Sandvik Drilling Tools Care And Maintenance

Sandvik Coromant rock drilling tools

Care and maintenance

Your Sandvik Coromant rock drilling tools have been specially developed to withstand the high stresses to which they will be subjected during drilling. The steel components are made of special alloys, with high resistance to fatigue and good rigidiry. Subsequent heat treatment ofthe steel gives high resistance to wear. The cemented carbide used in drill bits has been spec'ially developedfor"rock dt"illing. It can be said that all the ingredients needed Jbr good drilling results are "built-in" to your Sandvik Coromant products during the manufacturing process. But remember - if you want to make the most of this quality, it is important that you use the rock tools in the correct way.This booklet contains afew practical tips on how you can erploit the full capabilities of Coromant rock drilling tools, and so produce your drill holes in the most fficient and reliable way.

Setting-up the

drill rig

The feed beam (or mast) must be braced firmly against the rock, so that it does not move during drilling. If the feed moves during drilling, the drillsteel will bend, which could result in fracture. In bench drilling, a stable set-up is obtained (on modern drill rigs) with the aid of hydraulic jacks. A firmly braced feed beam will enable you to exploit the feed force (or thrust) fully, so that you get the optimum penetration rate (or drilling speed).

Collaring (starting the

suddenly switched on - this could cause the cemented carbide in the

drill hole)

drill bit to crack. There would also be a danger of getting stuck, and/or blocking the flushingholes in the

Switch-on the flushing and run the

drill bit up against the rock face with the percussion (hammer) switched OFF. If you engage percussion too early, you will loosen the threads in the drill string, and possibly damage the drill bit.

drill bit.

With the drill bit pressed lightly against the rock face, engage reduced feed force, reduced percussion and normal rotation. Make sure that flushing is effective. Collaring the hole without flushing would cause the temperature of the drill bit to rise. Rapid cooling would then occur when the flushing was

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Increase the percussion energy once the drill bit is bedded in the rock. In drifting, it is sometimes necessary to realign the feed beam after the first few centimetres of drilling, so that the feed and drill steel are exactly parallel. This

corrects any bending in the drill steel and minimises deviation of the drill hole. In bench drilling, no further adjustment of the feed is necessary after collaring, provided that the drill rig was given a stable set-up to begin with.

In bench drilling, the drill-steel support (or centralizer) must be closed during collaring. In drifting, do not allow the play between the drill steel and the bushing in the centraltzer to become too great.

Check the centralizer bushings regularly, and change them as soon as they become wom.

If you were to continue drilling with bending stresses in the drill steel, the risk of fracture would increase considerably.

Drilling Always adjust the percussion energy to suit the type of rock in which you are drilling. If the rock is soft or cracked, reduce the pressure to the percussion mechanism. Modern drill rigs are equipped with a variety of devices for adjusting the pressure.

Feed

and couplings to wear-out very quickly.

By applying the correct feed pressure, you will obtain the best drilling economy. If the feed force is too low, the rate of penetration will also be low, and the threads in the drill string will loosen. Drilling with loose threads interferes with the transmission of energy through the drill string. This causes high stresses which can lead to premature fatigue in the drilling equipment. It can also cause fatigue damage to the cemented carbide in the drill bit.

A sign of poor feed force is hot and rattling coupling sleeves. The high temperature causes the threads

ofthe shank adapter, rods

The feed force can also be too high, which causes the rotation speed of the drill bit to drop. This increases the risk of jamming and, at the same time, reduces the rate of penetration. You will also notice an increase in bending stresses in the drill steel.

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Rotation The speed of rotation must be set to suit the diameter of the drill bit and the frequency of the percussion mechanism (the number of blows per minute). The inserts (or buttons) in the drill bit must be tumed a suitable distance between each blow from the hammer, so that they always break fresh rock. The larger the diameter of the drill bit, the slower the permissible rotation speed. Ifthe rotation speed is too high, the cemented carbide inserts will wear rapidly, particularly at the periphery (outer edges) of the drill bit. For drill bit diameters between 51 and 102 mm, the normal speed of rotation is 160-80 rev/min (faster rotation for smaller

bit diameters). This would correspond to turning a peripheral button through 9-10 mm between each

blow from the hammer. For lar-eer drill bit diameters, the rotation speed can be even slower than 80 rev/min, especially in abrasive rock. In the case of bit diameters greater than 51 mm, the rotation speed for a button bit should be lower than that for a corresponding cross- or X-bit.

(Example: Using T38 drill steel equipment and a m l6 mm drill bit in granite with a compressive strength of 150-220 MPa: feed pressure 65 bar; percussion pressure 180 bar; rotation speed for button bit, 100 rev/min; rotation speed for cross- or X-bit, 130 revlmin).

Flushing The bottom of the hole must be kept free of drill cuttings. Good flushing gives quick removal of the drill cuttings, and keeps the drill hole open. Insufficient flushing reduces the rate of penetration and increases the risk of jamming. If you have pneumatic drilling equipment that is designed for central flushing, using water as the flushing medium, the water pressure must be kept lower (approx. 1 bar) than the air pressure. If this is not done, there is a danger that water will leak into the rock drill. If you think you need a higher flushing-water pressure, use equipment designed for separate flushing. To avoid damaging the seals

in a rock drill of this type, the shank adapter must be clean, and free of excessive anti-rust wax.

