Surface Drilling

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Surface Drilling

in Open Pit Mining

First edition 2006

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Thinking young takes practice

Atlas Copco Construction & Mining Technique

www.atlascopco.com/cmt

Working with Atlas Copco gives you access to more than a century of rock drilling innovation. It ensures that you work with a solutions provider who delivers the best-performing systems, products, and people available – today and tomorrow. Our success in construction and mining is based on the com-bination of young minds and long experience. Get your free copy of Success Stories at www.atlascopco.com/rock Committed to your superior productivity

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Drilling in open pit mining

Foreword

2 Foreword by MarkkuTerasvasara,

Vice President Marketing Surface Drilling Equipment at Atlas Copco Rock Drills AB

Talking Technically

3 Open Pit Mining

7 Principles of Rock Drilling

10 Principles of Rock Blasting

12 Putting Rotary Drilling Into Perspective

15SmartRigTakes Control

17 Correct Selection of

Tophammer Rock DrillingTools

20COPROD Combines the Best of DTH

andTophammer Drilling

25Increased Productivity with DTH Drilling

28 Selecting the Right DTH DrillingTools

34Economic Case for Routine Bit Grinding

38Ergonomics & Safety

41Rotary Club Expands WithThiessen

42In Search of the Right Balance

45Protection by ROC Care

Case Studies

47 A Viper for the Copper King

49Best Practice at Bingham Canyon

51Rotary and DTH Work Side by Side

54 The GoldenTwins of Southern Mexico

58 Minera Maria’s Pre-Split Pioneer

61ROC L8 Gains Favour in Brazil

65HydraulicTophammers Exceed Soquimich

Expectations

67Winning Iodine from the Atacama Desert

69 Replacing Rotary in Iron Ore

70 MultipleTasking in Western Australia

73 ROC L8 Outperforms in Assmang Iron Ore

74 Greater Geita

78DTH Choice Cuts Costs at Navachab

80 Iron Ore From Erzberg Mountain

82 MoreThan a Match for Scotland’s Coal

84Apatite for Extraction

– Coprod Solution for Siilinjärvi

Product Specifications

88 Drilling Method Guide

90 Tophammer Drill Rigs

97 Tophammer Rock Drills

99 Tophammer DrillingTools

110 COPROD System

111 COPROD Crawlers

114 COPROD Drill Rigs

115 COPROD DrillingTools

118 DTH Drill Rigs

124DTH Hammers

126Rotation Units

127Rotary Drill Rigs

131SecorocTricone Bits

133Secoroc Grinding

138Drill Rig Options

147Service Workshops

148ConversionTable

Front cover: Blasting time at Aitik Copper Mine in Sweden. SmartRig, COPROD, ROC and COP are Atlas Copco trademarks. Atlas Copco reserves the right to alter its product specifications at any time. For latest updates contact your local Atlas Copco Customer Center or refer to www.surfacedrilling.com

Produced by tunnelbuilder ltd for Atlas Copco Rock Drills AB, SE-701 91 Orebro, Sweden, tel +46 19 670-7000, fax –7393. Publisher Ulf Linder [email protected] Editor Mike Smith [email protected] Senior Adviser Hans Fernberg [email protected] Picture and Specifications Editor Lisa Boyero [email protected]

Contributors Alf Stenquist, Bo Persson, Brian Fox, Göran Nilsson, Gunnar Nord, Hans Fernberg, Jan Jönsson, Jean Lindroos, Jessis Ng, Joanna Jester, Leif Larsson, Lennart Lundin, Lennart Söderström, Lorne Herron, Mathias Lewen,Therese Blomster, all [email protected], Adriana Potts [email protected], Maurice Jones [email protected]

Copies of all Atlas Copco reference editions can be ordered in CD-ROM format from the publisher, address above, or online at www.atlascopco.com/rock. Reproduction of individual articles only by agreement with the publisher.

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Foreword

T

o be perceived as the global market leader in providing rock drilling products, it is important that our product and service offering has the competitive edge: that we can assist our customers to generate high profit leading to business expansion. Our growth is a consequence of our customers’ success.

We have also grown by making strategic acquisitions such as the drill rig division from Ingersoll Rand, and by launching new, efficient products and aftermarket programmes during the past five years. Today, our range of drill rigs comprises more than 40 models suited for various surface drilling applications.

Our modern products are equipped not only with key components such as powerful rock drills, engines, pumps and compressors, but also with the latest computer based technology.

We have never before been committed to such a dynamic and intensive product development, giving a whole new dimension to quality and productivity in terms of directing and guiding the equipment to perform drill holes as close as possible to plan, to planned depth and hole bottom locations. This is a prerequisite for optimum fragmentation of blasted rock, even benches and rock wall contours.

High productivity, as a result of outstanding equipment availability and drilling capacity, leads to better utilization of the investment. Our long-standing relationship with Secoroc has allowed us to develop drill string components, bits, and high pres-sure DTH hammers to match the potential of our rock drills and rigs, together with bit grinding to maintain high performance with economy. The second generation of Coprod for straighter small-diameter holes is a prime example of how successful this partnership has been.

Our new computer based rigs are known as the SmartRig concept, emphasizing that they have incorporated state-of-the-art functionality, making them easy to use and maintain. Additionally, we have spent a lot of effort in providing a good working environment inside the operator’s cabin.

We trust that this book, presenting not only our current product offering, but also some examples of best practice at selected operations, will stimulate technical interchange between people having an interest in surface drilling in open pit mining. Those engaged in mining projects, technical consultancy, universities and our own sales and marketing efforts should, hopefully, find a lot of valuable reading material.

Markku Terasvasara

Vice President Marketing Surface Drilling Equipment

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Talking Technically

Drilling in Open pit Mining

Open Pit Mining

Finding the Best

combination

Large quantities of raw material are produced in various types of surface operations. Where the pro-duct is rock, the operations are known as quarries. Where metallic ore or non-metallic minerals are in- volved, they are called open pit mines. There are many common parameters in design and choice of equipment, and in the process of finding the best combination of drilling and blasting methods. Atlas Copco has the advantage of long experience in all types of sur- face drilling operations, with a pro-duct range to match. With its his-tory of innovative engineering, the company tends to think forward, and is able to advise the user on improving design elements of the operation that will result in overall cost savings.

Surface or Underground

Mining

Mining carried out underground can follow and be tailor-made to suit the mineralization zones on a selective higher metal content basis, thereby minimizing the amount of waste rock, which has to be extracted. The amount of ore to be left behind varies depend-ing on mindepend-ing method between 10-35 percent. Waste to ore ratio is typically 1 to 4.

As no orebodies have the perfect co- nical shape, vast quantities of waste have to be removed from both the hangingwall and the footwall to get access to the ore as it progresses in depth. Waste to ore ratio varies extensi- vely depending largely on the geomet-ry of the orebody. Many open pit ope-rations excavate more than 5 times the amount of waste compared to ore. Figu- re 1 shows a sectional layout of a typi-cal pit. The waste to ore ratio increases as the pit gets deeper. Eventually, for

economic reasons, the open pit will be abandoned, or underground mining will take over.

