thedesignanddevelopment ofa rockcutting machineforgoldmining

The design and development of amachine for gold mining

rockcutti ng

J. P.M.HOJEM*, (Visitor), N. C. JOUGHINt, (Member), and C. DIMITRIOUt, (Visitor)

SUMMARY

This paper describes the design and construction of the first experimental underground rockcutting machinefrom measurements of cutting forces determined in tests on small rock specimens in the workshop. Thedeficiencies of this design, revealed by stoping with almost identical rockcutting machines, are described. Anaccount is given of the design and construction of further experimental machines incorporating substantialimprovements which made them technically feasible for use in production.

INTRODUCTION

One of the most serious problems in South Africangold mining is that during rockbreaking by means ofexplosives the goldbearing portion of the reefs, whichis often less than 15 cm thick, becomes intimately mixedwith the adjacent waste rock. Other major problems alsoresult from the use of explosives. Most mining takesplace at great depths, and the severe rock mechanicsand thermal control problems which arise can be shownto be proportional to the effective stoping width1, 2.With explosive rockbreaking, the effective stoping widthor milling width is almost identical to the actual stopingwidth of about 1 m, since so little of the waste rock canbe sorted and packed. In addition, the use of explosivesprecludes continuous mining because virtually allpersonnel have to be evacuated from the mine beforeblasting and are not permitted to re-enter until a fewhours after the blast.

Economic assessments3 have shown that substantialbenefits would result from the use of an alternativemethod of rockbreaking which would permit the reefto be mined more selectively. With the view to thedevelopment of a continuous, selective mining processan investigation was made of most known methods ofrockbreaking4. These included ultra high pressure waterjets, mechanical wedging with or without impact,diamond sawing, roller bits, percussive tools, drag bits,various thermal methods such as jet piercing, lasers,electron beams and electrical techniques. A drag bitmethod with a tungsten carbide tipped tool for cuttingthe rock was selected as the most feasible of thetechniques applicable within the near future for developinga machine which could be made to mine selectively underthe conditions encountered in deep-level, hard rockmining.

Rockcutting investigations were carried out in ashaping machine and in a lathe with the aid ofinstrumented tool shanks to measure the cutting forcesand tool wear on several types of rock for various feeds,depths of cut and cutting speeds4. In order to simulatethe cutting of a long slot a large drill core 15 cm indiameter was set up in a lathe and a rectangular sectiongroove was cut using a feed, as in thread cutting, ofmore than twice the width of the cutting edge.

These experiments showed that the force acting onthe cutting tool consisted of two roughly equal com-ponents, namely, the cutting force parallel to the directionof the movement of the tool relative to the rock and thepenetrating force acting at right angles to the rock

surface. The cutting and penetrating forces on a toolwere estimated to be of the order of 40 kN whenmaking a cut 20 mm wide and 9 mm deep in moderatelyhard rock. Wear on the tool was found to be moderateprovided the cutting speed was less than 1.5 m/sec.Specific tool wear was less than 1 part of tungstencarbide in 25 000 parts of quartzite by weight whendeep cuts, 2 mm or more, were made at speeds ofless than 0.5 m/sec.

These results indicated that it was possible to designa machine capable of cutting at a rate of 450 cm2/minusing a cutting speed of 15 cm/sec and a depth of cutof 5 'mm, which would result in a rate of mining of8 centares per six-hour shift, cutting for, say, half of thetime.

This paper discusses the various configurations forsuch cutting machines and describes the design of thefirst prototype, underground rockcutting machine usinga linear, reciprocating cutting action. The testing of thisand other similar rockcutting machines showed that thisdesign formed the basis of a feasible system of continuousselective mining, and revealed deficiencies in detail whichhad to be overcome to yield a machine capable ofmining at an economic rate. The design of a secondprototype, incorporating the necessary improvements,is described and the development of each of the sub-assemblies towards a reliable rockcutting machinesuitable for use in production is detailed.

CONCEPTUALDESIGN

Selective mining of the substantially plane gold-bearingreef could be achieved by cutting parallel slots in thewaste rock immediately above and below the reef. Theseslots would separate the reef from the waste rock andrelieve the vertical stress acting on the reef, whichotherwise prevents its easy removal from the stope face.At depth the high vertical stress in the face would causethe reef between the slots to become detached from theface in the form of slabs much in the same manner asdiscing of diamond drill cores occurs when drilling inrock subjected to high stress transverse to the drillaxis5. In the event of the stresses in the face being

*Chief, Engineering Division, Mining Research Laboratory,Chamber of Mines of South Africa.

tChief, Simulation Division, Mining Research Laboratory, Chamberof Mines of South Africa.

:j:Senior Research Officer, Simulation Division, Mining ResearchLaboratory, Chamber of Mines of South Africa.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY FEBRUARY 1971 135

insufficient to cause slabbing, the reef between the slotscould be detached from the rock ahead of the slots bywedging or jacking. The waste rock on either side ofthe slots would then be removed to restore the faceshape and stoping width. Breaking of this waste rockwould also be greatly facilitated by the removal of thevertical stress. The reef would be transported from thestope and the waste rock would be packed in the mined-out area immediately behind the face.

The gold-bearing portion of the most important reefsin South African gold mines is 15 cm or less in thickness4and deviates less than 10 cm from flat over distancesof about 3 m. If the slots could be made to follow theundulations of the reef, a separation between them of20 cm would be sufficient to embrace the gold-bearingportion but, if the slots had to be straight over adistance of about 3 m due to machine configuration, aseparation of about 25 cm would be needed to embracethe gold-bearing portion.

Whenever an excavation is extended, the weight of theoverlying strata causes substantial and immediate con-vergence of the roof and floor. Removal of the verticalstress by slotting gives rise to elastic closure of the slotsat the rate of about 2 mm for every 100 mm depth ofslotting. This closure had to be allowed for in the designof a slot-cutting machine and imposed a practical limitof about 30 cm on the depth of slot which would be cutwithout removing the surrounding rock.

In principle, it was decided to use a tungsten carbidedrag bit mounted at the end of a thin blade which wouldbe used to deepen the slot by up to half the width of thedrag bit at each pass. The bit could be made to generatethe required slot while moving in a path which wascurved, linear, or a combination of curved and linearmotions. Accordingly, three main, cutting configurationswere considered and these are shown in Fig. I.

A machine using a rotary action, Fig, la, was rejectedbecause it suffered from three major drawbacks. Firstly,the varying depth of cut inherent in this configurationwould cause excessive tool wear by abrasion during theshallow portion of the cut. Secondly, the need for thetool to re-enter the slot at each revolution would causewear on the sides of the bit as a result of the continuousclosure of the slot as it is deepened. Thirdly, the con-siderable engineering difficulties in building a rotarymachine of acceptable weight and bulk capable ofexerting a torque of about 70 kN/m to drive thecutter were evident at an early stage of the design study.

