An Insight into Grinding from a Materials Viewpoint

An Insight into Grinding from a Materials Viewpoint

CIRP Reports and News An Insight into Grinding from a Materials Viewpoint E. D. Doyle and S. K. Dean. Materials Research Laboratories. Defence Scienc...

669KB Sizes 0 Downloads 0 Views

CIRP Reports and News

An Insight into Grinding from a Materials Viewpoint E. D. Doyle and S. K. Dean. Materials Research Laboratories. Defence Science and Technology Organisation/Australia Submitted by L. E. Samuels (1)

Surnnary: t\ study has bl-' C'n mat.h.• o f thost:" aspects of th e metallurgical/eng ineering interface which relate to metal removal 1n grlndin b with in c rt!ds1.ug in-it!eJ u1 lh c grinding wht!cl. the results being interpreted in terms of the material behaviour

during grf.nding.

It is pointed out that there is a marked difference in the relation between depth of cut and wheel

in-feed on the one hand and d('pth of cut Olnd normal force on the other as the depth o f cut increases 1n both external

cylindrical and surface

grindin~.

It is shown that at large wh eel depths of cut chip formation, ploughing and sliding all

contribute In varyln~ de~rces to metal rcnvval and that temperature s generated 1n grinding can have an important effect on

chip formation.

Also, chip formation Is obs erved to be the maj o r mechanism of met,,1 removal as the wheel depth

of cut

is redu c .... d. This trend iN a c c ounted Cor in terms of the influ(! n c e of th e damaged la y er on metal removal proces ses and on the degree of mlcro-unevenne~M of the abra sive grit s . These ob s ervations of material behaviour are discussed in relation

to considerntions of specific energy 1n grinding.

I.

INTRODUCTI()N

A balan(c d approa c h to an und l' r s tanding of the total grinding syst e m must in c ludl~ .J knowledgl~ of the behaviour of

the workpiec e milterial during grinding. enl;lneer has not provlde cJ

Ll

To date, the mdterials

clJfDplete description a nd, while t h is

information 1~ mt~sLngt the production engineer remains l a rg<,I y unconscious of the influen ce of materials on the grinding process. On the other hand, thll: primary conc('rn of the production engineer is with removin g metal frum the workpiece

even though hls ultimate objective might be to produce hi gh qua lity surface fin(sh, high diruenaional accura c y or a hi gh stock removal rate. 11,18 study i s c oncerneJ with tho~c aspects o f the metlll1urglcal/e~1neerlng int e rface which relate t o

me tal removal In grinding wi th increasing in-f eed of the grinding wht!t'l, and Wilh the interpretation of this relati on in term.'1 of the materilll b(>haviour during grindinb'

2.

IlELATlllNS lIEnlEEN HETAL RJ-:~ll)Vm ANU 1~-t'EED IN GRINUING

2.1

Cylindrical l;rinding

in prc vi o u:-i studh' H by lil(" auth o rs (1,2), the rel.ati on bptween depth o r "ut LIonu whe,'l In-f,·(>d. f 11wHratf'd 1n Fig. I, was establishe d lor hardened stt.'t'l w(,) rkpiece s . nlree re gIons can be identified. In region A, no metal 1:-1 rt.!tnOved for

in-feeds of iess than 0.5 um.

The depth of cut/normal force relation is contained within

the depth of cut/wheel in-feed relation shown in Fig. I, since the difference between the wheel in-feed and the depth of cut is equal to the elastic deformation of the machine/wheel/workpiece system which, 1n turn, is proportional to the normal force .

This difference appears in Fig . I as the vertical distance between the depth of cut and the 45 0 line. The nomal force, therefore, can be calculated from Fig. I, given a knowledge of the stiffness of the grinding s ystem. The result is shown in HS. 2, whi ch indicates that a normal force of some 2N was necessary before matal 1s removed, and that the normal force

increased steadily to about 6.5N as the depth of cut increased to about 0.9 ~m. For further increases in depth of cut to over ) ~m. however, the normal force remained almost constant. Since the previous results referred to the grinding of only one type of workpiece material, it is important to consider the influence of a change in workpiece material. Consequently, the same cylindrical grinding procedure (1,2) was carried out on two other workpiece materials, namely, a medium-carbon steel in the annealed condition and a nickel of commercial purity. The results of these tests are shown in Fig. 3. The bphaviour of the medium-carbon steel during grinding was

In region B the wheel in-feed

Ib f

from 0 to 0.9 um, I .e. the increase in depth of cut was le8s than half the increase in wheel in-feed. Beyond wheel in-feeds of 2.7 urn, region C, the increase in depth of cut Is almost equal to the increase in ill-feed.

