Indian Journal of Engineering & M aterials Sciences Vol. 10. Apr il 2003. pp. 123- 130

Effects of tool feed rate in single point diamond turning of aluminium-6061 alloy

Gufrall Sayccu Khan;!, Rama Gopal V Sarcpaka". K D Chattopacl hyay". P K lain" & V M L arasimham"

"Advanced Optica l Processing Di vision. hlndo-Swiss Training Cen tre Central Sc ientilic Instruments Organ isati on. Sec tor 30-C. Chandigarh. 160030. India

R('('('il'ed 21 .lIllIe 2002: a('cepted 14 .IallllillT 2003

During single po int dia illond turning. the effects of too l feed rate on surface fi gure and fini sh o f aluminiulll -606 1 alloy are eva luated. The studi es are conductcd on 60 mm diameter optica l fl at surfaces. Twenty-s ix specimens are diamond turned for va ry ing too l feed rates (0.3 ~lin/rev to 30.0 ~lin/rev) keep ing all the other machining IXlrameters constant. The alumin iulll surfaces thus machined. show a typica l roughness of 15 nm. ,md surface fi gure of better than a half wave peak-to-va lley (PV) for optimum too l feed rate. The tool feed rate between of I .S-8.0 ~lin/rev is fou nd to be optimum for a given sc t of other machining parameters to get the acceptab le surface fi gure as well as roughness va lues. A n empirica l forilluia w ith best fit has been formulated to find out the practica l ro ughness ach ievab le for the machining condit ions under study.

Single point diamond turning (S POT) of optics can be defi ned as the use of a diamond tool on a precision lathe under very precise ly controlled machine and environmental conditions to fabricate a fi ni shed opti cal co mponent!.:!. As is in the case of turned surfaces, SPOT also yields onl y rotationall y sy mmetri c surfaces. albeit of op ti cal quality'.

The diamond turning machine is sophi st icated eq uipment that produces the final surface, which typica lly docs not need the trad iti onal poli shing operation. There are advantages of using diamond turning: incl uding the abi lity to produce good opti ca l surfaces up to the edge of th e element , to fabri cate soft ductil e materials that are difficult to po li sh , and to fabrica te shapes that are difficu.lt to do by other methods. Thi s process produces finished surfaces by very accurately cutting away a thin chip or layer of the surface . It is generall y app licable to ducti le materi als th at machine well rather than hard brittl e materi als traditi onally used for opti cal elements. However, by usi ng a gri nding head on a diamond­turning mach ine in place of th e too l, hard brittle material s like glasses and ceram ic. can be machined~.

In diamond turning. the intended shape and surface produced depend on the machine tool accuracy and other machining parameters. In traditional optical fab ricat ion, lappi ng and poli shing wi th an abras ive­loaded lap produce the final shape and surface of optical elements. The fundamental di ITerence bet ween diamond turning and traditional optical fab ri cation is that diamond turning is a di splacement-contrnlled process while conventional optical fab ri cation is a

force-co ntroll ed process. The operating parameters of a prec ision machining

process on a g i ven pi ece of material wi II vary considerab ly depending on production rates req uired, work-p iece and machine characteri stics, and all ot her process variables such as coo lant, too l cond iti on. depth of cut, tool feed rates. Tool feed rates, cutting speeds, and depths of cut are typicall y much lower in diamond turning process compared to turning with conventi onal machining too ls. For a given material, under different co mbinations of machining parameters (w ithin their optimum range), similar surface figure and fini sh results ca n ofte n be obtained. The main machining parameters are too l feed rates, spindle speed and depth of cut('. The tool feed rate is normall y expressed in terms of either di stance tra ve ll ed by the too l per unit time (mll1 per min ) or di stance travelled per unit rotation (mm per revo lution). It is most co mmon to see the di stance per revo lution as it is direct ly related to the anticipated theoreti cal surface finish. For a given too l feed rate, larger the tool nose radius. lower the roughness and better the optical surface fini sh. The surface quality depends to a great ex tent on the materia l characteri sti cs like: grain size, microstructure of crystal boundary, crys lal uniformity and ann ea ling procedures adopted 7

.

In thi s paper, the effec ts of tool feed rate on the surface figu re and surface roughness are studied under similar mach ining conditions. The materia l under study is Aluminium-6061 all oy. It is proposed that for optical quality !lat componen t fab ri cation, it is always advisable to explore ancl optimise the tool feed rate

124 INDIAN 1. ENG. MATER. SCI. . APRIL 2003

range and other corres ponding machining parameters before batch production.

