UNIT 1: Geometry of single point turning tools:

Transcription

1 UNIT 1: Thery f Metal uttig: Sigle pit cuttig tl meclature, gemetry, Merchats circle diagram ad aalysis, Erst Merchat s sluti, shear agle relatiship, prblems f Merchat s aalysis, tl wear ad tl failure, tl life, effects f cuttig parameters tl life, tl failure criteria, Taylr s tl life equati, prblems tl life evaluati. Gemetry f sigle pit turig tls: 7 Hrs Istructial bjectives: At the ed f this less, the studet shuld be able t : (a) ceive rake agle ad clearace agle f cuttig tls (b) lassify systems f descripti f tl gemetry (c) Demstrate tl gemetry ad defie tl agles i : Machie eferece System Orthgal ake System ad Nrmal ake System (d) Desigate cuttig tl gemetry i ASA, OS ad NS Gemetry f sigle pit turig tls: Bth material ad gemetry f the cuttig tls play very imprtat rles their perfrmaces i achievig Effectiveess, Efficiecy ad Overall ecmy f machiig. uttig tls may be classified accrdig t the umber f majr cuttig edges (pits) ivlved as fllws: 1. Sigle pit: e.g., turig tls, shapig, plaig ad slttig tls ad brig tls 2. Duble (tw) pit: e.g., drills 3. Multipit (mre tha tw): e.g., millig cutters, brachig tls, hbs, gear shapig cutters etc. 1

2 (i) cept f rake ad clearace agles f cuttig tls. The wrd tl gemetry is basically referred t sme specific agles r slpe f the saliet faces ad edges f the tls at their cuttig pit. ake agle ad clearace agle are the mst sigificat fr all the cuttig tls. The ccept f rake agle ad clearace agle will be clear frm sme simple peratis shw i Fig. 1.1 Defiiti: Fig. 1.1 ake ad clearace agles f cuttig tls. ake agle (γ): Agle f icliati f rake surface frm referece plae. learace agle (α): Agle f icliati f clearace r flak surface frm the fiished surface ake agle: is prvided fr ease f chip flw ad verall machiig. ake agle may be psitive, r egative r eve zer as shw i Fig (a) psitive rake (b) zer rake (c) egative rake Fig. 1.2 Three pssible types f rake agles 2

3 elative advatages f such rake agles are: Psitive rake helps reduce cuttig frce ad thus cuttig pwer requiremet. Negative rake t icrease edge-stregth ad life f the tl Zer rake t simplify desig ad maufacture f the frm tls. learace agle: is essetially prvided t avid rubbig f the tl (flak) with the machied surface which causes lss f eergy ad damages f bth the tl ad the jb surface. Hece, clearace agle is a must ad must be psitive (3 ~ 15 depedig up tl-wrk materials ad type f the machiig peratis like turig, drillig, brig etc.) (ii) Systems f descripti f tl gemetry Tl-i-Had System where ly the saliet features f the cuttig tl pit are idetified r visualized as shw i Fig There is quatitative ifrmati, i.e., value f the agles. Fig. 1.3 Basic features f sigle pit tl (turig) i Tl-i-had system Machie eferece System ASA system 3

4 Tl eferece Systems Orthgal ake System OS Nrmal ake System NS Wrk eferece System WS (iii) Demstrati (expressi) f tl gemetry i : Machie eferece System: This system is als called ASA system; ASA stads fr America Stadards Assciati. Gemetry f a cuttig tl refers maily t its several agles r slpe f its saliet wrkig surfaces ad cuttig edges. Thse agles are expressed w.r.t. sme plaes f referece. I Machie eferece System (ASA), the three plaes f referece ad the crdiates are chse based the cfigurati ad axes f the machie tl ccered. The plaes ad axes used fr expressig tl gemetry i ASA system fr turig perati are shw i Fig Fig. 1.4 Plaes ad axes f referece i ASA system feed The plaes f referece ad the crdiates used i ASA system fr tl gemetry are : 4

5 π - π - π ad X Y - Z X Y m m m Where, π = eferece plae; plae perpedicular t the velcity vectr (shw i Fig. 1.4) π = Machie lgitudial plae; plae perpedicular t π ad take i the directi f assumed X lgitudial feed π = Machie Trasverse plae; plae perpedicular t bth π ad π [This plae is take i the directi Y X f assumed crss feed] The axes X m, Y m ad Z m are i the directi f lgitudial feed, crss feed ad cuttig velcity (vectr) respectively. The mai gemetrical features ad agles f sigle pit tls i ASA systems ad their defiitis will be clear frm Fig Fig. 1.5 Tl agles i ASA system Defiiti f: 5