A machine using both rotary and linear cutting actionsin a manner of a chain saw, Fig. 1b, appeared to offermany advantages over the rotary cutter.

Used with a plunge feed normal to the stope face, asshown in the figure, this machine would make possiblethe attainment of near optimum drag bit operatingconditions. Cutting depth along the base of the slotwould be constant and the variation in cutting depthduring the curved portions of the cut would notnecessarily be excessive. The use of two tools as shownwould result in almost continuous cutting and a greaternumber of tools could be used to increase the rate ofcutting if needed. Furthermore, this machine would beable to cut into a straight face.

Unfortunately the engineering difficulties involved inthe design and manufacture of such a machine areformidable. The mechanism for driving and guidingthe tools together with a means for feeding them intothe face are likely to make this machine bulky andexpensive, and it would certainly take longer to develop

136 FEBRUARY 1971

To,qu. - 70 kNm

T,o,m.

Va"ing depthof cut

300

~Sid.woo,on tools ~ :duo to slo' closu..No p.n.,..1ion

..pod ab,asi,. WO",

la)

F..d

.-.-.

Ib)

le)

Fig. 1-Alternative methods of applying a drag bit for cuttingrock.

(a) Rotary cutter(b) Combined rotary and straight line cutter(c) Simple straight line cutter

than a machine based on a simpler configuration. Thisconcept was, therefore, also rejected.

A simpler configuration, based on the action of ashaping machine (Fig. Ic) was then considered. Amachine of this type offers several important advantagesover the previous type in ease of design and con-struction. Important among these is the ease with whichreciprocating linear motion of adequate force can begenerated using a screw, a hydraulic cylinder, or a rackand pinion. The problem of guiding the drag bits tofollow a straight line can be solved easily by mountingthem on a simple slide or saddle guided on a rail or bed.The linear motion produces the constant depth of cutfound to be important in the preliminary laboratoryexperiments.

There are three main objections to this simplifiedmachine. Firstly, the drag bit requires a free face fromwhich to commence cutting; a free face is also requiredat the end of the slot to prevent the bit from bumpingrepeatedly against solid rock at the end of each stroke.Secondly, the reciprocating motion entails a time con-suming return stroke between cutting strokes. Finally,in its return down the slot the drag bit is exposed toexcessive wear to its sides and to the danger of damagefrom jamming against fragments of rock in the slot,unless the additional complication of completely with-drawing the tool from the slot on the return stroke isadded.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY

In spite of these disadvantages, the shaping machinetype of cutter was chosen because of its simplicity andthe constant depth of cut which it gives. This choicewas influenced by the need to establish the practicabilityof a rockcutting machine using tungsten carbide dragbits to cut a pair of parallel linear slots in the rock faceof a gold mine with the least possible delay and expense.

DESIGN OF FIRST PROTOTYPEThe layout of the first prototype rockcutting machine

is shown in Fig. 2. The reciprocating head consisted ofa saddle or slide A on which were mounted the cuttingtools B, the manual tool feed mechanism and theclapper mechanism to relieve the forces on the toolduring the return stroke C. The saddle fitted over, andwas guided by, the bed D which was of circular section.A web E was welded to the bed to stiffen it in atransverse direction and to restrain the saddle fromrotating. It also provided mounting points for the threehydraulic mine props F to which the machine would beclamped in the stope.

The cutting force was generated by a double-actinghydraulic cylinder, with a stroke of 3 m, housed withinthe bed. Hydraulic pressure was chosen as the means forgenerating the cutting force because of the ease withwhich a light and simple cylinder can provide largeforces and the convenience of generating hydraulic powerin a remote power pack connected to the machine byflexible hoses. The cylinder was anchored by a nut andthreaded spigot to a diaphragm G which was welded

c E/

F

0--

Stroke 3m

into the end of the bed. The piston rod was connectedby a hemispherical joint H to the saddle through thesplit cylindrical column I. Hydraulic fluid from theremote high pressure, electrically powered pump unitwas piped to the cylinder through flexible hoses enteringthe web at J after passing through a control unit K,which incorporated a control valve, a pressure gauge,and a pressure relief valve.

A disadvantage of the hydraulic cylinder comparedwith the alternatives of a screw or rack and pinion typelinear actuator is the long reciprocating piston rod andconnecting column. It is necessary to provide a clearancein the stope, at the one end of the machine, equal to theworking stroke of the piston. A machine using ahydraulic cylinder coupled direct in this manner has anoverall length varying between a minimum of its strokeplus the saddle length, and a maximum of twice thestroke plus the saddle length. Using other types ofactuator it is possible to design a machine of constantlength equal to one stroke plus the saddle length. Tothese lengths must be added a few centimetres of deadlength for items such as connections and attachments.In practice, difficulties due to the reciprocating piston rodand connecting column have been minor.

Because of the large forces imposed on the machineduring cutting, anchoring of the machine in the cuttingposition is of primary importance. Two methods ofanchoring the machine were considered:

(i) The machine could be anchored to bolts andhooks set into holes pre-drilled into the rock

B\ c --F

E

G0///

A/ f\, j---lM---

LM L

H

.+

K

Fig. 2-Elevation, section and plan of the first prototyperockcutting machine, installed at a stope face. A is the saddlewith tools B and tool feed C. D and E are the bed and web

of the frame mounted on hydraulic props F.


footwall, hangingwall or stope face. The problemsof accurate drilling for this purpose made thisidea very unattractive.

(ii) Anchoring the machine between the hanging- andfootwall by friction forces generated by jacks.This method was considered most likely to besuccessful. Tests have shown that a prop set atabout 2oo kN is capable of sustaining a transverseforce of about 40 kN at either or both ends. Asprops required no pre-drilled holes and could bere-used indefinitely, it was decided that themachine should be anchored in the stope byclamping it to three 200 kN hydraulic propswhich would form an abutment capable ofrestraining a total reaction force of about 120 kNin a plane substantially parallel to that of thereef.

The machine was designed to rest on removablecradles on a sled L, so that it could be moved in thestope. To the sled was fixed a lever-type hydraulic jack Mfor lifting the machine into the cutting position.

Handling heavy equipment in the narrow confines ofa stope is usually difficult and the first machine was builtto be light so that final positioning could be donemanually. The procedure for moving in the stope wasto be as follows: With the machine resting on removablecradles of the sled and the collapsed props lightly clampedwith their lower ends clear of the footwall, the sled wasto be pulled parallel to the face by means of a chainblock fastened to one of the pulling eyes and anchoredto a prop some distance up the stope near the face. Whenthe machine was adjacent to the section of face whichit was intended to cut, the sled was to be manhandledclose up to the face with the help of pinch bars.