Change damaged shank packings at once. Otherwise the film of lubricating oil on the shank adapter will be washed away very quickly. This would result in speedy damage to both the rock drill and the shank adapter. (Drilling back and

forth while attempting to free a jammed drill string causes heavy wear to the shank packing.) Use suitable tongs that do not have sharp edges when changing a shank packing. Excessive flushing pressure can cause abnormally high steel wear, and reduce the service life of the equipment.

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Rod circulation If you couple a new

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est way to avoid this is to make

sure that all the rods involved in the circulation system are turned "upside-down" regularly, e.g. once a shift.

thread to an

old thread, the new thread will wear abnormally fast. In order to obtain an even rate of wear among all drill rods in the drill string, it is therefore important to establish a system of rod circulation. This will ensure that all rods do the same amount of drilling, and that all threads are subjected to the same amount of wear. Ideally, the rods should be circulated so that, when you change the drill bit, the leading rod is removed and placed last in the drill string. Also, it is a good idea to include a few spare rods in the circulation system. The rate of thread wear can be different at either end of a drill rod. The easi-

Rod circulation is easier to administer if each rod is marked with a different colour (gloss paint will do), and if some kind of rod rack is used. Problems can arise on modern drill rigs that are equipped with mechanrzed rod handling. Howevef, even on these, you should switch the order of the rods according to some kind of routine, e.g. once a shift or once a day.

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or with the aid of some kind of straightening press. But remember that you cannot expect to obtain a full service life from a drill steel that has been bent and then straigh-

Thread grease In order to obtain maximum service life from the threads, it is important to keep them clean. Thread grease reduces wear and makes it easier to uncouple the rods. However, the grease must be kept free of drilling dust, otherwise it will simply work as an abrasive rather than a lubricant. Make sure therefore that the lid is kept on the grease tin when it is not being used.

tened.

Bent and blocked

drill

steels

It is not always necessary to scrap bent drill steels. They can often be straightened, either in the drill hole 11

Drill steels can become blocked due to poor flushing. If you can clear the blockage, the component can be put back into service. A copper tube and water flushing can be used to clear blockages. Copper tubing is recommended, since there is always a danger the material blocking the drill steel might contain explosive. The process of clearing the blockage can damage

Loosening threads The easiest way to loosen a drill bit is to use the percussion mechanism of the rock drill. Press the drill bit against the rock face, e.g. at the bottom of the drill hole, and engage the percussion for a few seconds, without rotation, and using very little feed.

the anti-rust treatment on the inside of the flushing hole. For this reason, drill steels that have been unblocked should be put straight back into circulation rather than being put back into the stores.

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If this

does not work, bit and rod spanners will have to be used. Never use a sledge hammer!

Note that percussion must not be engaged when a spanner is attached to the spanner-flat of a rod, since it would damage the rod. Difficulty in loosening threads is often caused by excessive thread wear.

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Wear It is important that thread wear be

Note that when changing to new drill rods, it is often more economical to replace the coupling sleeves as well, even if they are not completely worn. Mixing old and new threads causes the new threads to wear out more quickly.

checked regularly, so that components can be discarded when it is

economical to do so. To continue drilling with wom threads is to invite expensive breakdowns. Use Coromant thread gauges to measure wear. Male threads should be discarded when the thread gauge can be passed over the tops of the threads without catching. Always check the most badly worn part of the thread, since the end of the thread could be less wom, e.g. if it has worked in the clearance behind the drill bit. Female threads should be discarded when it is possible to slide the thread gauge all the way into the thread.

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There is a special gauge for checking wear in the chuck bushing of the rock drill. Drilling with a wom bushing will cause deformation of the shank or shank adapter. In the case of a shank adapter, the splines will be subjected to heavy wear. Excessive play between the drill bushing and the shank adapter can also cause the piston to strike the shank obliquely. This will result in heavy wear and upsetting on both the shank and the piston. Damage to the shank often results in breakage.

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Transportation and storage A side-blow to the surface of

a

steel component can become the

starting point for eventual fracture, since the steel is subjected to very high stresses by the shock waves from the piston of the rock drill. Drillsteels, rods, coupling sleeves and drill bits should therefore be handled with care, as theY often have a hard but brittle outer surface that is sensitive to lateral impact.

(the original damage), at the centre of a so-called "fatigue rose". Pack drill bits and cemented carbide components in such a way that the cemented carbide is not damaged during transpofiation. Even though cemented carbide is highlY

resistant to impact from anY other material, it is easily damaged bY impact from another cemented carbide product.