6 ake agles: [Fig. 1.5] i ASA system: γ = side (axial rake: agle f icliati f the rake surface frm the referece plae (π ) ad easured x Machie ef. Plae, π. X γ = back rake: agle f icliati f the rake surface frm the referece plae ad measured Machie y Trasverse plae, π. Y learace agles: [Fig. 1.5]: α = side clearace: agle f icliati f the pricipal flak frm the machied surface (r V) ad measured x π plae. X α = back clearace: same as α but measured π plae. y x Y uttig agles: [Fig. 1.5]: φ = apprach agle: agle betwee the pricipal cuttig edge (its prjecti π ) ad π ad measured s Y π φ = ed cuttig edge agle: agle betwee the ed cuttig edge (its prjecti π ) frm π ad measured e X π Nse radius, r (i ich): r = se radius : curvature f the tl tip. It prvides stregtheig f the tl se ad better surface fiish. Tl eferece Systems Orthgal ake System OS: This system is als kw as ISO ld. The plaes f referece ad the c-rdiate axes used fr expressig the tl agles i OS are: π - π - π O ad X - Y - Z which are take i respect f the tl cfigurati as idicated i Fig

7 Fig. 1.6 Plaes ad axes f referece i O Where, π = eferece plae perpedicular t the cuttig velcity vectr, V π = cuttig plae; plae perpedicular t π ad take alg the pricipal cuttig edge π O = Orthgal plae; plae perpedicular t bth π ad π ad the axes; X = alg the lie f itersecti f π ad π O Y = alg the lie f itersecti f π ad π Z = alg the velcity vectr, i.e., rmal t bth X ad Y axes. The mai gemetrical agles used t express tl gemetry i Orthgal ake System (OS) ad their defiitis will be clear frm Fig

8 Fig. 1.7 Tl agles i OS system Defiiti f ake agles [Fig. 1.7] i OS: γ = rthgal rake: agle f icliati f the rake surface frm eferece plae, π ad measured the rthgal plae, π λ = icliati agle; agle betwee π frm the directi f assumed lgitudial feed [π ] ad measured X π learace agles [Fig. 1.7]: α = rthgal clearace f the pricipal flak: agle f icliati f the pricipal flak frm π ad measured π α = auxiliary rthgal clearace: agle f icliati f the auxiliary flak frm auxiliary cuttig plae, π ad measured auxiliary rthgal plae, π as idicated i Fig

9 uttig agles [Fig. 1.7]: φ = pricipal cuttig edge agle: agle betwee π ad the directi f assumed lgitudial feed r π X ad measured π φ 1 = auxiliary cuttig agle: agle betwee π ad π X ad measured π Nse radius, r (mm): r = radius f curvature f tl tip Fig. 1.8 Auxiliary rthgal clearace agle Nrmal ake System NS: This system is als kw as ISO ew. ASA system has limited advatage ad use like cveiece f ispecti. But OS is advatageusly used fr aalysis ad research i machiig ad tl perfrmace. But OS des t reveal the true picture f the tl gemetry whe the cuttig edges are iclied frm the referece plae, i.e., λ 0. Besides, sharpeig r resharpeig, if ecessary, f the tl by gridig i OS requires sme additial calculatis fr crrecti f agles. 9

10 These tw limitatis f OS are vercme by usig NS fr descripti ad use f tl gemetry. The basic differece betwee OS ad NS is the fact that i OS, rake ad clearace agles are visualized i the rthgal plae, π, whereas i NS thse agles are visualized i ather plae called Nrmal plae, π. The rthgal plae, π is simply rmal t π ad π irrespective f the icliati f the cuttig edges, N i.e., λ, but π (ad π fr auxiliary cuttig edge) is always rmal t the cuttig edge. The differeces betwee N N OS ad NS have bee depicted i Fig The plaes f referece ad the crdiates used i NS are: π - π - π ad X Y Z N N where, π = rmal referece plae N π = Nrmal plae: plae rmal t the cuttig edge N ad X = X Y = cuttig edge Z = rmal t X ad Y It is t be ted that whe λ = 0, NS ad OS becme same, i.e. π π N, Y N Y ad Z Z. Defiiti (i NS) f ake agles: γ = rmal rake: agle f icliati agle f the rake surface frm π ad measured rmal plae, π N α = rmal clearace: agle f icliati f the pricipal flak frm π ad measured π N α = auxiliary clearace agle: rmal clearace f the auxiliary flak (measured π plae rmal t N the auxiliary cuttig edge. The cuttig agles, φ ad φ 1 ad se radius, r (mm) are same i OS ad NS. 10