Machine Description Base machine

In thi s study, the work-pi eces arc turned on Nanoform-250, a preci sion diamond turning Illachine fro lll Taylor-Hobson (Fi g. I). The machine is having 8.6 nm positi on feedback with the X hori zontal strai ghtn ess 0.30 ) .. lin over 350 Illm traverse. Z horizontal strai ghtness 0.20 ~lm over 250 mill traverse, X ve rti cal strai ghtness 0.75 ~ll1l over 350 mm traverse, Z vertical strai ghtness 0.50 ~lm over 250 mm trave rse, and X to Z squareness 0.50 arc seconds.

L VDT tool set station

The LVDT tool setting stati on is used to adjust the height of the di amond too l as well as X and Z pos itions relati ve to the spindle centreline. The ve rti cal LVDT air-bearing probe is used to accurat ely set diamond too l height. The hori zontal LVDT air­bearing probe is used to autolllati ca ll y calculate the di amond too l's nose radiu s and relati ve X and Z pos iti ons, in relati on to the spindle's ce ntreline.

Iiero height adjust tool holders

The mi cro-height adjuster incorporates an adjustment mechani sm for initial loca ti on of the tool , and in additi on, accurately es tab li shes the exact height 01" the too l relati ve to the centreline. The tool ho lder asse mbl y includes a Venturi type chip ex tracti on system uti I izi ng the co mpressed ai r be i ng suppl ied to the machine. A seco nd too l holder with height adjus tment , fitted alongs ide the firs t too l holder. permits simultaneous set-up of two tools. One too l

...

Fi g. l - Nanororm 250 lathe (Taylor-Hobson)

can be used for rough cuts and to produce any reference surfaces, while the other tool is used for finishin g the optical surface.

Spray mist coolant system

Most diamond machinable materi als require a spray mi st coolant of so me type. For those material s (like PMMA). whi ch do not require a spray mi st coo la nt. a hi gh-pressure jet of air can be supp li ed fro m thi s system.

Single-crystal diamond tool

When combined with an ultra- prec ision vibration­free machine, a compact ri gid too l ho ld er. stable and well -balanced fi xturing, single crys tal natural diamond cutti ng tool s wi II remove material fro m substrates cleanl y and effec ti ve ly. Because of the ex treme level of sharpness on the diamond-cutting too l, very slllall forces are ge nerated during the machining process. The end result is a surface th at ex hibits opti cal qualiti es in both surface fini sh and form accuracy.

Extreme leve l of sharpness , minimal tool wear. cutting edge quality, edge waviness control. low coeffici ent of fri cti on and thermal ex pansion and high thermal condu eti vi ty, make di amond the bes t cUlli ng too l for ultra prec ision machining. There are two types of too ls: with control led wav iness and non­controll ed wav iness at the too l tip . Controll ed wav iness di ctates that the radius shape o f" the tool ti p dev iates from a true circle by a guaranteed va lue ranging between 1.0-0.05 mi crons.

[n these inves ti gati ons, single-crys tal natural cliamond too ls ha ving a 0.5 mm nose radius. 100° arc and 0° rake angle are used. The wav iness o f" too l is des ignat ed to be 0.40 ~m.

SUI-face Eva[uation There are three types of errors that may occur on

the machined surface, i.e., f"orm, fi gure and fini sh. [t is impossible to say, at what point does fini sh error become fi gure error. [t is better to separate fini sh. fi gure and form according to their cause, as thi s relates to the performance factors. Roughness is due to the irregularities, which are inherent in the production process (e.g. cutting tool and feed ratex

) . The roughness also depends on the materi al composi tion and heat treatment 7.'). Figure error may result from vibrations, chatter or work deri ee ri ons and stra ins in the material. Form is genera l shape of the surface, neglecting variations due to roughness and

KHAN el 01.: EFFECTS OF TOOL FEED RATE IN SINGLE POINT DIAMOND TUR ING . 125

fi gure error. The most commonly accepted parameters to evaluate surface figure and surface finish are peak­to-va lley and roughness (Ra) respecti ve ly. The term peak-to-valley is the di fference between the highest and lowest poi nts in any surface trace, whereas Ra is the arithmetic mean of the departures of the profi Ie from the mean line over the sampli ng length II). There are other stati st ica l parameters for roughness measurements like Rz., Rq , skewness, and kurtos is also l l

. Rz is the average of f ive highest peaks and the average of five lowest valleys. Skewness describes the asy mmetry of a profile, while kurtosis describes the peakedness or spikiness of a profile.