To set the machine up in its cutting position, first theprop clamps were to be slackened off allowing the propsto rest on the footwall, though still in an upright positionguided by the clamps. Next, the machine would be raisedclear off the cradles which would then be removed. Thelifting socket on the machine was located at the centreof gravity so that the machine could be positionedmanually while the height was controlled by theelevating jack. At this stage it was intended that thehydraulic props be set and clamped tightly one at a time,securing the machine in position. Lowering of the jackarm would then allow the cutting operaton tocommence. If the clearance remaining between thecollapsed jack and the machine were insufficient for thereciprocating saddle to clear it would be necessary toremove the sled. But it was preferred that the sled remainin position beneath the machine during cutting. Aftercutting of the slots had been completed, the weight ofthe machine was to be taken by the jack, allowing theprops to be released, after which the cradles would bereplaced on the sled and the machine would be loweredinto them before moving to the next position.

DEVELOPMENTOF THE ROCKCUTTING MACHINE

The first machine was intended mainly to test thefeasibility of cutting rock underground. Nevertheless,it was designed to be the basis of a mining machine.Critical to the success of this new method of mining werethe answers to a number of important questions:

(i) Would an economic cutting tool life be attainable?

(ii) Would it be possible to advance the face andmaintain the desired stoping width?

138 FEBRUARY 1971

(iii) Would the handling of a machine of this sizeand weight in the stope prove too difficult?

(iv) Would it be possible to achieve an economicmining rate?

The success of the first prototype was such that, forthe initial development, the basic machine concept wasretained and design changes were made only wherenecessary to facilitate continued testing. At an earlystage it became apparent that the first basic conceptcould, in fact, form the basis of a workable miningmachine and that there was no need to deviate from thisconcept, though there was no shortage of ideas foralternative machine designs or for alternative methods ofmining. The development process was followed throughstep by step, the worst defects remaining at each stagebeing eliminated.

To hasten the ultimate manufacture of productionprototypes, the Chamber of Mines decided to involvemanufacturing companies in the development of machinesat the earliest possible stages, so that they could derivethe benefit of experience in testing these machines rightfrom the start. Three companies, namely, AndersonMavor (South Africa) (Pty) Limited, Delfos and AtlasCopco (Pty) Limited, and Klockner-Ferromatik (S.A.)(Pty) Limited, have collaborated actively with the Chamberof Mines in the development of machines, and twoothers, Boart and Hard Metal Products S.A., Limitedand Sandvik (Pty) Limited, have collaborated in thedevelopment of cutting tools.

A brief history of the machines which were built mayassist the reader to keep the developments referred toin perspective.

The first machine described above was designed andbuilt by the Chamber of Mines and was tested under-ground at Luipaardsvlei Estate and Gold MiningCompany, Limited.

Thereafter, four machines were built by two of thecompanies to the order of the Chamber of Mines. Themachines were designed in collaboration with the Cham-ber of Mines, and the most significant changes were inthe shape of the bed, the tool feed mechanism, theprovision of increased cutting force, and the additionof a fourth prop. One machine built by AndersonMavor was installed at West Driefontein Gold MiningCompany, Limited, but testing was curtailed by theflooding of that mine. The other three machines werebuilt by Delfos and Atlas Copco. These were used atDoornfontein Gold Mining Company, Limited andunderwent many modifications in the course of testing.Mining with these machines showed that it was feasibleto obtain a satisfactory tool life and that the face couldbe advanced over large distances.

Early in 1969, another machine was designed andbuilt by the Chamber of Mines. This machine, shown inFig. 3, incorporated significant changes in the design ofthe tool holding arrangement, the hydraulic drivecontrol, and the anchoring and moving of the machine.KlOckner-Ferromatik collaborated in the design of thenew props while the saddle and tool feed mechanismfrom a Delfos and Atlas Copco machine was used.Experience with this prototype showed that it could behandled with much greater ease and speed in a stopedespite its greater weight, and that a satisfactory rateof mining could be achieved. In the meantime, continuedimprovements in cutting tool design and performancewere being made.


~ --

-- -.

.

--- -"

-- -- -----

FiV. 3-Section, elevation and plan of the second prototyperockcutting machine, showing the offset twin anchoring

props and fixed sleds for moving.

The Chamber of Mines then entered into developmentcontracts with Anderson Mavor and KI6ckner-Ferro-matik for the design and construction of a series of sixmachines from each of them, suitably spaced in time toallow the incorporation of improvements, as indicatedby the underground trials being carried out at Doorn-fontein Gold Mining Co., Ltd. The emphasis in thisseries of machines has been on improving machinereliability. The first of the machines delivered under thesecontracts is shown in Fig. 11.

Fig. 11-0ne of the first machines supplied by AndersonMavor.

The initial design and subsequent development of themany parts or sub-assemblies of the machines will nowbe described in detail.

Cutting too/ bit

The tool bit (Fig. 4a) as designed originally, wassimilar to those used in metal and coal cutting. Itconsisted of a nearly rectangular tungsten carbide bitbrazed into a notch in a hard steel body. The bit wasmade wider than the tool body and blade so that theycould be advanced into the slot without becomingpinched between the closing edges of the deepening slot.The front and side faces were given clearance anglesof 5° and 3° respectively and the leading face had zerorake angle, that is, it was perpendicular to the directionof tool travel.

Many types of brazing material were investigated andcopper-based materials containing a high proportionof copper proved most successful. An important factorin selecting braze materials is that the brazing tem-peratures must be sufficiently high to cause hardening ofthe steel of the tool body near the bit which must behard enough not to yield when cutting. The high melting

B

A

0 ~'" ",'Side face

B

__m,("J

c

0[][I

Ib)

Fig. 4-Plan, front and end views of cutting tool bodies.(a) Original drag bit shape.(b) Latest drag bit shape showing reduced front face

width, bevelled corners and improved method ofinserting bit into holder.


temperature of 1 OOO°C to 1 1O0°C of the copper-basedbrazing alloys results in hardening of the EN30B steelbody near the bit to RHC50.

The cutting action in hard, brittle rocks is verydifferent from that in metal or coal. The front facebears on the rock and is subjected to heavy wear whilethe leading face of the bit virtually never comes intocontact with the rock. As a result a flat surface, almostparallel to the base of the slot forms on the front faceand increases in area as wear progresses. The area ofthis worn surface is a measure of the bluntness of thetool. Experience obtained in cutting underground hasshown that the corner at A, Fig. 4a, remains rectangular.With tungsten carbide bits containing less than about9 per cent cobalt thin flakes tended to break away fromthe leading face, whereas when the bits contained morethan 9 per cent cobalt, plastic flow of the tungstencarbide occurred in the forward cutting direction sothat a small projecting lip was formed on the leadingface at A.