N

.

inc reases from 0.5 to 2.7 ~tD while the depth of cut increases

8

... (.)

a::

6

-' ..: :::E

4

0

2

... 0

-5

xl0 in

• Workpiece: M6 Tool StB!"1 Wheel : 38A 320 JBE

a::

z

1Jm 6

0

20

5

4

~

::l

Workpiece: M6 Tool Steel Wheel : 38A 320 JBE

00~L-L-L-L-~L-L-L-L-~~L-~~~pm I ~....I....----L.-"""'....I....--!-~-"""'....I....---l--"""'-:'l~~.l--LXI05 in 0 DEPTH Of CUT

(.)

"0

:z::

~

HII. 2.

3 10

CL

2

w

c

1

o

6

pm

~-L~~'-L'~'-L,~,~I~~L'~'-LI~'~-L~LJ-L~'~x10 5 in

10

WHEEL IN·FEED Relation b~tw"en depth of cut and wheel In-feed for cyUndrical grindlns wloll .. traverslnll with conotaM in-feed . Workpiece material i. HI> tool oteel.

Annals of the CIRP Vol. 29/2/1980

of cut for

similar to the 1'16 tool steel except that the slope discontinuity (i.e. the transition from regiona 8 to C) occurred at a lower value of wheel in-feed. Indeed, the two metals behaved in almost identical manner at wheel depths of Cut below I um. On the other hand, the behaViour of the nickel workpiece was quite different in that no region 8 was detected, although a region A was observed. These results highlight the influence of workpiece material on the relation between depth of cut and wheel in-feed, demonstrating that the slope discontinuities are related to the behaviour of the workpie~e materials rather than to some peculiarity of the machine/wheel system. 2.2

Fig. 1.

Relation between normal force and depth cylindrical grinding as in Fig. I.

Surface Grinding

Further tests were carried on a surface grinding machine to determine the effect of grinding technique on the relation berween wheel in-feed and depth of cut. Surface grinding, however, does not so readily lend itself to the same types of measurements. Consequently, the relation between normal force and depth of cut (which, as shown in section 2.1, is related to the wheel in-feed/depth of cut relation) was established by recording the grinding forces during spark-out. The workpiece material was again H6 tool steel which was attached to a piezo-

571

J.l

electric dynamometer mounted on the machine table.

-5

x10 in pm

.NI,ko' ~ o Medium carbon steel

6

20

WhH"'2

5

...

4

::::I (J

~

3

0

...a. 10

Large Wheel Depths of Cut

Scanning el~ctron ~icrographs of the grindin~ debris obtained from a medium-carbon steel at relatively large wheel depths of cut (i.e. corresponding to ret;lon C in Fig. 1) are shown In Figs. SA and B. In general, three types uf grinding debris particlt!s are distinguishable: (a) partlcl~s of metal with little or no characteristic shape or surface structure, (b) spheres and (c) chips.

.'

J:

2

w

0

0

_L __ L

00 I,

,

!

0

!

,

, ,

I

10

--L

lpm

3 5 , , , , , 4, , , I,

WHEEL IN-FEED

6 -5 , x10 in

20

Fig. 5. Fig. 3.

Relation between depth of cut and wheel in-feed for cylindrical grinding while traversing with constant in-feed. Workpiece materials are a nickel of commercial purity and a medium carbon steel.