A standard Form T alysurf Series 2 (PG I) Profilometer from Tay lor-H obson is used for the surface evaluation. A diamond conical sty lus havi ng

the tip radius 2 J1.m and 10 mm vertical range is used to measure the figure error and surface roughness. Droughts and airborne vibrations are avoided during the measurements. The profi lometer is mounted on an epoxy granite construction on anti-vibration mounts, and provides a finn support for the column and work­piece. Measurements are taken w ith the sty lus movement speed of 0.5 mm/s.

Theoretical surface fini sh during tuming

The resultant roughness produced during turning operation is due to the combinati on effec t of two independent quantities such as ideal roughness and natural roughness. Ideal roughness is due to the tool geometry and its feed . It is a geometri ca l phenomenon and is the minimum possible magnitude of the unevenness, which results from a machining operat ion.

Movement of a cutting tool across the surface o f the turning component produces the diamond turned surface, therefore, it always has some period ic surface roughness. The surface texture is directly related to the combination of the tool shape and radius, and the tool's path over the su rface. The machine ' s operating cond itions affect this surface f igure and fi ni sh in the direct quan ti ta tive manner indicated in the theoret ica l sur face finish equat ion given as4

:

Maximunt he ight or unevenne ss

(feed/rev) 2 =

8x (tool nose radi us)

." ( I )

In add ition to the theoretical fini sh based on the ' cusp' structure (Fi!! . 2), the measured surface fin ish

on di amond-turned parts is influenced by other factors such as: slides straightness error, sp indle rotati on error, external and self-induced vibrations, material impurities, roughness of the edge of the tool.

Natural roughness in actual turning operation is the result of formation of a built up edge and vibrati on, which adversely affect the surface finish. When the cutting conditions are properl y chosen, the vibrati on may be avoided. Since the built up edge formati on depends whether coolant is being used or dry machining is performed and the proper cutting speed is used or not, it is expected that natural roughness to vary with actual cutting speeds.

Cutting speed can be defined4 as the speed by which the tool moves over the length o f th e job per unit time.

n x D xN Cutting speed (m/mi n) = ----

1000 .. . (2)

where D is diameter of the work -piece in mm, N is sp indle's rotational speed (rpm).

In the experimental set-up, as the cutting tool is approach ing nearer to the centre, the CUlling speed becomes lower as the diameter is approach ing zero. It is expected that for a given set of cutt ing conditions, the natural roughness w ill vary with the CUlling speed. It is also expected that except for low cutting speed. the intensity of built up edge formation decreases with the cutting speed, so the maximum height of su r face unevenness is also expected to decrease with the cu tting speed.

Experimental Results Twenty-six alu1l1inium-606 1 work-pieces. each o f

60 mm diameter are . ingle-point-diamond-turned to study the effects of tool feed rate on surface f igure and roughness. A 11 the speci mens are machi nt'd under

Fig. 2-Expanded view or "Cusp" surface of dianlll lld- llII'ncd opti c:." e k ment

126 INDIA J. E G. MATER. SCI.. APRIL 2003

the same conditions with the exception of tool feed rate. The tool feed rate is selec ted to vary from 0.3

11m/rev to 30.0 I1mlrev. The depth of cut is kept at 2 11111 and spindle's rate at 3000 rpm. The depth of cut

was vari ed from I I .. un to IS 11111 , and sp indle's rate frol11 1000 rpm to 5000 rpm. It was observed th at vari ati on of depth of cut does not affect the su rface roughness much but it affects the surface figure. The

depth of cu t of 2 11m and sp ind le's rpm 3000 is found to be opt i l11 unl for 60 mm dial11eter aluminiul11 work­piece to get the desired surface roughness and surface figure. All the specimens are (urned while tool is moving fro m edge to centre. The surfaces thus turned are evaluated for their surface fi gure (P-V and rms P­V) and surface roughness (Ra). These results are presented in Table I . The practical optimum range of tool feed rate is found to be 1.5-8.0 )..lmlrev. The surface finish is almost constant in this range. Average of the rough ness in thi s range is around I S nm. Standard deviation from the mean roughness is 1.24824. The surface f ini sh is affected signifi cantl y outside thi s prescribed range. This variat ion occurs due to process conditions e.g., v ibrati on, chattering, built -up-edges, and wear oul at tool tip . Figs 3a, 3b and 3c show the surface figure P-V at the tool feed

