WO2008133581A1

WO2008133581A1 - Method and arrangement for grinding button drill bits, and an abrasive wheel for carrying out the method

Info

Publication number: WO2008133581A1
Application number: PCT/SE2008/050368
Authority: WO
Inventors: Göran STENBERG; Stefan Forsgren
Original assignee: Atlas Copco Secoroc Ab
Priority date: 2007-04-25
Filing date: 2008-03-31
Publication date: 2008-11-06
Also published as: SE0700991L

Abstract

The invention concerns a method for grinding button drill bits, in which an abrasive wheel is used to restore the original form of a button in a button drill-bit. The grinding is carried out according to the invention at a temperature in the grinding region of approximately 250-700 °C. The invention concerns also an arrangement for the grinding of button drill-bits in which an abrasive wheel is used to restore the original form of a button in the button drill-bit, which abrasive wheel is provided with upright side flanges and an intermediate groove, where the arrangement comprises an arrangement for directing a stream of a cooling agent onto the side flanges and groove of the abrasive wheel. The invention concerns also an abrasive wheel that comprises diamonds with a mean particle size of 200 μm or greater, embedded in a matrix.

Description

METHOD AND ARRANGEMENT FOR GRINDING BUTTON DRILL BITS, AND AN ABRASIVE WHEEL FOR CARRYING OUT THE METHOD.

The present invention concerns a method for grinding button drill-bits, in which an abrasive wheel is used to restore the original form of a button in a button drill-bit. The invention relates also to an arrangement for the supply of a cool^¬ ing agent during the grinding of button drill-bits. Further^¬ more, the invention relates also to an abrasive wheel with particular properties m order to carry out the method according to the invention efficiently.

An abrasive wheel is known through, for example, European patent 0 397 955, for grinding button drill-bits for the purpose of restoring a worn shape of a button to its original form, for example a semi-ballistic form as is shown in the drawings m the above-mentioned patent document. Such buttons are manufactured from hard metal, normally a material that consists of tungsten carbide grains bound together by a bmd- ing agent consisting of cobalt, or a material having similar properties .

The invention thus relates to the grinding of buttons that have a cylinder-shaped part that is partly embedded m steel and that have a cupola-shaped end that protrudes from the steel, the form of which is to be restored by the grinding operation .

The abrasive wheel according to the above-mentioned patent has the form of a wheel body, with a groove located between two flanges with the form that the drill button originally had, which form is to be restored by the abrasive wheel. Not only the groove on the abrasive wheel but also the outer edges of the flanges of the wheel are coated with an abrasive agent, a diamond coating, intended to be able to grind down the hard metal of which the drill button is manufactured.

There several problems associated with restoring the form of the hard metal button. It contains hard and sharp particles that rapidly wear out the tool that is used to restore the form of the button.

Furthermore, the situation is made worse by the fact that the hard metal button is embedded in steel, which, when the hard metal button is heavily worn or when it has been previously ground once or several times, entails a necessity that both the hard metal and the steel must be ground on the same occa- sion. This is, however, a difficult combination of materials to grind at the same time, and it is for this reason that a serious wear is experienced of the edges of the abrasive wheel using the tools and grinding methods that are currently in use. These edges are caused to grind down the surrounding steel, and also this effect reduces the lifetime of the abrasive wheel.

Instead of considering solely the properties of the tool, an attempt is made with the present invention to study m more detail the various effects that arise during the grinding operation, m order to attempt to change, through changing the operating conditions, the material properties of the hard metal such that the reduction in material wears down the tools less severely. It has also at the same time been at- tempted to discover a tool that withstands the new grinding conditions better than currently available tools. A balance has previously been sought for the grinding load applied to the abrasive wheel, while supplying at the same time sufficient cooling to the grinding region in order to hold the temperature m the grinding region at as low a value as possible, m order m this way to attempt to obtain an increased lifetime for the abrasive wheels. The grinding load has in this case been reduced, while the supply of cooling agent has been increased or maintained at the same level, which has led to an increase m the time required for gπnd- ing each drill button.

The present invention concerns the solution of the above- described problems such that the lifetime of the abrasive wheels can be increased, while at the same time the time required for grinding the drill buttons can be reduced.

The above-described problems can, according to the invention, be solved through - m direct opposition to what has previ^¬ ously been thought to be obvious - increasing the grinding load on the abrasive wheel while at the same time the supply of cooling agent is reduced, m order to achieve a temperature m the grinding region of 250-700 ⁰C.

