FIELD OF THE INVENTION

This invention relates to a drill steel, and a method of forming a drill steel, and a method of percussive drilling utilising a drill steel.

In percussive drilling, energy is transmitted from a rock drill through a drill steel and the drill bit to the rock where the energy is used to perform crushing work. The rock drill is provided with a piston which is thrown forwards to strike the shank of the drill steel, the energy of the piston passing into the drill steel and through the drill bit in the form of an impact wave.

Conventional drill steels are formed of elongate steel rod, and are often provided with a central throughbore to carry flushing fluid, such as water, air or foam from the drilling rig to the bit to flush out the loosened rock chippings. Drill steels may be of a variety of sizes, typically being between 2 and 22 feet long. Smaller, less substantial drill steels or extensions between a driver and a bit are used in many other industries, notably in the construction industry.

BACKGROUND OF THE INVENTION

According to a first aspect of the present invention, there is provided a method of forming a drill steel for use in percussion drilling comprising providing a drill steel and applying a twist to the drill steel.

Preferably, the twist is applied along a substantial length of the drill steel and most preferably, along at least 30% of the length of the drill steel.

The twisted drill steel exhibits improved longevity when compared to a conventional, untwisted drill steel of corresponding size. Further, it has been found that vibration at the drill bit and at the drill rig chuck

or coupling is reduced when using a twisted drill steel in place of a conventional, untwisted drill steel. The reduction in vibration facilitates handling of the drill, reduces energy losses and decreases wear and damage to the drill coupling components.

According to a second aspect of the present invention these is provided as a method of percussive drilling comprising utilising a drill steel having a twist along a substantial portion thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

ϊigure 1 shows a drill steel in accordance with a preferred embodiment of the present invention;

Figure 2 shows a machine tool for producing the drill steel of Figure 1;

Figure 3 is an enlarged end view of the clamp, indicated as area 3, of the machine tool of Figure 2; Figure 4 is a schematic representation of a drill rig utilising a drill steel; and

Figures 5A and 5B illustrate, somewhat exaggerated, the deformation characteristics of a prior art drill steel and a drill steel in accordance with the present invention, respectively, under difficult drilling conditions.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention relates to a percussive drill steel provided with a twist along a substantial portion thereof, and a method of forming such a drill steel. Reference is first made to Figure 1 of the drawings which illustrates a drill steel in accordance with a preferred embodiment of the present invention. The drill steel, generally indicated at 10, is an elongate rod having an intermediate

twisted portion 12 and two end coupling portions 14, 16 for connection to a drill rig coupling and drill bit, respectively. In this particular example, the drill steel 10 is formed of a 12' length of 1.1/8" diameter hexagon cross-section steel. The twisted portion 12 has a helical pitch which decreases from 41" adjacent the rig coupling portion 14 to 36" adjacent the bit coupling portion 16. The coupling portions 14, 16 in the illustrated example are in the form of a 1.1/4" rope thread coupling portion 14 and a 1.1/8" rope bit thread coupling portion 16, the thread being formed after the plain drill steel was twisted over its length.

Reference is now also made to Figure 2 of the drawings, which illustrates the machine 20 which is used to apply the twist to the steel. Mounted on the machine bed 22 is a longitudinally moveable carriage 24 provided with a rotatable chuck 26 for gripping one end of the steel 10. In order to assure that the steel is held firmly by the chuck 26, it may be necessary to machine or forge a chuck connection at the end of the steel. The connection could be in the form of a thread, collar or any other appropriate connection.

The intermediate portion of the steel 10 extends through a clamping device 28 provided with a hexagonal clamp 30 in the form of rollers 42 (Figure 3) of complementary diameter to the untwisted intermediate portion. The clamping device is illustrated in greater detail in Figure 3 of the drawings, and comprises a frame 32, a lower, fixed roller assembly 34, and an upper, moveable roller assembly 35, which is mounted in vertical guides 38. The clamping force for the movable force roller assembly is provided by a hydraulic clamping cylinder 40.

An induction coil 39 (Figure 2) is provided adjacent the clamping device and is used to heat the steel prior to twisting.

At the start of the twisting operation, the

carriage 24 is positioned adjacent the clamp 30 and then is moved longitudinally along the bed 22, away from the clamp 30 (towards the left as seen in Figure 2), while the chuck 26 is rotated. The intermediate ^" portion; 12 of the steel 10 passes through the induction coil to be heated and then passes through the clamp 30, which does not allow rotation of the part of the portion 12 within the clamp 30. Accordingly, the portion is twisted as it leaves the clamp. The operation is continued, with the longitudinal feed and rotation of the chuck 26 being maintained at predetermined rates to provide a desired pitch, varying or uniform, until all of the intermediate portion 12 has passed through the clamp 30. This is the position shown in Figure 2. If the machine 20 and drill steel permit, the twisting process may be performed when the steel is cold, but is more likely to be carried out when the steel is warm or hot, with feed and rotation speeds being varied accordingly, as will be obvious to a person skilled in the art. In this particular example, the drill steel was heated to 1550°F to 1600°F and mounted on the machine 20 set for longitudinal feed of 38 inches per minute (ipm) and chuck rotation of .83 rpm.