11 Fig. 1.9 Differeces f NS frm OS w.r.t. cuttig tl gemetry. (b) Desigati f tl gemetry The gemetry f a sigle pit tl is desigated r specified by a series f values f the saliet agles ad se radius arraged i a defiite sequece as fllws: Desigati (sigature) f tl gemetry i ASA System γ, γ, α, α, φ, φ, r (ich) y x y x e s OS System λ, γ, α, α, φ, φ, r (mm) 1 NS System λ, γ, α, α, φ, φ, r (mm) 1 11

12 Quiz Test: Select the crrect aswer frm the give fur ptis : 1. Back rake f a turig tl is measured its (a) machie lgitudial plae (b) machie trasverse plae (c) rthgal plae (d) rmal plae 2. Nrmal rake ad rthgal rake f a turig tl will be same whe its (a) φ = 0 (b) φ = 0 (c) λ = 0 (d) φ = Nrmal plae f a turig tl is always perpedicular t its (a) π plae (b) π plae (c) π plae (d) e f them X Y 4. Pricipal cuttig edge agle f ay turig tl is measured its (a) π (b) π (c) π Y X (d) π 5. A cuttig tl ca ever have its (a) rake agle psitive (b) rake agle egative (c) clearace agle psitive (d) clearace agle egative 6. Orthgal clearace ad side clearace f a turig tl will be same if its perpedicular cuttig edge agle is (a) φ = 30 (b) φ = 45 (c) φ = 60 (d) φ = Icliati agle f a turig tl is measured its (a) referece plae (b) cuttig plae (c) rthgal plae (d) rmal plae 8. Nrmal rake ad side rake f a turig tl will be same if its (a) φ = 0 ad λ = 0 (b) φ = 90 ad λ = 0 (c) φ = 90 ad λ = 90 (d) φ = 0 ad λ = 90 Aswer f the bjective questis 1 (b) 2 (c) 3 (c) 4 (a) 5 (d) 6 (d) 7 (b) 8 (b) 12

13 Failure f cuttig tls ad tl life: Istructial bjectives: At the ed f this less, yu will be able t i) State hw the cuttig tls fail ii) Illustrate the mechaisms ad patter f tl wear iii) Ascertai the essetial prperties f cuttig tl materials iv) Defie ad assess tl life v) Develp ad use tl life equati. (i) Failure f cuttig tls: Smth, safe ad ecmic machiig ecessitate Preveti f premature ad catastrphic failure f the cuttig tls educti f rate f wear f tl t prlg its life T accmplish the afresaid bjectives e shuld first kw why ad hw the cuttig tls fail. uttig tls geerally fail by: i) Mechaical breakage due t excessive frces ad shcks. Such kid f tl failure is radm ad catastrphic i ature ad hece are extremely detrimetal. ii) Quick dullig by plastic defrmati due t itesive stresses ad temperature. This type f failure als ccurs rapidly ad are quite detrimetal ad uwated. iii) Gradual wear f the cuttig tl at its flaks ad rake surface. The first tw mdes f tl failure are very harmful t ly fr the tl but als fr the jb ad the machie tl. Hece these kids f tl failure eed t be preveted by usig suitable tl materials ad gemetry depedig up the wrk material ad cuttig cditi. But failure by gradual wear, which is ievitable, cat be preveted but ca be slwed dw ly t ehace the service life f the tl. 13

14 The cuttig tl is withdraw immediately after it fails r, if pssible, just befre it ttally fails. Fr that e must uderstad that the tl has failed r is gig t fail shrtly. It is uderstd r csidered that the tl has failed r abut t fail by e r mre f the fllwig cditis : (a) I &D labratries Ttal breakage f the tl r tl tip(s) Massive fracture at the cuttig edge(s) Excessive icrease i cuttig frces ad/r vibrati Average wear (flak r crater) reaches its specified limit(s) (b) I machiig idustries Excessive (beyd limit) curret r pwer csumpti Excessive vibrati ad/r abrmal sud (chatter) Ttal breakage f the tl Dimesial deviati beyd tlerace apid wrseig f surface fiish Adverse chip frmati. (ii) Mechaisms ad patter (gemetry) f cuttig tl wear: Fr the purpse f ctrllig tl wear e must uderstad the varius mechaisms f wear, that the cuttig tl uderges uder differet cditis. The cmm mechaisms f cuttig tl wear are : i) Mechaical wear Thermally isesitive type; like abrasi, chippig ad delamiati Thermally sesitive type; like adhesi, fracturig, flakig etc. ii) Thermchemical wear Macr-diffusi by mass dissluti 14