According to the invention, an arrangement is also achieved during the grinding of button drill-bits in which an abrasive wheel is used to restore the original form of a button m the button drill-bit, which abrasive wheel is provided with up^¬ right side flanges and an intermediate groove, where the arrangement comprises an arrangement for directing a stream of a cooling agent onto the side flanges and groove of the abrasive wheel. The current drill button can m this way reach the required temperature region. According to the invention, a new abrasive wheel is also achieved having improved properties, in order to be used during the method according to the invention, where the abrasive wheel comprises diamonds having a mean grain size greater than 200 μm, where the outermost layer is only par^¬ tially embedded m a matrix. It is principally improved steel grinding that is achieved m this manner.

Several experiments have previously been carried out m which it has been studied how the hard metal changes under at least one of an increase in pressure and an increase m temperature, i.e. conditions that often arise during the grinding of hard metals. Thus, Milman et al . , for example, m "Result from bending test on submicron and micron WC-Co grades at elevated temperatures", International Journal of Refractory Metals & Hard Materials 20 (2002) pp. 71-79, have shown that there is a ductile-brittle transition for hard metals (with a 6% cobalt content) m a region of temperature of 200-300 ⁰C, and these authors have also shown that plastic deformation does not occur until temperatures of over 600-700 ⁰C are reached. Their experiments, however, were carried out at normal pressure (1 bar) and no external forces were applied to the specimens .

A study has also been carried out by Zelwer et al . m "Grind^¬ ing of WC-Co Cemented Carbides", J. of Eng. for Ind., Aug. 1980, vol. 2 (102) pp. 209-220. These authors studied the wear of tools of tungsten carbide-cobalt for use within the mining industry, and found that plastic flow of the tungsten carbide is an important factor in the grinding operation. Johannes Bernardus Jan-Willem Hegeman, m a doctoral thesis presented at the University of Groningen m 2000, "Fundamen^¬ tals of Grinding: surface conditions of ground materials", investigated the grinding of hard metals with 10% Co and with tungsten carbide grain sizes of 3, 5 and 10 μm. Most of the material in this case was removed by pulverisation and fragmentation of the tungsten carbide, although plastic deforma- tion of both the tungsten carbide grains and the cobalt bind^¬ ing agent also occurred.

It has also been shown that cobalt undergoes a phase transi^¬ tion from hexagonal close packed to cubic close packed at approximately 420 ⁰C, and this makes it easier for the bind^¬ ing agent to be deformed. On the other hand, it has been noted with respect to diamond grains that these start to oxidise in air above 300 ⁰C, and that the diamonds start to combust at a temperature above 480 ⁰C. It has also been dem- onstrated that a mixture of water and oxygen causes diamonds to oxidise above 400 ⁰C.

The tests and experiments mentioned above suggest that there should exist an optimal temperature for the grinding of hard metal when using diamond tools. It is necessary to achieve a balance of temperature m order to achieve a long lifetime of the abrasive wheels, whereby a compromise must be created between high temperature for the processing of the hard metal and a low temperature to avoid oxidation of the diamond coat- mg on the abrasive wheels. A simple method for increasing the temperature during the grinding operation is to reduce the degree of water cooling applied during this operation. A second method is that of increasing the load on the abrasive wheel, or a combination of these measures.

The invention will now be described in more detail m the form of experiments carried out and examples, the results of which are shown m the attached drawings, m which Figure 1 shows how the temperature at the edge of the flat end of a drill button developed as a function of the grinding time, Figure 2 shows temperature curves as a function of load and the rate of supply of cooling agent, Figure 3 and Figure 4 show curves corresponding to that shown in Figure 2 but using different cooling agents, Figure 5 shows the lifetime for the abrasive wheels as a function of load and rate of supply of cooling agent, Figure 6 shows the grinding time as a function of load and the rate of supply of cooling agent, Figure 7 shows how the grinding time changes with the number of drill buttons that have been ground using the abrasive wheel, Figure 8 shows a temperature curve corresponding to that m Figure 2 but for a drill button with a larger diameter, Figure 9 shows, in a manner corresponding to that of Figure 5, the lifetime for abrasive wheels with a larger diameter, Figure 10 shows, in a manner corresponding to that of Figure 7, how the grinding time changes with the number of drill buttons that have been ground with the abrasive wheels of larger diameter, Figure 11 shows a comparison between new and prior art technology m a practical experiment showing the number of buttons per abrasive wheel, during which experiment the button drill-bits were severely worn and had been re- ground several times previously, which entailed much grinding of steel, and Figure 12 shows the result from grinding but- tons of different sizes using not only abrasive wheels of current standard products m which currently used technology was used, but also abrasive wheels that comprise the new sizes of diamonds and types of diamond suggested according to the invention, ground using the grinding method suggested according to the invention, m which frontal or peripheral buttons had been ground. Experiments

