By R.B. Wilson

Logitech LimitedErskine Ferry Road, Old Kilpatrick,Glasgow G60 5EU, Scotland, U.K.

Tel: +44 (0) 1389 875444Fax: +44 (0) 1389 890956Email: info@logitech.uk.com

www.logitech.uk.com

Preparation of microscope slides of rocks, minerals andother research materials

First face lapping on a singlestation PM5 Precision

Lapping & PolishingMachine

Abstract:Requirements for production of high quality rock sections are discussed and a new machine system technology whichmeets these requirements is described. Operator / sectionmaker skill is still necessary but the emphasis is movedaway from manual activities towards those of machine gauging and control and process techique. The new approachis justified by the results, in that sections of first class quality are produced with increased efficiency whilst theoperator’s conditions of working life are distinctly improved. The machine system technology is versatile and adaptableto meet a wide variety of materials geometry specifications.

Contents:1. Introduction2. The Standard Section3. Relative Precision and Machine Processing Requirements4. Basic Section Making Processes5. Machine Technology6. Typical Results7. Extension Processes8. Conclusions

1. INTRODUCTIONPreparation of Rock Sections for microscopic study has been a major laboratory activity throughout the world for manydecades - indeed, during the history of geological research which dates back more than 100 years. Much of thispreparation work was, and in the majority of laboratories still is, of an essentially manual nature: tedious and slow forthose who actually do the work; expensively inefficient for those who pay for it. Many articles describing the develop-ment of the thin section making process have been written by authors throughout the world and central to so much ofthe thought and work has been a range of questions. These, and the generally accepted answers are tabulated inFigure 1.

CENTRAL QUESTIONS IN THIN SECTION PRODUCTION

GENERALLY ACCEPTED ANSWERSTHROUGHOUT THE WORLD PRIORTO THE LOGITECH SYSTEM

Can mechanical devices and machine tools- improve cost efficiency?- reduce operator tedium?- improve product quality?- improve job interest?

Only to a limited extent- primarily manual- necessarily tedious & repetitive- rock saws will slice- grinders will remove bulk rock at the risk

of section damage

- actually finish sections of high qualitywithout compromise?

- absolutely not: many have tried. All havefailed.

- all at the same time? - NO: Presumed not possible

FIG 1. Questions and answers in the Thin Section Laboratory

Now, however, the above answers are incorrect. Study of the traditional methods of thin section production,understanding of basic principles of geometry, consideration of the control process errors - has led to a major advancein machine based thin section technology. This is the subject of the present paper.

It is not suggested that skill has been eliminated, for it has not. Nor is it expected that all manual operations or theneed for skilled technical experience can be avoided, unless all realistic cost limitations were to be removed: “Rock” isfar too variable a material; microscope slide glasses as supplied by manufacturers are really quite inadequate forsection substrates (from the precision mechanical aspect); cover-slipping is a separate problem in its own right (notconsidered in the present context). Nevertheless, the accent has been changed from pure manual skill to machinecontrol skill, and the general processing from rock “chunks” through to uncovered slide sections has now beenintegrated into a machine based technology with results which fully justify the method. The advantage of the integratedmachine system technology are summarised in Figure 2.

LABORATORY PERFORMANCEUPRATED

QUALITY OF OPERATORS LIFEIMPROVED

- improved section quality- accurate section geometry & thickness

with minimal damage- reduced need for impregnation- system flexibility for very large &

ultra-thin sections- increased output per operator (typically

100%)

- much less tedium & physical weariness- interest of machine system control

procedures- considerable satisfaction from obtaining

unexpectedly improved results

FIG 2. Advantages of integrated machine system technology

This paper describes the degree of precision necessary to produce sections of good quality; the basic processesnecessary to make standard rock sections by hand or machine; and the way in which a suitable machine system isdesigned and correctly operated to deal with each stage in a reliable and adequately accurate manner in order toobtain the desired results.

The same type of equipment and process, with appropriate detail modifications, may also be applied to SectionPolishing (for example, flatness and geometric control) and for other materials outside the normal geological field: cal-cified tissue, refractory, concrete, coal, electronic research crystals and so on. Some of these alternative modifiedprocesses are outlined at the end of this paper.

