GB2284370A

GB2284370A - Drilling machine

Info

Publication number: GB2284370A
Application number: GB9424038A
Authority: GB
Inventors: Kuniaki Shibuya
Original assignee: Nitto Kohki Co Ltd
Current assignee: Nitto Kohki Co Ltd
Priority date: 1993-12-03
Filing date: 1994-11-29
Publication date: 1995-06-07
Anticipated expiration: 2014-11-29
Also published as: GB2284370B; JPH0733509U; MY116480A; GB9424038D0; KR950017018U; JP2598174Y2; KR0116869Y1

Abstract

A drilling machine comprises a base frame 1 fixed to a workpiece 21, a disc 3 mounted on the base frame 1 and rotatable around its center of rotation, a drill stand 22 mounted on the disc 3, and an electric drill 31 provided on the drill stand 22 and movable in parallel with the rotational axis of the disc 3. A locking device 12 detachably connects the disc 3 to the base frame 1, and a supporting portion 50 is formed on the disc 3 for rotatably and axially slidably supporting the rotary drill shaft 51. By supporting the rotary drill shaft 51 on the supporting portion 50, the cutter 24 is prevented from being displaced laterally and broken. <IMAGE>

Description

DRILLING MACHINE The present invention relates to a drilling machine suited for drilling a workpiece such as groove shape steel or H shape steel and more particularly to a drilling machine for drilling a workpiece after the drilling machine has been fixed to the workpiece, a cutter such as a drill has been placed at the required position of the workpiece and then drill stand has been firmly held on the workpiece.

With the conventional drilling machine of the type in which the portion to be drilled of a workpiece by rotating the electric stand around the rotational axis of the drill stand after the drilling machine has been fixed to the workpiece, the rotary drill shaft is also rotated around the rotational axis of the drill stand.

Thus, a supporting portion for holding and guiding the rotary drill shaft which is rotatable around its own axis and slidable axially cannot be provided on the machine bed on the workpiece. There has been developed a drilling machine of this type in which the rotary drill shaft is held only at its upper end by the electric drill.

When, however, the rotary drill shaft makes a long stroke or a heavy cutting work is carried out by a large cutter, the tip end of the cutter is displaced laterally from its center, resulting in inaccurate cutting and/or breakage of the cutter. Further, as the surface of the workpiece is hardened by an oxide film formed thereon, the cutter is vibrated at the beginning stage of the drilling operation, also resulting in breakage of the cutter and/or lateral deviation of the hole.

There has also been developed a drilling machine in which a drill stand on which a drill stand is attracted by a magnet to a workpiece and then the workpiece is drilled, whereby positioning accuracy and firmness of fixture to the workpiece are improved.

However, the intensity of magnetic attraction by means of the magnet depends on the thickness of the workpiece. In particular, when the workpiece is relatively thin, drilling torque exerted on the workpiece such as groove shape steel increases and thus the drill stand is likely to slide on the workpiece.

As disclosed in Japanese Laid-open Utility Model Registration Application Publication No. 61-96609, the inventor of the present invention proposed a drilling machine which realizes a magnetless drilling machine which is fixedly placed in position on a workpiece accurately and performs accurate drilling operation without sliding of the drilling machine on the workpiece caused by torque due to increase of the cutting resistance although the workpiece is relatively thin.

This conventional magnetless drilling machine will be described with reference to Figs. 1 to 3.

As shown in Fig. 1, a drilling machine 100 comprises a holding member 102 for holding a workpiece 101, an upright drill stand 103 provided on the holding member 102 and rotatable around its own axis, a slider 104 vertically slidably provided on the drill stand 103, and an electric drill 105 fixed to the slider 104.

A cutter (not shown) is connected to the lower end of the rotary drill shaft of the electric drill 105.

Replacement of the cutter allows for forming various kinds of holes in the workpiece. Vertical adjustment of the slider 104 can make the vertical adjustment of the cutter connected to the rotary drill shaft of the electric drill 105. By rotating the drill stand 103 around a disc 113 which will be described later, the position of the hole to be drilled in the workpiece 101 by the cutter on the electric drill 105 can be changed.

Referring to Fig. 1, the holding member 102 comprises a pair of right and left holding plates 106a and 106b for fixing the drilling machine 100 to a workpiece 101 and a base frame 108 for vertically holding the drill stand 103. The holding plates 106a and 106b threadably engages a lead screw shaft 107 so as to approach each other and be separated from each other and hold the workpiece 101 as they approach.

Thus, the holding plates 106a and 106b and the lead screw shaft 107 constitute a vice.

As shown in Fig. 2, the base frame 108 having a U-shape in cross section comprises a horizontal main portion 108c and a pair of side wall portions 108a and 108b having upper portions fixed to the both sides of the main portion 108. The upper portions of the holding plates 106a and 106c are placed in a space 108d defined by the base frame 108.

