3 Mechanism of Chip Removal. Production Technology

Transcription

1 3 Mechanism f Chip Remval Prductin Technlgy

2 Instructinal Objectives At the end f this lessn, the student wuld be able t (i) describe with illustratin the mechanism f chip frmatin in machining ductile materials and brittle materials (ii) illustrate and assess gemetrical characteristics f ductile chips : chip reductin cefficient & cutting rati shear angle and cutting strain (iii) Identify and state the causes, characteristics and effects f built up edge (BUE) frmatin. (iv) Classify chips and identify the cnditin fr different chip frms. (i) Mechanism f chip frmatin in machining Machining is a semi-finishing r finishing prcess essentially dne t impart required r stipulated dimensinal and frm accuracy and surface finish t enable the prduct t fulfill its basic functinal requirements prvide better r imprved perfrmance render lng service life. Machining is a prcess f gradual remval f excess material frm the prefrmed blanks in the frm f chips. The frm f the chips is an imprtant index f machining because it directly r indirectly indicates : Nature and behaviur f the wrk material under machining cnditin Specific energy requirement (amunt f energy required t remve unit vlume f wrk material) in machining wrk Nature and degree f interactin at the chip-tl interfaces. The frm f machined chips depend mainly upn : Wrk material Material and gemetry f the cutting tl Levels f cutting velcity and feed and als t sme extent n depth f cut Machining envirnment r cutting fluid that affects temperature and frictin at the chip-tl and wrk-tl interfaces.

3 Knwledge f basic mechanism(s) f chip frmatin helps t understand the characteristics f chips and t attain favurable chip frms. Mechanism f chip frmatin in machining ductile materials During cntinuus machining the uncut layer f the wrk material just ahead f the cutting tl (edge) is subjected t almst all sided cmpressin as indicated in Fig a 1 : chip thickness (befre cut) a 2 : chip thickness (after cut) Wrk V f V C Tl π F R N Fig. 5.1 Cmpressin f wrk material (layer) ahead f the tl tip The frce exerted by the tl n the chip arises ut f the nrmal frce, N and frictinal frce, F as indicated in Fig Due t such cmpressin, shear stress develps, within that cmpressed regin, in different magnitude, in different directins and rapidly increases in magnitude. Whenever and wherever the value f the shear stress reaches r exceeds the shear strength f that wrk material in the defrmatin regin, yielding r slip takes place resulting shear defrmatin in that regin and the plane f maximum shear stress. But the frces causing the shear stresses in the regin f the chip quickly diminishes and finally disappears while that regin mves alng the tl rake surface twards and then ges beynd the pint f chip-tl engagement. As a result the slip r shear stps prpagating lng befre ttal separatin takes place. In the mean time the succeeding prtin f the chip starts underging cmpressin fllwed by yielding and shear. This phenmenn repeats rapidly resulting in frmatin and remval f chips in thin layer by layer. This phenmenn has been explained in a simple way by Piispannen [1] using a card analgy as shwn in Fig In actual machining chips als, such serratins are visible at their upper surface as indicated in Fig The lwer surface becmes smth due t further plastic defrmatin due t intensive rubbing with the tl at high pressure and temperature. The pattern f shear defrmatin by lamellar sliding, indicated in the mdel, can als be seen in actual chips by prper munting, etching and plishing the side surface f the machining chip and bserving under micrscpe.

4 The pattern and extent f ttal defrmatin f the chips due t the primary and the secndary shear defrmatins f the chips ahead and alng the tl face, as indicated in Fig. 5.3, depend upn [1] Piispannen V., Thery f frmatin f metal chips, J. Applied Physics, Vl. 19, N. 10, 1948, pp wrk material tl; material and gemetry the machining speed (V C ) and feed (s ) cutting fluid applicatin chip V C Tl (a) Shifting f the pstcards by partial sliding against each ther (b) Chip frmatin by shear in lamella. Fig. 5.2 Piispanen mdel f card analgy t explain chip frmatin in machining ductile materials primary defrmatin (shear) zne WORK CHIP secndary defrmatin zne V C Fig. 5.3 Primary and secndary defrmatin znes in the chip. The verall defrmatin prcess causing chip frmatin is quite cmplex and hence needs thrugh experimental studies fr clear understanding the phenmena and its dependence n the affecting parameters. The feasible and ppular experimental methds [2] fr this purpse are: Study f defrmatin f rectangular r circular grids marked n the side surface as shwn in Fig. 5.4

