2 90 MT 90And OB = R cos = cos T 2 T MT MT 90Hence h= +M - cos 2 2 T MT MT 90 = +M - cos ] (2) 2 2 T MT 90 =M+ [ 1- cos ] 2 T T 90For diametral-pitch gears, (1) becomes w = sin P T 1 T 90And (2) becomes h= [1+ ( 1 – cos ) P 2 T 27
28. 28. msharizanJJ204Example: To calculate the gear tooth vernier setting to measure a gear of33T, 6 modul. 90 90 w = MT sin = 6 x 33 sin T 33 = 198 sin 2o 43.5‟ = 198 x 0.0476 = 9.42 mm. T T h= M[1+ ( 1 – cos )] 2 2 33 90 =6[1+ ( 1 – cos )] 2 33 33 =6[1+ (0.0011) ] 2 = 6.11 mm 28
29. 29. msharizanJJ2042.4. PLUG METHOD OF CHECKING FOR PITCH DIAMETER AND DIVIDE OF TEETH The tooth vernier gives us a check on the size of the individual tooth,but does not give a measure of either the pitch diameter or the accuracy ofthe division of the teeth. Figure 2.13 Fig. 2.13 shows a rack tooth symmetrically in mesh with a gear toothspace, the curved sides of the gear teeth touching the straight rack tooth atthe points A and B on the lines of action. O is the pitch. If now we considerthe racktooth as an empty space bounded by its outline, a circle with centre at O andradius OB would fit in the rack tooth and touch it at A and B (since OA andOB are perpendicular to the side of the rack tooth). Since the rack touches 29
30. 30. msharizanJJ204the gear at these points, the above circle (shown dotted) will rest against thegear teeth at points A and B and will have its centre on the pitch circle. In triangle OBD: OB = radius of plug required. 1 OD = circular pitch 4 m = 4 < B = 90o, <O= OB = OD cos m = cos 4Dia of plug = 2OD m = cos 2 This is the diameter of a plug which will rest in the tooth space andhave its centre on the pitch circle. Notice that the plug size remains thesame for all gears having the same pitch and pressure angle. With such plugs placed in diametrically opposite tooth spaces, it is asimple matter to verify the gear pitch diameter. The accuracy of the spacingover any number of teeth may be found as shown in chordal calculations. 30
31. 31. msharizanJJ204Example:Calculate for a 36Tgear of 5 mm module and 20o pressure angle, (a) plug size(b) distance over two plugs placed in opposite spaces, (c) distance over twoplugs spaced 10 teeth apart.Solutions: m(a) Dia of plug = cos 2 5 = cos 20o 2 = 7.854 x 0.9397 = 7.38 mmPitch dia of gear = mT = 5 x 36 = 180 mm(b) Distance across plugs in opposite spaces = 180 + 7.38 = 187.38 mm(c) Distance across plugs spaced 10 teeth apart (Fig.2.14) 31
32. 32. msharizanJJ204 Figure 2.14 360 Angle subtended by 10 teeth = 10 x 36 = 100o. In triangle OAB: AB = OA sin 50o = 90 x 0.766 = 68.94Centre distance of plugs = 2 x AB = 2 x 68.94 = 137.88 mm.Distance over plugs = 137.88 + 7.38 = 145.26 mm. 32
33. 33. msharizanJJ2042.5 MEASURE AND INSPECT OF SPUR GEARMengukur tebal perentas dengan angkup vernier gigi gear Rajah di atas menunjukkan sebuah angkup vernier gigi gear. Angkup tersebut dilengkapkan dengan plat penahan yang boleh dilaraskan mengikut ukuran adendum gear yang hendak diukur. Kemudian hujung plat yang terletak di antara rahang angkup itu dikenakan pada puncak gigi gear. Rahang angkup vernier dilaraskan untuk mendapat ukuran tebal perentas gigi. 33
34. 34. msharizanJJ2042.6 KAEDAH PERENTAS MALAR Perentas malar ialah satu garis rentas yang panjangnya sentiasa sama bagi semua gigi gear yang mempunyai pic di garis pusat dan sudut tekan sama , walaupun bilangan gigi bagi gear mungkin berbeza. 34
35. 35. msharizanJJ20435
36. 36. msharizanJJ2042.7 KAEDAH TANGEN TAPAK Alat pengukur seperti angkup vernier yang besar, tolok tinggi vernier, pembandingan tangen tapak atau mikrometer tebal gigi iaitu sejenis mikrometer yang dipasang dengan andas yang besar berbentuk plat bulat,boleh lah digunakan untuk mengukur jarak rentang itu. 36
37. 37. msharizanJJ20437
38. 38. msharizanJJ2042.8 MEMERIKSA GARIS PUSAT PIC BAGI GEAR TAJI Dalam kaedah ini sepasang guling (rola) atau palam piawai digunakan bersama mikrometer luar. Garis pusat guling hendaklah bersesuaian dengan pic dan sudut tekanan bagi gear hendak diuji. Jika gear bergigi genap, guling guling itu di letakkan dalam lurah yang bersetentangan. Jika gear bergigi ganjil, kedudukan guling mestilah pada lurah-lurah yang paling hampir bersetentangan. 38
40. 40. msharizanJJ204 Long, slender work held between centers is prevented from bendingby the adjustable centre rest. Figure 3.9. A universal dividing head set Figure 3.10 Section through a dividing head, showing the worm wheel and worm shaft 40
41. 41. msharizanJJ2042.11 METHODS OF INDEXING The main purpose of the indexing or dividing head is to divide thework piece circumference accurately into any number of divisions. This maybe accomplished by the following indexing methods: direct, simple, angular,and differential. However, this modul will only cover direct and simpleindexing. Direct indexing Direct indexing is the simplest form of indexing. It is performed by disengaging the worm shaft from the worm wheel by means of an eccentric device in the dividing head. Some direct dividing heads do not have a worm and worm wheel but rotate on bearings. The index plates contain slots, which are numbered , and a spring-loaded tongue lock is used to engage in the proper slot. Direct indexing is used for quick indexing of the work piece when cutting flutes, hexagons, squares, and other shapes. The work is rotated the required amount and held in place by a pin which engages in to a hole or slot in the direct indexing plate mounted on the end of the dividing head spindle. The direct indexing plate usually contains three sets of hole circles or slots: 24, 30, and 36. The number of divisions it is possible to index is limited to numbers which are factors of either 24, 30, or 36. The common divisions that can be obtain by direct indexing are listed in Table 3.3 41