2. THE STANDARD SECTIONThe Standard Section is illustrated in exaggerated form in Figure 3. - from the microscopist’s point of view and require-ments. This specification will satisfy the geologist / microscopist and, indeed, deviations from a perfect specification willnot be apparent. However, no machine can work to this type of requirement. The specification must be “tightened up”geometrically for machine processing.

F

12

3T

1. Microscope glass slide: wide variety of dimensions in use.2. Cement: optically clear, R.I. = 1.54 preferred.3. Rock section: free of gross holes, cracks and damage.

Ready for polish or coverslip.

T = Correct rock thickness, edge to edge; 30µm +/- ‘some microns’, or as chosen.F = Flatness within microscope depth of focus.

FIG 3. The standard rock section

3. RELATIVE PRECISION AND MACHINE PROCESSING REQUIREMENTSIt is a fundamental in physics and engineering that absolute accuracy does not exist. For any process acceptable errortolerances must be assigned - relative to the end application of the process or product. Whilst excessive errors giveunacceptable results it must be appreciated that unnecessarily tight tolerances lead to a waste of time and money.

In geology, rock sections are usually of 30µm nominal thickness - what is actually required? A glance at the MineralsInterference Colour Chart, and discussions with geologists, indicates the true answer:- 30µm +/- 3 to 5µm is highlyacceptable. It follows that the machine system must account for this in dealing with individual process stages and thatthe (statistical) sum of process stage errors should not exceed the allow error band. This implies that all geometrycomponents - flatness, parallelism, absolute thickness and uniformity of thickness - must be held to an accuracy ofabout +/-2µm during machine processing. The process will then be (geometrically) successful provided that on releasefrom the machine the specimen does not distort in flatness beyond the depth-of-focus range of the user’s microscope.In summary, therefore, ‘accuracy’ is a relative quality. Geometric tolerances of 2µm are accurate and adequate withinthe present context. (This would be mediocre within the field of Optical Polishing, but on the other hand OpticalPolishing is generally concerned with materials of known reproducible characteristics and is, in any case, expensiveand slow). The resulting Rock Section Geometric Specification for successful machine processing is illustrated inFigure 4.

Whilst Figure 4 provides an adequate geometric specification, the machine process must still satisfy the requirementsfor optical transmission and low bulk and surface damage throughout the entire glass slide-bond-rock layer sandwich.There is no point in using a machine system of extreme geometric accuracy (e.g. diamond wheel toolroom surface

grinder) if the rock sections are badly damaged by cracks and plucked minerals.

12

T3

1. Glass slide - accurately flat, parallel, uniform thickness T1 to about +/-2µm.2. Bond - of thickness T2, parallel and uniform to about +/-2µm. Aim for zero as only practical

way to achieve this.3. Rock - both faces flat, parallel and with uniform chosen thickness T3 +/-3µm.

FIG 4. Rock section geometric specification for machine processing.

3

T2

T1

4. BASIC SECTION MAKING PROCESSESBy hand or machine, rock sectioning involves five basic steps. These are summarised in Figure 5.

a) It is noted in particular that, for a long time, it has been known that free abrasive lapping by hand givesa section of adequate smoothness and low damage - whereas diamond wheel grinding does not.Therefore, the successfull machine system aslo uses free-abrasive lapping - it is not a diamond wheelgrinding machine.

b) Machine processing introduces one further basic requirement - glass slides of accurate, uniform knownthickness. These are not available commercially and therefore the machine system must itself be usedto produce them from standard commercial slides - this is annoying but necessary, and the resultsachieved show that it is in fact worthwhile.

BASIC PROCESS MANUAL OPERATIONS MACHINE SYSTEM OPERATIONS

Slice rock & trim Diamond Saw:Choice of cut & trim by eye & experience. A manualoperation due to rock variety.

Saw

Smooth lap firstface of rock slice

One specimen at a time:Smooth lapping with freeabrasive on cast iron or glass- doubtful flatness

Multiple specimens:Smooth & flat 2µm accuracyeasily obtained.

PrecisionMachineLap

Bond rock slice toglass slide

Main aim is lack of bubbles &bond thickness:- May be 1µm or 100µm

tapered

One specimen at a time:Smooth lapping with freeabrasive on cast iron or glass- doubtful flatness

Jig bond

Pre-section toremove bulk excessrock

Saw, grind, coarse lap:- Often too thin with resultant

section damage

Diamond saw:- At 500µm (approx.) to avoid

damage

Saw,AutomaticFeed

Finish section,smooth & uniformto 30µm

Single specimen:- Rub down on glass or Cast

Iron, skilled judgementrequired with frequent opticalchecks

Multiple slides on precisionjig(s) to finish:- Final optical check only

PrecisionMachineLap

FIG 5. Basic section making processes - manual & machine interpretations.