Spanners that are used to loosen threads must be in good condition, without burrs. It is often easy to

find the starting point of

a fracture

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Wear to cemented carbide Cemented carbide in inserts and buttons becomes worn during drilling. Most of the wear is caused by abrasion against the bottom of the drill hole, and also by abrasion against the hole wall, as the bit rotates. If wear is allowed to become too great, the penetration rate drops and both the cemented carbide and the other steel components in the drill string are subjected to

abnormally high stresses. The cemented carbide must therefore be ground regularly, to restore its

original shape. Different types of rock cause differing rates of wear, and different wear patterns.

Frontal wear occurs when drilling in hard rock, such as granite and gneiss. The height of the cemented carbide insert or button wears down, and begins to show a wearflat. In the case of cross- or X-bits, the wear-flat becomes wider towards the edge of the drill bit, owing to the greater distance covered by the periphery during rotation. In the case of a button bit, the periphery buttons show more wear than the frontal buttons, for the same reason.

Peripheral wear occurs in abrasive rock types, which usually have a high quartz content. The cemented carbide at the periphery of the drill bit wears heavily, causing a socalled "anti-taper" to develop. This 17

diminishes the clearance of the drill bit.

of fatigue damage. Under such circumstances. the recommended grinding interval for buttons is 300 m, and for blade-type inserts,

Snakeskin is a wear pattern that develops when drilling in soft, non-abrasive rock such as limestone. After a while, the surface of the cemented carbide begins to suffer from fatigue, with evidence of micro-cracks that resemble snakeskin, particularly at the comers of the inserts. At the first sign of snakeskin, the cemented carbide must be ground, otherwise the micro-cracks will penetrate more deeply into the insert, eventually causing chunks of cemented carbide to loosen and drop out. In certain types of rock, no visible wear occurs at all. Even so. the cemented carbide must be ground to prevent the occurrence

l-50 m.

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Grinding intervals A button bit need only be reground if the penetration rate drops, or if damage begins to occur in the cemented carbide. Small button bits, especially if they are used with hydraulic rock drills, do not generally need to be reground at all. Larger button bits can however benefit

Cross- and X-bits must be reground when the wear-flat across the insert reaches a width of 3 mm, measured 5 mm from the periphery of the drill bit. This is best checked with a grinding template.

from re-grinding. The penetration rate normally begins to drop when the width of the wear-flat becomes equivalent to half the diameter of the button. However, in order to obtain good grinding economy and make grinding quicker and easier, it is advisable to re-grind the buttons when they are worn to about U3 of their diameter.

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The height of anti-taper, i.e. the distance between the highest point of the drill bit and the point at which the clearance begins, must not be allowed to exceed 8 mm on an integral steel, and 6 mm on a cross-bit or X-bit. A button bit has an anti-taper when the clearance of the buttons has disappeared. This is best checked using an ordinary ruler.

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Fixed grinding routines

Grinding

the wear-flats on each and every drillsteel or drill bit at the worksite. On larger worksites, it is therefore essential to establish fixed grinding routines, in which all

Detailed instructions on grinding drill bits can be found in separate Sandvik publications. Grinding templates are useful for checking the grinding result. The easiest way to regrind integral steels and drill bits is to use grinding machines built specifically for this pur-

drill bits are reground after a pre-

pose.

It is often impractical to measure

determined number of holes, or, e.g. at the end of every shift. Since less cemented carbide needs to be removed at each regrind, this will not have a negative effect on drill bit economy.

A correctly ground integral steel should have a fresh cutting edge along 315 of the cemented carbide insert. All sharp edges must be honed, using, e.g. a discarded

grinding wheel or a grinding stone. The insert angle should be 110", and the insert radius, 80 mm.

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On cross-bits or X-bits, a wear-flat equivalent to 1/10 of the bit diameter should be left at the outer edge of each insert.

Worn cemented carbide buttons must be ground to restore their original shape. The easiest way to do this is to use a grinding cup impregnated with synthetic diamonds. If the body steel needs to be removed first (to expose more of the button), special boron-nitride grinding pins are available for this purpose. Today it is also possible to obtain grinding cups that remove steel and cemented carbide at the same time.

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An anti-taper must be removed as soon as possible, by means of frontal grinding. Frontal grinding at

After an integral steel has been reground, the bit diameter must be checked to see if it still fits into the drill steel series.

short intervals can prevent the development of excessive antitaper. Peripheral grinding of crossor X-bits (to restore the clearance) should be employed only when frontal grinding alone is insufficient. In this way, premature reduction of the drill bit diameter is avoided.

To remove the anti-taper on a button bit, it is usually necessary to grind-down the diameter of the bit at the same time as the buttons are re-dressed.

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Naturally, you should use wheels intended for wet grinding when water is used, and wheels intended for dry grinding

It is of great importance to drilling economy that the grinding wheels, grinding cups etc; used to re-dress drill bits are suitable for grinding cemented carbide. Grinding wheels must be self-sharpening, ceramically bonded, carborundum wheels, of the correct hardness and grain size. Manufacturers give all grinding wheels an alpha-numeric designation, which indicates the properties of the product. It is especially important to avoid the use of grinding wheels that are too hard. While they may give good grinding economy, they tend to destroy the cemented carbide, which results in poor total economy.

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