42. 42. msharizanJJ204 Table 3.3. Direct Indexing Divisions Plate Hole Number 24 2, 3, 4, -, 6, 8, - ----- 12 …………24 30 2, 3, -, 5, 6, -, -, -, 10, -, -, 15, ……….30 36 3, 4, -, 6, -, 9, -, 12, -, 18,…………… 36Example:What direct indexing is necessary to mill eight flutes on a reamer blank?As the 24 hole circle is the only one divisible by eight (the required ofdivisions), it is the only circle which can be used in this case. 24 Indexing = = 3 holes on a 24-hole circle. 8Note: Never count the hole or slot in which the index pin is engaged. Simple Indexing In simple indexing, the work is positioned by means of the crank, index plate, and sector arms. The worm attached to the crank must be engaged with the worm wheel on the dividing head spindle. Since there are 40 teeth on the worm wheel, one complete turn of the index crank will cause the spindle and the work to rotate one-fortieth of a turn. Similarly, 40 turns of the crank will revolve the spindle and 42
43. 43. msharizanJJ204 work one turn. Thus there is a ratio of 40:1 between the turns of the index crank and the dividing head spindle. To calculate the indexing or the number of turns of the crank for most divisions, it is necessary only to divide 40 by the number of division (N) to be cut, or 40 Indexing = NExample:The indexing required to cut eight flutes would be: 40 = 5 full turns of the index crank 8If, however, it was necessary to cut seven flutes, the indexing would be 40 5 =5 turns 7 7Five complete turns are easily made; however, the five seventh of a turninvolves the use of the index plate and sector arms. 43
44. 44. msharizanJJ204 ACTIVITY1. State three (3) characteristics of the following gears i. helical gear ii. spur gear2. Sketch and name six (6) parts of a spur gear3. Calculate the diameter of plug which will lie in the tooth space of a 5 mm module gear with its centre on the pitch circle. If the gear has 50T, find (a) distance over two such plugs spaced in opposite spaces, (b) distance over two plugs spaced 12 spaces apart ( = 20o) (J: 1. 7.38 mm (a) 257.38 mm (b) 178.52 mm)4. Determine the diameter of a plug which will rest in the tooth space of a 4 mm module 20o rack, and touch the teeth at the pitch line. Calculate (a) the distance over two such plugs spaced 5 teeth apart. (b) The depth from the top of the plug to the top of the teeth. (J: 5.9 mm (a) 59 mm (b) 10.664 mm) 44
45. 45. msharizanJJ204SURFACE TEXTURE General Objectives: To understand the importance of surface texture in engineering. To understand the methods of calculating the surface roughness.Specific Objectives : At the end of this unit you will be able to:  Identify the surface finish symbols that appear on a drawing.  Identify the surface texture terms/ definitions.  Calculate the arithmetic mean value, Ra.  Calculate the root-mean-square average, Rq.  Calculate the maximum roughness height, Rt.  Compare Ra and Rq. 45
46. 46. msharizanJJ2044.0 DEFINITION Surface Texture is defined as a degree of finish conveyed to themachinist by a system of symbols devised by a Standards Association, eg.ASA – American Standards Association, BS – British Standards Modern technology has demanded improved surface finishes to ensureproper functioning and long life of machine parts. Pistons, bearings, andgears depend to a great extent on a good surface finish for proper functioningand therefore, require little or no break-in period. Finer finishes oftenrequire additional operation, such as lapping or honing. The higher finishesare not always required on parts and only result in higher production costs.To prevent overfinishing a part, the desired finish is indicated on the shopdrawing. Information specifying the degree of finish is conveyed to themachinist by a system of symbols devised by Standards Associations, eg.ASA American Standards Association and BS British Standards. Thesesymbols provide a standard system of determining and indicating surfacefinish. The inch unit for surface finish measurement is microinch (µin), whilethe metric unit is micrometer (µm) 46
47. 47. msharizanJJ2044.1. SURFACE TEXTURE TERMS AND DEFINITIONS Lay direction Flaw Waviness height Roughness Height, Rt Roughness Roughness spacing width cutoff Waviness width Surface profile Error of form WavinessRoughness Figure 4.1. Standard terminology and symbols to describe surface finish Regardless of the method of production, all surfaces have their owncharacteristics, which are collectively referred to as surface texture, Fig. 4.1.Certain guidelines have been established to identify surface texture in termsof well-defined and measurable quantities (Figure 4.2) 47
48. 48. msharizanJJ204 4.1.1. Flaws Flaws or defects, are random irregularities, such as scratches,cracks, holes, depression, seams, tears or inclusions. These defects canbe caused during the machining or production process such asmolding, drawing, forging, machining, eg, holes cause by air bubblesduring casting, crack and tears by forging and drawing process. 4.1.2. Lay Lay or directionality, is the direction of the predominantsurface pattern caused by the machining process and it is usuallyvisible to the naked eye. 4.1.3. Roughness Roughness is defined as closely spaced, irregular deviation on ascale smaller than that of waviness. It is caused by the cutting tool orthe abrasive grain action and the machine feed. Roughness may besuperimposed on waviness. 4.1.3.1. Roughness height Roughness height, Ra is the deviation to the centre line in micro inches or micrometers. 4.1.3.2. Roughness width Roughness Width is the distance between successive roughness peaks parallel to the nominal surface in inches or millimeters. 48
