AU2011202932A1 - A drilling apparatus - Google Patents

Abstract

A drilling apparatus (10) comprises an outer cylinder (12); a drill bit (30); and a piston (32) disposed within the outer cylinder (12) and driven by a working fluid for impacting the drill bit (30). The piston (32) is reciprocable relative to the outer cylinder (12) during operation of the drilling apparatus (10) and has a top end (39) and a bottom or front end (34), each piston end having a bore (33, 35). At least one of the bores (33, 35) has a tapered portion to facilitate movement of the piston (32) over a sealing tube (14, 41) controlling pressure of the working fluid driving reciprocation of the piston (32) relative to the outer cylinder (12) of the drilling apparatus (10). The drilling apparatus may be a down the hole, in the hole or reverse circulation hammer drill (10).

Description

pool Section 29 Reguation 3,2(2) AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: Invention Title: A drilling apparatus The following statement is a full description of this invention, including the best method of performing it known to us: P1 11H#AU/0610 1 A DRILLING APPARATUS FIELD OF THE INVENTION The present invention relates to a drilling apparatus suitable for use in mineral exploration. 5 BACKGROUND TO THE INVENTION Down-hole drilling for geological sampling is generally conducted using a pneumatically operated hammer drill comprising a drill string with a drill bit at its bottom end. Such drills use a conventional down-the hole ("DTH") mechanism, with live compressed air being supplied to operate a reciprocating piston near the 10 base of the hole being drilled. Two forms of operating cycle are possible: reverse circulation and normal circulation, some of the principles of which are described in Australian Patent No. 682640, the contents of which are hereby incorporated herein by reference. In "normal" circulation, air is exhausted to the bottom and sides of a drill 15 bit. This exhaust air, entraining cuttings and other debris, passes the outside of the drill string. Such an operating cycle is not suited for accurate sampling because of the sample contamination issues. In "reverse" circulation operation, exhaust air is returned through a passageway extending through the drill string to a collector at the surface, 20 carrying with it drilled rock cuttings and debris which can be analysed in surface laboratories. Reverse circulation ("RC") allows accurate sampling without contamination by contact with the wall of the hole which is of great importance in mineral exploration. Sampling is more accurate where there is good sealing between drill and hole, this causing substantially all air to be passed with cuttings 25 through the drill string. The drill string includes a piston which acts against a drill bit at the base of the hole, providing repeated high frequency impacts to the drill bit to cause rock fracture and formation of rock particles which are returned to the surface for sampling. 30 Additionally, drills or DTH hammers for conventional down-hole drilling whether of normal or reverse circulation type - include a fluid pathway through which the compressed air flows. Such a fluid pathway includes sealing tubes that enable pressure buildup in chambers, located above and below the piston, 2 dependent on the state of operation. Such pressure build up drives piston movement or reciprocation. A sealing tube located at the bottom of the piston may be described as an exhaust tube. This exhaust tube typically includes a plastic tube portion, inserted into the top (strike end) of the drill bit with the piston 5 being supported around this tube. The plastic tube portion also operates as a foot valve, providing a region in which air pressure can build to assist in reciprocation, through driving lift of the piston. The higher that this air pressure can build, the better the efficiency and speed of drilling operation and the lower the associated exploration costs. A sealing tube, serving a similar function to the 10 exhaust tube but located at the top of the piston, may be described as a control tube. With conventional DTH hammers, the majority rely on piston lift by sealing (as such) on the above described plastic sealing tube portion or foot valve. The ideal clearance for good compression on the piston to cylinder bore generally falls 15 in the range between 0.004" and 0.008". However, such clearance is not feasible on the piston inner diameter (ID) and exhaust tube portion outer diameter (OD) as allowance has to be made for drill bit to drive sub clearance; and drill bit to guide bush clearance. Therefore, more clearance is necessary (than ground to size piston in honed to size cylinder) to avoid breakage of the exhaust tube portion or 20 foot valve. Such clearance usually ranges from 0.012" to 0.020", a compromise that does not provide good sealing. Without good sealing, compression and drilling performance are also likely to be compromised. SUMMARY OF THE INVENTION It is an object of the present invention to provide a drilling apparatus with 25 better sealing and better drilling performance. With this object in view, the present invention provides a drilling apparatus comprising: an outer cylinder; a drill bit; and 30 a piston disposed within the outer cylinder and driven by a working fluid for impacting the drill bit, the piston being reciprocable relative to said outer cylinder during operation of the drilling apparatus and having a top end and a bottom end wherein each piston end has a bore and at least one of said bores has a tapered 3 portion to facilitate movement of the piston over a sealing tube controlling pressure of the working fluid driving reciprocation of the piston relative to said outer cylinder of said drilling apparatus. One tapered bore is a front bore located at a bottom or front end of the 5 piston. The front bore may have a portion tapering inward in a direction away from the front end of the piston. Put another way, the front bore may taper inward in a direction away, that is rearward, from the drill bit. The front bore may be provided with sections of varying taper. In one section, a first section, the front bore may 10 taper inward in a direction rearward from a head of the drill bit, particularly in a region where sealing between the front bore and the sealing tube is required to maximize compression for driving the piston upstroke. In this region, the taper measured as an angle from a centerline of the piston - is very gradual, being less than 0.2 degrees, more preferably less than 0.16 degrees. This angle or taper is 15 preferably constant, In another section, a second section, the front bore may terminate in an opening or port with wider dimension than the first section of the front bore. Alternatively, in the second section, the front bore may taper outwards to assist movement of the sealing tube within the front bore during operation of the drilling apparatus. 