As the ore, compared with under-ground mining, is more diluted and in-termixed with waste and lower grade ore, crushing, screening, milling, flotation etc need high capacity. As mining progresses at depth large quanti-ties of side rock have to be excavated in stages, so called pushbacks. See fig. 2 Aitik.

Open Pit Mining

A typical work cycle in an open pit mine consists of a number of work ele-ments. Exploration drilling is conduct-ed to define ore boundaries for future planning. This is commonly combined with in-pit reverse circulation drill-ing to confirm the mineral contents, which is important for optimizing the blasting and the mineral processing. Drilling of blastholes is undertaken in

Figure 1 General principles of open pit mining.

Figure 2 Aitik open pit mine. Hangingwall extractions in stages..

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a predetermined pattern, followed by plugging the drill holes with wooden or plastic plugs to prevent debris from falling down into the holes.

When an adequate number of holes have been drilled, preparations for blasting will start. The holes are blown clean with compressed air to remove water and rock fragments, and are then charged with booster bottom charges, detonators and explosives. Stemming (decking) is inserted into the top of each hole, and the detonator leads are connected. Where electric detonators are used, the circuit resistance is checked with an ohmmeter. See fig. 3 charging the blast holes at Aitik The area is evacuated, equipment is moved away, and the round is fired. The size of blasts in an open pit is nor-mally much larger (up to a couple of million tonnes) compared to a quarry

is cleaned of fly rock with a wheel loader. Blasted rock is then loaded and removed with huge sized equipment such as drag line bucket loaders into trucks, and transported to the crusher station. Large boulders are pushed aside, and stockpiled for subsequent secondary breaking. Rock is discharged directly, or via a grizzley for size con-trol, into the primary crusher. There- after, it is transported by conveyor belts for secondary, and possibly tertia-ry crushing.

The different products, comprising rock fragments of certain size ranges, are recovered from the process by vib- rating screens, and transported to stor-age silos or bunker piles on the ground. A major difference between open pit mining and quarries is the geological conditions and the demand characteris-tics on the blasted material. Whereas quarries deliver the majority of rock via the crushing and screening plant in various size fractions, the open pit mine attempts to deliver the ore as pure as possible via crushers to the dressing plant, consisting of mills, separators, and/or flotation, and/or biochemical systems, and finally to smelters, in order to convert minerals to metals.

Optimum Fragmentation

and Pit geometry

Without jeopardizing slope stability, it is of prime importance to keep the pit slope angle as steep as possible, maintaining excavated waste at a minimum.

The demands on fragmentation of the waste, as it will not pass through the Production holes Ø 315 mm 1 tonnes explosives/hole Two Boosters Decking 6 m Pipe charge 6,5 m Bottom charge 4 m

Figure 3 Charging the blast holes at Aitik.

where blasts are restricted to a couple of ten thousand tonnes. The individu-al holes and the rows are delayed with short intervals to get good swell for efficient loading.

Before loading, the lower pit floor

Quantity Drillhole diameter Boulders Fragment elongation % fines in blast Micro cracks in fragments Figure 4 Charging the blast holes at Aitik. Metso Minerals.

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Drilling in Open pit Mining

crushing/dressing system, are simple. It should merely suit the loading and trucking equipment used for subse-quent removal to the waste dump. On the other hand, good fragmentation of the blasted ore will make great savings in the total costs of the mineral dress-ing process. By contrast to quarries, where the fines fraction are regarded as reject (especially at aggregates), open pits delivering ore to the dressing plant get savings in the disintegration process in case high amount of fines are present. Large holes generate a higher spread of size distribution from more boulders to higher amount of fines-see fig. 4

Blasting will not only break the rock that is planned to be excavated, but

will also cause damage to the slopes that form the boundaries of the pit. The extent of this overbreak is mainly dependent on the size of the individual charge and its proximity. A common means of minimizing overbreak is to use smaller diameter holes, making provision for restricted blasting in the zone next to the planned bench slope. Figure 6 shows two different blast designs.

Figure 5 shows a typical drilling pattern to be applied in connection with pre-split blasting, to achieve increased slope stability with reduced back break. The huge Chuquicamata open pit (pro-duction rate 650,000 tonnes per day) in Chile being 8 km long and 2.5 km wide and progressing towards a final

Figure 5 Drilling pattern for presplit in open pit mining.

Figure 6 Savings in waste extraction by increasing the pit slope.

depth of 1,000 metres saves a stagger-ing 150 million metres of rock excava-tion by just making the pit slopes one degree steeper. Consequently there are large savings to be made if drilling and blasting is carried out in an optimum way.

Drilling Patterns and Type

of Drill Rigs

Traditionally, focusing on the drill-ing cost parameter and productivity only, the predominant method in open pit mining is large hole rotary drill-ing usdrill-ing hole sizes in the 250 to 400 mm (10-15.75 in) range. No doubt this implies lower cost for drilling, ignor-ing the expense of excess waste, more explosives and less controllable frag-mentation. A survey of 36 open pit operations in Chile using hole sizes between 75 and 345 mm (3.0-13.75 in) reveals that hole sizes above 200 mm (8 in) do not generate any substan-tial savings in total drilled metres per tonne. This indicates that burden and spacing cannot be increased indefinite-ly. One important reason for replacing rotary with other methods is the inflex-ibility of the heavy rotary rigs, which are restricted to vertical benches and single pass drilling only.

Figure 6 shows the advantages relat-ed to using an Atlas Copco ROC L8 rig, having the ability to drill inclined holes close to the bench wall, compa-red to a traditional rotary rig. The best combination of drill rigs might well

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Figure 8 Soufflé blasting at Björkdal gold mine. Figure 9 Charging for soufflé blasting.

prove to be large rotary rigs for waste rock and high productivity in the pit centre coupled with flexible DTH rigs for selective mining, pre split of slopes as well as for in-pit grade control drill-ing. See fig 7 Aitik.

Soufflé Blasting

Mining of rich, narrow and irregularly stratified ore zones, such as gold mine-ralizations, requires extra attention, in order not to introduce unnecessary

Figure 7 Drill pattern at Aitik open pit, Sweden.

Production holes 315 mm 8-9 m drillpattern 15+2 hole depth 2 rows 140–165 mm

quantities of waste into the ore stream. Consequently, this type of mining has to be progressed on a selective basis, in close liaison with surveyors and geologists, by taking frequent samples before and after each individual blast. Short benches and small holes are used to cope with ore zone irregularities. A recently-developed method to ensure maximum recovery from each blast is called soufflé blasting. Figures 8 and 9 illustrate this principle of cautious blasting with a minimum of dilution as

practised at the Björkdal gold mine in Sweden. To a depth of merely 5 m, 100 vertical holes are blasted in one round, without free surface for expansion. The firing sequence starts in the centre and, thanks to 2.5 m of stemming, the blasted ore material just swells on site like baking a soufflé. Selective extrac-tion by backhoe loaders facilitates maximum recovery of the rich, narrow gold-bearing zones.