5 [2] Bhattacharya, A.., Metal cutting Thery and Practice, Bk, New Central Bk Agency, Klkata. Micrscpic study f chips frzen by drp tl r quick stp apparatus Study f running chips by high speed camera fitted with lw magnificatin micrscpe. (a) rectangular grids (b) circular grids Fig. 5.4 Pattern f grid defrmatin during chip frmatin. It has been established by several analytical and experimental methds including circular grid defrmatin that thugh the chips are initially cmpressed ahead f the tl tip, the final defrmatin is accmplished mstly by shear in machining ductile materials. Hwever, machining f ductile materials generally prduces flat, curved r ciled cntinuus chips. Mechanism f chip frmatin in machining brittle materials The basic tw mechanisms invlved in chip frmatin are Yielding generally fr ductile materials Brittle fracture generally fr brittle materials During machining, first a small crack develps at the tl tip as shwn in Fig. 5.5 due t wedging actin f the cutting edge. At the sharp crack-tip stress cncentratin takes place. In case f ductile materials immediately yielding takes place at the crack-tip and reduces the effect f stress cncentratin and prevents its prpagatin as crack. But in case f brittle materials the initiated crack quickly prpagates, under stressing actin, and ttal separatin takes place frm the parent wrkpiece thrugh the minimum resistance path as indicated in Fig Machining f brittle material prduces discntinuus chips and mstly f irregular size and shape. The prcess f frming such chips is schematically shwn in Fig. 5.6.

6 Initial minute crack crack prpagatin in case f brittle materials V C Fig. 5.5 Develpment and prpagatin f crack causing chip separatin. (a) separatin (b) swelling (c) further swelling (d) separatin (e) swelling again Fig. 5.6 Schematic view f chip frmatin in machining brittle materials. (ii) Gemetry and characteristics f chip frms The gemetry f the chips being frmed at the cutting zne fllw a particular pattern especially in machining ductile materials. The majr sectin f the engineering materials being machined are ductile in nature, even sme semiductile r semi-brittle materials behave ductile under the cmpressive frces at the cutting zne during machining. The pattern and degree f defrmatin during chip frmatin are quantitatively assessed and expressed by sme factrs, the values f which indicate abut the frces and energy required fr a particular machining wrk.

7 Chip reductin cefficient r cutting rati The usual gemetrical features f frmatin f cntinuus chips are schematically shwn in Fig The chip thickness (a 2 ) usually becmes larger than the uncut chip thickness (a 1 ). The reasn can be attributed t cmpressin f the chip ahead f the tl frictinal resistance t chip flw lamellar sliding accrding t Piispannen s (a) (b) Fig. 5.7 Gemetrical features f cntinuus chips frmatin. The significant gemetrical parameters invlved in chip frmatin are shwn in Fig. 5.7 and thse parameters are defined (in respect f straight turning) as: t = depth f cut (mm) perpendicular penetratin f the cutting tl tip in wrk surface s = feed (mm/rev) axial travel f the tl per revlutin f the jb b 1 = width (mm) f chip befre cut b 2 = width (mm) f chip after cut a 1 = thickness (mm) f uncut layer (r chip befre cut) a 2 = chip thickness (mm) thickness f chip after cut A 1 = crss sectin (area, mm 2 ) f chip befre cut The degree f thickening f the chip is expressed by a2 ζ = > 1.00 (since a 2 > a 1 ) (5.1) a 1 where, ζ = chip reductin cefficient