20 At the top end of the piston may, alternatively or advantageously additionally, be located another bore, this top bore typically being adjacent a top sub of the drill string. This top bore may also advantageously be tapered as similar considerations, with respect to sealing and compression to create a driving force for piston movement (in this instance downstroke) arise as for the bottom 25 bore and sealing tube. The top bore may taper inward in a direction away from the top end of the piston and towards the bottom end of the piston. This top bore receives another sealing tube during drilling operation. Again, this bore also has a gradual and constant taper which may be less than 0.1 degrees, preferably less than 0.08 degrees, angle from the centerline of the piston. It will be noted that 30 this taper is less than the taper of the bore at the bottom or front end of the piston. The sealing tube may be termed an exhaust tube in the case of the front bore at the bottom or front end of the piston. It functions as a foot valve. The sealing tube is a control tube at the top end of the piston. The exhaust tube is 4 fixed in the top end of the drill bit. The control tube may be fixed in a top sub of the drilling apparatus. In either case, fixing may be by interference fitting. The control tube and exhaust tube may have the same or different design. So, the exhaust tube may be generally cylindrical being formed in the shape of a cylinder 5 of uniform outer diameter and extending a certain calculated distance from the strike face of the drill bit. The inner tube and/or exhaust tube is desirably provided with smooth walls to reduce frictional losses. The control tube may have a different design, though still generally cylindrical, with top and bottom ends having higher diameter portions at its ends 10 than along an intermediate portion. Wall thickness of the control tube may be higher than for wall thickness of the exhaust tube or foot valve recognizing the severe working conditions and with a view to maximizing service life. The control tube may require to be of metal or heat resistant and durable polymer to handle the high working temperatures, potentially higher than 200"C. While the control 15 tube may be a metallic part to handle these working conditions, a polymer which provides excellent wear resistance and a low coefficient of friction is expected, by the Applicant, to be suitable for the application. Suitable polymers include woven nylon and a polyphenylene sulfide polymer available under the trade mark Techtron@ HPV PPS. A control tube made of this polymer forms another aspect 20 of the present invention. As the piston moves over a sealing tube, to enable pressure build up in a chamber surrounding that sealing tube as the piston approaches a reversal in direction of travel, the clearance between piston bore inner diameter (ID) and exhaust or control tube outer diameter (OD), gradually reduces until a minimum 25 clearance is reached. This minimum clearance may occur over a length of the piston bore to form a region of minimum clearance. This region of minimum clearance sets the maximum degree of sealing and compression, ideally achieved - in the case of an exhaust tube - at the point where the piston impacts the drill bit. The higher this maximum pressure, the more efficient the drilling 30 operation. The degree of taper of a piston bore receiving the sealing tube is also set to achieve the required degree of tube clearance as the piston bore moves over the sealing tube. A desirable clearance range for piston ID to exhaust tube OD is 5 between 0.004" and 0.008", for example between 0.006" and 0.008". Clearance of 0.004" or lower, even size for size, may also be achievable. The piston may have a reduced diameter front end portion of the piston. The strike face which impacts an upper end of the drill bit is defined by the reduced diameter front end 5 portion of the piston. The drilling apparatus is suitable for reverse circulation and normal circulation operation and may be provided with a range of porting arrangements. In a further aspect of the present invention, there is provided a drilling piston having a top end and a bottom end wherein each piston end has a bore 10 and at least one of said bores has a tapered portion to facilitate movement and sealing of the piston over a sealing tube for controlling pressure of a working fluid driving the piston within a drilling apparatus. Most conveniently, both bores will be tapered in the manner above described. The drilling apparatus, by providing a tapered bore on the piston to receive 15 the sealing tube enables a good to excellent degree of sealing between sealing tube outer diameter and piston bore inner diameter when required during reciprocation of the piston. This enables increased compression of working fluid and consequent significant increase in drilling performance over prior drilling apparatus. In addition, use of tapered bore(s) in the piston reduces the 20 occurrence of exhaust tube breakages. DESCRIPTION OF THE FIGURES The drilling apparatus of the present invention may be more fully understood from the following non-limiting description of a preferred embodiment which is described with reference to the following drawings in which: 25 Fig. I is a side section of portion of a drilling apparatus in accordance with one embodiment of the present invention and showing inner or exhaust tube disengaged from a bore of a piston of the drilling apparatus at a commencement of a downstroke of the piston. Fig. 2 is a side section of portion of a drilling apparatus, constructed in 30 accordance with the first embodiment, and showing the exhaust tube received in a first position within the piston bore at a first later point in the downstroke of the piston.