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Rotary Drilling

Rotary drilling can be subdivided into rotary cutting and rotary crushing.

Rotary cutting creates the hole by

shear forces, breaking the rock’s ten-sile strength. The drillbit is furnished with cutter inserts of hard metal alloys, and the energy for breaking rock is provided by rotation torque in the drill-rod. This technique is limited to rock

with low tensile strength, such as salt, silt, and soft limestone not containing abrasive quartz minerals.

Rotary crushing breaks the rock

by high point load, accomplished by a toothed drillbit, which is pushed downwards with high force. The bit, being of tricone roller type fitted with tungsten carbide buttons, is simul-taneously rotated, and drill cuttings are removed from the hole bottom by blowing compressed air through the bit.

Drillrigs used for rotary drilling are large and heavy. The downwards thrust is achieved by utilizing the weight of the drillrig itself, and the rotation, via a hydraulic or electric motor, applied at the end of the drill pipe. Common hole diameters range from 8 to 17.5 in (200-440 mm) and, because adding the heavy drill pipes is cumbersome, most blasthole drillrigs use long masts and pipes to accommodate single-pass drilling of maximum 20 m (65 ft). Electric power is usually chosen for the large rigs, whereas smaller rigs are often powered by diesel engines.

Rotation rates vary from 50 to 120 rev/min, and the weight applied to the bit varies from 0.5 t/in of bit diameter in soft rock, to as much as 4 t/in of bit diameter in hard rock.

Recent technical advances include: improved operator cab comfort; automatic control and adjustment of optimum feed force and rotation

speed to prevailing geology and bit type and diameter; and incorporation of the latest technology in electric and hydraulic drive systems.

Rotary drilling, which is still the dominant method in large open pits, has limitations in that the rigs are not suited to drilling holes off the vertical line. As blasting theories and practice have proved, it is generally beneficial to design, drill and blast the bench slopes at an angle of approximately 18 degrees off vertical.

Many rotary rig masts have pin-ning capabilities permitting drilling at angles as much as 30 degrees out of the vertical. However, the inclined hole drilling capabilities in rotary drilling are limited by the heavy feed force required, since part of this force is directed backwards. This causes rig stability problems, reduced penetra-tion, and shorter life of drilling con-sumables. Consequently, most blast hole drilling using rotary drillrigs is for vertical holes.

Percussive Drilling

Percussive drilling breaks the rock by hammering impacts transferred from the rock drill to the drillbit at the hole bottom. The energy required to break the rock is generated by a pneumatic or hydraulic rock drill. A pressure is built up, which, when released, drives the piston forwards. Figure 1

Principles of Rock Drilling

Drilling for

excavation by

Blasting

This reference edition deals with surface rock drilling used for the purpose of excavating rock in qu-arries and construction projects by means of blasting. Other types of drilling, such as for oil and water, mineral exploration and exploita-tion, and grouting, are excluded.

The reader is given a brief ex-planation of prevailing drilling me- thods, together with an introduc-tion to blasting techniques and the interrelation of drilling and bla- sting. Also discussed are the main parameters involved when plan-ning and executing blasthole dril- ling at quarries and civil engine-ering projects.

The range of Atlas Copco pro-ducts, with references to the Atlas Copco websites, are presented and discussed by comparing their suit- ability and expected productivity related to a selection of applica-tions. Case studies from worksites around the world should prove in- teresting and beneficial, especially when planning and selecting me-thods and equipment for blasthole drilling applications.

Blastholes have certain unique and important characteristics. These are: hole diameter, depth, direction, and straightness. Drilling produces a circular hole in the rock, the strength of which must be overcome by the drilling tool. Depending upon rock properties, there are several ways to accom-plish this, as shown in the follow-ing article.

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illustrates the principle of top hammer percussive drilling.

The piston strikes on the shank adapter, and the kinetic energy of the piston is converted into a stress wave travelling through the drillstring to the hole bottom. In order to obtain the best drilling economy, the entire system, rock drill to drillsteel to rock, must harmonize.

Stress Wave

Theoretically, the stress wave has a rectangular shape, the length of which is twice that of the piston, while the height depends on the speed of the piston at the moment of impact, and on the relationship between the cross-sectional area of the piston and that of the drillsteel.

The total energy that the wave contains is indicated diagramatically in Figure 2. To calculate the output power obtained from a rock drill, the wave energy is multiplied by the impact frequency of the piston, and is usually stated in kW. Rock drill designers seek to find the best com-binations of various parameters, such as the piston geometry, the impact rate and the frequency. Two rock drills having the same nominal power rating

might therefore have quite different properties.

The shock waves that are generated by hydraulic (Figure 3) and pneumatic (Figure 4) rock drills are significantly different in shape. Drillrods used with hydraulic rock drills will normally show substantially longer service life, compared with pneumatic rock drills, because of the higher stress level obtained with the pneumatic driven piston.

The reason is the larger section needed when operating at substantially lower pressure, which is 6-8 bars, compared to the 150-250 bars used with hydraulic systems. The slimmer the piston shape, the lower the stress level.

Figure 5 compares the stress level generated by three different pistons having the same weight, but with dif-ferent shapes and working difdif-ferent pressures. The lowest stress, or shock wave amplitude, is obtained with the long slender piston working at high pressure.

efficiency and losses

The shock wave loses some 6-10% of its energy for every additional cou-pling, as it travels along the drillstring. This loss is partly due to the differ-ence in cross-sectional area between the rod and the sleeve, and partly due to imperfect contact between the rod faces. The poorer the contact, the greater the energy loss.

When the shock wave reaches the bit, it is forced against the rock, there-by crushing it. The efficiency at the bit never reaches 100%, because some of the energy is reflected as a tensile pulse. The poorer the contact between the bit and the rock, the poorer the efficiency (Figure 6).

To optimize drilling economy, the drilling parameters for percussion pressure, feed force, and rotation must harmonize.

Percussion Pressure

The higher the percussion pressure, the higher will be the speed of the piston, and consequently, the energy. Where the bit is in good contact with hard and competent rock, the shock wave energy can be utilized to its maximum. Conversely, when the bit has poor con-tact, the energy cannot leave the drill-string, and reverses up the drillstring as a tensile wave.

It is only when drilling in sufficient-ly hard rock that the maximum energy per blow can be utilized. In soft rock, to reduce the reflected energy, the per-cussion pressure, and thus the energy, will have to be lowered (Figure 7).

For any given percussion pressure, the amplitude, and hence the stress in the drillsteel, will be higher with reduced cross-section of the drillrods.

Figure 3 Shock wave generated by hydraulic rock drill.

+ – s

Figure 2 Stress wave energy.

5200 m/s

2 x piston length +

– s

Figure 4 Shock wave generated by pneumatic rock drill.

+ – s

Figure 5 Stress level generated by different pistons of same weight.