Of the grades of tungsten carbide used the typescontaining about 9 per cent cobalt, as used in percussivedrilling, proved best. Grades containing less cobalttended to fracture and grades containing more cobaltwore rapidly. Even with the optimum grades, fracturetended to develop at B when the flat worn surface onthe front spread towards B. This fracture pattern waspossibly due to the trailing part of the edge of the bitat B coming under load and, since this edge wasrelatively poorly supported by the steel, flaking away asthe tool was dragged over the rock.

In the initial design of tool, the most common typeof failure was over the brazed joint where the entire bitbroke away from the body. There were two majorcauses of this type of failure: (i) the brazed joint wasput into tension during the return stroke when the bitfrequently snagged on irregularities in the slot or whensmall chips of rock wedged between the bit and thebase of the slot. (ii) Because of irregularities in the rockand especially inclined rock faces at the commencementof cutting, side loads developed on the bit and, underextreme conditions, pushed it out by shearing thebrazing material.

Many difficulties were overcome in a series ofdevelopments which resulted in the design shown inFig. 4b which incorporates all the successful changes.Essentially the bit has been put into a socket thusimproving resistance to side loads and to damageoccurring during the return stroke. The leading facehas a negative rake of 100 which reduces the plasticflow and the fracturing of this face. The long axis ofthe bit is approximately in the direction of the resultantof the forces acting on it. The trailing edge, C, of thefront face is well supported, so that the bit itself is moreresistant to fracture. The tool, in effect, remains sharpfor longer since the front face is smaller and becomesequal to the area of the front face of the initial designonly after considerable wear has taken place. Sharpedges have been removed from the front face tominimize stress concentrations in the bit.Blade and tool body

Tool performance was unpredictable and it wasdecided to provide means for fitting and replacingcutting tools easily in the stope. Clamping and wedgingdevices used to hold the tool body in position in theblade had, like the blade itself, to be narrower than thetungsten carbide bit by at least 6 mm, which was theclosure expected when a single slot was cut to a depthof 0.3 m.

140 FEBRUARY 1971

A 12.5 mm thick by 150 mm wide blade was originallyconsidered sufficiently strong to carry the cutting forcesgenerated in a tool bit 18 mm wide, but experiencewith the prototype indicated that the cutting forcesacting on the bit were about twice those expected bylinear scaling of the forces measured in the laboratoryexperiments. These greater forces made it necessary toincrease the dimensions of the blade to 14.5 mm by250 mm for the machines which were built by Delfosand Atlas Copco. At this stage, the blade material waschanged from EN9 to 'Abrasalloy', a proprietary high-strength abrasion-resistant steel. The early trials hadproved that, under conditions of high vertical stress,slabs of rock separate from the face during cutting andthe effective slot depth remains small so that thisthicker blade would not be pinched by closure in deepslots.

The original tool body shown in Fig. 5a was retainedin the blade by a wedge drawn in by an AlIen screw.The large penetrating forces tended to rotate the toolbody forward, generating a high reaction force on thefinger retaining the wedge; this resulted in distortionof the finger and loosening of the tool body in theblade. When this weakness became apparent, the designin Fig. 5b was adopted. The acute angle between thetwo seating faces of the tool body ensured that the toolbody was seated securely in the blade by the cuttingforces, and the forces on the finger retaining the wedgewere relieved considerably.

In all the designs, the tool body was seated in theblade on V-shaped edges so as to resist forces in adirection transverse to the plane of the blade. These

t'~s_ullant of cutting forces

I ~x""cted to fall within

Tool,

'/i1'\se

I,~t~

Penetroting

'

,'

-~,

-,

',

'//'/:;11

force'5~ L,- /'_

I",I f'" ."

-_JIg

"- \ \~ t Wedge Blode

g>~

H 101

~.~ rn //

I17

0120'

17

=c---~=I~

""left hand, right hond stud

I b)

Icl

Fig. 5-Methods of fastening tool body into blade:(a) Original design.(b) Improved design with reduced loading on wedge.(c) Present wedgeless design with resilient retaining

finger.


transverse forces were caused by uneven rock surfacesand sometimes were responsible for a high proportionof tool and blade failures.

Retaining the wedge by means of an AlIen screw wasnot satisfactory, as the hexagon socket was oftendamaged or became packed with dirt, making itstightening or removal difficult. The use of double-ended,left and right-hand threaded studs resulted in only aslight improvement (Fig. 5b). Finally, the screw waseliminated by providing a resilient finger and drawingthe wedge into place where it was held by a softtransverse rivet. However, this modification requiredhigher standards of manufacturing accuracy. Also, theflat contact face between the wedge and the tool bodyallowed transverse movement between these parts.

A design without a wedge was the next logical step(Fig. 5c). To reduce the degree of accuracy required inmanufacture, the resilient finger was made more flexibleby increasing its length. The wedge angle was calculatedto give an adequate pre-load when the tool body wasdriven into the blade. The tool body was secured inposition by means of a soft rivet through the V-shapedjoint on the resilient finger.

The wedgeless design is still in use but may bemodified to provide greater strength against transverseforces. A tongue-and-groove seating would providegreater resistance to bending than does the V-shape.

Tool feed mechanism

The tool is fed into the slot manually by means of ascrew. Initially, the screw engaged with a nut fastenedto the back end of the blade. The blade was mountedin a dapper or pivoted member which was free to rotateby 5° to provide automatic withdrawal of the tool fromthe base of the slot of about 5 mm during the returnstroke (Fig. 6a). The penetrating force on the tool bit,acting through the blade, was taken in tension by thescrew. The screw was anchored at its other end to aswivel, co-axial with and above the dapper pivot, whichallowed the screw to swing with the dapper. The cuttingforce on the tool bit was balanced by reactions at thedapper pivot and the dapper stop.

The high penetrating force encountered in the under-ground trials resulted in severe bending of the bladeand screw. This was due mainly to the eccentricitybetween the axis of the screw and the plane of the blade.Also, the tool did not withdraw sufficiently on the returnstroke, especially in the early stages of cutting a slotwhen the radius of rotation of the bit was smallest.Furthermore, this mechanism was completely exposedand quickly became covered with abrasive grit.