6 Fig. 3 is a side section of portion of a drilling apparatus, constructed in accordance with the first embodiment, and showing the exhaust tube received within the piston bore at a second later point in the downstroke of the piston. Fig. 4 is a side section of portion of a drilling apparatus, constructed in 5 accordance with the first embodiment, and showing the exhaust tube received within the piston bore at a third later point in the downstroke of the piston. Fig. 5 is a side section of portion of a drilling apparatus, constructed in accordance with the first embodiment, and showing the exhaust tube received within the piston bore at a point in the downstroke of the piston where it impacts 10 the drill bit. Fig. 6 is a detail section view of the portion of the drilling apparatus in a position having minimum clearance between exhaust tube and piston bore of the drilling apparatus shown in Figs. 1 to 6. Fig. 7 is a detail section view of the relationship between exhaust tube and 15 piston bore of the drilling apparatus, at minimum clearance, as shown in Fig. 6. Fig. 8 is a detail section view of the portion of the drilling apparatus showing clearance between exhaust tube and piston bore at the point in the piston downstroke illustrated also in Fig. 2. Fig. 8a is a detail section view of the relationship between exhaust tube 20 and piston bore of the drilling apparatus at the clearance as shown in Fig. 8. Fig. 9 is a side section of portion of a drilling apparatus in accordance with a second embodiment of the invention in whIch tapered bores are provided at top and bottom ends of the piston. Fig. 10 is a detail section view of a top sub end of the piston showing its 25 relationship with the top sub and an inner tube in the form of a control tube. Fig. 11 is a partial schematic section view of a drilling apparatus showing the piston and its relationship between the top sub, control tube, drill bit and exhaust tube. Fig. 12 is a partial perspective view of a drilling apparatus showing the 30 piston and its relationship between the top sub, control tube, drill bit and exhaust tube.