The inventors have carried out a large number of experiments in order to investigate the relationships between various factors. These experiments are described below. The correlations between grinding temperature, grinding time and the lifetime of the tool have been investigated in. these experi^¬ ments by varying the flow of water and the load on different types of abrasive wheel. The experiments were carried out on a traditional, but somewhat modified, Atlas Copco Secoroc GMBQ2 grinding machine with a speed of rotation of the sbut- tondle shaft of approximately 15,000 rpm, in all experiments carried out. The load, the grinding load, during the expeπ- ments was varied between 15-20 kg, i.e. a grinding force of 147-196 N. This is to be compared with a previous normal load of 2-6 kg, i.e. a grinding force of 19.6-58.9 N. The rate of flow of water for cooling of the grinding region was varied between 100-400 ml/mm, to be compared with the previously used flow of water of approximately 1,800 ml/mm. The water was added through a nozzle with a flat spray pattern in the direction of the rotation.

Hard metal buttons were used during the experiments with a diameter of 9.0 mm or 14.5 mm, buttons of both dimension having a flattened cupola "worn down" by a value of approximately 70%. The hard metal buttons were ground with semi- ballistic abrasive wheels having dimensions of 9 mm, denoted "Ball 9", or 14 mm denoted "Ball 14". The grinding capacity m steel was checked by using strengthened steel tubes of the type that is usually used for button drill-bits (having a diameter of 20 mm and a length of 60 mm) . In this case, how- ever, abrasive wheels for a spherical capped form for 14.5 mm buttons were used, denoted "GMB 14".

Also different sizes of diamond grains were investigated in order to determine their effects on the grinding result.

The lifetime of abrasive wheels when grinding hard metal was defined as the number of drill buttons that it was possible to grind using the same abrasive wheel to give an acceptable cupola form. Scratches could be detected m the surface of the cupola at the end of the wheel lifetime, and the grinding was then interrupted. The lifetime for the grinding of steel was defined as the length of steel pipe, measured m millimetres, that it was possible to grind with an acceptable rate of removal. Also the time taken for the grinding operation was noted.

The temperature was measured during the hard metal grinding using a thermographic camera. The camera operated within the temperature interval 100-800 ⁰C, and 3 images per second, each having a resolution of 320x280 pixels, were taken during the complete grinding operation. The distance to the camera was 30 cm. The recorded temperature was subsequently cor^¬ rected for the emissivity of WC/Co (6%), (E=O.32), and for the absorption of the lens cover (3%) .

The temperature was measured during the complete grinding cycle, and the maximum temperature during the operation was defined as the highest mean temperature that was experienced for 3.5 seconds m the hottest region. This region during the experiments was the edges of the "worn down" flat end of the hard metal button. Three different concentrations of the cooling agent were used during the temperature measurements, namely pure water, 5% and 10% Binol solution (where "Binol" is the tradename of a cooling and lubricating additive manufactured by Aarhus Karl- shamn Sweden AB, Karlshamn, Sweden) .

It has been traditional when grinding button drill-bits that the cooling agent is supplied as a stream from above that flows down over the abrasive wheel at the grinding location. In order to obtain better control of the amount of cooling agent supplied, it is instead suggested, according to the invention, that a stream of cooling agent is directed towards the abrasive wheel, and in particular towards the side flanges and groove of the abrasive wheel. It is preferable that the stream is directed m the same direction as the direction of rotation of the abrasive wheel and directed towards a point that lies in front of the region at which the abrasive wheel makes contact with the button that is to be ground. It is appropriate that a nozzle is used to direct the cooling agent towards the abrasive wheel.

Example 1

Drill buttons with a diameter of 9.0 mm were ground according to the parameters described above. Figure 1 shows m a draw^¬ ing how the temperature at the edge of the flat end of a drill button developed as a function of the grinding time, under a loading, a grinding load, of 17.5 kg, i.e. a grinding force of approximately 170 N, and a rate of supply of cooling agent (pure water) of 250 ml /mm. The drawing shows that the temperature at the beginning of the grinding operation rose very rapidly to approximately 300 °C, and then subsequently planed out to reach a maximum value of 343 ⁰C, and then - once the grinding was finished after approximately 7 seconds - to rapidly fall.