rates of 0.3 11m/rev, 1.0 11m/rev and 4.0 ~lIn/rev

respectively. At 0.3 I1l11lrev the surface figure is

1.9480 11m (nns PV 0.5796 ~lm ) and it improves at 4.0 I1mlrev to 0. 1882 )..lm (rms PV 0.0428 11m ). Figs 4a, 4b and 4c show the roughness at the tool feed

rates o /" 0.3 )..lm/ rev, 4.0 )..lmlrev and 30 )..lmlrev respect ively . At 0.3 11m/rev tool feed rate, the

2.0

15

1 0

05 V> c e 00 ~

·05

·10

·15

·20

roughness was 26.5 nm and it improves at 4.0 ~lInlrev

to 13.6 nm and deteri orate after 8 I1mlrev. Fig. 5 shows the surface fi gure (P-V and its rms value) versus tool feed rate.

Tah le I- E ITecls of feed rale on surface fi g ure c,nd surface lini sh

Feed rale

( ~l11/rev)

0.3 0.5 075 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 X.O 10.0 12.0 14 .0 15 .0 16.0 IX.O 20.0 22.0 25.0 30.0

Su rface fi g ure (P-V)

( ~lIn)

1.!l322 1.1 !l54 0.878 1 0.6884 0.423D 0.3439 0.3625 0.4270 O.30n 0.2228 0.2558 0.248 1 0.234 1 0.5963 0.4790 0.3205 0.2782 0.2 199 0.2474 0.2875 0.3367 0.2725 0.3254 0.310 1 0.3299 0. 1976

Surface figure

( rlns 1'- V )

(~lIn J

0.551 3 0.2832 0.26 18 0 .1 657 0.0569 OJlS09 0.0656 D.I 037 0.0665 0.0462 0.0568 0.0671 D. IOn 0.1 534 0.1 329 0.06!l6 00596 O.D504 0.0525 0.060 I O.OXM 0.0572 0.0697 0.0702 O.OMI 0.0347

R()ughlles~

(in nl11 over 5.6 111111 range)

25.30 18.05 19.25 18.XO 17.60 16.35 16.28 16.·Hl 15.33 14.45 14. 10 1390 14.10 13.55 15.DO 15.20 17 .35 19.55 24.35 24.DO 26.65 30.90 35 .30 40.45 49.70 65.70

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 11 5 120 125 millimetres

Pa

1, 9480 urn

o. 5196 urn I

Fig . 3a-Surfaee fi g ure (1'- V) a l feed rUle 0.3 ~lInlrev

'" ~ .~

Ra

KH AN et at.: EFFECTS OF TOOL FEED RATE IN SINGLE POI T DIAMOND TUR ING

p,

~ c e

.~

P.

1.2

1.0

08

06

04

02

00

-0 .2

-0 .4

0.5 0.5

04 0.4

0.3 0.3

02 02

o 1 01

00 00

-0 1 ::} -01

-/ -02 -jr.

/ -0;

// ./

-03 .·f -0 :

25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 millimetres

0.1351 ut: :: :::: t: I Fi g. 3b- Surfaee fi gure (P-V) at feed rate 1.0 ~t11llrev

04

03

0.2

01

00

-0 1

-0 2

/ -03 -03

-04 -0.4

30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 11 5 120 125 130 mllhmetres

0.Ol69 ut; Fig. 3e- Surfaee fi gure (P-V) at feed rate 4.0 ~11l1/ rev

~-------~--'--------'----------/0/;-------~ 12

/

./ 10

/ /.

08

06

/ --

" /. ~'. - 02

t-----------~·~~~~~~~~~~~~~~~~/T·c~C-------_+OO

25 30 35 40

o. 0265 Uml Rg

/ ./ --:.