7 DETAILED DESCRIPTION OF THE INVENTION Figs. I to 8 are illustrative of a drilling apparatus in the form of a pneumatic down the hole (DTH) hammer drill 10 for mineral exploration. The drill 10 comprises an outer cylinder or wear sleeve 12 and a piston 32 disposed within 5 the outer cylinder 12. The piston 32 is concentric with the outer cylinder 12 and has as adjacent components a top sub 61 (see Figs. 9 to 13) and a drill bit 30. Piston 32 reciprocates within outer cylinder 12, its movement being driven by a working fluid as understood in the drilling art. Typically, the working fluid is compressed air. 10 A sealing tube 14 is assembled to drill bit 30 and it may be appreciated that such assemblies may be separately supplied for use in drills such as drill 10. Sealing tube 14 defines at least part of a passageway 20, through which compressed air flows, during operation of the drill 10. Sealing tube 14 is of suitable durability plastic for drilling service in accordance with current practice, 15 and the pressures and temperatures encountered during such service. The passageway 20 is also located centrally and coaxially of the outer cylinder 12. The sealing tube 14 is an exhaust tube and acts as a foot valve. Exhaust tube 14 also has a tapered portion 14a, tapering inward in the direction rearward of the front end portion 34 of piston 32. Such taper facilitates movement of 20 exhaust tube 14 relative to a bore 33 located in the bottom or front end of piston 32, Exhaust tube 14, of cylindrical shape, has a smooth surface to reduce frictional losses during reciprocation of piston 32 over it, it is not threaded. Exhaust tube 14 extends a calculated longitudinal distance, D, from a strike face 38 of drill bit 30. Distance D is not arbitrary; it must be selected during the drill 10 25 design stage, dependent on drill diameter and other design parameters. Piston 32 is slidably movable relative to the exhaust tube 14 during its reciprocation. Exhaust tube 14 is fixed, in interference fit, by flange 142 in the central passageway 20 as formed in drill bit 30 at its shank end. Flange 142 is formed by a thicker portion of the wall of the exhaust tube 14. Piston 32 has a 30 reduced diameter front end portion 34 which provides a piston strike face 35. The piston 32 is mounted over the exhaust tube 14 to reciprocate - in response to variable air supply pressure in variable volume chambers 63 and 66 above and below the piston 32 - so as to repeatedly strike, through impacts of 8 piston strike face 35 on corresponding strike face 38 of the drill bit 30. Strike face 38 is located at the upper or rear end of drill bit 30. The consequential impacts of the drill bit 30 on the drilled mineral deposit in a hole, fracture rock into cuttings and other debris to be removed to the surface. The piston 32 strikes drill bit 30 5 during its downstroke and Figs 1 to 7 show RC drill 10 at various points in a piston 32 stroke. Piston 32 has a front bore 33, located at the front end portion 34 of the piston 32, this front bore receiving exhaust tube 14 during reciprocation of the piston 32. Front bore 33 is designed to achieve two objectives. First, it must facilitate 10 movement of piston 32 over exhaust tube 14. Front bore 33 must readily receive exhaust tube 14 within bore 33 when the piston 32 moves over it. Front bore 33 must provide sufficient clearance for the piston 32 to achieve this. Second, the front bore 33 is designed to achieve a minimum clearance and better sealing, higher compression in chamber 66, a higher driving force for piston 32 lift and 15 more efficient drilling performance. To achieve these ends, the wall 33a of front bore 33 is provided with sections of varying taper. Bore 33 is not of plain cylindrical geometry. In a first section, rearward of the opening or port 37 to piston bore 33, front bore 33 tapers linearly inward in a direction away from the front end of the piston 32. In a 20 second section, front bore 33 terminates in an opening or port 37 with wider dimension than the front bore 33. A step or land 37a, with rounded edges, is formed in the opening or port 37 providing a rapid transition from front bore 33 diameter to a wider diameter. Bore 33 tapers inward in a direction towards the top sub end 39 of the 25 piston 32. At an upper point of the piston 32 upstroke, exhaust tube 14 is disengaged from the piston front bore 33 (Fig. 1), pressure in chamber 66 falls and this taper has no function. During a piston 32 downstroke, proceeding toward striking of drill bit 30, exhaust tube 14 is received within piston front bore 33 (Figs. 2 to 5). From the point the exhaust tube 14 is first received within the 30 piston front bore 33, a process assisted by the wider opening or port 37 of piston front bore 33 and the inward taper - at an angle of about 0.13 degrees to the centerline of piston 32 - of a rearward portion of exhaust tube 14, the clearance between exhaust tube 14 outer diameter (OD) and piston front bore 33 inner 9 diameter (ID) gradually decreases. This clearance linearly decreases to a minimum at the point illustrated by Figs. 6 and 7, this minimum being 0.006" (0.15mm). At this point, a good to excellent degree of sealing between exhaust tube 14 and piston bore 33 results. This enables increased compression in 5 chamber 66 between the piston 32 and drill bit 30 which drives lift of piston 32. There is a consequent significant increase in lift performance and drilling performance over prior drilling apparatus. Reduced incidence of breakage of exhaust tubes is also observed with the drill 10 as described above. Referring now to Figs. 6 to 8, the taper of front bore 33 and exhaust tube 10 14 is clearly shown. Fig. 7 shows the region of minimum clearance between exhaust tube 14 OD and front bore 33 ID. It will be noted that this region of minimum clearance 145 is a short distance forward of the rear portion or end 14a of the exhaust tube 14 due to the taper of a rearward portion or end 14a of exhaust tube 14. The degree of taper of rearward portion or end 14a of exhaust 15 tube 14 is greater than the degree of taper of wall 33a of front bore 33. In this region of minimum clearance 145, the clearance is between 0.006" (0.15mm) and 0.008" (0.20mm) maximum, a clearance allowing a high degree of sealing, compression and drilling performance. In Fig. 8 is illustrated the contrasting clearance between exhaust tube 14 OD and front bore 33 OD as the exhaust 20 tube 14 commences to be received within front bore 33. The clearance range at this point is between 0,015" (0.38mm) minimum clearance and 0.017" (0.43mm) maximum clearance, significantly greater clearance than at maximum sealing. Referring now to Figs. 9 to 12, at the top end 39 of the piston 32, adjacent a top sub 50 of the drill 10, is located another bore 35. Bore 35 is also tapered, at 25 an angle of about 0.07 degrees to the centerline of piston 32. Bore 35 is not of plain cylindrical geometry. Therefore, the degree of taper is less than for the bore 33. Bore surface 35a tapers inward in a direction towards the drive sub end of the piston 32. This bore 35 facilitates movement and sealing of the piston 32 over another sealing tube, control tube 41. Control tube 41 is interference fitted 30 within a bore 51 of top sub 60. The control tube 41 has a a different design to exhaust tube 14, with top and bottom ends 41 a and 41 b having higher diameter portions than along the length of intermediate portion 41c. Wall thickness of the control tube 41 is higher than for the exhaust tube 14 recognizing the severe 10 working conditions (temperature of greater than 200 0 C and high pressure in chamber 63, between the piston 32 and top sub 60, as the piston 32 moves upward toward the top sub 60 during an exhaust stroke), in comparison with the exhaust tube 14, and with a view to maximizing service life. Control tube 41 is 5 made of a polymer which provides excellent heat and wear resistance and a low coefficient of friction. This polymer is a polyphenylene sulfide polymer available under the trade mark Techtron@ HPV PPS and its use avoids the need for a metallic sealing tube component. The result of a higher degree of compression in chamber 63 is a greater 10 driving force for the piston 32 downstroke. This improves lift performance and increases drilling performance. Modifications and variations to the drilling apparatus of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations may be understood by the skilled reader of this 15 disclosure and are deemed within the scope of the present invention.

