Technical area of the invention This invention relates to a method, in connection with rock drilling, for measuring an air flow through a rock drill bit of the type which during drilling is connected to an extensible drill string and comprises one or more nozzles and/or flush channels for air that is fed through the drill string and the drill bit from a compressor, including the steps to measure the pressure of the flushing air in a reference area, to mathematically calculate a nominal value of the air flow measured as volume with the aid of said measured pressure and to correct that nominal air flow value by considering variable parameters, such as temperatures of entrance air to the drill bit and its environment, respectively, the height of the drilling site above the sea and the like, all during obtaining of a corrected air flow value.

Often also the air flow rate between the drill string and the bore wall is calculated in a mathematical way with the aid of the corrected flushing velocity, which flow rate is compared with established values for requisite force to lift the dust.

Background of the invention

At drilling of blast holes (for example with a depth of 15-30 m) often rotary drill bits are used, which works with pressurised air as flushing medium. A primary object for the flushing air is to lift the drill cuttings crushed by the drill bit, from the drilled hole; something which is done by blowing air out through one or more nozzles terminating downwards in the drill bit with comparatively large diameter (for example 8-26 mm), whereafter it is forced to rise up through the hole at fairly high speed. Another object of the flushing air is partly to cool the bearings for the hubs which are included in the drill bit, and partly to keep these bearings clean. To this end the so called legs on which the hubs are mounted, includes channels which terminate in areas of the bearings and through which a

certain amount of air is conveyed from the total air flow through the drill bit. In order to obtain good drilling results it is vital that the air flow rate through the drill bit and the drilled hole is optimally adjusted in each individual case. This rate depends on many different variable factors and must therefore be controlled at repeated occasions during each individual drilling work. Some minerals can give coarse and heavy drill cuttings, which require a very high flow rate (for example about 50 m/s), to be lifted up from hole while other minerals give fine-grained and light drill cuttings which require a considerably more moderate flow rate (for example about 25 m/s). The rate should on the other hand not be too high, since then blasting damages on different components in the drill bit may occur. Usually the compressor capacity deteriorates with increasing life. For that reason it is not practically possible to rely on nominal ratings for the compressor when the real flow rate is assessed. It is consequently necessary to determine the flow rate in each individual case in accordance with the method as generally indicated in the preamble.

Prior art

At previously known methods for air flow determination a particular measuring set has been used, which includes measuring tube as a main component, which can be connected to drill strings with different diameters via particular adapters. More exactly, the measuring tube can be connected to the drill string first when the drill bit has been removed from the string, wherein a fitting adapter first is placed on the drill string, whereafter the measuring tube is firmly screwed into the adapter. The measuring tube is either directly, or indirectly via hoses, connected to a manometer by which the air pressure inside in the tube can be detected. In the measuring set a variety of interchangeable additional discs with different holes, whose cross cross-sectional areas correspond to the total cross sectional areas of the nozzles and/or bearing flush channels which are included in different drill bits. The hole of the additional disc forms in this manner a reference area which is characteristic to different types of drill bits and to different nozzles to one and the same drill bit. After the pressure

over this reference area has been measured the measuring tube removed is from the adapter and the adapter is removed from the drill string. In a final step the drill bit is mounted on the drill string, whereafter the drilling work can be resumed. Such measurement of the flushing velocity is in practice performed frequently in connection with removing the drilling equipment from a finished hole to a new heading.

A definite drawback, however, with the above mentioned measuring method is that all the operation can take not less than 1-2 hours. Such a long down time implies that the expensive drilling equipment as well as manpower is utilised in a economical ineffective manner. Since personnel generally regards the measurement work only as a necessary evil there is a risk of negligence in case that control measurements are made too seldom. This can in its turn lead to bad drilling results in both economical as technical terms.

To measure pressure directly in a drill bit is previously known per se through US-A-4,754,818 without removing the bit from the drill string and without exchanging the bit for a measuring device. In the known method, however, only one measurement is performed, namely one absolute pressure measurement without regard of different variable parameters and their influence on air flow. Furthermore, the known measurement is conducted by means of a manometer device that requires removal of a plug and mounting of an adapter. To assemble and reassemble such threaded components are very awkward and time consuming.

Objects and features of the invention The present invention aims to eliminate above mentioned drawbacks and to create an improved method for air flow determination in connection with rock drilling. A basic object of the invention is in this manner to achieve a method which enables a quick and efficient measurement of pressure in the reference area and thereby obtaining a quick measurement of the actual air flow through the drill bit. According to the invention this object is achieved by the features which are indicated in the characterizing part of claim 1.

In a particular aspect the invention also relates to a measurement means for accomplishing the method. The features of this measurement means are defined in claim 2.

Detailed description of the invention

In the drawings are:

Fig. 1 a partly sectioned perspective view showing a conventional rotary drill bit,

Fig. 2 a partial longitudinal section through the drill bit according to Fig. 1 , and

Fig. 3 a similar section showing a measurement means according to the invention inserted in a nozzle in the drill bit.