The first modifications to the tool feed mechanismwere made in collaboration with Delfos and Atlas Copco(Fig. 6b). The thicker and wider blade was housed in arobust dapper and the whole mechanism was protectedby a cover plate. The feed screw was' positioned in theplane of the blade and engaged with a rack machinedinto the side of the blade. The screw was anchored to thedapper and took the penetrating force in compression,transferring it ultimately to the dapper pivot. The otherreactions were taken up by the machine frame at A andby the dapper pivot. This mechanism worked betterthan the previous one but the components deterioratedrapidly, and cutting the rack on the blades was anexpensive operation. The cutting force caused highengagement loads between the blade rack and the screw,which increased as the blade was fed into the slot,with the result that the screw became indented. Rockfragments sometimes lodged between the stop at A and

Co,e' plate Clappe' stops

Screw onc ho,

Clappe' pi,o!

la)

A B C

IClappe, pi,o! 0

0

~Co,", plate~ 'I

I

'------

I,II-,

Ib)

Retu,n spring------

Stop block

\\

Co,", ptate -; :b-

n _n_~Icl

Fig. 6-Clapper layout.(a) Original design with offset feed screw and narrow

blade.(b) Improved design with feed screw in the plane of

the blade.(c) Present design with simplified blade.

the dapper box with the result that the large unsupportedreaction at B permanently deformed the dapper. Whenrock fragments lodged in front of the leading edge of thedapper at C, they prevented rotation of the dapper andtool withdrawal, resulting in tool damage during thereturn stroke.

An optimum position for the dapper pivot wasdetermined after much experimentation. The withdrawalof the tool bit during t~return stroke is determinedby the position of the pivot and the angle of rotation ofthe dapper. A withdrawal of approximately 13 mm canbe obtained with a rotation of 10°, when the pivot is250 mm from the front face of the machine and 40 mmbehind the bit, as shown in Fig. 6c. The withdrawal canbe increased by increasing the 40 mm separating the bitand the pivot centre line. However, the penetrating forceacting on the bit can then cause the dapper to rotateand the tool to withdraw on the forward stroke. Thiswas found to leave a lump at the base of the slot, whichinvariably caused damage to the bit on the subsequentreturn stroke. Experience has shown that, with thepresent position of the pivot, the tool never withdrawson the forward stroke and a withdrawal of 13 mm onthe return stroke is adequate.

Fully automatic feed mechanisms have been considered,but very robust and elaborate arrangements would berequired to make them foolproof. Furthermore, due tovariations in rock stress and hardness, the rock cut-tability on a stope face varies from place to place and acontinuously variable feed increment is desirable. Con-sequently, further developments have been continuedwith the dapper and normal screw feed mechanism.


.The latest arrangement, developed in collaborationwIth Anderson Mavor and KlOckner-Ferromatik isillustrated in Fig. 6c. A return spring has been addedto act on the clapper to ensure that the tool bit is notin the withdrawn position at the commencement of thec.utting .stroke. Replacement of damaged tools is some-tImes dI~cult at the stope face, and the mechanism hasbeen desIgned so that the blade can be removed quicklyand replaced by a spare blade fitted with a new tool.The assembly of the lower feed mechanism was difficultbecause its weight had to be supported in the stope.!he c°I!lpone~~s have now been arranged so that eachIS held m posItIon separately and the lower mechanismc~n be assembled one part at a time. The screw engageswIth t~e ?lade, as in the original design, through a nut,but thIS IS now arranged so that the screw lies in theplan~ of ~h~ blade. The nu~ is allowed a slight degree ofrotatIOn InsIde a block whIch is fixed to the back of theblade~ and the screw pulls on the clapper through aspherIcal washer. These arrangements eliminate thedanger of bending the screw. A lip, D, has been addedto t.he clapper t<;>prevent the ingress of rock fragmentsdurIng the cuttIng stroke. The main reaction' to thecutting force is taken through the blade by a replaceablehardened stop block. The other reaction is taken by theclap per at a block which is also subjected to the directpull of the screw. The design of this block, as part of theclapper, has been the main subject of the latest modi-fications.

Bed and saddle

. During slotting the cutting tools are guided in theirlm~ar path by the bed along which the saddlerecIprocates.

The bed on the prototype consisted of an unmachined165 mm diameter circular steel tube of 6 mm wallthickness. To stiffen this tube, a 50 mm thick webfabricated from 6 mm steel plate was welded to it (Fig. 7a).The saddle was made from thickwalled mechanicaltubing. For high stiffness, consistent with lightness, itwas bored eccentrically before slitting. Two flat plateswere gussetted on either side of the slot to enclose a portionof the web. Brass bearing material was fused directly ontothe internal surfaces of this saddle over an axialdistance of 15 cm at each end. The' axial separationbetween the two end bearings was formed to provideand maintain a moment arm and allow the saddle tofollow the uneven, unmachined bed. To reduce theweight of the saddle further, the tube was bored outbetween the bearings, concentric with the outer surface.

Saddle

. Bed

/

Hydraulic cylinder Brass bearing

la)

Fig. 7-Bed and saddle bearings.(a) Prototype, unmachined bed.

142 FEBRUARY 1971

To facilitate the mounting of the tool feed mechanismthe outer section of the saddle was made square bywelding on 50 X 50 X 8 mm steel angles.

Tool guidance with this arrangement was inadequatebecause of large clearances and forces greater than hadbeen expected. To improve tool guidance the bed on thefirst. Delfos and Atlas Copco machine was of rectangularsectIOn and was machined on the bearing surfaces(Fig. 7 b). This bed consisted of a hollow rectangularsteel section 250 X 154 X 9.5 mm thick to which waswelded a 62 mm fabricated web. The improved toolguidance obtained with this beam was satisfactory andthe rectangular form remained basically unaltered untilrecently.

Hydraulic cylinder Bran.. bearings

Saddle

Fig. 7(b) Improved machine with rectangular machinedbed.

Because of its open C-shape, the saddle is quiteflexible, particularly when transverse forces act on thetool tips and apply torsion about the axis of the bed.The ~ectangular s~ddles of the first three Atlas CopcomachInes were fabrIcated from 13 mm plates and stiffenedby three encircling ribs made by welding on 38 X 76 mmrectangular bar. This appeared adequate at the timebecause the overall lack of rigidity of the machine wasdue mostly to the method of supporting the machine inthe stope on standard mine props. However with the~ntroduction of the fixed twin-props, which gave muchImproved support to the frame, the excessive flexibilityof the saddle became evident. Furthermore, no allowancehad been made for taking up wear of the phosphorbronze liners between the saddle and the bed (Fig. 7b).As these wore a considerable amount of slack developedand the direction of the cutting tools became controlledby the rock rather than by the machine.