In the spark-out process, the depth of cut Is not controlled directly but is a function of the decay of normal force with each spark-out pass. Consequently, from a knowledge of the system stiffness and the values of the average normal force for each successive spark-out pass one can estimate the actual depth of cut for each pass. The relation obtained between the average normal force and depth of cut for spark-out Is shown In Fig. 4. this relation was similar to that obtained for cylindrical grinding (see Fig. 2), again showing a slope discontinuity. Furthermore, it is evident from this comparison that the slope discontinuity occurs at very much smaller wheel depths of cut in surface grinding. This marked difference in the two grinding procedures can be largely explained In terms of the difference in the number of grit cutting edges which remove metal (active cutting edges). The number of active cutting edges is thought to be very much larger in the cylindrical grinding tests, because of the much finer control of the wheel dressing procedure. Also, the grinding method in cylindrical grinding was to progress from small to large wheel depths of cut, whereas the surface grinding tests were started at relatively large depths of cut (5 ~m) during which time the grits unavoidably pick up workpiece metal. thereby reducing the number of active cutting edges.

N

Ib f

0.3

1.2

...CJ .......0 iii:

1.0 0.2 Workpiece: M6 Tool Steel

c

:E iii:

0

z

0.1

0.4 0.2

0

Fig. 4.

3.

00 I 0

I 0.1

0.1 I 0.4 0.2 0.3 DEPTH OF CUT

I 0.5

Relation between normal force and depth of cut for surface grinding with spark-out. Workpiece material is M6 tool steel.

MECHANISMS OF MATERIAL

RE~VAL

Having established certain patterns of material behaviour in relation to wheel in-feed, it remains to provide a physical description of the mechanics of the proce8s. A convenient starting point 1s to examine the grinding debris and to assess, from its shape and surface structure, how this debris is generated.

572

Scanning electron micrographs of the grinding debris obtaIned from a medium carbon steel at large wheel depths of cut: A: general view of debris B:

three particle types are distingUished; particles with little or no characteristic shape or surface structure (arrow a). l::Ipheres (b) and chips (arrow c).

The first type of grinding debris particles, which are characterised by their absence of any distinguishing topographical features, are thought to be fanned as a result of the general rubbing wear occurrint; between the grits and the workpiece and by prow remova.l in ploughing. The rubbing wear aspect of the grit/workpiece interaction was recently highlighted by Turley and Uoyle (3) as being largely responsible for the poor surface finishes obtained In grinding at larlle wheel depths 01 cut with normal speeds and feeds. They re{err~d to the phenomenon as redeposition, that Is. the continual pick-up of workpiece metal by the grits and its redeposition back onto the ground surface. TI,is workpiece material that is picked up by the grits builds up with repeated rubbing Interactions and eventually foI1DS part of the grinding debris, either by becoming mechanically unstable and breaking off from the grits, or following breakdown of the grinding grits (i.e. wheel wear). Next, prow removal in ploullhing has been fully described by Doyle and Samuels (4). Rriefly, this refers to the fact that grits of high negative rake angle may not form chips but displace metal laterally on the workpiece surface. A prow then forms immediately ahead uf the grit and this prow will be removed as the grit loses contact with the workpiece surface. As pointed out by Doyl~ and Samuels, this is an inefficient means of metal removal.

The second type of grinding debris particles, namely, spheres, are formed as a result of chips or rubbing wear particles undergoing surface oxidation at such a rate that combustion takes place. It has been reported (5) that these spheres do not form when steel is ground in an stmosphere of pure nitrogen or helium under conditions which have produced abundant spheres in ai r. These spheres are more commonly observed as the familiar spark stream in grinding and, although the nature of the spark stream has been well understood for many years (6), investigators (7) stIll ml.t~kenly interpret the presence of spheres as an indicdtion that ~lting hilS taken place at the abra.lve/workpiece interface. TI,e third type of grinding debris, namely, chips, are identified by their characteristic lamellar structure (8). The surprising thing, however, is the large number of chips that form. The contacting abrasive grit. In a grinding wheel present largely negative rake angle. to the workpiece surface (9), and it is well established in slow speed abrasiun (9) that grits with negative rake angle. do not form chips in steel workpieces but displace ...,tal laterally with 11ttle metal removal. nle question that haa to be- answered, therefore, 121 why any chips are produced at all, let alone chips in such profusion. TI,i. is a fundamental question because the first objective in grinding is to remove materlal. The obvious difference in sliding .peed between grinding and abrasion suggests that strain rate and/or temperature may influence the critical rake angle at which the transition from cutting to ploughing occurs. It I. unlikely that strain rate alone 1s the domin.nt factor becau.e abraston And grinding are both carried out at relatively .lUll depth. of cut, where Larsen-Basse and Oxley (10) have demonstrated that strain rate Increases with decreasing depth of cut so that the strain rates