Corresponding experiments were carried out with different loads within the range 15-20 kg, and with different supply rates of cooling agent in the range 100-400 ml/mm. Figure 2 shows temperature curves, with numerical information, as a function of load and the rate of supply of cooling agent. In this case, however, the cooling agent is a solution that contains 5% Bmol^c. The temperature varied within the range 210-480 ⁰C. It could be concluded that the effects on the temperature have a mainly linear correlation with the load and the rate of supply of cooling agent. A reduction in the rate of supply of cooling agent from 400 ml/mm to 100 ml/mm gave in response an increase of the temperature of approximately 170 ⁰C. An increase of the load from 15 kg to 20 kg gave an increase in the temperature of approximately 100 ⁰C.

Figure 3 and Figure 4 show experiments corresponding to those shown in Figure 2, using other cooling agents. The cooling agent m Figure 3 is pure water, while the cooling agent m Figure 4 contains 10% Binol⁰. These drawings, m comparison with Figure 2, show that the composition of the cooling agent does not have a major effect on the temperature behaviour: it is principally the same, although there are certain differences .

Figure 5 shows the lifetime for the abrasive wheels, according to the definition given above, as a function of load and rate of supply of cooling agent. The drawing shows that a lower rate of supply of cooling agent and a higher grinding load give a longer lifetime for the grinding wheel, measured as the number of drill buttons that can be ground with the abrasive wheel, according to the definition given above. The numerical values m the drawing thus specify the number of drill buttons that can be ground before the abrasive wheel is worn out .

A comparison with the parameters that have been used up until now during the grinding of drill buttons shows that the method according to the invention can give a significant improvement in the lifetime of the abrasive wheels. A maximal increase in lifetime from approximately 275 buttons to ap^¬ proximately 800 buttons is achieved in the example illustrated m Figure 5.

Figure 6 shows instead the grinding time as a function of load and the rate of supply of cooling agent. The times specified are obtained towards the end of the lifetime of the grinding wheel. The grinding time is somewhat shorter when the grinding wheel is new. It is, however, clear also here that a lower rate of supply of cooling agent and a higher grinding load give a more rapid rate of removal, and thus a shorter grinding time.

Figure 7 shows how the grinding time changed for different loads and different supply rates of cooling agent with the number of drill buttons that had been ground with the abrasive wheel m question. It is clear that the grinding time is very short at the beginning of the lifetime of an abrasive wheel independent of the load and rate of supply of cooling agent, while the grinding time rises rapidly with the number of drill buttons that have been ground for an abrasive wheel that uses grinding loads and rates of supply of cooling agent that were previously common, i.e. 6 kg and 1,800 ml/mm, respectively, and it reaches a maximum grinding time of ap- proximately 38 seconds after just under 200 drill buttons have been ground. In contrast, with a moderate increase in load to 10 kg and using a rate of supply of cooling agent of 250 ml/mm, a grinding time is obtained that does not m- crease as rapidly, and these parameters give a grinding time of approximately 18 seconds after 200 drill buttons have been ground. A maximal grinding time of approximately 20 seconds is reached at the end of the lifetime of the abrasive wheel. If the load is further increased to 20 kg and the rate of supply of cooling agent reduced to 100 ml/mm, the grinding time after 200 drill buttons have been ground is significantly reduced, and it then lies at around 8 seconds, in order subsequently to plane out at approximately 10 seconds at the end of the lifetime of the abrasive wheel, which at this time has ground as many as 800 drill buttons.

The grinding time could be reduced on average to approximately 1/3 of the time required by the previously used pa^¬ rameters .

It was noted during the experiments that the change m lifetime for the abrasive wheel mainly followed the change in grinding load and rate of flow. A higher grinding load gives a longer lifetime, as is also the case with a lower rate of supply of cooling agent. The effects for the grinding time are equivalent, but mversed. A higher grinding load gives a shorter grinding time, as is also the case with a lower rate of supply of cooling agent.