49. 49. msharizanJJ2044.1.4. Waviness Waviness is a recurrent deviation from a flat surface, much likewaves on the surface of water. It is measured and described in termsof the surface between adjacent crests of the waves (waviness width)and height between the crests and valleys of the waves (wavinessheight). Waviness can be caused by: a) deflection of tools, dies or the work piece b) force or temperature sufficient to cause warping c) uneven lubrication d) vibration e) any periodic mechanical or thermal variations on the system during manufacturing operations.4.1.5. Profile The contour of a specified section through a surface.4.1.6. Microinch and micrometer The unit of measurement used to measure surface finish. The microinch is equal to 0.000 001 inch and the micrometer equals to 0.000 001 meter. 49
50. 50. msharizanJJ2044.2. STANDARD SYMBOLS TO DESCRIBE SURFACE TEXTURE/FINISH 0.02 – 2 6.3 1.6 0.01 Figure 4.2 A sample of a surface texture/finish designation Symbols‟ definition: 0.02 – Maximum waviness height (mm) 2 - Maximum waviness width (mm) 6.3 - Maximum roughness height ( m) 1.6 - Minimum roughness height ( m) 0.01 - Maximum roughness width (mm) - Lay symbol (Lay perpendicular to the line representing the surface to which the symbol is applied) Sometimes, the roughness number is used as a substitute for theroughness value eg. N7 is equals to 1.6 µm, (Table. 4.1). Table 4.2 shows anaverage surface roughness produced by standard machining processes. 50
51. 51. msharizanJJ204 Table 4.1. Roughness number and valueµm 50 25 12.5 6.3 3.2 1.6 0.8 0.4 0.2 0.1 0.05 0.025Roughness N12 N11 N10 N9 N8 N7 N6 N5 N4 N3 N2 N1number Table 4.2 Average surface roughness produced by standard machining processes PROCESS MICROINCHES MICROMETERS Turning 100 - 250 2.5 - 6.3 Drilling 100 - 200 2.5 - 5.1 Reaming 50 - 150 1.3 - 3.8 Grinding 20 - 100 0.5 - 2.5 Honing 5 - 20 0.13 - 0.5 Lapping 1 - 10 0.025 - 0.254 51
52. 52. msharizanJJ2044.3. SYMBOLS FOR SURFACE ROUGHNESS The following symbols indicate the direction of the lay (Table 4.3)Lay Interpretation ExamplesSymbol Lay parallel to the line representing the = surface to which the symbol is applied Lay perpendicular to the line representing the surface to which the symbol is applied. Lay angular and both direction to line representing the surface to which symbol is X applied M Lay multidirectional Lay approximately circular relative to the centre of the surface to which the symbol is C applied C Lay approximately radial relative to the R centre of the surface to which the symbol is R applied 52
53. 53. msharizanJJ204 Pitted, protuberant, porous, or particulate P non-directional lay P Figure 4.3. Standard lay symbols for engineering surfaces4.4 SURFACE ROUGHNESS Surface roughness is generally described in two methods: arithmeticmean value and root-mean-square average. 4.4.1 The Arithmetic Mean Value, Ra. Ra, formerly identified as AA for arithmetic average or CLA for centre-line average is based on the schematic illustration of a rough surface, which is shown in (Figure 4.4). The arithmetic mean value, Ra, is defined as a b c d e f ... Ra = (4.4.1) n Where, all ordinates, a, b, c, …, are absolute values, and n is the number of readings 53
54. 54. msharizanJJ204 4.4.2. The Root-Mean-Square Average, Rq. Rq, formerly identified as RMS is defined as a2 b2 c2 d 2 ... Rq = (4.4.2) n The datum line AB in figure 4.4 is located so that the sum of the area above the line is equal to the sum of the areas below the line. The units generally used for surface roughness are µm (micrometer, or micron) or µin (microinch). ( Note, 1µm = 40 µin and 1µin = 0.025 µm ). A f g h i j k B a b c d e Centre line (datum line)Figure 4.3. Coordinates used for surface – roughness using equations 4.4.1 &4.4.2 3.4.3. Maximum Roughness Height, Rt Maximum roughness height, Rt, is defined as the height from the deepest trough to the highest peak. It indicates how much material has to be removed in order to obtain a smooth surface by polishing or other means 54
55. 55. msharizanJJ204 (h1 h3 h5 h7 h9 ) (h2 h4 h6 h8 h10 ) Rt = 5 h1 h3 h6 h5 h7 h9 h2 h4 h8 h10 Where, h1, h2…......hn - height of ordinates in mm M - magnification4.5. COMPARISON OF Ra AND Rq The arithmetic mean value, Ra was adopted internationally in themid-1950s and is used widely in engineering practice. Equations 4.4.1 and4.4.2 show that there is a relationship between Rq and Ra, as shown by the Rqratio . The table 4.4 below gives this ratio for various surfaces: Ra 55
56. 56. msharizanJJ204 Table 4.4 Ratio for various surfaces Rq Surface Ra Sine Curve 1.1 Machining by cutting 1.1 Grinding 1.2 Lapping and honing 1.4 In general, a surface cannot be describe by its Ra and Rq value alone,since these values are averages. Two surfaces may have the same roughnessvalue but have actual topography which is very different. A few deeptroughs on an otherwise smooth surface, for example, do not affect theroughness values significantly. However, the type of surface profile can besignificant in terms of friction, wear and fatigue characteristics of amanufactured product. It is therefore, important to analyze the surface in great detail,particularly for parts used in critical applications. Some 130 parametershave been identified thus far for measuring surface roughness. 56