The side wall portions 108a and 108b are bridged by a pair of parallel front and rear guide rods 109a and 109b which slidably pass through the holding plates 106a and 106b.

As shown in Fig. 1, the left end portion of the lead screw shaft 107 passes through the left side wall portion 108a so as to be rotatable around its own axis.

A tandem arranged stop collars 110a and llOb are fixed to the left end portion of the lead screw shaft 107 so as to sandwich the left side wall portion 108a. The right end of the lead screw shaft 107 is pivotally supported by the right side wall portion 108b. The right and left half portions of the lead screw shaft 107 form a right male screw portion 107a and a left male screw portion 107b, respectively. The right male screw portion 107a threadably engages the proximal portion 106c of the right holding plate 106b, and the left male screw portion 107b threadably engages the proximal portion 106d of the left holding portion 106a so that the lead screw shaft 107 and the holding portions 107a and 107b form a vice. This vice structure allows the holding plates 106a and 106b to approach each other along the guide rods 109a and 109b when the lead screw shaft 107 is rotated as seen upward from the paper of Fig. 1 and to the right side in Fig.

2 by means of a holding lever 111 fixed to the stop collar ilOa and to be separated from each other along the guide rods 109a and 109b when the lead screw shaft 107 is rotated reversely by the holding lever 111.

A pin 112 vertically erects on the main body portion 108c of the base frame 108 and passes through the supporting shaft of a disc 113 in such a way that the disc 113 can be rotated around the supporting shaft. A locking nut 115 threadably engages the top end portion of the pin 112 with a flat washer 114 interposed between the locking nut 115 and the disc 113 so as to prevent the disc 3 from dropping from the base frame 108. As shown in Fig. 2, an elongate arcuate hole 116 extended through a rotation angel S at the central axis of the pin 112 as its center of curvature is formed in an edge portion 113a of the disc 113.

As shown in Fig. 1, the screw portion 118 of a fixing lever 117 which is threadably engages the main portion 108c of the base frame 108 is loosely inserted in the elongate hole 116. Thus, the disc 113 can rotate around its supporting shaft within the range of the rotation angle S. After the disc 113 has been rotated to the predetermined position, the fixing lever 117 is turned in the tightening direction. The flange portion 119 of the screw portion 118 is pressed against the upper surface of the disc 113, and the disc 113 is tightened to the main body 108c of the base frame 108.

By doing so, the disc 113 is set to the required position on the base frame 108.

As shown in Fig. 2, a pair of stops 120a and 120b is provided in the elongate hole 123 formed in the base frame 108 so as to be slidable therealong in accordance with the rotation of the disc 113 in the left or right direction. The stops 120a and 120b are tightened to the base frame 108 in the elongate hole 123, and a projection 122 extending radially outward from the outer peripheral wall of the disc 113 is fixed by the stops 120a and can set the disc 113 at the required position.

The workpiece 101 is drilled by the magnetless drilling machine 100 in the following way.

First, the drilling machine 100 is placed on the workpiece 101 and moved lengthwise to be set in a predetermined position thereon when required. Then, the disc 113 is rotated around its supporting shaft to align the front end of the pilot pin of the electric drill 105 with a punch mark on the intersection of marking lines formed on the workpiece 101. In this condition, the disc 113 is firmly tightened to the main portion 108c of the base frame 108. The projection 122 of the disc 113 is fixed to the required position, whereby the cutter is aligned with the position to be drilled on the workpiece 101. Thereafter, the lead screw shaft 107 is rotated rightward by the holding lever 111, as shown in Fig. 2. The holding plates 106a and 106b approach each other along the guide rods 109a and 109b, and the facing inner walls 106e and 106f of the holding plates 106a and 106b hold the outer side walls lOla and 101b of the workpiece 101, whereby the drilling machine 100 is fixed to the workpiece 101.

When the electric drill 105 is driven to lower the cutter, the workpiece 101 is drilled by the cutter in a state in which the center of the cutter aligns with the punch mark on the intersection of the marking lines, which mark corresponds to the center of the hole to be drilled. Since the holding plates 106a and 106b hold the outer side walls lOla and 101b of the workpiece 101, the drilling machine 100 does not slide on the workpiece 101 even if rotational torque is produced by the cutter on the disc 113 due to cutting resistance upon the drilling operation.

With the conventional magnetless drilling machine 100, a locking device provided with the fixing lever 117 having the screw portion 118 rotates horizontally and thus requires a too large area as a drilling machine of a portable type. In addition, since the screw portion 118 of the fixing lever 117 which is at a position D at which the disc 113 is tightened to the base frame 108 is much separated from a straight line C formed by connecting the center of rotation A of the disc 113 to the center B of the cutter (the drilling position) connected to the rotary drill shaft of the electric drill 105 as shown in Fig. 3, a large amount of moment is produced on the disc 113 due to a reaction from electric drill 105. This moment lifts the base frame 108 from the base frame 108 and/or causes the drill stand to resonate. Either one or both these phenomena are likely to deteriorate the drilling accuracy and/or to break the cutter.