5. MACHINE TECHNOLOGYThe five individual stages are now described. In essence the machine system process uses three main pieces ofequipment:

a) A Diamond Wheel Trim Saw; for normal slicing and trimming, with an additional vacuum chuck forautomatic saw reduction of slide mounted bulk specimens to about 500µm thickness - a low accuracy of+/-100µm or so is quite adequate.

b) A Bonding Jig / Press: for establising a “zero” - typically <1µm - thickness of cement bond betweenrock specimen and glass slide.

c) An Automatic Free Abrasive Lapping Machine with Precision Lapping Jigs: The Lapping machine, whencorrectly controlled, prepares specimen surfaces which are as flat as required - in a reliable and repeat-able manner. Additionally, for section finishing (and glass slide preparatory lapping), the precision lap-ping jig(s) replaces the section maker’s hand, holding multiple glass slides by vacuum; maintaining slidesections accurately flat and parallel during finish lapping; stopping the sectioning process automaticallywithin 2 or 3µm of the pre-selected thickness.

5.1 SLICE ROCK & TRIMThis process is well known. The dual purpose trim saw is illustrated in Figure 6.

Cover for use duringautomatic pre-sectioning

Precision sliding armvacuum chuck for useduring auto pre-sectioning

Normal trim sawfacility

FIG 6. Dual purpose Trim Saw

At this stage the trim saw is used in normal fashion though there are several points to stress in relation to laterprocess stages:

The rock is cut 4 to 6mm in thickness so that it has adequate mechanical rigidity for the following stages of first-faceflat lapping and zero bonding. The current machine system will deal with slices up to about 100mm x 70mm in area -for 110mm x 75mm glass slides.

The slices are trimmed so that when bonded to the glass slide there is a margin of 2mm or more all round - for con-venience in bonding such that adhesive does not run over the edge of the glass and onto the back face (the machine’sreference face which must be absolutely clean).

The rock is impregnated only after machine system experience shows it to be absolutely necessary. This machineprocess is unexpectedly gentle at the section finishing stage and impregnation may usually be omitted.

5.2 SMOOTH LAP FIRST FACE OF ROCK SLICEThe LP50 Precision Free Abrasive Flat Lapping Machine is illustrated in Figure 7.

Automatic AbrasiveFeed

Flat Cast Iron LappingPlate, 35cm diameter (0-70rpm variablespeed)

Retaining rings hold rock slicesas they slide on rotating lap

FIG 7. LP50 Precision Free Abrasive Flat Lapping Machine

Abrasive slurry (usually 600 grit Silicon Carbide / Water) is fed automatically to the rotating lap. With rock slices ofabout 25mm x 30mm, the three retaining rings hold about 24 pieces and the machine will lap them truly flat andsmooth on one face in a timed half hour run. Figure 8 illustrates the way in which rock slices (of non-uniformthickness) are loaded within the retaining rings. The sponge rubber is most important.

Cast iron lapping plate Rock slices

Pressure block

Lead weight

Retainingring

Sponge rubber

FIG 8. Rock slices within retaining ring

It is also most important to understand that a “flat” lapping plate must be gauged and controlled by the operator if it isactually to be flat. In practice the lap tends to wear into a concave or convex spherical shape and the lapped rockslices have a surface which is the converse to the lap. The operator aims to wear the lap towards perfect flatness,continually, during use. He uses a Dial Gauge system to measure the error regularly. If the lap is convex he wears thelap down at the centre by moving the work stations towards the centre. If the lap is concave, or saucer shaped, hemoves the work stations towards the periphery. The accuracy is in the hands of the operator and with practice it isrelatively easy to maintain a flatness of about 2µm across 100nm - well within process requirements.

The Flatness Gauge system is illustrated in the photograph of Figure 9 and commentary diagrams of Figure 10.

Master FlatTwo DialGauge

Test Block

FIG 9. Components of the Flat Lapping Gauge System

Exaggerated Convex Lap

A Test Block is lapped for 10 minutes or soat regular intervals (once per hour or onceper day according to operator experience).