/

45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 millimetres

o. 0395 um I

Fi g. 4a- Surfaee roughness at feed rate 0.3 ~lInlrev

-02

-0 4

127

128

1.8

.. 1.6 c: ~ 1.4

I 1.2

~

" '"

(I) c g f:

Ra

04

03

-0 .2

-03

04

0.3

0.2

0.1

0.0

-0 .1

-0.2

-0 .3

-0.4

INDI AN J. ENG. MATER . SCI. , APR IL 2003

./ ~ I

.. j I

/ 1

'j-'

/ ;,-

.... ;

30 35 40 45 SO 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 millimetres

O. 0118 vm I Fig. 4b-Surface roughness at feed rate 4.0 J..llll/rev

-35 -30 -25 -20 -15 -10 -5 5 10 15 20 25 30 35 40 45 50 55 60 65 milllmetres

o. 0768 urn I

Fig. 4c- Su rfaee roughness at feed rate 30.0 J..llll/rev

Relationship of surface figure with feed rate

0 .4

03

0.2

0.1

00

-01

-02

-0 .3

04

0.3

0.2

0.1

0.0

0.2

03

Comparison of Theoretical and Pract ical Surface finish

-+- Peak-Ie-Valley

i--Peak-Ie-Valley (rms) 1000

i 100 j e u 10 I

..........- Theo~et jCal finish I r --- Practical fimsh

u:: 0.8

j 06~' ., 0.4

0: _ ~ -+-+-:: ----; • 30

.!1 ~

~ u. '0 0.1 o ....

om I

10 15 20 25

Feedrate (microns/rev.)

Fig. 5- Sul face figure (P-V ;md rms va lue) versus tool feed rate

35 0.1 10 100

Surface Fi nish (microns)

Fi g. 6- Theoreti cal and prac ti ca l surface fin ish versus tool feed rate

KH AN ef al.: EFFECTS OF TOOL FEED RATE IN SINGLE PO I T DIAMOND TU RNING 129

A compari son o f the theoreti cal surface fini sh and the obtained surface fini sh is presented in Table 2. It is observed that smalle r the tool feed rate, bette r the surface roughness. The surface roughness, however, becomes almost constant when the tool feed rate is between 1.5-8 .0 /-lin/rev as shown in Fi g . 6. The roughness va lue increases as the too l feed rate goes

dow n to I /-lin/rev or less. Beyond 8 /-lin/rev tool feed rate theoretical roughness is hi gher than practical one. There fore fro m Fig. 6, it can be conc luded that theoreti ca l formul a does not ho ld good beyond 8.0

J..un/ rev tool feed rate.

Empir ical fo rmula for practical roughness

An empiri cal fo rmul a is deri ved fro m the prac ti cal surface fini sh versus the tool feed rate (Fi g. 6 and Table 2) as:

( 2 Roughness = A + a~ -' -

- 8R

a l where A = a o +-f

... (3)

... (4)

Table 2-Comparison of theoretica l and obtained surface fini sh

Feed rate Theoreti cal fini sh Prac tica l fini sh (~lIll lrev) (Roughness in nm) (Roughness in nl11 )

0.3 0.0225 25.30 0.5 0.0625 18.05

0.75 0. 1406 19.25 1.0 0.2500 18.80 1.5 0.5625 17.60 2.0 1.0000 16.35 2.5 1.5625 16.28 3.0 2.2500 16.40 3.5 3.0625 15.33 4.0 4.0000 14.45 4.5 5.0625 14. 10 5.0 6.2500 13.90 5.5 7.5625 14. 10 6.0 9.0000 13.55 6.5 10.5625 15.00 8.0 16.0000 15.20 10.0 25.0000 17.35 12.0 36.0000 19.55 14.0 49.0000 24.35 15.0 56.2500 24.00 16.0 64.0000 26.65 18.0 8 1.0000 30.90 20.0 100.0000 35.30 22 .0 12 1.0000 40.45 25.0 156.2500 49.70 30.0 225.0000 65 .70

f is the too l feed rate : coeffic ients ao, al and a2 are de fined as:

ao= 12.4109; a I = 4 .0529 and a2 = 0.23 17

The first term A has been added to the exis ting theore tical formul a ( I). Thi s re lationship (3), instead of the o ne given by Eq . ( I) will sati sfac to ril y provide the practical ro ughness under the machin ing

Table 3-Confidcnce in terva ls for the values of variab les (I f) . (I , . (I ,

Variab le Value Confidence inte rva ls

en en .. c: ~ Ol :J o 0::

(10

(I,

(I ,

70 •

i 60 .

50 I

10

o o

12.4 109

4.0529

0.23 17

5

68% 90%

0.435836 0.734926

0.420552 0.709 153

0.00549 0.00926

10 15 20

Feedrate in micron/rev.