Piston 1 – 0,8 MPa Piston 3 – 20 MPa Piston 2 – 12 MPa Shock-wave amplitude 1 2 3 Piston 1 – 8 bar Piston 2 – 120 bar Piston 3 – 200 bar

Figure 6 Poor contact between bit and rock results in poor efficiency.

+ – s

Primary wave

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To get the longest possible service life from shank adapters and rods, it is important to ensure that the working pressure is matched to the drillstring at all times.

Feed Force

The purpose of the feed is to maintain the drillbit in close contact against the rock. However, the bit must still be able to rotate. The feed force must always be matched to the percus-sion pressure. Figure 8 illustrates this relationship.

Rotation

The purpose of rotation is to turn the drillbit to a suitable new posi-tion for the next blow. Using button bits, the periphery is turned about 10 mm between blows. Consequently, the rotation rate is increased using higher impact frequency and reduced bit diameter. Using insert bits, the recommended rotation rate is 25% higher.

Setting Parameters

In practice, the driller sets the percus-sion pressure that the rock can cope

with, and then sets the rev/min with regard to the percussive frequency and the bit diameter.

When drilling starts, the feed is adjusted to get even and smooth rota-tion. In case this is not achieved, which will show up in low shank adapter life, the percussion pressure can be progressively reduced, until even and smooth rotation is reached.

The temperature of the adapter sleeve can be checked to ensure that the drilling parameters are correctly set. Immediately after drilling, the temperature should be 60-70 degrees for dry drilling, and approximately 40 degrees for wet drilling.

Drilling problems, mainly related to loose couplings, may arise what-ever parameters are used. In order to tighten the couplings during drill-ing, the friction of the bit against the hole bottom has to be increased. This can be done by increasing the feed, increasing the rotation rate, or chang-ing the bit.

Flushing

Drill cuttings are removed from the hole bottom to the surface by air blow-ing or water flushblow-ing. As the power output from rock drills increases, accompanied by increased penetra-tion rate, efficient flushing becomes gradually more important. The flush-ing medium is normally air for

surface drilling, and water for under-ground drilling. The required flushing speed will depend on:

specific gravity– material having a density of 2 t/cu m requires at least 10 m/sec, whereas iron ore, for example, having a density of 4 t/cu m, requires an air speed of 25-30 m/sec;

particle size – the larger the particles, the higher flushing speed required; particle shape – spherical particles require more speed than flaky, leaf shaped particles.

Productivity and

Methodology

During the past century there has been a rapid and impressive increase in efficiency and productivity related to tophammer drilling. Starting from hitting a steel manually by a sledge hammer 100 years ago, today’s hydrau-lically powered rock drills utilize the latest state-of-the-art technology.

Every drilling method has its pros and cons, making an objective com-parison quite cumbersome. In view of this, the table in Figure 9 can serve as a guideline when comparing the various percussion drilling alternatives which Atlas Copco can offer. The choice of best drilling method to apply depends on hole size and type of application.

by hans Fernberg

Figure 8 Feed force must be matched to percussion pressure.

Low percussion

pressure High percussionpressure Feeding

Figure 7 To reduce reflected energy, percussion pressure is lowered.

Percussion pressure

Soft rock Hard rock

Percussion pressure _Hydraulic

Drilling method Tophammer DTH COPROD

hole diameter, mm 76-127 85-165 105-165

penetration rate 2 1 3

hole straightness 1 3 3

hole depth 1 3 3

production capacity (tons rock/shift) 2 1 3

fuel consumption/drill metre 2 1 2

service life of drillstring 1 2 3

investment in drillstring 2 2 1

suitability for good drilling conditions 3 2 2

suitability for difficult drilling conditions 1 3 3

simplicity for operator 2 3 1

adjustability of flushing capacity 1 2 3

Figure 9: Comparison for 20 m bench drilling in a limestone quarry. Ratings: fair = 1, good = 2, very good = 3.

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Blasting

To understand the principles of rock blasting, it is necessary to start with the rock fragmentation process that follows the detonation of the explosives in a drillhole.

The explosion is a very rapid combustion, in which the energy con-tained in the explosives is released in the form of heat and gas pressure. The transformation acts on the rock in three consecutive stages (figures 1-3).

Compression: a pressure wave

prop-agates through the rock at a velocity of 2,500-6,000 m/sec, depending on rock type and type of explosives. This pres-sure wave creates microfractures which promote rock fracturing.

Reflection: during the next stage, the

pressure wave bounces back from the free surface, which is normally the bench wall or natural fissures in the rock. The compression wave is now transformed into tension and shear waves, increasing the fracturing process.

Gas Pressure: large volumes of gas

are released, entering and expanding the cracks under high pressure. Where the distance between the blasthole and

the free face has been correctly calcu-lated, the rock mass will yield and be thrown forward.

Benching

Bench blasting is normally carried out by blasting a large number of paral-lel holes in each round. Considering the blasting mechanics, with a com-pression-reflection-gas pressure stage in consecutive order for each charge, it is of vital importance to have a proper delay between each row, and even between individual holes in each row. A proper delay will reduce rock throw, improve fragmentation, and limit ground vibrations. The blast should be planned so that the rock from the first row of holes has moved about one third of the burden, when the next row is blasted (figures 4 and 5).

The horizontal distance between the

hole and the free face is the burden, and the parallel distance between ho-les in a row is the spacing. The ratio between spacing and burden will have great impact on the blasting result, and 1.25 can be considered as an average ratio. The optimum burden depends upon a number of parameters, such as rock type, required fragmentation, type of explosives, hole deviation, and hole inclination. Nevertheless, as large drill- holes can accommodate more explosives, there is a distinct relationship between burden and hole diameter (figure 6).

As the bottom part of the blast is the constricted and critical part for suc-cessful blasting, it is used as a basis for deciding all other parameters. The bottom charge, normally 1.5 x burden, from where the initiation should start, requires well-packed explosives of higher blasting power than is needed in the column charge (figure 7).

Principles of Rock Blasting

combination of

Factors

Blasting by design results from a large number of factors, all of which need to be brought under control in order to achieve the right result. These include the choice of drillrig and tools, the layout of the holes, the explo- sive, and the skill of the opera- tors. Geology is the governing factor, and experience is a major ingredient. Atlas Copco produces drillrigs and systems to suit all rock types, and has the experi- ence to recommend the correct approach to all ground condi-tions in order to achieve the optimum result. The following outline of the principles involved in rock blasting is a logical start point in the quest for the perfect round.

Figures 1-3 Rock breaking sequence in a normal blast.

Compression Reflection Gas Pressure

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Drilling in open pit mining 11

Stemming of the top part of the hole is used to ensure that the energy of explosives is properly utilized. It will also reduce and control the fly rock ejected from the blast. This tends to travel long distances, and is the main cause of on-site fatalities and damage to equipment. Dry sand or gravel having a particle size of 4 to 9 mm constitutes the ideal stemming material.

Inclined holes give less back break, safer benches and less boulders, when compared to vertical holes.