57. 57. msharizanJJ204ACTIVITY 4A4.1. Explain why present-day standards relating to surface texture are very important to industry .4.2. List and explain the types of defects found on surfaces.4.3. Explain the following terms: a) roughness b) waviness c) lay4.4. What do Ra, Rq and Rt stand for?4.5. Describe how you would use the surface roughness comparator gauge.4.6 Define the symbol on figure below. 0.03 – 1.5 3.2 0.01 1.6 57
58. 58. msharizanJJ204COMPUTER NUMERICAL CONTROL General Objective :To understand the concept and principles of computer numerical control (CNC) system. Specific Objectives : At the end of the unit you will be able to :  Understand the main components of the CNC system,  Understand the point-to-point system (positioning),  Understand the contouring system (continuous system), and  Write a simple CNC milling program. . 58
59. 59. msharizanJJ2046.0 INTRODUCTION Computer numerical control is a system in which a controlmicrocomputer is an integral part of a machine or a piece of equipment(onboard computer). The part programmes can be prepared at a remote siteby programmer, and it may incorporate information obtained from draftingsoftware packages and from machining simulations, in order to ensure thatthe part programme is bug free. The machine operator can, however, easilyand manually programme onboard computers. The operator can be modifythe programs directly, prepare programme for different parts, and store theprogrammes. Because of the availability of small computers having a large memory,microprocessor(s), and programme-editing capabilities, CNC systems arewidely used today. The availability of low-cost programmable controllersalso played a major role in the successful implementation of CNC inmanufacturing plants. Numerical Control is a system where machine action is created fromthe insertion of Numeric Data. The Numeric Data is, in the beginning,written words in an easily understood code of letters and numbers(alphanumeric characters) known as a programme, which in turn isconverted by the machine control unit (MCU) into the electrical signals usedto control the machine movements. 59
60. 60. msharizanJJ204 The relationship between the words "Numerical" and "Control" isshown below. NUMERICAL CONTROL An instructional expression, To control such machine in a language of numbers, actions as: which represents a series of Directing Altering commands for specific Commanding Timing Prescribing Ceasing machine tool movements Sequencing Guiding Initiating Two important points should be made about N.C. First, the actualN.C. machine tool can do nothing more than it was capable of doing before acontrol unit was joined to it. There are now new metal removing principlesinvolved. N.C. machines position and drive the cutting tools, but the samemilling cutters, drills, taps, feeds, and other tools still perform the cuttingoperations. Cutting speeds, feeds, and tooling principles must still beadhered to. Given this knowledge, what is the real advantage of numericalcontrol? Primarily, the idle time or time to move into position for new cuts islimited only by the machines capacity to respond. Because the machinereceives commands from the machine control unit (MCU), it respondswithout hesitation. The actual utilisation rate or chip making rate istherefore much higher than on a manually operated machine. The second point is that numerical control machines can initiatenothing on their own. The machine accepts and responds to commands fromthe control unit. Even the control unit cannot think, judge, or reason.Without some input medium, e.g., punched tape or direct computer link, the 60
61. 61. msharizanJJ204machine and control unit will do nothing. The N.C. Machine will performonly when the N.C. tape is prepared and loaded and cycle start is initiated.6.1. NC OPERATION CNC stands for Computer Numerical Control. An N.C. system inwhich a dedicated stored program computer is used to perform basic controlfunctions. The functions of a CNC Controller are:1. To read and store programme information. 2. To interpret the information in a logical command sequence.3. To control the motion of the machines mechanical members.4. To monitor the status of the machine. The interpretation of programme commands by a machine control unitand its conversion of those commands into machine motion is complex. Thebasic elements and operation of a typical NC machine are shown in Fig. 6.1.The functional elements in numerical control and the components involvedfollow: a. Data input: The numerical information is read and stored in the tape reader or in computer memory b. Data processing: The programmes are read into the machine control unit for processing. c. Data output: This is information is translated into commands (typically pulsed commands) to the servomotor (Fig. 6.2 and 6.3). The servomotor then moves the table (on which the work piece is mounted) to specific positions, through linear or rotary movements, by means of stepping motors, leadscrews, and other similar devices. 61
62. 62. msharizanJJ204 Computer: Input command, Processing, Output command Limit switches Position feedback Drive signal Figure 6.1. A schematic illustration of the major component of a computer numerical control machine tool Work table Pulse train Stepping motor Gear Lead screw Figure 6.2. An open-loop control system for a numerical-control machine Work tableInput Dc Comparator DAC servomotor Gear Feedback signal Lead screw Position sensor Figure 6.3. A closed-loop control system for a numerical-control machine 62