8 a 1 = s sinφ (5.2) where φ = principal cutting edge angle Larger value f ζ means mre thickening i.e., mre effrt in terms f frces r energy required t accmplish the machining wrk. Therefre it is always desirable t reduce a 2 r ζ withut sacrificing prductivity, i.e. metal remval rate (MRR). Chip thickening is als ften expressed by the reciprcal f ζ as, 1 a1 = r = (5.3) ζ a2 where, r = cutting rati The value f chip reductin cefficient, ζ (and hence cutting rati) depends mainly upn tl rake angle, γ chip-tl interactin, mainly frictin,μ Rughly in the fllwing way [3] π μ ( γ ) 2 ζ = e [fr rthgnal cutting] (5.4) π/2 and γ are in radians The simple but very significant expressin (5.4) clearly depicts that the value f ζ can be desirably reduced by Using tl having larger psitive rake Reducing frictin by using lubricant The rle f rake angle and frictin at the chip-tl interface n chip reductin cefficient are als schematically shwn in Fig Chip reductin cefficient, ζ μ (frictin cefficient) Rake angle, γ Fig. 5.8 Rle f rake angle and frictin n chip reductin cefficient Chip reductin cefficient, ζ is generally assessed and expressed by the rati f the chip thickness, after (a 2 ) and befre cut (a 1 ) as in equatin 5.1. But ζ can als be expressed r assessed by the rati f

9 [3] Krnenberg, M., A new apprach t sme relatinships in the Thery f Metal Cutting, J. Applied Physics, Vl.6, N. 6, Ttal length f the chip befre (L 1 ) and after cut (L 2 ) Cutting velcity, V C and chip velcity, V f Cnsidering ttal vlume f chip prduced in a given time, a 1 b 1 L 1 = a 2 b 2 L 2 (5.5) The width f chip, b generally des nt change significantly during machining unless there is side flw fr sme adverse situatin. Therefre assuming, b 1 =b 2 in equatin (5.5), ζ cmes up t be, a2 L ζ 1 = = a (5.6) 1 L2 Again cnsidering unchanged material flw (vlume) rati, Q Q = (a 1 b 1 )V C = (a 2 b 2 )V f (5.7) Taking b 1 =b 2, a VC = 2 ζ = a (5.8) 1 V f Equatin (5.8) reveals that the chip velcity, V f will be lesser than the cutting 1 velcity, V C and the rati is equal t the cutting rati, r = ζ Shear angle It has been bserved that during machining, particularly ductile materials, the chip sharply changes its directin f flw (relative t the tl) frm the directin f the cutting velcity, V C t that alng the tl rake surface after thickening by shear defrmatin r slip r lamellar sliding alng a plane. This plane is called shear plane and is schematically shwn in Fig Shear plane: Shear plane is the plane f separatin f wrk material layer in the frm f chip frm the parent bdy due t shear alng that plane. Shear angle: Angle f inclinatin f the shear plane frm the directin f cutting velcity [as shwn in Fig. 5.9]. V C ' B a 1 A Shear plane β (β - γ ) a 2 V C O C π γ Fig. 5.9 Shear plane and shear angle in chip frmatin

10 The value f shear angle, dented by β (taken in rthgnal plane) depends upn Chip thickness befre and after cut i.e. ζ Rake angle, γ (in rthgnal plane) Frm Fig. 5.9, AC = a 2 = OAcs(β -γ ) And AB = a 1 = OAsinβ Dividing a 2 by a 1 a2 cs( β γ ) = ζ = (5.9) a sin β r, 1 tan β csγ = (5.10) ζ sinγ Replacing chip reductin cefficient, ζ by cutting rati, r, the equatin (5.10) changes t r csγ tan β = (5.11) 1 r sinγ Equatin 5.10 depicts that with the increase in ζ, shear angle decreases and vice-versa. It is als evident frm equatin (5.10) as well as equatin (5.4) that shear angle increases bth directly and indirectly with the increase in tl rake angle. Increase in shear angle means mre favurable machining cnditin requiring lesser specific energy. Cutting strain The magnitude f strain, that develps alng the shear plane due t machining actin, is called cutting strain (shear). The relatinship f this cutting strain, ε with the gverning parameters can be derived frm Fig Due t presence f the tl as an bstructin the layer 1 has been shifted t psitin 2 by sliding alng the shear plane. Frm Fig. 5.10, Cutting strain (average), Δs ε = = Y NM PM ON PN + NM PN r, ε = = + ON ON ON r, ε = ct β + tan( β γ ) (5.12) (iii) Built-up-Edge (BUE) frmatin Causes f frmatin In machining ductile metals like steels with lng chip-tl cntact length, lt f stress and temperature develps in the secndary defrmatin zne at the chip-tl interface. Under such high stress and temperature in between tw clean surfaces f metals, strng bnding may lcally take place due t adhesin similar t welding. Such bnding will be encuraged and