Types of explosives

The geology frequently has more effect on the fragmentation than does the explosive used in the blast. The properties that influence the result of the blast are compressive strength, tensile strength, density, propagation velocity, hardness and structureIn general, rock has a tensile strength which is 8 to 10 times lower than the compressive strength.

The tensile strength has to be exceeded during the blast, otherwise the rock will not break. High rock density requires more explosives to achieve the displace- ment.

The propagation velocity varies with different kinds of rock, and is reduced by cracks and fault zones. Hard, homo- geneous rocks, with high propagation velocity, are best fragmented by an explosive having high velocity of detona-tion (VOD).

An extensive range of different types and grades of explosives is available to suit various blasting applications. A breakdown is presented in Table 1.

In dry conditions, ANFO has become the most used blasting agent, due to its availability and economy.

The blasthole diameter, together with the type of explosive used, will deter-mine burden and hole depth. Practical

hole diameters for bench drilling range from 30 to 400 mm. Generally, the cost of large diameter drilling and blasting is cheaper per cubic metre than using small holes. However, rock fragmentation is better controlled by higher specific drilling.

The explosives are initiated with detonators which can be electric or non-electric. Electric systems have the advantage that the complete circuit can easily be checked with an ohmmeter to ensure that all connections and detonators are correct before blasting. To eliminate the risk for spontaneous ignition from lightning, non-electric systems, including detonating cord, are used.

by hans Fernberg

Firing pattern

This firing pattern provides separate delay time for practically all blastholes and gives good fragmentation as well as good breakage in the bottom part of the round.

Figure 6 Burden as a function of drill hole diameter. Burden as a function of

Drill Hole Diameter

Hole Diameter, mm Spacing Equal to 1.25 x Burden

Figure 7 Charging for optimum fragmentation. Boulders and flyrock come from this zone

Back break

Subdrilling = 0.3 x burden Bottom charge

requires well packed high blasting power Column charge only light charge needed for good fragmentation

Stemming

(length ~ burden) Burden

Table 1 Features of common types of explosives.

base type detonation velocity m/s features

nitro-glycerine dynamite 5500-4500 highly adaptable cartridged

gelatin excellent in smaller holes

ammonium- ANFO 2500 low cost, high safety, easy

nitrate to pour or blow

no water resistance, contains 5-6% fuel oil water slurry 4000-3000 watergel basically ANFO made water

resistant gel

5000 emulsion stable oil/water emulsion – heavy ANFO

range depends on packaged or pumpable storage time

Figure 5 Firing sequence in delay blasting.

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Putting Rotary Drilling into

Perspective

Rotary or DTH

Atlas Copco now offers a complete range of rotary as well as DTH and tophammer drill rigs for most types of open pit mining and quar-rying applications. But how do the- se technologies complement each other and how do drillers know which method to choose, and when? As readers of M&C are well acquainted with DTH drilling, this article puts rotary drilling into per-spective.

complete Range

With the acquisition of the Ingersoll-Rand’s Drilling Solutions and Baker Hughes Mining Tools (BHMT) busi-nesses, there is now another way to break rock within the Atlas Copco fa-mily of products. Much of the world’s mining output begins through drilling of holes with rotary drills. Ingersoll-Rand built air-powered rotary drills for many years prior to the introduction of their first fully hydraulic unit, the T4, in 1968.

about Rotary Drills

It is important to note that rotary drills are capable of two methods of drill-ing. The majority of the units operate

as pure rotary drills, driving tricone or fixed-type bits. The fixed-type bits, such as claw or drag bits, have no moving parts and cut through rock by shearing it. Thus, these bits are limit-ed to the softest material. The other method utilized by rotary drill rigs is down-the-hole (DTH) drilling. High pressure air compressors are used to provide compressed air through the drill string to drive the DTH hammer. The main blasthole drilling methods are shown in Fig 1. The primary differ-ence between rotary drilling and other methods is the absence of percussion. In most rotary applications, the pre-ferred bit is the tricone bit. Tricone bits rely on crushing and spalling the rock. This is accomplished through transfer- ring down-force, known as pulldown, to the bit while rotating in order to drive the carbides into the rock as the three cones rotate around their respec-tive axis. Rotation is provided by a hydraulic or electric motor-driven gear- box (called a rotary head) that moves up and down the tower via a feed system. Feed systems utilize cables,

chains or rack-and-pinion mechanisms driven by hydraulic cylinders, hydrau-lic motors or electric motors. Pulldown is the force generated by the feed system. The actual weight on bit, or bit load, is the pulldown plus any dead weight such as the rotary head, drill rods and cables.

More Weight with Rotary

It only takes one look to see that the biggest DTH and tophammer drill rigs are very different to the biggest rotary blasthole rigs. In fact, the Pit Viper 351 rotary drill rig weighs in excess of nine times that of our largest DTH hammer drill rig, the ROC L8. Yet it is drilling a hole that is generally only twice the diameter. Take a typical medium formation tricone bit with a recom-mended maximum loading of 900 kg/ cm of bit diameter (5000 lbs per inch of diameter). With a 200 mm (7-7_/₈_”)

bit, you could run about 18,000 kg (40,000 lbs) of weight on the bit. The laws of physics dictate that for every action, there is an equal and opposite

By Brian Fox, Vice President Marketing, Atlas Copco Drilling Solutions, USA.

TONS Fig 1. Drilling

meth-ods (1) Down-the-Hole (DTH); (2) Tophammer; (3) COPROD; (4) Rotary tricone.

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Drilling in Open pit Mining 13

Surface Drilling Applications

Construction Aggregate Industrial Minerals Gold Coal Copper Iron Hole Diameter 2" 3" 4" 5" 6" 7" 8" 9" 10" 11" 12" 13" 14" 15" 16" 51mm 76mm 102mm 127mm 152mm 178mm 203mm 229mm 254mm 279mm 305mm 330mm 356mm 381mm 406mm 125,000 LB Bit Load PV-351 110,000 LB DM-H2 90,000 LB DM-M3 75,000 LB PV-271/PV-275 75,000 LB DM-M2 50,000 LB _DMLSP 60,000 LB _DML 45,000LB _DM45 30,000 LB _T4BH 30,000 LB _DM30 25,000 LB_DM25SP ROC L8 CM780D CM760D KEY: ROC L6 ROC L7/L7CR Rotary ECM720 ROC F9 Rotary or DTH ROC F6 ECM660III DTH ROC F7 ROC D3/D5/D7 Tophammer/COPROD ECM585MC ECM470

complete Range

With the acquisition of the Ingersoll-Rand’s Drilling Solutions and Baker Hughes Mining Tools (BHMT) busi-nesses, there is now another way to break rock within the Atlas Copco fa-mily of products. Much of the world’s mining output begins through drilling of holes with rotary drills. Ingersoll-Rand built air-powered rotary drills for many years prior to the introduction of their first fully hydraulic unit, the T4, in 1968.