In the current series of machines built under contractby Anderson Mavor and KlOckner-Ferromatik the saddlehas been. stiffened considerably by fabricating it from50 ~m thICk steel plate o~ by using a casting of equivalentsectI<;>n.Means for takIng up liner wear have beenprovIded as follows: Wear in the horizontal direction istaken by.means of screws and locknuts pushing the twonarrow hn~rs at th~ back of the bed (Fig. 7c). Thesescrews are m a relatIvely protected position. A differentarra!1gem~nt ~ad to be used for taking up wear in thevertIcal dIrectIon because any screws extending above orbelow ~he saddle would .be damaged very quickly duringoperatIon and the workIng loads on the bearings wouldbe concentrated on the adjusting screw ends which wouldbe overstressed. A system of wedges pulled together bymeans of screws in the body of the saddle was usedinstead.


Ve";<o1 adj,,'ment to< wear

,'earlng plate' "F;,h lall" mt;an web

Saddle

\Bed 'ab"cated by wetd;n,tagether two channel,

Fig. 7(c) Later version of (b) with adjustable bearingclearance.

The wedges operate quite well but the screws at theback of the beam are not easily accessible. Also, the costof manufacture and internal machining of the saddle ishigh. New methods of achieving adequate rigidity of thesaddle and of taking up linear wear were thereforesought.

The solution proposed by KlOckner-Ferromatik (Fig.7d) and incorporated in their latest machine is promising.The bearing surfaces are arranged in the form of threeV's and adjustment for wear is effected by pushing the pairof V's at the back of the bed. This is done by means ofwedges, as is the vertical adjustment shown in Fig. 7c,except that these wedges are pushed in a direction parallelto the axis of the bed by screws which are readilyaccessible at the open ends of the saddle. It is intendedin the next machine to replace this manual-mechanicalmethod of adjustment by an automatic method.

Fab",ated bed

Adhe,," bood lIne' fa "ddle

Saddle

Fig. 7(d) Latest type with simplified bearing adjustment.

If an automatic method of taking up wear provesreliable, it will be possible to rely on the re-entrantV's at the back of the bed to increase the stiffness of thesaddle, allowing its section to be reduced. Meanwhilethe manufacture of the saddle had been simplified byusing two heavy forged steel channel sections which aremachined on their internal surfaces before being weldedtogether along their two edges. After stress-relieving, oneof the welds is cut away to form the opening of theC-shape. No further machining is required on the internalsurfaces.

Frame structureThe machine frame, of which the bed forms a major

part, transmits the reactions due to the cutting forcesto the supporting hydraulic props. The frame should bevery rigid so that the bits are deflected as little as possiblefrom their intended linear path during cutting.

The props should be disposed so that support isprovided as close as possible to the cutting tools. It isnecessary, however, to allow a clearance of 0.7 m for theblades in their fully withdrawn position and for the feedscrews which must pass between the rock face and thesupporting props. Fortunately the end supports can beplaced much closer to the face as they are beyond thetravel of the saddle. The disposition of the props out of astraight line increases the rigidity of the frame. It alsoprovides stability during positioning, as the centre ofgravity is within the boundary of the base formed by thefour downward pointing props on which the machine israised.

The weakest link in the transmission offorces from bedto props is the relatively thin straight line junctionbetween the bed and the web. This is unavoidable becauseof the need to accommodate the bearing strips in thelips on the C-shaped saddle. When the twin-prop systemwas introduced it became necessary to reinforce the webbecause the eccentricity of the twin-props introducedlarge couples when the machine was set in position inthe stope. The web was then fabricated from parallel flatplates separated by a zig-zag spacer while the gussetssupporting the prop mounting plates were external tothe web (Fig. 7b). In all subsequent machines a fish-tailsection has been used (Fig. 7c, d) in which all bracing andgussetting is internal. In addition to providing greaterstrength and rigidity this arrangement provides an areain which hydraulic tubing can be protected. However, theclearance between the web and the reciprocating bladesand feed-screws has been reduced. This may causedifficulties when fallen pieces of rock accumulate in thetrough between the bed and the upper slope of the web.

Anchoring and moving of the rockcutterMoving the prototype rockcutter on its sled parallel to

the face ~ith a chain block proved to be surprisingly easy,but movIng it towards the face was difficult. To facilitatepositioning of the machine close to the face the sled wasmodified to permit the jack to slide transversely on thesled after removal of a holding pin. This rendered theoperation dangerous because, after lifting the machineoff the .sled and removing the pin holding the jack, it wasvery difficult to control the movement of the machineon top of the sliding jack.

The original concept of the setting-up procedure forthe machine was not successful as the balance of themachine, while it was being lifted at its centre of gravitywas easily disturbed by the weight of a prop. It wa~intended that the props should slide easily in their clampsso that their weight would not affect the balance, but thiscould not be achieved. To overcome this difficulty, theprops had to be removed completely from the clampsbefore the machine was set up.

To improve control of the machine, the prop clamp atthe tail end was modified so that it would swivel. Thesetting-up procedure was then to lift the machine, positionthe tail end by hand and insert and set the tail end prop.Since the tail end prop clamp could swivel, it was possibleto manoeuvre the other end of the machine while thetail end was held in position. The worst feature of thismodification was that the swivel at the clamp was amechanically weak point.

JOURNAL OF THE SOUTH AFRICAN INSTITUTE OF MINING AND METALLURGY FEBRUARY 1971143

The use of ordinary props for anchoring the machineresulted in much wasted labour since the props had to beput in and taken out of the clamps every time the machinewas set in a new position. On occasion, when the machinewas set up near the hangingwall the props had to beinverted since the clamps could grip only on the externalcylinder of the props. It was necessary to move out thesled and jack from under the machine while it was cuttingbecause rock cut from the face would fall onto the sledand could not be removed easily. From the foregoing itwill be evident that the process of moving the machinefrom one cutting position to the next was both lengthyand arduous. In practice the operation was seldomachieved in less than an hour.

This system of anchoring proved to be too flexiblebecause the props were not designed to withstand sideloads and had excessively large bearing clearances. Also,there was some clearance between the clamps and theprops which could never be set up exactly parallel toone another. On the prototype only three props wereused and were arranged nearly in line. Consequentlytransverse forces on the tools caused large verticalmovements of the machine. The introduction of a fourthprop, offset from the line of the other three, reduced theextent of this movement considerably.

When it was realized that the machine would have tobe supported more rigidly and that moving it in thestope took more time than cutting, a complete re-designof the moving and anchoring system was undertaken.

The standard props were replaced by twin props keyedand bolted permanently to the machine. These twin propsconsisted of two parallel adjacent prop cylinders rigidlyconnected together, with one ram extending upwardsand the other downwards. This eccentric twin proparrangement made it possible to have large bearings inthe props to carry side forces and still have a goodexpansion ratio. The closed height of the assembly is0.48 m and the fully expanded height 0.99 m. Theexpansion can be increased when necessary by the additionof extension pieces which fit into female conical surfacesin the rams providing bending stiffness so that side forcescan be transmitted through the extension pieces. Whenthe extension pieces are not required they are replacedby short caps which fit into the same female conicalrecesses.