Claims (15)

1. A drilling apparatus comprising: an outer cylinder; a drill bit; and 5 a piston disposed within the outer cylinder and driven by a working fluid for impacting the drill bit, the piston being reciprocable relative to said outer cylinder during operation of the drilling apparatus and having a top end and a bottom end characterized in that each piston end has a bore and at least one of said bores has a tapered portion to facilitate movement of the piston over a sealing tube 10 controlling pressure of the working fluid driving reciprocation of the piston relative to said outer cylinder of said drilling apparatus.

2. A drilling apparatus as claimed in claim 1 wherein one tapered bore is a front bore located at a front end of the piston.

3. A drilling apparatus as claimed in claim 2 wherein said front bore has a 15 portion tapering inward in a direction rearward from the drill bit.

4. A drilling apparatus as claimed in claim 3 wherein said front bore has sections of varying taper wherein, in a first section, said front bore tapers inward in a direction rearward from a head of the drill bit in a region where sealing between front bore and sealing tube is required to maximize compression for 20 driving a piston upstroke and, in a second section, the front bore terminates in an opening with wider dimension than said first section of said front bore.

5. A drilling apparatus as claimed in claim 4 wherein taper of said first section, measured as an angle from a centerline of said piston, is less than 0.2 degrees, preferably less than 0.16 degrees. 25

6. A drilling apparatus as claimed in claim 4 or 5 wherein, in said second section, the front bore tapers outwards. 12

7. A drilling apparatus as claimed in any one of the preceding claims wherein, at the top end of the piston, is located a top bore, said top bore being tapered.

8. A drilling apparatus as claimed in claim 7 wherein said top bore receives a sealing tube and tapers inward in a direction away from the top end of the piston 5 and towards the bottom end of the piston.

9. A drilling apparatus as claimed in claim 8 wherein said taper - measured as an angle - is less than 0.1 degrees, preferably less than 0.08 degrees, from a centerline of said piston.

10. A drilling apparatus as claimed in claim 9 wherein said taper is less than 10 taper of a front bore of said piston.

11. A drilling apparatus as claimed in claim 2 wherein said sealing tube is an generally cylindrical exhaust tube fixed in the top end of the drill bit, said exhaust tube being cylindrical and extending a calculated distance from a strike face of the drill bit. 15

12. A drilling apparatus as claimed in claim 7 wherein said top bore receives a sealing tube being a generally cylindrical control tube having top and bottom ends, said top and bottom ends having higher diameter portions than along an intermediate portion of said control tube.

13. A drilling apparatus as claimed in claim 12 wherein said piston has a 20 sealing tube being an exhaust tube fixed in the top end of the drill bit and a wall thickness of said control tube is higher than wall thickness for said exhaust tube.

14. A drilling apparatus as claimed in claim 12 or 13 wherein said control tube is of heat resistant and durable polymer. 13

15. A drilling piston having a top end and a bottom end wherein each piston end has a bore and at least one of said bores has a tapered portion to facilitate movement and sealing of said piston over a sealing tube for controlling pressure of a working fluid driving the piston within a drilling apparatus. 5 DRILLING TOOLS AUSTRALIA PTY LTD WATERMARK PATENT & TRADE MARK ATTORNEYS P33352AU00