in bOlh pnln's s t.'~ wllulu uppt:.Ir to bt' speed . fh h Il',I Vl':-; Ll:lIlJ,h,' r,ltu1'4,' ,md the crLtil';.t t LIKl' ,Ing!&...·, lJ~lyh~ ..lad inVc!:iLigaL"J this ;t~Pl·l.:t by l.:.lrryint;

hlt!lL n· ~.tr.Jlt:~s llf sliding

its pl ,ss ibl ... Int

LUl~ llc~

on

S, uUtll,ls ( :,) lhlv ...• ,Jut !il,1\oj sp"''''d abrasiun expc:rimt!nls ,)II :-;lel,l ..... orkpll·L:~s hl·,lll'.J III ll'mpl:r.Hurt!s up Lo 4S0oC. Lllll ..~ Ch.lnhL' was l)iJservt'd up t(1 I.. U{)"C but at 4 50' ) ( they fo und lIJ :ll lll .., ...: rl1(l" .1 r. l k t ' .m b ll.· dl .lIlt:)l'd lh)m (1\..1 to _40 0 which, they subgc!':iLl'd, would h.I \' I ' ,I s lgnlfh: ;JI\L cfft:'t.:l un llIt'tal rcml)val. in addition, tlLt 'Y I\Oll,J ll&oIl l,u' bl' prl.,.....s JOllllt.'J .llll'ad lIt g,rlts wi til neg.at i Vt' t' .1)..,(' ~1I1~',l t · !-) bt' t Wt't'll _ ':'0 11 t lJ - lUI) ..1lld t ha t these pr()w~ tbenl.s l.')vt·s ,h't,·J .lS c ut tint; LVI}ls ot I!\lll'l' ldvllurablc rakt' an~le f ornd I\h lil i II r t hhon-ll kt' l.'hl ps •

Netalltlgr lnl~ ~'n .Illllllln~l Wllt'l' l .. It .. li.lr~L' Wlll!t'J ot."pth of cut; l.· X, tt·U .... tVl· fl.·..;t· ys t ..ll11z;'l!l11\ h . IS llt:t:url',,·J in th e chi p, Further, Fh.;. bU is .111 llPlil' ..ll ml(' r~l or~Iph slHlwin~ iI chip prouuceJ from a hibh SpCI.·J Htl.·el .... ~'I·kplt.·(:t' (:Oi2) gruunJ uIlJ"r !'>iml L.1r conditiuI19 Lt.) tlhlt oJ Fi~:. bA; tit" l>ulk I)t til" chip has t>tched darkly but ,""Ilil l· -I·tehlng h .mds .lr ..., IH .... ~t·l1l 11,\ thl;' rt"'git\ns uf primary ..mJ Sl,t..:t1Iul .. r y 6 h",', lr. Hils l."l..:hltl b t''''·:-, p, ' nst.-" s ugbcsts that tcmp,,·f' .:lltl f' l·H 01 ,lb' IUt t:SUU "1C II ,IV,,' tlet'l\ rea,:hl.·d in tlH;' bulk of th~ ddp and th,lt t ... . mpl~ f'aturcs l,l lLlUOoC or more haVe he-en reached in til ..• whltt·-t.·t c hin~ b.lnds.

wheel depths of cut

t

and this is

cov~rl:'d

in the next se c tion.

Small I.ll.e 1 Dep ths of Cu t

J. Z

Sinct! most investigations have been focussed on metal remova l

at relatively large depths of cut, there is little information

avallable on what happens at small depths of cut. A notable cxccptlun to this is the work of Hahn (lJ) who, In a Ct1nsideration of the nature of the grinding process from work done on the internal grinding of steel, concluded that the mechanism of metal r:~moval changes from chip formation to ploughing to rubbing with nu metal removal as the wheel de-pth of cut is reduced. Simi lar hyputhese~ have been arrived at by Okamura and Nakajima (12) and Halkin and Anderson (13).