about Rotary Drills

It is important to note that rotary drills are capable of two methods of drill-ing. The majority of the units operate as pure rotary drills, driving tricone or fixed-type bits. The fixed-type bits, such as claw or drag bits, have no moving parts and cut through rock by shearing it. Thus, these bits are limit-ed to the softest material. The other method utilized by rotary drill rigs is down-the-hole (DTH) drilling. High pressure air compressors are used to provide compressed air through the drill string to drive the DTH hammer. The main blasthole drilling methods are shown in Fig 1. The primary differ-ence between rotary drilling and other

methods is the absence of percussion. In most rotary applications, the pre-ferred bit is the tricone bit. Tricone bits rely on crushing and spalling the rock. This is accomplished through transfer- ring down-force, known as pulldown, to the bit while rotating in order to drive the carbides into the rock as the three cones rotate around their respec-tive axis. Rotation is provided by a hydraulic or electric motor-driven gear- box (called a rotary head) that moves up and down the tower via a feed system. Feed systems utilize cables, chains or rack-and-pinion mechanisms driven by hydraulic cylinders, hydrau-lic motors or electric motors. Pulldown is the force generated by the feed system. The actual weight on bit, or bit load, is the pulldown plus any dead weight such as the rotary head, drill rods and cables.

More Weight with Rotary

It only takes one look to see that the biggest DTH and tophammer drill rigs are very different to the biggest rotary blasthole rigs. In fact, the Pit Viper 351 rotary drill rig weighs in excess of nine times that of our largest DTH hammer drill rig, the ROC L8. Yet it is drilling a hole that is generally only twice the diameter. Take a typical medium formation tricone bit with a recom-mended maximum loading of 900 kg/ cm of bit diameter (5000 lbs per inch of diameter). With a 200 mm (7-7_/₈_”)

bit, you could run about 18,000 kg (40,000 lbs) of weight on the bit. The laws of physics dictate that for every action, there is an equal and opposite

reaction, meaning that if you push on the ground with 18,000 kg (40,000 lbs), the same force will push back on the unit. Therefore, the weight of the ma-chine must be over 18,000 kg (40,000 lbs) at the location of the drill string to avoid the machine “lifting off” the jacks. To achieve a stable platform through proper placement of the tracks and levelling jacks, the distribution of weight results in an overall machine weight that approaches or exceeds twice the bit load rating. This weight does add cost to the machine, but the size of the components also translates to long life. Even smaller rotary blast-hole drills are built to run 30,000 hours of operation.

The importance of air

A key parameter of rotary drilling is flushing the cuttings from the hole. In most rotary blasthole drills, cuttings are lifted between the wall of the hole and the drill rods by compressed air. Sufficient air volume is required to lift these cuttings.

Many types of tricone bits have been developed to meet various drilling needs. Softer formation bits are built with long carbides with wide spac-ing on the face of the bit. This design yields large cuttings which increase drill speed and reduce dust. It is important to have sufficient clearance bet-ween the wall of the hole and the drill rods in order for such large cuttings to pass. If this clearance, known as annu-lar area, is not sufficient, the cuttings will be ground between the wall of the hole and the rods or by the bit itself

Fig 2: The Atlas Copco product range by application and method.

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0 2000 4000 6000 8000 10000 12000 14000 $-$1,00 $2,00 $3,00 $4,00 $5,00 $6,00 Footage/24 Hours Bit Life (ft) Overall Cost/Ft 299 ft/hour, 1500' bit life High Production 74 ft/hour, 12,000' bit life

Great Bit Life 218 ft/hour,

5300' bit life Lowest Cost

Footage/24 Hours & Average Bit Life

Overall Cost/Foot

of air is required. Take for example a Secoroc QL80 203 mm (8”) DTH hammer that is designed to operate at 25 bar (350 psi). Even with our larg-est high pressure compressor with 686 litres per second (1,450 ft3_{/min), the}

pressure will only build to 23 bar (325 psi), thus providing less impact energy. In real terms, each blow of the piston is about 45 kg (100 lbs) less than it is designed for. In some cases, this method will still out-perform rotary drilling. For most large diameter blasthole drilling, there is simply not enough air on-board for a DTH to be as cost effective as rotary drilling with a tricone bit.

Rotary drilling is still the predomi-nant method of drilling 230 mm (9”) diameter or greater. This is driven pri-marily by the current limitations of tophammer units and rig air systems. Tri-cone bits also become more cost effec-tive as the larger bits are equipped with larger bearings which in turn can hand-le higher loads. These higher loads translate to improved drill rates. Another advantage of rotary rigs is the length of the drill rods that can be carried on board. Longer rods mean fewer connections. Further, some rota-ry rigs are large enough to handle a long tower that enables drilling of the entire bench height in a single-pass. At the largest open pit mines, rotary units are drilling 20 m (65 ft) deep holes in a single-pass to match the bench heights dictated by the large electric shovels which can dig a 17 m (55 ft) bench.

Productivity versus cost

Studies have shown that pure penetra-tion rate will increase linearly with increased pulldown. The same has also been said of rotation speed. So why doesn’t every operation use more of each? Unfortunately, higher pulldown and rpm usually results in increased vibration and lower bit life. The vibra-tion causes increased wear-and-tear on the rig, but more importantly, it cre-ates a very unpleasant environment for the operator. What invariably happens is that the operator reduces the weight or rpm until the vibration returns to a comfortable level. Some operations limit bit load and rpm even if there is

no vibration in order to improve bit life. This is often the wrong strategy as the overall drilling cost per unit, also known as Total Drilling Cost (TDC), should be considered.

TDC is calculated using the bit cost per metre/foot and the total rig cost per hour. The unit cost per hour includes labour, maintenance and power, and possibly the capital cost. The drilling speed really doesn’t impact this cost per hour figure. What it does impact though is the cost per unit produced (cost/metre/foot, cost/ton, etc…). You generally want to push the rig harder to reduce the cost/foot, but there will be a point where the rig overloads the bits (see fig's 3 and 4).

large versus Small

There are some drawbacks to rotary rigs. Smaller crawler rigs are more flexible with many advantages such as articulating and extendable booms and guides that allow drilling at many dif-ferent angles. Some models also offer

significantly more technology with automated rod handling systems and automatic drilling. The components on rotary rigs are not enclosed. They are mounted onto the frame in an open layout which makes them extremely easy to service. Looks are not of pri-mary concern for a rig that is subjected to the rigors of breaking rock for more than 60,000 hours.

The general trend for 165 mm (6-1_/₂_”)

or less is towards the smaller, more flexible units. However, many large scale quarries and small mines still favour the durability, life and simplici-ty of the larger rotary rigs for these small diameters. For the large scale open pit operations that yield a high percentage of the total worldwide mi-neral production, it is anticipated that rotary drilling will remain the primary method for years to come.

Fig 4. The impact of bit life and productivity on overall cost/foot (1 ft = 0.305 metres).