95% 9'Y,I

O.88706() 1.20378

0.855957 1.1 6 1565

O. III X (>.0 15 18

__ Practical Roughness

___ Best fi tted curve

25 30 35

Fig. 7- Best-fit curve for surface roughness in term s of tool feed rate

Percentage Error of curve fitted data w ith practica l roughness

15

1 10

~ 5 ! ~

~ w 0 .. OJ ~ -5 , c: .. ~ I .. -10 ' Q. I

-15 1

-20 I 0 5 10 15 20 25 30 35

Feedrate in m icron/rev.

Fig. 8- Percentage error between the best-fit curve and the prac tical roughness

130 IND IAN J. ENG. MATER. SCI.. APRIL 2003

condit ions described. As the feed rate is less than 1.5

~lIn/rev th e first term of the empirical formula dom inates thus deteriorat ing the surface roughness.

Beyond 8.0 )..tm/rev tool feed rate theoretical roughness is higher than practical one. Therefore from Fig. 6, it can be conc luded that theoretica l formula

does not hold good beyond 8.0 )..tm/rev tool feed rate. Confidence intervals at 68%, 90%, 95 %, and 99% for thc constant va lues o()= 12.4 109, 01 = 4.0529 and 0 2 =

0.23 17 are presented in Table 3. Since surface roughness value remarkably increases

as the tool feed rate goes down to I )..tm/rev or less, thi s region has been excluded while fitting the curve. Fig. 7 depicts the bes t-fitted curve on the plot for roughness w ith too l feed rate. The bes t- f itted curve is as per the relationship (3). It shows that the practi ca l roughness and the roughness from the deri ved form ul a are in agreement. Fig. 8 shows the percentage error of the best-fitted curve.

At thc med ium tool fced rates 1.5-8.0 )..tm /rev, the other machining parameters such as tool condit ion, spray mist coolant, chip extract ion and vibration dominate over theoretical fini sh resulting in higher actua l surface roughness. A lso it is observed that at higher tool feed rates , the other machini ng parameters lose their dominance resulting in the theoretica l and actua l surface fini shes close to each other.

Conclusions The study has been carr ied out on aluminium-6061

alloy rIat work-pieces o f 60 mm diameter w ith the 2 11m depth of cut and 3000 rpm of spindle speed. It has been observed that the tool feed rate of 1.5 to 8.0

pm/rev provides opt imum surface f igure and f ini sh under the above said machining parameters. The theoreti ca l and practi ca l surface fini shes are compared. It is observed that. smaller the feed rate, better the surface roughness. The surface roughness, however, becomes almost constant when the feed rate

is in between 1.5 to 8.0 )..tm/ rev . The roughness va lue significantly increases as th e feed rate reduces to I

)..tm/rev or less. I t has also been observed that the slower cutting speeds produced by facing to the cen tre of a j ob do not affect the surface fini sh, however they affect the surface fi gure. An empiri ca l formula is deri ved to best f it the practi ca l roughness obtained in thi s inves ti gation. Thc pract ical roughness and roughness from the empiri ca l formul a are In agreement.

There are two approaches to write the CNC program for flats and regul ar conics. One is based on ISO code (G39 facing cycle) and another by using the Tool Path Generator (TPG) software. T he work-piece has been turned by both the programs, and it was observed that G39 cycl e gives better surface fi ni sh compared to TPG, giv ing lesser scattering and dispersion.

Acknowledgement The authors are grateful to Dr R P Bajpai , Director,

CSIO, Chandigarh , for hi s constan t encouragement throughout the inves ti gati on.

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6 Sange r G M. in Applied Oplics alld Op liCil/ Ellgilleerillg. Vol. X. edited by Robert R. Shannon & James C \Vyant (Academi c Press, Inc .. New York ). 1987.25 1-390.

7 Zhang X & Zhang Y. Opl Ellg. 36 ( 1997) 825-830. 8 Sugnno T & Takeuchi K. 11 1111 C/R P. 36 (I <JR7) 17-20. <) Taylor J S, Syn C K. Sailll T T & Donaldson R R. Opl Ellg.

25 ( 1986) 10 13- 1020. 10 Church E L & Takacs P Z. Opl Ellg. 24 ( 1985) 396-403. II Stout K J. Obray C & Jungles J. Opl Ellg. 24 ( 1985) 4 14-41 8.