11 accelerated if the chip tl materials have mutual affinity r slubility. The weldment starts frming as an embry at the mst favurable lcatin and thus gradually grws as schematically shwn in Fig β τ s Δs Y O P N M Δs π γ Y shear strain Fig Cutting strain in machining Built-up-edge F V C Fig Scheme f built-up-edge frmatin

12 With the grwth f the BUE, the frce, F (shwn in Fig. 5.11) als gradually increases due t wedging actin f the tl tip alng with the BUE frmed n it. Whenever the frce, F exceeds the bnding frce f the BUE, the BUE is brken r sheared ff and taken away by the flwing chip. Then again BUE starts frming and grwing. This ges n repeatedly. Characteristics f BUE Built-up-edges are characterized by its shape, size and bnd strength, which depend upn: wrk tl materials stress and temperature, i.e., cutting velcity and feed cutting fluid applicatin gverning cling and lubricatin. BUE may develp basically in three different shapes as schematically shwn in Fig (a) psitive wedge (b) negative wedge (c) flat type Fig Different frms f built-up-edge. In machining t sft and ductile metals by tls like high speed steel r uncated carbide the BUE may grw larger and verflw twards the finished surface thrugh the flank as shwn in Fig chip V C Fig Overgrwing and verflwing f BUE causing surface rughness

13 While the majr part f the detached BUE ges away alng the flwing chip, a small part f the BUE may remain stuck n the machined surface and spils the surface finish. BUE frmatin needs certain level f temperature at the interface depending upn the mutual affinity f the wrk-tl materials. With the increase in V C and s the cutting temperature rises and favurs BUE frmatin. But if V C is raised t high beynd certain limit, BUE will be squashed ut by the flwing chip befre the BUE grws. Fig shws schematically the rle f increasing V C and s n BUE frmatin (size). But smetime the BUE may adhere s strngly that it remains strngly bnded at the tl tip and des nt break r shear ff even after reasnably lng time f machining. Such detrimental situatin ccurs in case f certain tl-wrk materials and at speed-feed cnditins which strngly favur adhesin and welding. S 03 > S 02 S 02 > S 01 S 01 Size f BUE Cutting velcity, V C Fig Rle f cutting velcity and feed n BUE frmatin. Effects f BUE frmatin Frmatin f BUE causes several harmful effects, such as: It unfavurably changes the rake angle at the tl tip causing increase in cutting frces and pwer cnsumptin Repeated frmatin and disldgement f the BUE causes fluctuatin in cutting frces and thus induces vibratin which is harmful fr the tl, jb and the machine tl. Surface finish gets deterirated May reduce tl life by accelerating tl-wear at its rake surface by adhesin and flaking Occasinally, frmatin f thin flat type stable BUE may reduce tl wear at the rake face.

14 (iv) Types f chips and cnditins fr frmatin f thse chips Different types f chips f varius shape, size, clur etc. are prduced by machining depending upn type f cut, i.e., cntinuus (turning, bring etc.) r intermittent cut (milling) wrk material (brittle r ductile etc.) cutting tl gemetry (rake, cutting angles etc.) levels f the cutting velcity and feed (lw, medium r high) cutting fluid (type f fluid and methd f applicatin) The basic majr types f chips and the cnditins generally under which such types f chips frm are given belw: Discntinuus type f irregular size and shape : - wrk material brittle like grey cast irn f regular size and shape : - wrk material ductile but hard and wrk hardenable - feed large - tl rake negative - cutting fluid absent r inadequate Cntinuus type Withut BUE : wrk material ductile Cutting velcity high Feed lw Rake angle psitive and large Cutting fluid bth cling and lubricating With BUE : - wrk material ductile - cutting velcity medium - feed medium r large - cutting fluid inadequate r absent. Jinted r segmented type - wrk material semi-ductile - cutting velcity lw t medium - feed medium t large - tl rake negative - cutting fluid absent Often in machining ductile metals at high speed, the chips are deliberately brken int small segments f regular size and shape by using chip breakers mainly fr cnvenience and reductin f chip-tl cntact length.