63. 63. msharizanJJ2046.2. INDUSTRIAL APPLICATION 6.2.1. Metal Machining Lathes of all types Milling Machines of all types Drilling Machines Jig borers Electric Discharge Machining (including wire cut machines) Laser cutting machines Machining centres Turning centres All types of grinding machines Gear cutting machines 6.2.2. Metal Forming Punching and nibbling Guillotines Flame cut and profiling Folding Pipe bending Metal spinning 6.2.3. Finishing Plating Painting 63
64. 64. msharizanJJ204 6.2.4. Assembly Joining - Pick and place robots, spot and seam welding machines and robots, riveting, looming of wires and assembly of components into printed circuit boards.6.3. CNC AXIS CONVENTIONS CNC axis classification follows the three-dimensional Cartesiancoordinate system and is established in BS 3635: 1972: Part 1. Fig. 5.3 showsthe tree primary axes and the associated rotational axes. Most machines have two or three slide ways placed at right angles toone another. On CNC machines each slide is fitted with a control system,and is identified with either the letter X, Y or Z. Conventions have been adopted as to the naming of each axis. Theaxis of the main spindle, whether it is the axis of the tool spindle or the axisabout which the work piece rotates is called the Z axis. The X axis is the motion of the largest travel of the primary movement(in case there is more than one). The Y axis then makes the third motion and is the shorter primarymovement. In addition to these primary linear axes, there is provision for Rotaryaxes. They are designated A, B and C, with A rotary about the X axis, Brotary about the Y axis, and C rotary about the Z axis. It is often required to command a motion parallel to X, Y or Z axeswithin the realm of a secondary motion, or a tertiary motion within specialautomatic cycles such as describing the amount of finish allowance on aturned part, or to describe the distance of advancement of a drill during adrilling cycle etc. etc. 64
65. 65. msharizanJJ204 Table 6.1. NC axes Linear Axes X Y Z Rotary Axes A B C Secondary Linear U V W Interpolation I J K Tertiary motion codes differ considerably, but the address charactersvariously used are P, Q, R, D, L, E, and H. The z-axis is parallel to the main spindle of the machine. It will behorizontal on a lathe or horizontal machining centre and vertical on avertical machining centre. The x-axis is always horizontal and at 90o to z. The y-axis is at right angles to both the x and z axes. spindle rotation table z x y Figure 6.3. CNC axes 65
66. 66. msharizanJJ2046.4. NC MACHINE SUB-UNIT We have already seen the many and varied applications of numericalcontrol to the manufacturing and other industries, now we will look at themethods of controlling machines. There are three sub units to study: The machine tool itself. The control unit. The control system. 6.4.1. The Machine Tool A machine tool is a device designed to cut away surplus material and leave a component of the required shape and size. To do this a machine tool must be capable of: - Holding the work piece securely - Holding the cutting tool securely and driving it with suitable power. - Moving the tool and work piece relative to one another precisely enough to achieve accuracy of size and surface finish. In addition, provision must be made for altering the spindle speed and feed rates, tool changing, supply of coolant etc. On a conventional machine an operator controls these functions and sets or alters them when he considers it necessary, the decision resulting from his training, skill and experience. Obviously, the machine settings may differ between operators as will the time taken to read scales, set positions, change tools, alter speeds and feeds, engage drives and set up the work piece etc. CNC Automatic Control can be applied to these functions and so result in consistent and reduced 66
67. 67. msharizanJJ204machining times through optimised cutting data, fast accuratepositioning between cuts and fast automatic tool changing.6.4.2. The Control Unit The CNC Machine Control Unit (MCU) has to read and decodethe part programme, and to provide the decoded instructions to thecontrol loops of the machine axes of motion, and to control themachine tool operations.The main grouping of parts of a control could be considered to be: The Control Panel. The Tape Reader, The Processors The first part of the control panel is the human interface thatallows various modes of machine or control operation to be initiated,from switching on and homing, to programme loading and editing, tosetting work positions and tool offsets, manually controlledmovements and commencing the automatic cycling of a programme.Information about machine status and condition is available to theoperator via VDU screens, gauges, meters, indicator lights andreadouts. The tape reader is the device used to transfer the programmeinformation contained on a programme tape into the control unit.Most tape readers are of the photo-electric type which offers highspeed reading with reliability and accuracy providing the tape is ingood condition and the reader is kept clean and free of paper dustparticles. 67