15 Micr-diffusi by atmic migrati iii) hemical wear iv) Galvaic wear I diffusi wear the material frm the tl at its rubbig surfaces, particularly at the rake surface gradually diffuses it the flwig chips either i bulk r atm by atm whe the tl material has chemical affiity r slid slubility twards the wrk material. The rate f such tl wear icreases with the icrease i temperature at the cuttig ze. Diffusi wear becmes predmiat whe the cuttig temperature becmes very high due t high cuttig velcity ad high stregth f the wrk material. hemical wear, leadig t damages like grvig wear may ccur if the tl material is t eugh chemically stable agaist the wrk material ad/r the atmspheric gases. Galvaic wear, based electrchemical dissluti, seldm ccurs whe bth the wrk tl materials are electrically cductive, cuttig ze temperature is high ad the cuttig fluid acts as a electrlyte. The usual patter r gemetry f wear f turig ad face millig iserts are typically shw i Fig (a ad b) ad Fig respectively. 15

16 Fig (a) Gemetry ad majr features f wear f turig tls Fig (b) Phtgraphic view f the wear patter f a turig tl isert 16

17 Fig Schematic (a) ad actual view (b) f wear patter f face millig isert I additi t ultimate failure f the tl, the fllwig effects are als caused by the grwig tl-wear : Icrease i cuttig frces ad pwer csumpti maily due t the pricipal flak wear Icrease i dimesial deviati ad surface rughess maily due t wear f the tl-tips ad auxiliary flak wear (V s ) Odd sud ad vibrati Wrseig surface itegrity Mechaically weakeig f the tl tip. (iii) Essetial prperties fr cuttig tl materials: The cuttig tls eed t be capable t meet the grwig demads fr higher prductivity ad ecmy as well as t machie the extic materials which are cmig up with the rapid prgress i sciece ad techlgy. The cuttig tl material f the day ad future essetially require the fllwig prperties t resist r retard the phemea leadig t radm r early tl failure: 1. High mechaical stregth; cmpressive, tesile, ad TA 2. Fracture tughess high r at least adequate 17

18 3. High hardess fr abrasi resistace 4. High ht hardess t resist plastic defrmati ad reduce wear rate at elevated temperature 5. hemical stability r iertess agaist wrk material, atmspheric gases ad cuttig fluids 6. esistace t adhesi ad diffusi 7. Thermal cductivity lw at the surface t resist icmig f heat ad high at the cre t quickly dissipate the heat etered 8. High heat resistace ad stiffess 9. Maufacturability, availability ad lw cst. Tl Life: Defiiti Tl life geerally idicates, the amut f satisfactry perfrmace r service redered by a fresh tl r a cuttig pit till it is declared failed. Tl life is defied i tw ways : (a) I & D : Actual machiig time (perid) by which a fresh cuttig tl (r pit) satisfactrily wrks after which it eeds replacemet r recditiig. The mder tls hardly fail prematurely r abruptly by mechaical breakage r rapid plastic defrmati. Thse fail mstly by wearig prcess which systematically grws slwly with machiig time. I that case, tl life meas the spa f actual machiig time by which a fresh tl ca wrk befre attaiig the specified limit f tl wear. Mstly tl life is decided by the machiig time till flak wear, V reaches 0.3 mm r crater B wear, K reaches 0.15 mm. T (b) I idustries r shp flr : The legth f time f satisfactry service r amut f acceptable utput prvided by a fresh tl prir t it is required t replace r recditi. Assessmet f tl life Fr & D purpses, tl life is always assessed r expressed by spa f machiig time i miutes, 18

19 B Maufacturig Prcess II 06 ME45 Whereas, i idustries besides machiig time i miutes sme ther meas are als used t assess tl life, depedig up the situati, such as N. f pieces f wrk machied Ttal vlume f material remved Ttal legth f cut. Measuremet f tl wear The varius methds are : 1. By lss f tl material i vlume r weight, i e life time this methd is crude ad is geerally applicable fr critical tls like gridig wheels. 2. By grvig ad idetati methd i this apprximate methd wear depth is measured idirectly by the differece i legth f the grve r the idetati utside ad iside the wr area 3. Usig ptical micrscpe fitted with micrmeter very cmm ad effective methd 4. Usig scaig electr micrscpe (SEM) used geerally, fr detailed study; bth qualitative ad quatitative 5. Talysurf, specially fr shallw crater wear. Taylr s tl life equati: Wear ad hece tl life f ay tl fr ay wrk material is gvered maily by the level f the machiig parameters i.e., cuttig velcity, (V ), feed, (s ) ad depth f cut (t). uttig velcity affects maximum ad depth f cut miimum. The usual patter f grwth f cuttig tl wear (maily V B ), priciple f assessig tl life ad its depedece cuttig velcity are schematically shw i Fig