Following twisting, the drill steel is machined to form the thread on coupling portions 14, 16 and is then subjected to further hardening, by overall gas carburizing or by induction hardening.

To demonstrate the improved qualities of a twisted drill steel compared to a conventional non-twisted drill steel, two twisted drill steels, as described above, were used alongside two conventional untwisted drill steels of similar dimensions and specifications.

The drill steels were fitted to a Pneumatic 3-

Boom Jumbo Drill Rig, commonly referred to in the mining industry as a "Jumbo". The drill rig is capable of drilling three holes at the one time and provides rotation together with a percussive force. The particular rig used

operates at a rotation of 172 rpm with 100 psi air pressure and a percussive feed of 60 psi.

To facilitate understanding of the drilling operations. Figure 4 for the drawings illustrates, somewhat schematically, a drilling rig provided with a drill steel 10, engaged in a drilling operation. The Figure is not intended to accurately represent any specific drill rig. The drill rig coupling portion 14 is mounted in a cradle 46 which runs along a feed beam 48 which is braced firmly against the rock face 50. The feed force is mechanically transferred to the cradle by chain or screw. A rotation motor is provided on the cradle 46 as is the percussive drive. The drill steel 10 .is supported along its length by a travelling centraliser 52 and a collar 54 fixed to the end of the feed beam 48.

The inclination of the feed beam 48 is controlled by a hydraulic lifter 56 which extends from the rig to a mid point of the beam.

In testing, the drilling rig was fitted with the drill steels and used to drill wall slashes, flat drifting and down ramps in hard waste (30,000 psi) and soft ore (22,000 psi) .

The first of the twisted drill steels drilled 3,793 feet before breaking 30" from the bit thread coupling portion 16. While in use, the drill steel was bent on two occasions but was straightened and continued to be used. The second twisted drill steel provided 4,250 feet of drilling before the rope thread at the drill rig coupling portion 12 wore out. This drill steel was also bent at one point and straightened and put back in use.

Both untwisted drill steels broke at the drilling rig coupling portion, one drill steel after 756 feet of drilling, and the other drill steel after 608 feet of drilling. During drilling, it was observed that there was no significant difference in penetration rates between the twisted and non-twisted drill steels, and hole deviation

was much the same for both forms of drill steel. The removal of sludge (broken rock and flushing fluid) from the drilled holes was also substantially similar though the twisted portions slowed the sludge 'as it lef the hole such that the hydraulic lifters on the drill rig were kept cleaner than the lifters being used with the rig provided with conventional steels. It is also believed that the auger action of the twisted drill steel would provide improved penetration rates and better sludge removal where the sludge is particularly viscous.

Under difficult drilling conditions, when too high a feed was used in hard rock, the conventional drill steels bent in a _. single bend between the bit coupling portion and the drilling rig coupling portion, as illustrated in Figure 5A of the drawings. The particular bending illustrated would occur, for example, when the drill steel was "collared", that is, the bit held in position while starting a hole. Under these conditions, heat was generated at the drill bit threaded coupling portion, and this condition leads to premature breakage of the coupling portion. When collaring, the intermediate portion of the drill steel was supported by a travelling centralizer, and under similar conditions the twisted drill steels appeared to bend from the bit coupling portion though the bend only appeared to extend as far as the centralizer and did not effect the rig coupling portion.

Even when the twisted drill steels were elastically deformed over their length, the bend appeared as a "wave" of two or more bends, similar to the second or subsequent modes of vibration, as may be seen in Figure 5B of the drawings. As the misalignment of the coupling portions is reduced when the deformation takes this form, as opposed to the single bend, wear and damage to the coupling portions is reduced.

A further noticeable difference when using the twisted drill steels was that the vibration reaching the

drill rig was substantially lower than the vibration experienced under similar conditions when using a conventional drill steel. The vibration represents the "echo" shock waves returning up the length of ?the steel, the shock waves being produced by the portion of the percussive energy that is not utilised at the drill bit. The reduction in vibration serves to prolong the life of the coupling components of the drill rig, such as couplings, striking bars and chuck bushings, in addition to prolonging the life of the drill steel, and would reduce the necessity for vibration or shock absorbing features on the rig. In smaller, hand-held rigs such a reduction in vibration would also facilitate the use of the equipment and would reduce the likelihood of the operator suffering from "white hand", a circulation problem associated with the prolonged use of vibrating, hand-held machinery.

Although the drill steels described above are of relatively large dimensions and are described with reference to the mining and construction industries, drill steels of different dimensions, forms and for different applications could benefit from the advantages described above which are obtained through use of the present invention. It will be obvious to those skilled in the art that the above-described example is merely for purposes of illustration, and that various modifications and improvements may be made within the scope of the present invention.