A. B.

Master FlatBlock

“Perfect”FlatSurface

A Two Dial Gauge is used, thedials are zeroed on a “Master”Flat Block, sensitivity is +/-1µmacross 100mm.

The Gauge is placed on the cleanedTest Block, Dial A indicates theflatness error as a deviation fromoriginal zero setting, the operatormakes appropriate adjustment of workstation positions on the lappingmachine, and resumes lappingoperations.

Flatness error

FIG 10. Flatness Gauging

5.3 BOND ROCK SLICE TO GLASS SLIDEGlass slides of precise standard thickness are essential for a machine section finishing process. They are producedwith the same machine system by precision jig controlled lapping - as described later.

The clean smooth flat lapped rock face is bonded to the clean smooth flat lapped glass slide - using low viscosityepoxy resin of refractive index 1.54. The bonding Jig is shown in the photograph of Figure 11 and cross sectiondiagram of Figure 12.

9 Spring Loaded Bonding Stations

FIG 11. Bonding Jig Arrangement

Epoxy

Loadingpiston

Rock

Glass

Teflonblock

Tilting point of contactaccommodates wedgedrock slices

FIG 12. Zero bonding of rock to glass

The spring load for bonding is about 1kg/cm2. The operator learns to apply a very small quantity of low viscosity epoxyresin and assemble the rock / glass under pressure without bubbles in the resutling bond. On initial assembly, bypushing and pulling sideways on the loading piston, it will be felt that the rock is “floating” on epoxy clear of the glass.However, the epoxy is rapidly squeezing out and after a brief feeling of “rubbing contact”, the rock “locks” onto theglass and will no longer slip. The bond is truly zero. Bonds made in this way are extremely clear, free of bubbles, andintroduce zero dimensional error into the machine section-thickness control procedures. It is important to leave thebond under load whilst the epoxy cures. Further, it can hardly be emphasized that good bonds require rock and glasswhich have been properly cleaned, degreased and dried. Failures in cleaning are the usual cause of holes andplucking in the final rock section.

5.4 PRE-SECTION TO REMOVE BULK EXCESS ROCKThe Trim Saw in use for pre-sectioning is shown in Figure 13. The slides are held by vacuum on a universal chuck,and feed down into the saw blade automatically by gravity feed against a hydraulic damper. An adjustment nut enablesthe operator to set for a 500 +/- 100µm pre-sectioned rock layer - thick enough to avoid damage at the final 30µmsection rock layer. With the automatic finish sectioning process to follow, the pre-section accuracy is really quiteunimportant. The photograph shows four (76 x 52mm) slides being processed simultaneously and automatically in atime of 5 to 10 minutes. The chuck will equally well hold one 110 x 76mm slide, seven 25 x 76mm slides or twelve 28 x48mm slides, and so on.

5.5 SECTION FINISHING (AND INITIAL GLASS SLIDE PREPARATION)The Precision Lapping Machine in final Thin Section Mode is illustrated in Figure 14.

Vacuum chuckwith four slides

Adjustment forpresectionthickness

FIG 13. Automatic pre-section trim-saw

Slides held byvacuum chuck

Rotary VacuumAdaptor

Cast Iron lappingplate

PrecisionLapping Jigs

In this photograph the Precision Lapping Jigs are holding vacuum located slides on the rotating flat cast iron lap.(Regular flatness control remains a duty of the operator). The jig operation is explained by a series of diagrams on thefollowing page.

FIG 14. Thin Sectioning by Precision Jig controlled Lapping

It is worth emphasising - because it is so unusual - that the diamond bearing surface, or stop ring, is not an abrasivegrinding surface. At the end of the sectioning process it slides smoothly on the lap without significant wear on eithersurface.

The Vacuum Chuck is adjustable for step height as illustrated and, consistent with previously discussed accuracyrequirements, is flat and parallel to the diamond bearing plane within about 2µm.

The jig is first set to required step height Xµm, as illustrated in Figure 16, using the “Two Dial Gauge”.