20 Fig Grwth f flak wear ad assessmet f tl life The tl life bviusly decreases with the icrease i cuttig velcity keepig ther cditis ualtered as idicated i Fig If the tl lives, T, T, T, T etc are pltted agaist the crrespdig cuttig velcities, V, V, V, V etc as shw i Fig. 1.13, a smth curve like a rectagular hyperbla is fud t appear. Whe F. W. Taylr pltted the same figure takig bth V ad T i lg-scale, a mre distict liear relatiship appeared as schematically shw i Fig With the slpe, ad itercept, c, Taylr derived the simple equati as VT = 20

21 where, is called, Taylr s tl life expet. The values f bth ad c deped maily up the tlwrk materials ad the cuttig evirmet (cuttig fluid applicati). The value f depeds als the limitig value f V udertake ( i.e., 0.3 mm, 0.4 mm, 0.6 mm etc.) B Fig uttig velcity tl life relatiship Fig uttig velcity vs tl life a lg-lg scale 21

22 Example f use f Taylr s tl life equati Prblem : If i turig f a steel rd by a give cuttig tl (material ad gemetry) at a give machiig cditi (s ad t) uder a give evirmet (cuttig fluid applicati), the tl life decreases frm 80 mi t 20 mi. due t icrease i cuttig velcity, V frm 60 m/mi t 120 m/mi., the at what cuttig velcity the life f that tl uder the same cditi ad evirmet will be 40 mi.? Sluti : Assumig Taylr s tl life equati, VT = V1T 1 = V2T2 = V3T 3 =... Here, V 1 = 60 m/mi; T 1 = 80 mi. V 2 = 120 m/mi; T 2 = 20 mi. V 3 =? (t be determied); T 3 = 40 mi. Takig, VT = VT i.e., T 1 V 2 = T2 V1 frm which, = 0.5 Agai, VT = VT i.e, V 3 T 1 = V1 T3 ad V 3 = m/mi 22

23 Mdified Taylr s Tl Life equati I Taylr s tl life equati, ly the effect f variati f cuttig velcity, V tl life has bee csidered. But practically, the variati i feed (s ) ad depth f cut (t) als play rle tl life t sme extet. Takig it accut the effects f all thse parameters, the Taylr s tl life equati has bee mdified as, TL T = x y z Vc S0 t where, TL = tl life i mi T = A cstat depedig maily up the tl wrk materials ad the limitig value f V B udertake. x, y ad z - expets s called tl life expets depedig up the tl wrk materials ad the machiig evirmet. Geerally, x > y > z as V affects tl life maximum ad t miimum. The values f the cstats, T, x, y ad z are available i Machiig Data Hadbks r ca be evaluated by machiig tests. 23

24 Quiz Test Idetify the crrect aswer frm the give fur ptis. 1. I high speed machiig f steels the teeth f millig cutters may fail by (a) mechaical breakage (b) plastic defrmati (c) wear (d) all f the abve 2. Tl life i turig will decrease by maximum extet if we duble the (a) depth f cut (b) feed (c) cuttig velcity (d) tl rake agle 3. I cuttig tls, crater wear develps at (a) the rake surface (b) the pricipal flak (c) the auxiliary flak (d) the tl se 4. T prevet plastic defrmati at the cuttig edge, the tl material shuld pssess (a) high fracture tughess (b) high ht hardess (c) chemical stability (d) adhesi resistace Prblems Prblem 1 Durig turig a metallic rd at a give cditi, the tl life was fud t icrease frm 25 mi t 50 mi. whe V was reduced frm 100 m/mi t 80 m/mi. Hw much will be the life f that tl if machiedat 90 m/mi? Prblem 2 While drillig hles i steel plate by a 20 mm diameter HSS drill at a give feed, the tl life decreased frm 40 mi. t 24 mi. whe speed was raised frm 250 rpm t 320 rpm. At what speed (rpm) the life f that drill uder the same cditi wuld be 30 mi.? Aswers f the questis f Quiz Test Q. 1 : (d) Q. 2 : (c) Q. 3 : (a) Q. 4 : (b) Sluti t Prblem 1. As mi Sluti t Prblem 2 As. 287 rpm. 24