Example 2

Figure 8 shows a temperature graph corresponding to that m Figure 2, but for the grinding of a drill button with a di- ameter of 14.5 mm. The cooling agent in this example, however, is pure water. The grinding load was varied also in this case between 15-20 kg, and the rate of supply of cooling agent was varied between 100-400 ml/mm. It could be con- eluded also m this case that the effects on the temperature have a generally linear correlation with the load and the rate of supply of cooling agent. A reduction in the rate of supply of cooling agent from 400 ml/mm to 100 ml /mm gave m response an increase of the temperature of approximately 180 ⁰C. An increase m the load from 15 kg to 20 kg gave an increase in the temperature, but it was not as marked as that that occurred when grinding 9 mm drill buttons.

Figure 9 shows, corresponding to Figure 5, the lifetime for the abrasive wheels as a function of load and the rate of supply of cooling agent. Also in this case, a clear tendency for an increased grinding load and a decreased rate of supply of cooling agent to give a longer lifetime for the grinding wheel is seen.

Figure 10 shows, corresponding to Figure 7, how the grinding time changed for different loads and different rates of sup^¬ ply of cooling agent with the number of drill buttons that had been ground with the abrasive wheel in question, when grinding 14.5 mm drill buttons. In this case, however, only grinding at 15 and 20 kg grinding load and at a rate of supply of cooling agent of 400 ml /mm and 100 ml/mm are shown. It was noted that the difference was not so great when the abrasive wheels were new, and that differences started to appear when the wheels started to become somewhat worn, after grinding approximately 30 drill buttons. Also m this case, the higher grinding load and the lower rate of supply of cooling agent gave a shorter grinding time, while at the same time, significantly more drill buttons could be ground before the wheel became worn out.

In summary, it can be said for the 14.5 mm wheels that the lifetime could be doubled while the grinding time could at the same time be halved, when using the method according to the invention, compared to the values obtained when using the previously used parameters.

Other experiments

Experiments have been carried out into a further reduction in the rate of supply of cooling agent. A lifetime of 457 ground buttons was obtained at a rate of supply of cooling agent of 30 ml/mm, at a grinding load of 19 kg and a rate of revolu^¬ tion of approximately 13,000 rpm. A further reduction in the rate of supply of cooling agent, to 0 ml/mm, however, gave a significantly poorer lifetime: only 30 buttons in conditions that were otherwise equivalent. Without cooling, and at a load of 17.5 kg, a temperature of approximately 800 ⁰C was reached, a temperature at which the diamonds were combusted and at which the lifetime was therefore seriously reduced. Thus, a certain rate of supply of cooling agent is necessary m order to achieve the improved properties of lifetime.

Experiments have also been carried out changing the type of diamonds m the abrasive wheels, m order to see what the result of this might be. It became apparent that the particle size of the diamonds has a certain effect on the result, as do also the mechanical properties of the diamonds. This ef^¬ fect, however, is not particularly great for the grinding of the hard metal itself. In contrast, the size of the diamonds makes a significant difference when it comes to the grinding of the steel in which the drill buttons have been embedded. It proved to be the case that a larger particle size for the diamonds gives a greater capacity for grinding down the surrounding steel, but it did not, as has been mentioned above, have a major effect on the result of the grinding of the hard metal button itself. It became apparent, also, that the abra^¬ sive wheels when grinding 14.5 mm buttons had a significantly longer lifetime when grinding the steel than the 9 mm wheels had. It could also be established that the longest lifetime for grinding steel was achieved at a high load and at a high rate of supply of cooling agent. In this case, thus, the cooling has a different effect than it has when grinding hard metal .

Extensive investigations have been carried out in order to reveal the significance of the diamonds for the grinding of steel. These have been carried out not only on test specimens as described above, but also on actual button drill-bits with worn buttons.

The significance of the properties of the diamonds have prin^¬ cipally been investigated with respect to toughness, or to be more precise, the ability of the diamonds to resist mechani^¬ cal wear both before and after heating. Toughness is meas- ured m the form of a couple of indices: the toughness index, TI, and the thermal toughness index, TTI.

There are no generally accepted standards for determining TI and TTI, and it is for this reason that several different variants are found. These are often specific for one particu^¬ lar company. It has been decided m the present case to de^¬ termine TI and TTI m the following manner. The diamonds are sieved through a nominal sieve, and the sieved material is subsequently shaken in a Friagπt machine manufactured by Custodiam S.A., Brussels, Belgium. This machine measures the toughness index (TI) of the diamonds, or to be more accurate, it measures the time taken to crush 50% of the diamonds (by weight) . This time is here denoted the "halving time", abbreviated "HT".