68. 68. msharizanJJ204 The processors within a control are the electronic circuits thatpermit conversion of part programme data into machine motions andthey may be classified into two main sections. The data processingunit and the axis control processor. The function of the data processoris to decode the commands of the part program, process it and providedata to the axis control processor which then operates the slide drivesand receives feedback signals on the actual position and velocity ofeach axis.The Data Processing Unit includes the following functions: i. The input device, such a tape reader. ii. Reading circuits and parity checking logic. iii. Decoding circuits for distributing data to the controlled axes iv. An interpolator to supply velocity commands to the axes, either singly or in combination.The axis control processor consists of the following circuits: i. Position control loops for each and all axes. ii. Velocity control loops. iii. Deceleration and backlash take up circuits. An MCU is adaptable to virtually any machine, the differingcontrol motions and codes being a result of the way the control hasbeen programmed. This permanent resident program is known as anexecutive programme and resides in the read only memory (ROM) ofthe control, whereas the N.C. programme resides in the RandomAccess Memory (RAM). RAM allows external access and alteration ifnecessary, while ROM is programmed by the manufacturer andcannot be accessed through the control keyboard. 68
69. 69. msharizanJJ204 6.4.3. Control System There are two types of control systems used on NC machines. The point-to-point system and the continuous-path system. Point-to-point systems are not so common these days, but they operate only in straight lines, which are suitable for positioning moves on a drilling machine or limited use on a lathe or milling machine, where at best 45% cuts are possible with two axes running continuous path controls allow angular path and radius motion because the control interpolator has the ability to move the axis drive motors at varying velocities. The point-to-point controls were NC controls, while the continuous path controls could be NC or CNC controls.NOTE: NC is a general term used for Numerical Control and is also aterm used to describe controls that run directly off tape. CNC is a specificterm for Computer Numerical Control. CNC Machines are all NC machines,but NC controlled machines are not CNC machines. 69
70. 70. msharizanJJ2046.6. NC PROGRAMMING 6.6.1. Job Planning1. Sketch the part. Add incremental or absolute dimensions. 2. Ascertain fixturing. Select fixtures which have minimal projections above the part. 3. Identify a set-up point. Locate the set-up point near: 1. A corner of the part 2. A spot above the fixture Consider space requirements for: 1. Part loading and unloading 2. Tool change. 4. Plan operation sequence Mark sequence pattern of sketch. Test program data for accuracy. 5. Record necessary data for each movement of the table and tool on the program sheet.6. Record instructions for Identify, specific: the machine operator. 1. Tools needed. 2. Speed and feed data 3. Tool change points 4. Console switch setting 70
71. 71. msharizanJJ204 6.6.2. Incremental The word "incremental" may be defined as a dimension or a movement with respect to the preceding point in a prescribed sequence of points. Each positioning move is described quantitatively in distance and in direction from a previous point rather than from a fixed zero reference point. In incremental mode all moves are with respect to the last position reached. Y N10 G91 4 N15 G01 X10.Y10.F300. 0 4 0 N20 Y10. N25 X20.40 N30 X10.Y20.3020 N35 X20-Y-30. N40 X-10.Y-10.10 N45 X-50. X 10 20 30 40 50 60 N50 M02 71
72. 72. msharizanJJ204 6.6.3. Absolute The data in the absolute system describes the next location always in terms of its relationship to the fixed zero point. The zero point when used as a programme datum is known as the programme origin. The G90 code sets the control up in absolute mode. All moves are performed with respect to the axes zero. N10 G90 Y 4 0 N15 G01X10.Y10.F300. 4 N20 Y20. 0 N25 X30.40 N30 X40.Y40.3020 N35 X60.Y10. N40 X50.Y0.10 N45 X0. 10 20 30 40 N50 50 M02 60 72
73. 73. msharizanJJ204 6.6.4. Linear Interpolation Under this command the machine tool will move in a straight line at a defined feed rate. Combined axis motions (angled moves) will be executed at the programming feed rate as the control will reduce the velocity of both axes accordingly. E.g. G01 X200. F250. G01 Move in a straight line X200. A distance of 20O.mm F250. At a feed rate of 250.mm/min.NOTE: If a new line with G01 is listed again somewhere below, the F250does not have to be written again. This is called modal. 73
74. 74. msharizanJJ204Example: A block as shown below is to be machined, write a program in absolute mode.% 10 10 70 10G90 - 0 0 0G01 X100 F300 Y70 X90. Y80 90 X20. 80 80 X10. Y90. 70 XO. Y80 Y0. 100 M02 6.6.5. Circular Interpolation In circular interpolation mode G02 will cause the path to be transcribed in a clockwise direction and G03 will cause counter-clockwise motion. G02 - Clockwise G03 - Counterclockwise In circular interpolation there are a number of points to be remembered: The end point of the arc is defined as X and Y coordinates exactly the same as if commanding linear motion. The centre of the arc is defined with respect to the start point in the I and J words as an "increment" from this point. 74