X microns

Nut provides extremely accurateadjustment of chuck step height:X microns

127mm / 5 inchesVacuum chuck holds slidesflat with great accuracy

Smooth diamond bearingsurface provides a non-wearing reference stop

Underside view

Cross section

FIG 15. The Precision Lapping Jig

Xµm

Read gauge and setchuck within +/- 2µm

Lever for nutadjustment

Two Dial Gauge

FIG 16. Pre-setting the Precision Lapping Jig

The pre-sectioned rock slides - or plain glass slides of commercial quality - are then located by vacuum on the (clean)vacuum chuck and the jig is placed on the lapping plate as illustrated in Figure 17. Note that, initially, the jig standsentirely on rock (or glass).

Xµm

Cast iron lapping plate Slide specimens

Cast iron lapping plate

X UX - U

X = Chuck settingU = UndercutX - U = Resultant

specimenthickness

FIG 17. Precision Lapping Jig with slides for lapping

FIG 18. Jig lapped specimens at end of lapping cycle

Machine lapping proceeds, with specimen material being eroded away, until the diamond stop ring settles and slidessmoothly upon the rotating lapping plate. Lapping is continued for about 10 or 15 minutes thereafter in order to fullydevelop the “undercut” - Uµm. This is the same as relief (due to finite abrasive grain size) between the relatively softspecimen surface and extremely hard diamond bearing ring. Uµm is a function of abrasive size and, within 2 or 3µm,is constant for a given grade of abrasive, e.g. for 600 grit Silicon Carbide, U~28-30µm. After about 40 minutes lapping,the specimens are generally fully lapped to finish size and the Precision Jig on the lap is in the condition illustrated byFigure 18. No further section reduction will occur thereafter - i.e. the process stops automatically (even though themachine may continue to the end of a timed run). Full development of undercut - by sufficient lapping time allowance -permits simultaneous processing of mixed hardness materials. Hard and soft materials (all being “infinitely” softer thanthe stop ring) tend to an equality of selection thickness with minimal differential relief.

The operator learns to set the jig, allow for undercut effect, and control lap flatness, so that specimens of either plainglass or sectioned rock are made to desired thickness within about +/-2µm. In essence, if plain glass slides are lapped

at jig setting Xµm, the glass slide thickness will be (X - U)µm. If the jig is then reset to a chuck step height of (X + 30)µm, and presectioned rock slides are lapped at this setting, then the total slide thickness will be ([X + 30] - U)µm, i.e. the rock section itself will be the desired 30µm (as the bond is zero thickness) and will befinished ready for use. This condition may be checked, using the Two-Dial Gauge (the second “B” gauge is usefulhere) before release of vacuum.

As indicated in Figure 14, earlier, the LP50 machine will carry up to three Lapping Jigs, and each Jig will hold variouscombinations of slides, e.g. six U-stage 28 x 48mm, four 25 x 76mm, one 110 x 75mm. Therefore, in a typical machinerun for approximately 40 minutes the system will produce, unattended, up to 18 finished sections from a pre-sectionthickness of 500µm.

6. TYPICAL RESULTS

a) Rocks of extreme variety have been successfully and regularly processed to thin section with thismachine system. “Difficult” rocks are sectioned to high quality, without impregnation, with surprisingease.

b) Quality in terms of finish - low damage and plucking, smooth surface - is unexpectedly high, due to theundercut effect, i.e. the abrasive lapping action automatically reduces in severity to zero as the sectionapproaches the desired thickness.

c) Quality in terms of thickness, uniformity and lack of edge erosion is also extremely high. By normalstandards of laboratory assessment a correctly operated machine system will yield up to 80% sectionsjudged to be finished, ready for use. The operator should arrange that the 20% deviations tend to bethicker than desired so that they may be corrected by a minimal amount of manual rubbing.

d) Very large sections (on 110 x 75mm glass), multiple rock types mounted on one glass slide, ultra-thinsections (down to 10 or 15µm using 600 grit abrasive or finer) - all become quite straightforward withthis machine system.

e) Output rate per section-maker depends strongly upon the rock types being processed, laboratorysupport facilities and operator ability. However, a substantial increase in output is generally observed - ofthe order of 100%

f) The quality of the life of the section-maker is definitely improved by use of this machine system. Verylittle of the tedious manual processing remains - control of the automated machine system demandsskill, understanding and interest rather than wearisome routine and repetitive work. It is noted thatoperator training is important: the processes are quite detailed and the operator must understand thepurpose of each production stage and the accuracy required.