The measurement is carried out by placing approximately two carats of diamonds into a cylindrical steel container of length 22.9 mm, together with a steel ball (of diameter 5/16 inches, and of weight approximately 2.0 gram) . The diamonds are then shaken with a frequency of 40 Hz and an amplitude of approximately 4 mm. The test procedure is described in "Nu- merical Simulation of the Diamond Grit Friability Tester"; J. Sbuttonnewyn et al., published in "Diamond Tooling Proceedings of the Euro PM 2002 conference, Lausanne October 7-9 2002". The shaking is carried out until the diamonds have been damaged m such a manner that 50% (by weight) will be retained by a second nominal sieve. Electroformed sieves as defined by ISO3310-3 are used.

Diamonds with a mean particle size of 250 μm (D251 according to FEPA) are sieved before shaking through a nominal sieve in which the holes are 212±2 μm, and after the shaking a sieve is used m which the holes are 180±2 μm.

Diamonds with a mean particle size of 210 μm (D213 according to FEPA) are sieved before shaking through a nominal sieve in which the holes are 180±2 μm, and after the shaking a sieve is used m which the holes are 150±2 μm. Diamonds with a mean particle size of 149 μm (D151 according to FEPA) are sieved before shaking through a nominal sieve m which the holes are 125±2 μm, and after the shaking a sieve is used m which the holes are 106±2 μm.

The same procedure is used when the thermal toughness index (TTI) or the thermal halving time (THT) , is to be determined. The difference is that the diamonds from the same manufacturing batch are exposed to 1,100 ⁰C in a closed vessel for 30 minutes.

As previously mentioned, there are other methods for measuring TI and TTI: different systems and different sieving procedures are used. A defined time may be used, instead of crushing a defined quantity of diamonds. A weight fraction of diamonds is obtained in this case, rather than a time value. The weight fraction of diamonds is then normally presented as a percentage. Different names are thus used for the ability of the diamonds to withstand mechanical wear, depending on the measurement equipment and the measurement method that have been used.

Two different variants of diamond were used when testing diamond abrasive wheels according to the prior art technol- ogy:

Small wheels - for grinding buttons with diameters 7-12 mm. The mean size of the diamonds was 149 μm (D151) and the halv^¬ ing time, HT, was approximately 41 seconds before the heat treatment and approximately 28 seconds for diamonds that had undergone heat treatment, THT. Large wheels - for grinding buttons with diameters 13-22 mm. The mean size of the diamonds was 229 μm (D252) and the halving time, HT, was approximately 38 seconds before the heat treatment and approximately 33 seconds after the heat treatment, THT.

Corresponding experiments with abrasive wheels according to the present invention gave the following corresponding values :

All wheels, both large and small had a mean size of the diamonds of 210 μm (D213) or greater, and the halving time, HT, was in this case greater than 47 seconds before the heat treatment and greater than 40 seconds after the heat treatment, THT.

The results from the testing of the diamonds of two currently available types, and results from testing diamonds of a further size and type, are shown in Table 1.

Table 1

The term "type" is here used to denote the type of industrial diamonds that according to Diamond Innovations, Worthington, OH, USA, have been given the specified denotation. This denotation specifies both a measure of the crystal size and the crystal form. It is obvious that also diamonds from other suppliers having corresponding properties can be used.

TI and TTI can be measured also in other ways, such as, for example, the toughness index (TI) being measured by shaking the diamonds of a given "mesh size", i.e. a standardised mean diameter, for example 149 or 229 μm, in a cylindrical steel container with a steel ball. A fraction of the diamonds will break during this procedure. The amount of "broken" diamonds is determined by sieving the diamonds after the shaking pro- cedure and thereafter comparing the mass that has remained m the sieve with the original mass. The ratio of the mass that remains in the sieve to the original mass gives TI:

TI = the mass that remains in the sieve/the original mass.

Certain suppliers, including Diamond Innovation and Element 6, then specify the fraction of undamaged crystals as TI (most often expressed as a percentage) .

The thermal toughness index (TTI) is a toughness index meas^¬ ured in the same manner, with the difference that the dia^¬ monds are exposed to a temperature greater than, normally, 1,000 ⁰C, before the shaking procedure.