75. 75. msharizanJJ204 For G02 and G03 to function the feed rate "F." must be specified.Example:N5%N10 G90N15 G01 Y110. F200.N20 G02 X20. I10.N25 G03 X30. Y100. I10. 110N30 G01 X90. All radius – R10N35 G02 X100. Y90. J-10.N40 G01 Y10,N45 G02 X90. Y0. I-10.N50 G01 X0. 100N55 M026.7. PROGRAM DEFINITION To enable the machine to operate automatically it is necessary to putinto its memory a programme or set of instructions to carry out the requiredoperation. a) Programme. A programme is a series of instructions to the machine, set out in sequence to -produce a complete machining operation. A programme is made up of a series of blocks. b) Block.- A block or programme line is a set of instructions to the machine that are carried out simultaneously. A block is made up of 75
76. 76. msharizanJJ204 one or more Words and is terminated by an End of Block which is the Line Feed Character. c) Word. A word is a specific instruction to the machine that will affect a particular machine function. Every word consists of a Letter Code and a Numerical value.Examples of Dimensional Words: X100. Y2.345 F0.25Examples of Non-Dimensional Words: N25 G90 M03 S1200 Dimension words can be written in various ways, depending on thecontrol. Lets take the examples X100. Y2.345 some older controls cannotaccept decimal points, so both dimensional words would be written X100000Y2345, with Y showing all decimal places. With these controls, if the X wordwas written as X100, it would be interpreted as one-tenth of a millimeter,not one hundred millimeters. If a control accepts decimal points, then ALL dimensional wordsshould have a decimal point. On any control, non-dimensional must NOThave a decimal point. The method of writing words beginning with a letteris known as word address format and is now almost universally used. 76
77. 77. msharizanJJ204 6.7.1. Program(Start of Program)(Material 25.mm. dia.)(Grip 120.m.m. from Front of Jaws)N01 G71G90G95N02 G50X100.Z130.N03 S2000M03 N04 G00X26.Z119.T0101 BLOCK (Select Turning & Facing Tool) N05 GO1X2.F.O4 N06 GOOZ120. N07 X24. WORDS N10 GOOX100. Z130. T0100 N08 G01Z20. N09 X26. N10 G00X100.Z130.T0100 N11 M02WORD ADDRESS The letter at the beginning of each word is called the address character.e.g. XYZ for Axis designating word F for Feed rates G for Preparatory functions M for miscellaneous function N for Sequence numbers 77
78. 78. msharizanJJ204 CNC mills, drills and machining centers are all equipped with cyclesto perform drilling, reaming, counter boring, boring and tapping operations.Some others have pocketing cycles, slot cutting cycles, hole pattern cyclesetc, all of which are designed to save programming time and effort. CNC lathes usually have cycles to cover drilling, grooving/parting,screw cutting, repetitive cut (automatic roughing) operations and others.Each cycle has its own G code to control the sequence of motions and anaccompanying set of words to define the parameters of those motions. Thesewords have addresses such as: R,P,Q,D,E,I,K,H,B etc. 6.7.2. Program Preparation CNC programmes can be prepared manually, where the programmerusually roughs the programme out on paper, then produces it via a keyboarddevice of the type detailed below, or by assisted preparation in which acomputer plays a predominant role -such as when CAD/CAM packages havebeen installed for design and programming. The programmer must posses knowledge and skills in planningmachining sequences, fixturing, cutting data, cutting tools, calculations, aswell as being familiar with the machines he is programming. To implementthese skills to best effect a programmer should be prepared to observecritically his programs in use and modify them as necessary in order to gainmaximum machine utilization. 6.7.3. Operation of program Before a machine can set into automatic motion a program must bechecked for errors. A simple typing mistake - an incorrect code, a minus signinstead of a zero, the exclusion of a decimal etc, could cause and expensive 78
79. 79. msharizanJJ204machine crash. Anyone who considers their programmes to be without errorand not in need of careful and conscientious trialing has an attitude problemand is placing expensive machinery and operators safety at risk. There may be many ways in which a programme can be checked forerrors, but a programme can only be proved 100% by running the machineand producing a part. Error checking can be performed in a variety of ways:Verification: Read through the print-out (NOT the handwritten manuscript) carefully - sometimes mistakes can be seen easily.Trialing: This involves the execution of the programme without actually cutting the part and may be carried out in several ways depending on the type of machine, or control, or even the philosophy of the person in charge. Adhere to the later unless you can put up good reasons for alteration. Trialing usually consists of running the machinewith the single block switch active, that is, each block will only be executed by pressing cycle start, in conjunction with the programme being displayed on the screen. Quite often the dry run mode is switched on to hasten Proceedings. Dry Run results in all machine motion being executed at a preset rate, usually in the region of 50% to 80% of the rapid traverse capability of the machine. The actual axisvelocity can be overridden from 0% to 100%. The disadvantage of dry running a programme is that feed rates will be masked, and attention must be paid to determining theactual 79