Production Rate (Feet/Hour) 120 138 180

Bit Life (Feet) 10,000 8,000 4,000

Bit Cost $4,000 $4,000 $4,000

Bit Cost/Foot $0.40 $0.50 $1.00

Rig Cost/Hour $175 $175 $175

Total Drilling Cost/Foot $1.86 $1.77 $1.97

Operator Steady eddie Smart Sam Wild Jack

Fig 3. The table compares three operators on the same drill rig – Steady Eddie, Smart Sam and Wild Jack. The cost chart, using actual data collected at a major copper mine further illustrates the balance required.

acknowledgements

This article first appeared in Mining & Construction No 2, 2005. ■

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control System

Via PC software, the SmartRig control system generates electrical signals to control the hydraulic valves. This intro-duces the concept of a “dry cab”, with no hydraulic pipework and gauges, con-siderably reducing noise for the ope-rator. The number of hydraulic com-ponents has been reduced by 30%, compared with Hydraulic Control Sy-stem, HCS, resulting in higher effici-ency. The need for electrical cables is also reduced.

Control gauges and instruments are replaced by a display unit. This releases space in the cab, increasing visibility, and improving operator ergonomics.

The fundamentals of the rock drill control system are RPCF control and anti-jamming functions. RPCF, or Ro-tation Pressure Control of Feed pres-sure, adjusts the feed pressure according to the measured feed pressure. This keeps the joints correctly tightened and

saves drill steel. Anti-jamming uses the rotation pressure to detect a jamming situation, and will reverse the feed of the rock drill and initiate a replacement hole collaring.

Together, this advanced system of drilling control will give maximum life to the entire drill string, while ensuring high penetration rates and easy rod ex-traction.

automation in Surface

Drilling

Using the laser plane as a reference level, all holes are drilled to the same depth, reducing drilling, blasting and crushing costs by way of better frag-mentation, and cancelling the need for secondary blasting. A flatter, more uni-form bench surface results, making loa-ding and transportation easier. Automatic feed positioning reduces set-up time and cancels out operator error. More parallel holes result in better blasting and smoother bench bottoms. The longer the hole, the bigger will be the impact of even a small deviation on blasting. For instance, a one degree error will

produce a deviation of 36 cm at the bottom of a 20 m hole. Hence the im-portance of automatic feed positioning, which sets the feed to pre-defined angles at the touch of a button.

The automatic rod adding system, AutoRAS, enables the operator to drill a hole automatically to a given depth, allowing him to leave the cab to carry out other duties, such as maintenance checks or grinding bits, while keeping the drill rig in sight. The drilling is su-pervised by the drillsteel break detec-tion system, which shuts down the dril-ling operation if a breakage is detected. The result is better rig utilization, evi-denced by a couple of extra holes/shift.

MWD and ROc Manager

Measure While Drilling, or MWD, is an optional instrumentation and software package for recording and interpreta-tion of drilling data, and enhanced pres-entation of geomechanical variation of rock properties. A number of parame-ters, such as hole depth, penetration rate, and damper, feed, percussion and rota-tion pressures are logged at requested

SmartRig Takes Control

hcS to Plc to Pc

Acronyms are plentiful when it co-mes to automation, but PC-based will be the most important acro-nym in the years to come. Ever since Atlas Copco developed Hyd-raulic Control System (HCS) in the 1970s, the search has been on for its successor. Programmable Logic Control (PLC) saw us through the 1990s along with the VME-system, but in 2002 the first SmartRigs started to take over. SmartRig is a PC-based control system inten-ded for all kinds of automation in simple and advanced drill rigs. The hardware is designed to operate in every possible weather condi-tion, and the software can be up-graded at site. SmartRig has built-in loggbuilt-ing and monitorbuilt-ing func-tions, together with support for diagnostics and faultfinding. The control system is used in all Atlas Copco product families, in both un-derground and surface crawlers, making it easy to move functions and improvements between diffe-rent products. That’s smart!

Atlas Copco´s Silenced ROC D7C is a sound investment with a noise level of approximately 10 dB(A) below that of other rigs on the market.

• Hole Navigation System (HNS) • Measure While Drilling (MWD) • Automatic rod adding (AutoRAS) • Automatic feed inclination setup (AutoPOS) • Laser Plane

• Advanced Drilling System • Silenced

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intervals while drilling, and this pro-vides input to analysis of the rock properties.

Date, time, hole length, feed angle, and rig identity are logged once for each hole. MWD data can be recorded for every second centimetre up to a maxi-mum penetration rate of 3 m/min. In this way, data is extracted from every production hole to provide very high-resolution rock mass characterization. Typical parameters being reported are rock hardness and fracturing. Detailed information on rock mass properties is available immediately after drilling is completed, without disturbing produc-tion, since logging is carried out auto-matically during the normal drilling process.

ROC Manager is a stand-alone PC- based tool for making drill plans, mea-suring hole deviation, and logging, presenting and reporting drilling data graphically. This information can be presented individually or in combination with other parameters, and used both during drilling operations, and by trans- ferring logged data from the rig. Advan- ced MWD analysis is also possible as an option. Both the SmartRig and ROC Manager 2.0 support the IREDES format for data exchange on performance,

quality and MWD logs, and on drill plans.

In ROC Manager, the MWD data can be illustrated in slices through the bench, with the rock properties identified by contrasting colours, providing a map of the mineral qualities and types. This facility differentiates between good rock and poor rock, for instance, allowing the quarry or mine operator to select rock for excavation, and to prepare for loading and hauling before blasting takes place.

hole navigation System

Real-time satellite-based Global Posi-tioning System, HNS, has been chosen for the highest possible drillrig navi-gation accuracy, within 10 cm in most situations. With HNS there is no need to mark out holes, and the accuracy is such that all holes will be parallel, if required, resulting in a controllable product with better fragmentation and less boulders.

The focus is on road construction applications, but the system can be used in any type of drilling.

Using information on his display, the operator can navigate the rig to the co-verage position for a given hole, and

the computer will provide the informa-tion to place and align the feed exactly over the collaring position.

The drillplan can be provided by ROC Manager, transferred to the rig via a PC card. The time saved by not having to aim visually to set angles, and by being able to drill more than one hole from a single set-up, results in better rig utilization.

Silenced for noise

Sensitive areas

The sources and characteristics of noise are complicated, and have to be identi-fied and analysed in order to analyze their spectrum. Atlas Copco designed a concept rig in 2000, and a second prototype rig in 2004, both of which were used as testbeds for various sim-ulations.

These confirmed that noise was not just created by the drilling cycle, but also by elements of the carrier, such as cooling fans, hydraulic system compo-nents, and engine.

The recently introduced Silenced SmartRig is for use in areas where noise levels have to be controlled. Substantial efforts have been put into redesigning components, systems and soundproofing enclosures, resulting in a 10dB(A) ex-ternal noise reduction.

The most visible difference between the Silenced SmartRig and other Smart Rig rigs is its patented feed enclosure. The frame and panels of the enclosure are formed from lightweight aluminium. There are four access doors, which are hydraulically operated from the cab. A rubber sliding skirt at its base encloses the hole, and this can be hydraulically raised for collaring. The whole enclo-sure is designed for demounting when not needed.