In order to obtain values for the abilities of various abra^¬ sive wheels to grind down the steel that surrounds the hard metal buttons, experiments were carried out, as has been mentioned, using test specimens as those defined above. The rate of supply of cooling agent was varied in these experiments between 100-400 ml/mm, and the grinding load was varied between 15-20 kg. Table 2 shows the results for several different abrasive wheels for the grinding of steel using a rate of supply of cooling agent of 250 ml/mm and a grinding load of 17.5 kg.

Table 2

"Ball" in the table denotes a wheel used for grinding a half- ballistic cupola, and "GMB" denotes a wheel used for grinding a spherical cupola.

The results show that both the size of diamond and the type of diamond have significant effects on the result of the grinding .

The transition (for large products, 14 mm) from MBG600T to MBG620T gives an improvement of approximately 23%. The transition (for small products, 9 mm) from MBG600T/MBG620T to MBG620T gives an improvement of approxi^¬ mately 47%.

The transition (for small products, 9 mm) from MBG600T/MBG620T to MBG600T gives an improvement of approxi^¬ mately 200%. This means that a transition to MBG620T and a particle size of over 210 μm gives an expected improvement of between 23- 200%.

Experiments have also been carried out on actual button drill-bits. The new and the old techniques are both tested in this case, with respect to both diamonds and grinding technology. The lifetime depends on whether small or large wheels are used. The results using small wheels, 11 mm, are poorer than those currently obtained. The situation is slightly improved when using the new grinding technique and large wheels, 13 mm. The new selection of diamonds, however, solves the problem, and everything becomes better - or at least not worse - than the current situation. Figure 11 shows a comparison between the new and the old technologies during practical experiments m which the condition of the button drill- -bits was poor, i.e. they were severely worn and had been reground several times, something that m its turn means that much steel grinding is required, particularly for the frontal buttons. It should be noted that the frontal buttons, 11.0 mm, had been ground with 11 mm abrasive wheels, and thus with diamonds having a mean diameter of 149 μm, while the peripheral buttons, 12.7 mm, had been ground with diamonds having a mean diameter of 229 μm, in the experiments with currently used abrasive wheels. The new abrasive wheels had a mean diameter of 229 μm, and gave an equivalent lifetime for the wheels when grinding the frontal buttons with the new technique, but with a considerably shorter grinding time. Both a significantly improved lifetime for the abrasive wheels, and a shorter grinding time, were obtained for the peripheral buttons. In summary, this thus gives a clearly improved total economy . Finally, Figure 12 shows results from grinding buttons of various sizes not only with abrasive wheels of currently available standard products, but also with abrasive wheels that comprise the new types of diamond and sizes of diamond. It is clear in all cases that the new abrasive wheels give a better result than the prior art abrasive wheels, and thus is the case for button bits that have been reground several times, and where the abrasive wheels have been compelled to grind not only hard metal but also the steel m which the buttons are embedded.

Conclusion

It has been shown that by carrying out grinding with the invention in a certain range of temperature and with a re^¬ duced rate of supply of cooling agent it is possible to achieve not only a longer lifetime for the abrasive wheels, but also a shorter grinding time. Both effects thus contπb- ute to a considerably improved efficiency for the grinding operation, and furthermore, to lower direct costs.

While it is true that oxidation of the diamonds m the abrasive wheel is obtained when increasing the temperature m the grinding region, according to the invention, a shorter grinding time is obtained through the fact that the hard metal becomes easier to grind, and this ensures that the diamonds are exposed to the oxidising temperature for a shorter time, which results m an increase in lifetime for the abrasive wheels.

An advantageous balance of temperature for the grinding of 9 mm buttons was found to be located between 300 and 500 ⁰C. The diamonds are oxidised relatively slowly in this interval of temperature, while the hard metal is despite this held at a temperature at which it is relatively easy to grind. This gives thus a shorter grinding time and a longer lifetime for 5 the abrasive wheels.

An advantageous balance of temperature for the grinding of 14.5 mm buttons was found to be located between 250 and 400 ⁰C, and preferably m the range 300-400 ⁰C, where a rela- w tively short grinding time is obtained, while at the same time the lifetime increases. It is preferable that the grind^¬ ing load in this case be greater than 16.5 kg, m order to obtain a long lifetime combined with a short grinding time.

15 It is a requirement for the abrasive wheels that these must have properties that allow them to withstand the processing not only of the hard metal but also of the surrounding steel at the higher grinding load and at the lower rate of supply of cooling agent that has been suggested by the invention.