Separate controls are provided in a central controlvalve block for each of the eight rams in the four twinprops. The lower rams are used for raising, lowering andtilting the machine. To ensure stability the props aredisposed so that the centre of gravity of the machine fallswell within the area enclosed by the lower rams. Whenthe machine is in position the upper arms are expandedto anchor it. Each pair of cylinders in each twin prop isconnected hydraulically through a non-return valvewhich makes it possible to set the load by extending thetop ram without disturbing the position of the lower ramand, therefore, that of the machine. When pressure isapplied to set the prop the non-return valve opens and thesame pressure is applied automatically to both rams.The twin props are activated by an air powered hydraulicpump supplying a two per cent oil-in-water emulsion ata pressure of 250 bars.

A small skid plate is fixed under the body of each ofthe twin props and the machine rests on these plateswhen the lower rams are retracted. The machine is towedparallel to the face and positioned with two doubleacting hydraulic cylinders towards or away from the face.These two hydraulic cylinders are pin-jointed loosely toeach end of the machine and are activated through the

144 FEBRUARY 1971

central control valve block. They react against propspre-set in the stope at suitable positions.

A very rugged wrap-around bumper guides the move-ment of the machine and protects the hydraulic controlsand hoses. Standard hydraulic props are used in the stopefor face support and are placed in a straight line about1.5 m from the face. The machine is trapped between theprops and the face, and the bumper allows it to slideagainst the props.

This arrangement of twin props, fixed skid plates,permanently attached positioning jacks and wrap-around bumper, has resulted in a machine of greatlyimproved mobility which can be moved and set up inless than ten minutes. The resultant saving of nearlyone hour in the cutting cycle time has proved to be themost significant step towards the development of areliable production rockcutting machine.

The development of the twin props, carried out incollaboration with Kl6ckner-Ferromatik, has progressedin a number of small stages. The bearings and seals in theprops have been improved. The two cylinders have beencombined into a single forged rectangular block and allhydraulic conduits and connections are drilled into thisblock. The number of hose connections has been reducedto a minimum. They are of a patented type consisting ofa spud with an a-ring and a simple connecting staple.

Hydraulic driveThe hydraulic cylinder used to drive the saddle in the

prototype machine consisted of a cylinder of 76 mm dia,3 m long, with a 38 mm dia piston rod. It was originallyoperated at a maximum pressure of 200 bars giving amaximum cutting force of 68 kN. The slender piston rodhad to be protected against buckling in case the saddlestruck an obstruction during the return stroke. A by-passvalve was therefore incorporated into the piston to limitthe pressure difference between the two sides of thepiston during that stroke (Fig. 8a). This valve was alsoused to unload the hydraulic system at the end of thecutting stroke when the valve was opened by a probe,causing the oil to flow past the piston.

Very early in the testing programme it became obviousthat the maximum cutting force would have to beincreased. The cylinder was reinforced by winding itwith wire under tension and the maximum pressure wasincreased to 250 bars. However, the piston rod could notbe strengthened and its factor of safety was reduced toa Iow value.

All machines built after the prototype have hadcylinders of 110 mm dia with 48 mm piston rods. Amaximum operating pressure of 170 bars provides amaximum cutting force of 130 kN. The by-pass valve inthe piston has been dispensed with and hydraulic cushionshave been introduced to obviate impact at the end ofeach stroke. The danger of buckling the piston rods wasreduced by the adoption of a lower operating pressurebut protection against this danger was later achieved bythe introduction of a fast return stroke device whichreduces the maximum force automatically during thatstroke.

In the earliest machines a simple hydraulic circuit witha separate control unit was used which was connected tothe machine and to the power pack by flexible hosesprovided with self sealing, snap-on quick couplers. Thecontrol unit consisted of a lever operated, closed centre,two way control valve, a pressure relief valve set at themaximum working pressure and a pressure gauge, allmounted in a tubular steel protective frame (Fig. 8a).


Column coupling

Filter

(~\,_/ \. ./

(~\

'-' '_J

Hydraulic power pack

Quick couplers Pressure relief valve

-----

By - pass valve in piston

Control valve

Anchoring diaphragm

Flexible hoses

Return stroke pressure limiting valve

Fig. 8-Cutting cylinder hydraulic system.(a) Prototype without rapid return stroke.

These early machines were slower on the return strokethan on the cutting stroke because fluid was pumpedinto the crown side of the cylinder for the return stroke. Aquick return stroke was achieved by connecting bothends of the cylinder to the pump. The differencebetween the areas of the crown and annulus sides of thepiston providing the return force. The supply from thepump had only to displace the piston rod, the remainingflow into the crown side coming from the annulus sideof the piston. This system suffered from the resulting highrates of flow in the valves. The 68 litre/min supplied bythe pump combined with 292 l/min from the annulusside of the cylinder to provide a total flow of 360 l/mininto the crown side. Commercial high pressure valvesare not made for such high rates of flow and excessivepressure loss and heating of the fluid occurred.

A special rotary valve with large ports was thendesigned at the Mining Research Laboratory which madeit possible to obtain an efficient quick return motion bythe method described above6. The operation of this valveis illustrated in Fig. 8b. The annular side of the cylinderis always connected to the pump and:

(i) for cutting, the crown side is connected to thetank

(ii) in neutral, the crown side of the cylinder is shutoff and the pump is connected to the tank andfluid flows freely through the valve

(iii) for the return stroke, the crown side is connectedto the pump and fluid is then transferred fromthe annulus to the crown side as the piston rod isdisplaced.

On the return stroke the speed is increased and thestalling force decreased by a factor of 4.3.

The rotary valve was mounted direct onto the glandend of the cylinder and was connected to it withlarge bore steel tubes without any restrictions. The

Quick couplers /-

Flexible hoses /-

Hydraulic cylinder

VCushion pistons

-,/ Control valve.Rigidlyconnected to cylinder

Gauge FilterFlow,I---j =:1~, _1/

' 1'.~! \_-'

Tank 1I

, I'\ , I- -' 1// I

Pressure relief valveX- - - - - - - J

Hydraulic power pack

Cuttingstroke

Neutral RapidretUfnstroke

III.2 r::

E ~

J~) ~ .,6/ ~ 'I

~ P ~if W,5!

Connections: Pump to annulus Pump to annulusand tank

Crown to tank Crown blanked

Pump and annulusto crownTank blanked

(b) Present self-contained design with rapid returnstroke and simplified connection.

pressure relief valve, the pressure gauge and the filterare all placed on the power pack and hydraulic hosesfrom the power pack are taken direct to the machine.The cylinder and control valve are fitted to the machineand removed from it as a single unit. This arrangement


has caused difficulties regarding the strength of theframe but from an operational point of view it has provedreliable over more than a year of underground testing.