1t

The present findings, however, suggest a different picture.

has been established that chip formation, ploughing and

all contribute in varying degrees to metal removal at largt:! wht:!cl depths oi cut, i.e. 1n reglon C In Fig. 1. However. there is a marked change 1n th e relation between metal removal and wheel in-feed as the wheel depth of cut is reduced (region B, Fig. I). rubbin~

~rindlng debris particles produced in region B, such as that Illustrated In Hg. 7, have a lamellar structure indicative of a

chip forming mechanism. In addition. the microstructure within each lamella is not resolvable and electron diffraction patterns from these areas show fine continuous rings in a bce sequence indicating that the chips are composed of extremely small grains

of ferrite.

A further study on the nature of this fine grain

structure. carried out be Cashion et a1 (14) using

M~ssbauer

spectroscopy, confirmed that the structure was basically bec

with an estimated grain size of 10 run.

Cl e arly, the retention

of such a fine grain structure is indicative of unusually severe deformation conditions, although no evidence could be found that

high temperatures had been generated. In this regard, the theoretical analysis of Hastings and Oxley (lS) predicts a marked drop in temperature with depth of cut during fine grindit\ll.

TI\e conclusions from these observations are that. as the

depth of cut is reduced, the grit/workpiece interactions change from those involving chip formation, ploughing and rubbing to

chip furmatlon characterised by a marked lamellar structure and This trend, although contrary to some a very fine grain size. previous suggestions (11, 13), is not without precedent in metal r~moval op~ratlon8.

For example, Doyle et al (16) have

demonstrated that when machining a pre-machined workpiece surface the depth of cut can have a significant effect on chip formation. Thi. effect was shown to be related to the structure

Fill.

b.

Optical

mlcrok:uph~

t~n..,crllturt'H

A!

n:

o( chips Lormed at high workpiece

:

chlp fUI'mt!d from a IZK'dlum ca.rbon steel workp1t'cl! durln~ ~rtndlng with an alumina ",he~l "howlng r.xtentllve n'c f)'tI t.u 111 za t 10n l~hip

formed {rum u

hl~b

t:fpceJ

workpi.~ce

durlnll Krlndlnll with un alwnlna wh".l "howln~

... dark

~tchlng reHron~e

ln bulk

of th" chIp Indlc"tlnll ovcrteluperlnll.

...,hltC'! etching r~tlpon~u~ ln tht! primary and th.' cundary .. hear ZOntH. lndlcateti that tl!wplt?ratureM ln rei-Lon of LOUOoe were

111C

attained durlnk: grinding.

TIlU•• the tcmp,~r"turtl at the w4Jrkpl~t.:~ Murf8c~ could have an important lnf JUI.·nce on m~tal n:nnoval at large wheel depths of cut. lhJWtiver, healln~ and cool1n, of the durtace would occur very tllpiJJy amJ this may hAVe other inC tuences such 4s

produclnll pha •• chank: •• and/or 1I
alternlltively,

the rt!stdu.a.l wtre,HI pattern may be dumKeJ froUl compression to ten.lon. Thl! p&ttt!lrn. huwl!vt!r, 1. nOL cl,.lIl1plettl alnce the

pre8ent

M\lthor~

huvt!' found that chlp form..atlon increaaea with

decreosinv, wheel ..It·pth 01 cut while the tempen.tures genera.ted durins ~rlndln~ apPd~r tu be d~creA81ns. It turns out thut there 1" a lurtla·r cun .. ldcrdltt..>n which beel_DeS apparent at Bmall

Hg. 7.

Transmission electron micrograph showing segmental chip-type debris. Insert, typical ele"tron diffraction pattern obtained from the segmental chip-type debris showin& a continuous ring bodycentered cubic diffraction pattern.

of the damaged layer produced aa a consequence of the preceding machining pass. The general structure of the damaged layer produced on surfaces machined by all mechanical means has been fully described by Samuels (17) and is illustrated schematically in Fig. 8. The frllgmented layer is contiguous with the surface and is a layer that haa been very severely deformed resulting in a relatively fine 8ubgrain structure. TIle remainder of the damaged layer consists of material differing from the matrix only in that it haa been plastically deformed by comparatively modest amounts; this has been termed the deformed layer. Doyle et al (16) demonstrated that when ~-brass was machined using a large depth of cut in comparison ",!th the depth of fragmented Layer the chips tormed had a very low shear plane angle. However, when the depth of cut was of the aame order as the depth ot fragmented layer, the shear plane angle was much higher, i.e., there was a significant improvement in Chip formation.