The SmartRig system, because it delivers the right amount of power for each phase of the drilling operation, can re-duce fuel consumption by up to 30%. Add this to the productivity increase from automatic rod adding and auto feed alignment, and the Silenced Smart Rig is a really sound investment!

by Jean lindroos

With Silencing Kit Without Silencing Kit

200 400 600 800 1000 1200 1400 m 200 400 600 800 1000 1200 1400 m 55dB (A) area

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criteria of choice

Since button bits are used for 99% of surface drilling applications, a bit with guiding wings on the bit skirt, the Retrac-type bit, should be chosen to give the straightest possible hole.

The bit front should not be allowed to take on a convex shape during serv-ice life, since this convex front has been shown to give more deflection than a flat or drop centre front.

When a button bit is worn, it is the gauge buttons that always show the most wear. This means that the gauge but-tons lose more height during regrind-ing, leading to the tendency for the bit front to become convex. A drop centre bit, thanks to its lowered centre, will not become convex during the bit life, and is thus the best choice, wherever possible.

If the above criteria are taken into account, the choice of bit, in descend-ing order, should be: Retrac button bit with ballistic buttons and drop centre

front; Retrac button bit with spherical button and drop centre front; Retrac button bit with spherical buttons and flat front; Insert bit, only used when very straight holes are required and nothing else works.

Bit Designs

Flat Front, Standard

The standard bit is most suitable for medium-hard to hard rock, where it gives good performance and long service life in normal condi-tions. Standard bits are easy to regrind, as the front and gauge buttons usually are the same size.

Flat Front, heavy Duty

Hard rock, con-taining quartz and pyrites, often causes considerable wear to the gauge buttons. When drilling in rock with this characteri- stic, it is common practice to use Heavy

Duty bits, Model –20 or –21, where the gauge buttons are larger than the front buttons.

extra heavy Duty

Model –21 has better flushing characteris-tics, due to different clearance angle and shorter head.

Recommended in extremely hard and abrasive rock.

Drop centre

Excellent results are obtained with the Drop Centre bit in soft to medium hard rock, with high penetration potential. The Drop Centre bit has outstanding flushing characteristics, and the cuttings are dis-tributed evenly around the steel body, so minimizing steel wash. The drop centre part of the bit front produces a rock elevation or bump during drilling, which gives good guidance to the bit.

Correct Selection of

Tophammer Rock Drilling Tools

Optimizing

Penetration Rates

In order to achieve best possible penetration rate, a bit should be chosen where the total contact area between the cemented car-bide and the rock creates the best possible penetration per blow. As a rule of thumb, the following penetration rate index can be used: button bit with ballistic buttons, 130; button bit with spherical buttons, 115; insert bit, 100. However, when bits are compared for hole straightness, a different order emerges, with the insert bit on top, followed by the button bit with ballistic buttons, and lastly, the button bit with spherical buttons.

This article is intended to guide the driller through the range of bits, rods and shank adapters to assist with the best choice of rock tools for the particular job.

Bit designs and rock types

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Drop centre extra heavy Duty

Gauge buttons are larger than the front buttons, there are no side flushing holes, and the head is shorter. Recommended in extremely hard and abrasive rock, but can also be used in medium hard rock.

Retrac Skirt

The Retrac bit has cutting edges at the rear of the bit, allowing it to “drill” in reverse. This is an important feature when drilling in loose, broken or fissured rock, where it can be difficult to retract the drillstring due to hole collapse. In addi- tion, the Retrac bit has deep grooves mil- led along the bit body for efficient cut-ting removal. The good guidance of this bit gives straighter holes.

insert Bits

These are very seldom used, except when very straight holes are required. Cross bits normally have cemented carbide inserts with a carbide grade for high wear resistance. X-bits normally have a carbide grade for improved toughness,

and are preferred where there is a ten-dency for squaring of the hole.

Drillrod Selection

For bench drilling, three types of drillrods can be chosen: surface hardened rods, in which only the thread parts are hardened; carburized rods, where all sur- faces, including the inside of the flush-ing hole, are hardened; and carburized Speedrods, having integrated couplings with male and female threads at oppo-site ends.

Surface hardened Rods

Surface hardened rods are the toughest, and can take more abuse than the car-burized rods, but they have the lowest fatigue strength. They are a good choice when drilling in faulted or folded for-mations, when driller abuse, or lack of care and maintenance, are factors.

Tiger Rods

The new M-F Tiger Rods from Atlas Copco Secoroc are specially developed for surface drilling applications. They are composed of selected steels to give increased service life and better performance. The female thread is fully carburized, while the rest of the rod is sur-face hardened. The new Tiger Rods are friction-welded, so that the best steel grade can be selected for each section.

This improved production technology reduces the risks of pitting on the threads, and rod and thread end break-age, while tighter thread tolerance im- proves the total service life of the whole drill string.

Rigorous tests in various locations have shown that Tiger Rod is a drill string without any weak parts.

carburized Rods

A carburized rod has better wear resist-ance and a higher fatigue life compared to surface hardened rods. Demands good treatment, and hole deflection should be limited by putting guiding equipment in the string, at least when drilling holes deeper than 10 m. Their life will be 20%-30% longer if they are handled correctly, and guiding equipment is used when necessary. When lighter drillrods are required for manual rod handling, the carburized hexagonal rod is recom-mended.

When drilling with a number of rods in a string, using standard rods and couplings, the loss of energy in every joint is about 6%, if the connection is tight. If drilling with open threads, the energy loss at each joint can easily climb to 10%. Therefore, it is advantageous to use the maximum rod length possible.

Speedrods

If Speedrod carburized rods with inte-grated coupling are used, the energy loss per joint is less, since the mass (weight) of the joint is less than that of standard coupling joints. The energy loss is about 3.5%, which is 60% of that of standard joints.

In practice, the energy advantage of Speedrod joints compared to standard couplings is even greater, since it is easier to keep the Speedrod joint tight during drilling.

Field tests have shown that, when drilling 20 m holes with 4 m rods, the penetration increase is about 15% when using Speedrods.

The faster penetration and easier handling increase productivity, and due to better energy transmission, the joints are easily loosened.

From the point of view of drilling straight holes, rods that are as rigid as possible for the drilled hole diameter are best. For straight hole drilling, a Guide Rod or Guide Tube should be used as the first rod after the bit, to give the drillstring guidance.

Further information about rods is available in the yellow technical specifi-cation pages in this reference edition.

Shank adapters

The task of the shank adapter is to tran- smit rotation torque, feed force, impact energy, and flushing medium. It is made from specially selected material to with- stand the transmission of impact energy and rotation from the rock drill to the drillstring, and is hardened through car- burizing. Around 400 different shank adapters are currently available from Atlas Copco Secoroc. Shank adapters can be divided into three main types, based on the technique used to transfer the rota-tion motor torque to the drillrod.

Surface Drilling