The cylinder has normally been anchored to the bedby means of a threaded spigot at the closed end of thecylinder which passes through a diaphragm in the hollowbed and is secured by a nut. In machines supplied byAnderson Mavor a transverse pin passing through aneye in the end of the cylinder has been used. In futuremachines different means of securing the cylinder to thebed will possibly be used which will incorporate thehydraulic connections to the valve. These connectionswill be built into the frame with the tubing, rather thanbeing in unit with the cylinder as in current machines.It will then be possible to place the valve in the mostadvantageous position from an operating point of viewand without affecting the strength of the frame.

Drill jig

Initially the blade tended to become damaged at theend of the stroke because, as the slot became deeper, thefinger in the front bumped against the end of the slotand took the impact and full cutting force generated bythe cylinder. A 62 mm clearance hole is therefore drilledinto the face before cutting each slot, so that the tooland blade reach the end of the stroke in the hole.With the first machine this hole was located by eye, buton subsequent machines a drill jig, detachable from themachine, was used to locate the hole accurately.

The first design of jig weighed 180 N and carried twodrill rods for drilling a hole for each of the two slots.While this jig was relatively easy to instal in a workshop itwas very cumbersome underground. It was re-designed tocarry only one drill steel and had to be set up separatelyon top and then on the bottom of the machine frame.Setting up on the bottom side was awkward and timeconsuming.

The latest jig consists of a guide fixed permanently tothe frame on the far side from the face and a smalldetachable guide which is fitted into a hole in themachine bed. The drill steel carries one small bush whichfits into the detachable guide while the other end of thesteel passes through a slot in the fixed guide. With thisarrangement setting up is quick and easy.

Water sprays

Water sprays are needed for cooling the tools, cleaningthe slots and for dust suppression, although very littledust is produced. Experience in tunnelling applicationsof drag bits has indicated that, in order to minimizethermal shock loading of the tungsten carbide, the watershould not be sprayed directly onto the bits. Consequently,two water jets are used in line with each tool and aredirected into the slot immediately in front of and behindthe blade.

On the earlier machines, the water was carried in pipesto the nozzles projecting in front of the machine. Thesepipes and nozzles were very vulnerable to damage andon later machines the water was carried through internalducts in the saddle to counter sunk nozzles which areremovable.

FUTURE TRENDS

Underground tests at the Doornfontein Gold MiningCompany, Limited have shown that the basic conceptbehind the rockcutting machines can result in a workable

146 FEBRUARY 1971

mining system. Much effort is being devoted to improvingthe mechanical reliability of the machine and ancillaryequipment, and it is feasible that within the period ofabout a year satisfactory reliability will be achieved.However, the efficiency of mining would be greatlyimproved if the machines could work on a straightface7. In deep mines where there are high stresses on thefaces the rock fracture pattern is such that it tends tostraighten the faces and it is very difficult to maintainthe benches from which a rockcutting machine can startits cutting. Machines with different configurations suchas illustrated in Fig. 1b could effectively work a straightface. But it is possible to adapt a machine with the linearaction to work a straight face if oblate holes, largeenough to accommodate the blades at the beginning ofeach cut, could be made in the face. The addition ofdevices to the machine to generate these openings isfeasible and is receiving attention.

Some difficulty has been experienced in cutting veryhard rock and possibilities exist for supplementing thedrag bit cutting action with other methods of cuttingrock. It has been found that rapid heating of the rockin the path of the bit can cause sufficient damage to itto reduce the forces required to cut it by about a hairs.Fortunately, thermal damage is most effective in thoserocks which are hardest to cut. Also, the addition of avibratory force might assist in the cutting of the harderrocks.

CONCLUSION

An experimental rockcutting machine was designedprimarily to test the feasibility of cutting rock under-ground. It was also designed to form the basis of amachine for continuous, selective mining.

Extensive testing of similar machines has shown themto be capable of cutting rock underground and, after aseries of developments, to be capable of initiating miningby this method. The initial concept was sufficientlyworkable and further developments did not departradically from it. These developments have resulted in amachine which is highly maneouverable and able to cutrock effectively, but further minor developments are stillnecessary before the machine can be regarded assufficiently reliable for use in production.

Two major developments are being considered whichwould improve the efficiency of rockcutting machines.The first is a modification to the present machines or anew design which would permit the machine to workon a straight face. The second is a hybrid design,incorporating heat or vibration, which would enable themachine to cut the hardest rocks.

ACKNOWLEDGEMENTS

The work described in this paper forms part of theactivities of the Engineering and Simulation Divisionsof the Mining Research Laboratory of the Chamber ofMines of South Africa.

The authors are indebted to many of their colleaguesin the Laboratory, among the Manufacturers and on theDoornfontein Gold Mine for their help, but especiallyto Dr N. G. W. Cook and Mr J. H. Roux for theirinvaluable support and assistance.


REFERENCES

1. COOK, N. G. W. 'The design of underground excavations.'

Eighth Rock Mechanics Symposium, University of Minnesota,

1966, in "Failure and breakage of rock," C. Fairhurst, Ed.

2. HoDGSON, K. and JOUGHIN, N. C. 'The relationship between

energy release rate, damage and seismicity in deep mines.'

Eighth Rock Mechanics Symposium, University of Minnesota,

1966, in "Failure and breakage of rock," C Fairhurst, Ed.

3. COOK, N. G. W., IMMELMAN,D. A. and MRosT, M. 'Economic

factors affecting planning research and development for large

South African gold mines.' Ninth Commonwealth Mining and

Metallurgical Congress, London, May, 1969.

4. COOK, N. G. W., JOUGHIN, N. C. and WIEBOLS,G. A. 'Rock

cutting and its potentialities as a new method of mining.'

Jl. S.Afr. Inst. Min. Metal/., 68 (10), 1969, 435.

5. OBERT, L. and STEPHENSON,D. E. 'Stress conditions under

which core discing occurs.' Trans. Soc. Min. Engrs., 232, 1965,

227-34.

6. PALLISTER,G. F. and COOK,N. G. W. 'A note on a quick-return

valve for an hydraulic actuator.' S.Afr. Mech. Engr., 20 (7),

1970, 226-28.

7. JOUGHIN, N. C, et al. 'Progress towards a method of mining

gold reef selectively and continuously.' To be submitted to

Jl. S.Afr. Inst. Min. Metal/.

8. W AGNER,H., et al. 'Laboratory experiments on cutting hard

rock.' To be submitted to Rock Mechanics.

Fig. 9-Prototype machine operating in a stope.

Fig. 1G-lmproved twin-prop machine.


thedesignanddevelopment ofa rockcutting machineforgoldmining

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