80. 80. msharizanJJ204 programmed feed rate for each block. This may be displayed on the screen. Every movement the machine makes during programme trialing should be expected and accountable to the programmer, if not, those motions should be checked for viability, and if necessary, a more thorough understanding of the machine operation should be sought.Editing: Wherever errors are found, they should be corrected and rechecked, be it on the machine or at the programming station. Whenever a programme is edited on the machine, a note should be made on the print-out so the master or original programme can also be corrected. A better method is to punch out a programme from the control after successfully producing a component.6.8. TYPES OF CONTROL SYSTEM There are two basic types of control systems in numerical control:point-to-point and contouring. a. In a point-to-point system, also called positioning, each axis of the machine is driven separately by lead screws and, depending on the type of operation, at different velocities. The machine moves initially at maximum velocity in order to reduce non-productive time, but decelerates as the tool approaches its numerically defined position. Thus, in an operation such as drilling (or 80
81. 81. msharizanJJ204 punching a hole), the positioning and cutting take place sequentially (Fig. 5.4). After the hole is drilled or punched, the tool retracts upward and moves rapidly to another position, and the operation is repeated. The path followed from one position to another is important in only one respect. It must be chosen to minimize the time of travel, for better efficiency. Point-to-point systems are used mainly in drilling, punching, and straight milling operations. 15 10 45 C.P 15 (0,0) 10 1 2 4 3 45 Incremental (G90) Absolute (G91)Position Coordinate Coordinate Position Coordinate Coordinate (X) (Y) (X) (Y) C.P. -15 15 C.P. -15 15Point 1 10 -10 Point 1 25 -25Point 2 55 -10 Point 2 45 0Point 3 55 -55 Point 3 0 -45Point 4 10 -55 Point 4 -45 0 Figure 5.4. Point-to point system 81
82. 82. msharizanJJ204 b. In a contouring system (also known as a continuous path system), the positioning and the operations are both performed along controlled paths but at different velocities. Because the tool acts as it travels along a prescribed path (Fig. 5.5), accurate control and synchronization of velocities and movements are important. The contouring system is typically used on lathes, milling machines, grinders, welding machinery, and machining centres. Cutter Machined radius surface Cutter path Work piece Figure 5.5. Continuous path by a milling cutter6.9. PROGRAMMING CODES A number of standard codes are used to reduce the amount ofprogramming effort needed to command commonly used machiningoperations, instructions and conditions. These are commonly known as: G codes – call up machining commands M codes – call up machine control activities T codes – call up tool selection F codes – call up feed rates S codes – call 82
83. 83. msharizanJJ204 - modal codes remain active after being entered, unless they are cancelled by another G code; and - non-modal codes are only active in the programme block in which they appear. 6.9.1. G codes (preparatory codes) The majority of manufacturers follow the same practice in designationof codes, but their detailed implementation mav differ. Sample G codes GOO Rapid movement for position GOI Linear interpolation used for straight-line feed G02 Circular interpolation, clockwise G03 Circular interpolartion, counterclockwise G04 Dwell, a programmed stop to the tool movement G17 Circular interpolation xy plane G18 Circular interpolation xz plane G19 Circular interpolation yz plane G20 Inch units G21 Millimetre units G28 Return to home position G29 Return from home position G31 Reverses programmed direction of x axis G32 Reverses programmed direction of y axis G41 Tool radius compensation left G42 Tool radius compensation right G43 Tool length compensation-positive direction G44 Tool length compensation-negative direction G70 Imperial unit G71 Metric units G80 Cancel canned cycle G81 Drilling cycle G82 Drilling cycle with dwell G83 Deep hole drilling G84 Tapping cycle G85 89-boring cycles G90 Absolute mode G91 Incremental mode 83
84. 84. msharizanJJ2046.9.2. M codes These control the auxiliary functions of the machine.MOO Program stopM02 End of programM03 Spindle on, clockwiseM04 Spindle on, counter clockwiseM05 Spindle offM06 Tool changeM07 Oil mist coolant onM08 Flood coolant onM09 Coolant offM30 End of tape 84
85. 85. msharizanJJ204 6.10. WRITING A PROGRAM Figure 5.6. To cut a „S‟-slot/groove with a point-to-point method and a continuous path/contouring systemTable 5. Reference points and X and Y coordinates to cut a „S‟-slot/groove with a point-to-point method and a continuous path/contouring system Position Coordinate (X) Coordinate (Y) C.P. 0 0 P. 1 45.0 -25.0 P. 2 70.0 -25.0 P. 3 60.0 -65.0 P. 4 45.0 -50.0 P. 5 60.0 -50.0 P. 6 49.393 -75.607 P. 7 38.787 -65.0 P. 8 15.0 -65.0 85
86. 86. msharizanJJ204 To machine the above component (as in Fig.5.6), below is theprogramme that can be followed;N10 G71 G90 S1500 T1N20 G00 X0 Y0N30 G00 X70.0 Y-25.0 Z10.0N40 G01 Z-5.0 F250N50 G03 I-25.0 J0N60 X45.0 Y-50.0N70 G01 X60.0 Y -50.0N80 G02 I0 J-15.0N90 X49.393 Y-75.607N100 G01 X38.787 Y-65.0N110 X15.0 Y-65.0N120 Z10.0N130 G00 M00N140 G00 X0 Y0Description of The Above Programme NXX – block number Block No. 10 – set machine to use metric unit, incremental coordinate, spindle speed 1500 rpm, choose tool no. 1. Block No. 20 – rapid movement to centre point (C.P). Block No. 30 - rapid movement to point 1 (P. 1), cutting tool distance is 5.0 mm from the surface of the work piece. Block No. 40 – cutting tool cuts 10.00 mm deep, feed 250 mm/min 86