20 Thus, the diamonds should have a mean particle size of at least 200 μm, preferably 229 μm, and they should be of the MBG620T type, m order to achieve this. The diamonds may be also of a better type, such as of MBG640T, MBG660T or MBG680T type. Furthermore, the diamonds should have an HT value of at

25 least 40 s, and a THT value of at least 32 s.

The outermost diamond particles should be embedded m the matrix such that they have a protruding part that is greater than 1/5 of the mean particle size of the diamonds. The size 30 of this protruding part should not, however, be greater than 2/3 of the mean particle size.

Claims

1. A method for grinding button drill-bits, in which an abrasive wheel is used in order to restore the original form of a button m the button drill-bit, characterised in that the grinding is carried out at a temperature m the grinding region of approximately 250-700 ⁰C

2. The method according to claim 1, characterised in that the grinding is carried out at a temperature m the grinding region of approximately 250-500 ⁰C.

3. The method according to claim 1, characterised m that, when grinding small buttons, of diameter 7-12 mm, the gπnd- mg is carried out at a temperature in the grinding region of 300-500 ⁰C.

4. The method according to claim 3, characterised m that the grinding is carried out at a temperature in the grinding region of approximately 400-500 ⁰C.

5. The method according to claim 1, characterised m that, when grinding large buttons, of diameter 13-22 mm, the grinding is carried out at a temperature m the grinding region of 250-400 ⁰C.

6. The method according to claim 5, characterised m that the grinding is carried out at a temperature in the grinding region of approximately 300-400 ⁰C.

7. The method according to any one of the preceding claims, characterised m that the grinding is carried out at a tem^¬ perature m the grinding region lower than 480 ⁰C.

8. The method according to any one of the preceding claims, characterised in that the grinding is carried out at a grinding load of 15-25 kg, preferably 15-20 kg.

5

9. The method according to claim 8, characterised in that the grinding is carried out at a grinding load 17.5-20 kg.

10. The method according to any one of the preceding claims, w characterised m that the grinding is carried out at a rate of supply of cooling agent of 10-400 ml/mm.

11. The method according to claim 10, characterised in that the grinding is carried out at rate of supply of cooling

15 agent of 100-400 ml/mm.

12. The method according to claim 11, characterised m that the grinding is carried out at rate of supply of cooling agent of 100-200 ml/mm.

20

13. The method according to any one of claims 10-12, where the grinding is carried out with the aid of an abrasive wheel with upright side flanges and an intermediate groove, charac^¬ terised in that the cooling agent is supplied as a stream

25 directed towards the side flanges and groove of the abrasive wheel .

14. The method according to claim 13, characterised in that the cooling agent is supplied as a stream directed m the

30 same direction as the direction of rotation of the abrasive wheel, and m front of the grinding region.

15. The method according to any one of the preceding claims, characterised in that the grinding is carried out using an abrasive wheel that comprises diamonds with a mean particle size greater than 200 μm, embedded in a matrix.

16. The method according to claim 15, characterised in that the diamonds have an HT value greater than 47 s, and a THT value greater than 40 s.

17. An arrangement for the grinding of button drill-bits, m which an abrasive wheel is used to restore the original form of a button in the button drill-bit, which abrasive wheel is provided with upright side flanges and an intermediate groove, characterised in that the arrangement comprises an arrangement for directing a stream of a cooling agent onto the side flanges and groove of the abrasive wheel.

18. The arrangement according to claim 17, characterised m that the arrangement comprises a nozzle.

19. The arrangement according to claim 17 or 18, character^¬ ised m that the arrangement is arranged to direct the stream m the same direction as the direction of rotation as the abrasive wheel and at a location in front of the region at which the abrasive wheel meets the button that is to be ground.

20. An abrasive wheel for carrying out the method according to any one of claims 1-14, in which an abrasive wheel is used to restore the original form of a button m a button dπll- -bit, characterised m that the abrasive wheel comprises diamonds with a mean particle size of 200 μm or greater, embedded in a matrix.

21. The abrasive wheel according to claim 20, characterised in that the outermost diamond particles have a protruding part that has a dimension of 1/5 to 2/3 of the mean particle

5 size of the diamonds.

22. The abrasive wheel according to claim 20 or 21, charac^¬ terised in that the diamonds have an HT value greater than 47 s, and a THT value greater than 40 s. w

23. The abrasive wheel according to any one of claims 20-22, characterised m that the diamonds are of the types: MBG620T, MBG640T, MBG660T, MBG680T or equivalent.