A drilling apparatus boring vertical and horizontal holes in soil or rock, where the drill bit portion, which is mainly drilling the centre of the hole, is detachable from the drill bit portion drilling the outer portion of hole.

Formerly known is a drilling method, where there is in the head of the drilling apparatus a ring bit drilling the outer portion of hole. The drill head is also fitted with a drill bit to work the centre of the hole, whereat the working areas of the drill bits are covered by one another so that the front area of the entire hole gets worked. Both drill bits have a common hammer gear for drill bit hammering, usually operated by compressed air. When a hole is drilled downward and a protective tube put and left in the hole while dril-ling, it is impossible to get the bit that boring the hole outer portion of hole off the hole but it must be left on the bottom of hole. After drilling, the centre drill bit can be pulled out of the hole, if its dimensions are smaller than the diameter of the protective tube. The percussion hammer used as hammer gear is located at the mouth of bore and the stroke transfered to the drill bits by means of the drill rod.

The disadvantage of drilling apparatuses of this kind is that even though one has succeeded in pulling out the pilot bit and the drill rod from the hole after and even during drilling, their reinstallation for continued drilling is generally impossible, since the ring bit remains loose on the front side of the protective tube when it has got detached from the pilot bit and does not keep its position anymore. It can turn crosswise in any direction and even escape in soft soil and it cannot be placed in the tube anymore.

Further, a disadvantage of this kind of known drilling appa¬ ratus are the tandem percussion shoulders made in the pilot bit, the areas of which remain small due to structural

reasons. One shoulder hits the ring bit and another hits the protective tube. When the protective tube advances easily, for instance in drilling into rock, the ring bit is mainly hit by the shoulder, whereat the small shoulder area becomes damaged, eventually. In drilling into soft soil, the ring drill bit must be hit hardly at all and the percussions are even directed over another shoulder to hammer the protective tube, in this case needs the most of the impact energy. Further, when the area of this shoulder is small too, the shoulder or its counterface in the protective tube becomes damaged and wears out easily.

With the solution according to this invention a reliable joint between the pilot bit and the drill bit of the outer portion of hole is accomplished, whereat the pilot bit is detachable and also re-mountable with its hammer gear in the drill head. The invention is characterized in what is presented in the enclosed patent claims.

The major advantages of the invention are that the ring bit remains in its position with respect to the protective tube even after detachment of the pilot bit. It can neither turn crosswise nor escape, whereat the pilot bit, when it is being re-installed into drilling position, hits the inside of the ring bit using only slightly bevelled guides. With a construction according to this invention sufficient ring bit areas are achieved in regard to the thickness of the ring bit. The ring bit is hammered from its rear end, whereat the percussion impulse advances straight forward in the bit. When the hammer gear hammers the protective tube forward indirect¬ ly only over the bit, the stroke is suppressed a little and does not exert maximum force on the protective tube front edge even though there would be no significant counter-face to take the hit of that stroke. Usually, the protective tube is a thin wall construction and its front edge is under hard stress and can therefore tear under the force of the strokes. According to the construction as per this invention also a thin ring bit

construction has been achieved and does not cause any great costs if left in the deep borehole

Fig. 1 is a partial cross-section of the drilling apparatus.

Fig. 2 is an end view of the drilling apparatus.

Fig. 1 is a cross-section of another drilling apparatus.

Figure 1 shows the end of the drilling apparatus comprising a pilot bit 2 and a ring bit 5 outside of it. The hammer gear rod 1, which is hit by the hammer gear piston, has a thread onto which the pilot bit is screwed. A nore simple con¬ struction can be used also, whereat the rod and pilot bit are made of the same billet, i.e. one piece. Compressed air enters rod 1 and is conducted through holes 11 in bit 2 to the front side of this bit. In the outer edge of the pilot ring there are grooves 6, over which drill waste is transferred to the rear. In bit 2 there can be channels 22 even closer to the center running backward through the bit reducing unnecessary rambling of drill waste on bit front side and between bit and the surface being drilled.

A groove 8 parallel with the peripherie runs a certain distance out of each groove 6. In these grooves there are pins fixed to ring bit 5 and by means of this bayonet connection bits 2 and 5 are interlocked by screwing. Over this bayonet connection ring bit 5 is rotated by bit 2 on drilling in the used direction of rotation. Pins 21 of the ring bit are in groove 8 ends. On rotating in the opposite direction, the pins extrude from grooves 8 into channels 6, whereat the pilot bit can move backward while the ring bit remains in its position. The pins of ring bit 5 are loose in regard to grooves 8 so that the strokes do not get transferred over pins 21 to the ring bit but allow an axial movement as big as the clearance between bits 2 and 5. From pilot bit 2 the strokes travel over the shoulder face 7 to the ring bit. From ring bit 5 the suppressed stroke is transferred as a force pushing the protective tube forward when a ring 12 resting in groove 13 in

ring bit 5 has been fastened to the front edge of the protective tube. Edge 13 of ring bit 5 hits ring 12. Upon drilling the ring bit 5 is rotating, the protective tube 12 is not. Ring 12 is at the same time also a sliding ring. A construction as per fig. 1 allows also a sufficient shoulder face to hit ring bit 5. The stroke is transferred also from the pecussion gear 1 over the shoulder face to the pilot bit.

Figure 2 shows the front of the drill head, whereby holes 11 transmitting compressed air to the front of the drill bit, channels 10 arranged radially for compressed air in the bit front face and channels 6 arranged in the backward direction for drill waste and compressed air can be seen. Further, four grooves 8 are illustrated by a dash line which are on the periphery of pilot bit 2. The front faces of both drill bits 2,5 are furnished with possibly exchangeable working pins 9. Whether the thickness allows, there can also be in the ring bit channels, for instance grooves or holes directed to the rear of the bits.

Figure 3 shows another embodiment of the drill bit assembly. A shoulder 18 is connected to pilot bit 20 hitting both the shoulder face 17 of protecting tube 4 and the ring bit 14. Pilot bit 20 has brackets 19 with matching grooves in the ring bit. After detachment of the pilot bit, the ring bit is held in place for instance by a thick cord in groove 15 extruding from one or several holes 16. The ring bit can move axially a distance allowed by the width of groove 15. Also in this case the stroke is suppressed in bit 20 which only in the second stage is hammering the protective tube 4 forward while reducing its liability to damage. Drill waste and compressed air travel through bit 20 along channel 22.

When there are between the pilot bit and ring bit axial or almost axial guides/channels 19, both detaching and re- installation of the pilot bit is reliable, since possible rotation of the ring bit does not disturb locking or de-

tachment. In the solution shown in figure 1 the rotation of ring bit 5 may disturb detachment and locking if it can rotate easily on the edge of protective tube 3.

When the ring bit is dead and protected in protective tube 3,4 and only a part of its periphery is outside the protective tube while being, simultaneously, controlled and supported by the front edge of the protective tube, the periphery of the ring bit does not touch the tunnel bottom and is not then, due to its rotation, striving to change the direction of its forward travel during drilling.