573

o

Finally, the fact tllat redion H was nut ob~crved wilen grinding nickel is probably related to adhe~iun occurring between the alumina grits and the freshly exposed nickel surface once the initial oxide film has been penetrdted. This suggestion is supported by the fact that tile best ~urfacc finish value that could be obtained by fine grinding nickel .....as 100 n:n CLA whereas with steels values of less than 10 nm CLA were achieved, indicatin b that the adhesion between the alumina grits and freshly exposed iron was much less than wi til nickel.

~Surface ~T~ Fragmented Layer

5 10

~7.··.·ff·r·····.·.,··· 5% Compression

VV ' ' -",,'"

UM

~talkin and Joseph (22) have pOinted out that an understanding of the origin of the grinding energy can provide the basis for a physical description of the mechanics of the grinding process. The relation of the present findings to th~ variation of specific energy with decreasing wheel depth of cut will now be considered.

Depth

15

pm

The specific grinding energy (u) is given by:

~

Elastic-plastic Boundary

75

bdv where V is the wheel velocity, v the workpiece velocity, b the

width of cut and d the depth of cut.

Fig. 8.

Diagrammatic illustration of the general structure

of the damaged layer on a surface produced by

mechanical means, drawn as a transverse taper section. A similar effect may be occurring in grinding.

for

Values of specific energy

for di(ferent values of wheel depth of cut were obtained from the horizontal force cocponent (F ) recordt.. d during spark-out in surface grinding and are shownHin Fig. 9. Th~ relation shows that specific energy increa:ies with decrease in dt"pths of cut. which Is In accord with the general expectation.

The variation in specific energy over a wide range in depth

of cut is shown in Fig. 10.

This is a log-log plot of the

results shown in Fig. 9 and those obtained by

~alkin

and

example, Turley and Doyle (3) have shown that for surface grinding ~-brass at wheel depths of cut of 12 urn the average depth of the fragmented layer will be of the order of 2-3 um.

Joseph (22) for grindinll steel at larger wheel depths of cut.

in-feeds of 2 urn would mean that the individual grit depths of cut would be well within the depth of the fragmented layer, and hence increase the likelihood of chip formation even with highly negative rake angle grits. In reality, of course, these finer wheel depths of cut will produce smaller depths of fragmented layer which, in turn, have finer substructures. With regard to the latter, Turley and Doyle (18) have recently made a study of the dislocation substructures produced in chips and workpiece surfaces by various machining operations and have shown that the size of the subgrains within the fragmented layer is reduced when the undeformed chip thickness is reduced. In region B, it would appear that this process is reaching a limit since it is difficult to conceive of grains much smaller than 10 nm.

wheel depths of cut for steel and the specific energy rises with

In this particular case, further grinding passes of wheel

The observed trend for chip formation to increase with decrease in wheel depth of cut can be satisfsctorily accounted for in terms of the influence of the damaged layer on the metal removal process. It must be emphaSised, however, that the frequency distribution of the rake angles of abrasive grits at these very small wheel depths of cut cannot be quantified. Clearly, the width of the fine grinding chip, shown in Fig. 7, is an order of magnitude smaller than the nominal grit size of the grinding wheel, indicating that the active cutting edges represent only a very small fraction of the area of a single abrasive grit. In addition, Dean and Doyle (19) have shown that for fine abrasion on M6 tool steel an abrasive cutting tool with a high degree of micro-unevenness will produce an appreCiable quantity of segmental chips, while a smooth sphere causes plastic flow with no detectable amount of metal removed. It may be, therefore, that for abrasive grits with a high degree of micro-unevenness the mean of the rake sngle frequency

Considering the many differences in the exp~rimental conditions, the two sets of results are 1n good agreement. The results indicate that there is a minimum in the specific energy at large decreaSing depth of cut, eventually tending towards an upper

limit at the smallest wheel depths of cut.

The above trend of rising specific energy with decreasinll depth of cut was originally attributed by Backer et al (23) to the likelihood of having few strength-impairing dislocations

avai lable in a very small chip volume. Recent research (24), however, on carefully characterised metal surfaces gave no evidence of the very high strengths to be expected from a dislocation-free lattice.