Detailed description of the invention The in Fig. 1 shown rotary drill bit comprises a in its entirety a body designated by 1 , which shows three legs 2 which each carries a conical hub 3 which is equipped with inserts 4. Furthermore, wear protection means or wear pads 5 with large resistance against abrasion are applied on the hub and on the of the leg 2 surface. The individual hub is rotatably journaled relative to the leg 2 via roller and ball bearing means 6, 7, respectively. A main nozzle 8 is included in each leg 2 and has a comparatively large diameter. Via a branch channel 9 the nozzle communicates with a main channel 10 common for all branch channels. Furthermore, a somewhat more narrow branch channel 11 is included in each leg, which in its turn is connected to a number of still more narrow channels that may conduct flushing air to the different bearings with the purpose of cooling and rinsing these. A filtering means 12 is provided in the branch channel 11. On the surface of the tubular neck which delimits the main channel 10 a male thread 13 is formed with the assistance of which the drill bit can be connected to a drill string. The nozzle 8 consists of an ring which is releaseably mounted in the area of the orifice of the branch channel 9, said ring being held in place with the

aid of an appropriate locking means 8', a screw for example. An entire series of nozzle rings with different internal diameters are associated with the drill bit. In practice the smallest nozzle diameter is 8 mm and the biggest diameter is 26 mm. The difference in diameter between different rings is generally 1 ,5 to 2 mm (the next smallest nozzle has in this manner a diameter of 9,5 mm, the next nozzle a diameter of 11 mm, etc.).

As is illustrated by the arrows in Fig. 2 two branch flows for each of the three legs of the drill bit are conveyed from the greater main entrance channel 10, namely a relatively great flow through the branch channel 9 and nozzle 8, as well as a smaller flow via the branch channel 11 to the bearings of the hub.

As far as hitherto shown, the rotary drill bit until now described is wholly known. Such as described according to the preamble the earlier known technique for air flow measurement has required that the drill bit is removed from the drill string and is replaced by a particular measuring tube provided with an additional disc with a hole which serves as a reference area substantially corresponding to the total cross-sectional area of the nozzles of the drill bit and the bearing flush channels.

The present invention is based on a combination of two actions instead of the particular measuring tube, namely to utilise the drill bit itself and its nozzles and channels, respectively, as a reference area, and to accomplish the very measurement of the pressure with a measurement means which can be inserted into a nozzle of the drill bit without to considerably disturb an air flow through the drill bit. The first mentioned action is performed already in connection with the manufacture of the drill bit and the associated set of nozzles. Thus, the nominal air flows which are obtained for the different the nozzles 8 are measured (in laboratory) for the individual drill bit, all from the smallest nozzle to the greatest, as well as the total pressure in these and the flushing channels 11. The measurement results of the air flows and pressure measured in this manner

subsequently accompanies the individual type of drill bit such as characteristic ratings for this individual type of drill bit.

The second action consists in measuring pressure in and directly in the drill bit, more exactly with the aid of a simple measurement means 14 of the type shown in Fig. 3. Characteristic for this measurement means is that it comprises an elongated tube 15 which communicates with a manometer 16. In the schematically shown embodiment the tube 15 is directly connected to the manometer via a bent pipe 17. In practice the tube suitably has an outer diameter in the area of 3-5 mm and a wall thickness of about 0,5 mm. In a preferred embodiment the tube in this manner has an outer diameter of 4 mm and an inner diameter of 3 mm. At measurement the measuring pipe 15 is inserted into the branch channel 9 through the nozzle 8, such that the existing pressure in the flush channel propagates into the measuring pipe so that it can be directly detected by the manometer 16. Since the measuring pipe has a cross sectional area which amounts to less than 25% of the cross sectional area of the smallest nozzle 8 (φ = 8 mm) measuring pipe will in practice occupy only some few percent (less than 5%) of the total air conveying cross-sectional area of the drill bit. In other words, the measuring pipe does not interfere in any substantial way with the total air flow through the drill bit. At extremely high flow rates it can for safety reasons be appropriate to distance the manometer from the drill bit. This can be done by securing the very tube on the drill bit by means of a particular holder (not shown) which is connected to the manometer via a hose. In such a manner manometer can be read at a safe distance from the drill bit. After the pressure in the drill bit has been measured in the above mentioned manner a conventional method is used, i.e. a nominal value on the air flow measured as volume is calculated in a mathematical way based on the measured pressure and the nominal value for the air flow is corrected by considering different variable parameters. In particular, the temperature of the air entering into the drill bit is measured as well as the temperature in the drill bit environment, wherein the temperature difference can be used for modification of

that nominal value via a table. Another factor which requires correction is the height of the heading above the sea (on higher height the air is thinner, implying that compressor loses capacity). Finally the corrected value of the air flow be utilised to calculate the air flow rate between the drill string and the bore wall, which speed is compared with established values for force necessary to lift the dust.

A primary advantage with the invention is that the drill bit does not need to be removed from the drill string in connection with measurement. This means that the down time for the drilling equipment can be reduced to one or some minutes. A secondary advantage which follows on this short down time is that personnel without trouble can perform control measurements more often than earlier; something which in turn guarantees continuously favourable drilling results.