7. EXTENSION PROCESSESThe type of machine system processing described in this paper specifically for rock sectioning may be modifiedappropriately to deal with a wide range of other materials and surface finish specifications. In general, all moderatelyor extremely hard materials which are abrasive “lappable” or “polishable” are amenable to this type of treatment.

a) Very hard materials - such as refractories and ceramics containing alumina - are treated in the sameway as rock except that a little coarse diamond grinding paste (e.g. 14µm grade) is added to the castiron lapping surface from time to time. Alternatively, as slurry of appropriately graded cubic boron nitride(or even diamond) may be dispensed by the abrasive autofeed unit.

b) Very porous rock or friable materials, such as brick / unconsolidated sandstone / soil / concrete ......must be converted to rock-like characteristics by vacuum assisted resin impregnation. Thereafter, suchspecimens may be processed generally as previously described. A Vacuum Impregnation Unit providesa very solid impregnated specimen when used with suitable epoxy resin.

It is also worth noting that the use of the Precision Lapping Jig allows surface impregnation to be carriedout very close to the desired final section surface, i.e. lap section to (say) 10µm above final thickness;surface impregnate, wipe off excess resin and cure; lap off the final 10µm by jig control. This procedureis useful for low porosity specimen materials.

FIG 19. IU30 Vacuum Impregnation Unit and the PLJ2 Precision Lapping Jig

c) Smoother surface finishes are obtained with finer silicon carbide or alumina abrasives.

d) For producing polished surfaces a similar integrated approach - The WG2 Polishing System - is used.This is shown in Figure 20 and is fully described in other publications. Normally, a two stage processusing cloth polishing pads with fine diamond abrasive or fine alumina powder is preferred, although thesystem allows total flexibility in choice of polishing surface and abrasive.

Where extreme flatness, low intergranular relief and minimal edge rounding are necessary and for pro-ducing ultra-thin double sided polished sections, soft metal polishing plates are normally required. Theseare generallly lead or tin-lead alloy used with fine diamond polishing abrasive. The WG2 Polishing Headis replaced by the PP5GT Polishing Jig illustrated in Figure 21. The jig incorporates a vacuum chuckenabling extreme accuracy to be achieved and a dial gauge is fitted to indicate the rate of specimenstock removal. The plishing action is extremely gentle allowing fragile materials to be polished withoutdamage. The PP5GT has been used to prepare 5µm sections of slates and coal for fluid inspectionspecimens. Electronic crystals and semiconductor materials are also polished, chamfered and sectionedon this jig.

FIG 20. The WG2 Polishing Systemand the PP5GT Precision Polishing Jig

e) Certain materials, such as porous calcified tissue, must be kept particularly clean during processing asembedded abrasive will disturb the subsequent microscopic (or radiographic) analysis. Such materialsare difficult to impregnate, due to close pore structure, and the use of fixed-abrasive lapping plates maybe necessary. These may be of bonded diamond, abraded / roughened glass, or synthetic resin /abrasive. Jig controlled lapping will produce the required geometric accuracy of sections, but surfacefinish may not be as good as desired.

f) Many materials are far too delicate, or valuable, to cut roughly on a rock trim saw. More precise andgentle sawing techniques then become essential. For these materials an Annular or Peripheral Saw maybe the right answer to this particular material’s cutting requirements. Alternatively, an abrasive wire sawprovides the gentlest method of cutting.

FIG 21. The APD1 Annular & Peripheral Saw and the AWS1 Abrasive Wire Saw

g) Some specimen materials are damaged by standard processing methods. For example, concrete withepoxy resin impregnation should not be heated for bonding as a temperature above 50oC will damagethe structure and, in any case, may cause distortion of the epoxy impregnated slice on the glass slide.Furthermore, water based slurry can affect the cement structure. Accordingly, bonding is carried out ator near room temperature and oil based lapping slurry is used. Such process modifications may usuallybe devised.

8. CONCLUSIONSAfter one century of laboratory work, a study of process details has led to a major advance in rock sectioning method.The developed Machine System Technology is versatile and accurate, providing substantial improvements in sectionquality and production efficiency whilst, at the same time, removing much of the tedium and manual labour from thesection maker’s job.

The Machine System approach extends naturally, with detailed process modifications, to solve many other aspects ofmaterial shaping and surface finishing.