--

o 2·0

x

...Ie ~

>"

C)

0::

w

z w

1'0

distribution may be reduced to much smaller negative values,

o

and hence contribute to the increased likelihood of chip formation at small wheel depths of cut.

The observation of metal removal by chip formation in

region B does present the anomaly that the efficiency of metal removal in this region is low. It has previously been suggested (20) that this low rate of metal removal is related to the fact that the workpiece surface possesses very high

shear strengths by virtue of its fine grain size. There is, however, a further possible influence, namely, that of oxide formation during grinding. Recent work by Pethiea and Tabor (21) indicates that the local contact pressures, in the presence of oxide films about 5 nm thick, approach the theoretical strength of the substrate. Oxide films of this thickness would, in all probability, be forming continually in region B, particularly as there is nO evidence of high temperatures being

generated. Also the load per active cutting edge in cylindrical grinding in region B must be very small, hence the influence of these thin oxide films could be quite marked. Pethica and Tabor (21) also found that the load-unloading cycle at low loads is almost completely reversible prior to penetration of the oxide filma. This correlates remarkably well with the observation of a region A in Fig, I, which clearly implies some reversible or elastic interactions between the grits and

workpiece.

Fig. 9.

o~

0·05 0·10 DEPTH OF CUT, J.lm

I

0·15

Relation between specific energy and depth of cut for steel obtained und~r Hpark-out conditions.

More recently, Malkin and Joseph (22) have explained the trend by pointing out that the specific energy in grinding can be partitioned into chip formation, ploughing, and sliding energy components. They suggest that, at larKe stock removal rates, the ploughing and sllding energy components become

negligible and the minimum grindini energy ia equal to the specific chip fo.,...tion energy. It is ... s..-d that the ploughing and sliding components make an increasing contribution to the overall specific

eni!r~y 0111

the depth of cut is reduced.

The observed minimum in grindinil enefjlY WAS accounted for by showing that the specific shearing energy during chip formation in grinding is apprOXimately equal to th~ specific meltin~ energy,

which is the maximum allowable value.

The present results auggest the following alternative interpretation that the general trend of rialng specific energy

with decreasing depths ot cut 1. related to the fact that the

abrasive grits are defOrming increaSingly smaller deptha of

574

o

...

'e -, 12

>" 10 to!) 0:: w Z w

10

o DEPTH OF CUT, nm

Fig. 10. Lug-lu~ plut of the present results with those of Malkin and .Josl.'ph

sp('ciflc ('ncrgy

(~2)

l.wt.~r

K.S. Hdhn. Un the Nature of the Grinding Process, Proc. Third Mach. Tool lles. Res. Conf., 1962, p. 129.

12.

K. Okamura and T. Nakaj ima, The Sur face Generation ~lechanics in the Transitional Cutting l'rocess, in "~ew Developments in Grinding", ed. ~l.C. Shaw, Carnegie Press, 1972, p. 305 .

13.

S.

14.

J. Cashion, R.L. ;\ghan and E.D. Doyle, A Hossbauer Study of Fine Grinding Chips, Scripta !-tetallurglca, Vol. 8, 1974, p. 1261.

15.

W.L ilastings and P.L.J. Oxley, ~Iechanics of Chip Formation for Conditions Appropriate to Grinding. Proe. Seventeenth ~Iach. Tool Des. Res. ConL, 1976, p. 203.

lb.

E.D. Doyle, D.H. Turley and S. Ramalingam, Microstructural Phenomenology of Chip Formation, Fourth Tewksbury Symposium, Melbourne, 1979, p. 161.

17.

L.L Samuels, Hetallographic Polishing by Mechanical Hethods, 2nd edn. Pitman, London. 1971.

18.

D.H. Turley and E.D. Doyle, ~Ietallurgical Aspects of the Size Effect in some Chip Forming Processes. To be Published.

19.

S.K. Dean and E.D. Doyle, Significance of Grit Horphology in Fine Abrasion, Wear, Vol. 35, 1975, p. 123.

20.

E.D. Doyle and R.L. ;\ghan, Mechanism of Metal Removal in Polishing and Fine Grinding of Hard Metals, Metallur~ical Transactions, Vol, 6B, 1975, p. 143.

21.

J. Pethica and D. Tabor. Contact of Characterised Metal Surfaces at Very Low Loads: Deformation and Adhesion, Wear, Vol. 89, 1979, p. 182.

22.

S. Malkin and N. Joseph, Minimum Energy in Abrasive Processes, Wear, Vol. 32, 1975, p. IS.

23.

W.R. Backer, E.R. Marshall and M.C. Shaw, The Size Effect in Metal Cutting, Trans. ASHE, Vol. 74, 1952, p. 61.

24.

M. ilatherly and A.S. Malin, Deformation of Copper and Low

DOYLE and DEAN

• MALKIN

10

11.

showing thc variation in

and R.B. Anderson, Thermal Aspects of Grinding, Purt 1 - Energy Partition. ASME Paper 73 - WA/PROD. I, 1973.

~1.11kin

a wide rang(' In depth of cut.

fragmented layer which have increa.singly finer Rubstructures. Thus, the m.lximum Hpeclfic cncrsy may b(' governed by the llmitin~ yub-guln size which, as indic/Hed earller, is unllkely to be much fincr than the value of 10 nm repoftt>d by Cashion et al (14) fur fine grinding of oteel. It is noteworthy that recent reHearch (2J) on mlcrostructurea ClHsociated with large

strain defonnntion indicates that fine scale Rubstructures are produced in Rhcar bands in mctalH and that large plastic strains are acconunoda.ted by these bands. In fal.:t, shear strains of 50 to LOO have b('f:'n rt..'portt.~d (25) for Home shear bands indicating that the energy reqUired for plastic defomation of these fine grained shear bands i. very high. Finally, the minimum in grinding energy at large wheel d"l'ths uf cut certainly appears from pr~sent resu1 ts (~ee Figs. 6A and H) to be related to the influence of temperature; the shear resistance of the metal workpiece will fall sharply at temperatures not very much above one half the melting point.

Stacking-Fault Energy, Copper-Base Alloys, Metals Vol. 6, 1979, p. 308,

Technolo~y,

25.

L.E. Samuels and I.R. Lamborn, Failure AnalYSis of Armament Hardware, Metallography in Failure Analysis (1978) Ed. J.L. McCall and P.M. French, Plenum Publishing Co. New York.

REFERENCE~

1.

S.K. Dean and E.n. Duyle, Mechanisms In nne Grinding, Proc. Int. Conf. Prod. Eng .• Tokyo, Japan. 1974, Part 1. p. 655.

2.

S.K. Uean and E.ll. Duyle, Preci8ion Crinding without Sparkout, To be publi6ht~d in Proc. Int. Conf. on Manufacturing Engin~cring, ~tclbourne, Aug. 1980.

3.

D.H. Turley and Eoll. Duyle, Factors Affecting Workpiece lleformation During l;rinding, Haterials Science and Engineering, Vol. 21, 1975, p. 2bl.

4.

I.D. Doyle and L.E. Samue!Y, Further Developments of a Model of Grinding, Proc. Int. Conf. Prod. Eng. Tokyo. Japan. 1974, Part II, p. 45.

5.

H. Letner, Hodern Perspective of Grinding Process,

6.

E. PitoiH, SpurkinK of Steels, The Chemical Publishing Co.,

7.

P.J. Strachan, lliamond Grinding wi th Resin Bonded Wheels, tl "New llevelopments in Uiamond Grinding and Too11ng • lRt Ind. Diamond Seminar, Sydney, 1975.

8.

J. T. Black, Shear ~'ront-Lamella Structure in Large Strain Plastic Deformation Processed, Trans. ASHE, J. Eng. Ind. Vol. 94, 1972, p. 307.

9.

A.J. Sedriks and T.O. Mulhearn, Mechanics of Cutting and Rubbing in Simulated Abrasive Wear Processes Wear Vol. 6, 1963, p. 547.

and finishing, Vol. I, 1955, p. J6.

~rindln&

Eastun, U.S.A. 1929.

10,

J. Larsen-llasse Hnd I'.L.II. Oxley, Effect of Strain Rate Sensitivity on Scale Formation in Chip Formation. Proc. TIlirteenth Mach. Tool. Des. Rea. ConL, 1973, p. 209.

S7S