The object of the present invention is to provide a drilling machine which holds another portion of the rotary drill shaft than the upper end of the rotary drill shaft which is rotated by the electric drill although the electric drill is rotated around the disc, thereby preventing a drilling error of a workpiece due to lateral displacement of the center of a cutter.

The further object of the present invention is to provide a drilling machine in which the tightening position of a disc to a base frame is disposed on a straight line formed by connecting the center of rotation of the disc to the center of a cutter so as to prevent lift of the disc from the base frame and/or resonance of a drill stand, thereby elevating the drilling accuracy and avoiding breakage of the cutter.

In order to achieve the object, the drilling machine according to the present invention comprises a base frame fixedly provided on a workpiece, a disc rotatably provided on the base frame around its own axis within a predetermined rotational angle, a drill stand perpendicularly provided on the disc, an electric drill movable in parallel with drill stand, a rotary drill shaft rotatable around its own axis, the rotary drill shaft having one end supported by the electric drill and the other end provided with a cutter, a locking device for detachably fixing the disc to the base frame, and a supporting portion for rotatably and axially slidably supporting the rotary drill shaft.

The locking device is constructed in such a manner that its tightening position of the disc to the base frame is disposed on a straight line formed by connecting the center of rotation to the center of the cutter.

Fig. 1 is a front view of a conventional drilling machine with its locking device longitudinally cross sectioned; Fig. 2 is a perspective view of the main portion of the drilling machine of Fig. 1, which main portion includes the locking device; Fig. 3 is a plan view of the main portion of the drilling machine of Fig. 2; Fig. 4 is a perspective view of one embodiment of a drilling machine according to the present invention; Fig. 5 is a plan view of the drilling machine of Fig. 4 with the locking device longitudinally cross sectioned and with the supporting portion modified from the supporting portion of the drilling machine of Fig.

4; Fig. 6 is a longitudinal cross-sectional view of the locking device of the drilling machine of Fig. 4; Fig. 7 is a side view of the main portion of the drilling machine of Fig. 4 which main portion includes the locking device and the supporting portion of the rotary drill shaft, with the locking portion longitudinally cross-sectioned; Fig. 8A is a longitudinal cross-sectional view of the locking device of the drilling machine of Fig. 4 when the locking device is loosened; Fig. 8B is a longitudinal cross-sectional view of the locking device of the drilling machine of Fig. 4 when the locking device is in an intermediate state between the locked state and loosened state; and Fig. 8C is a longitudinal cross-sectional view of the locking device of the drilling machine when the locking device is locked.

An embodiment of a drilling machine according to the present invention will be described with reference to Figs. 4 to 7 and Figs. 8A to 8C.

As shown in Fig. 6, the same base frame 1 as the one of the conventional drilling machine comprises a horizontal plate-like main portion la, a pair of right and left side walls 1b and ic each having an upper end portion fixed to the respective side end of the main portion la so as to form an inverted U-shape.

The portions la, ib and ic define a space 1d in which are placed the upper portions of a pair of holding plates 2a and 2b for holding a workpiece 21 such as shape steel. Approaching and separation of the holding plates 2a and 2b are achieved by rotation of the holding lever 111 (Fig. 4). The approaching and separating mechanism is the same as that of the conventional drilling machine 100 as shown in Figs. 1 and 2. Only a holding lever 111 and a stop collar 110a are shown in Fig. 4, and the other elements which are the same as those of the conventional drilling machine are omitted from description and Fig. 4.

As shown in Fig. 5, a disc 3 is mounted on the upper surface if of the main portion la of the base frame 1 so as to be rotatable around the supporting axis 4 or the center of rotation A of the disc 3 in the same way as the one of the conventional drilling machine. As shown in Figs. 4 and 6, an upright drill stand 22 is provided on the upper surface 3a of the disc 3 with the lower end of the drill stand 22 fixed to the upper surface 3a of the disc 3 by means of suitable fixing means such as bolts or welding means.

As shown in Fig. 4, an electric drill 31 is vertically movably provided on the drill stand 22, and a rotary drill shaft 51 suspends from the electric drill 31 in parallel with the supporting axis 4 of the disc 3. The upper end of the rotary drill shaft 51 is fixedly held by the electric drill 31 and rotated thereby. A cutter 24 for forming holes in the workpiece 21 is fixed to the lower end of the rotary drill shaft 51. This structure is the same as that of the conventional drilling machine. The electric drill 31 and the rotary drill shaft 51 are driven by a motor 60.

As shown in Fig. 5, an elongate plate-like supporting portion 50 extends radially outwardly from the outer peripheral wall of the disc 3. A circular hole 52 is formed in the forward end portion of the supporting portion 50. A bush 53 formed with a circular hole 53a having the same inner diameter as the outer diameter of the rotary drill shaft 51 is fitted in the central hole 52 of the supporting portion 50 and fixed to the supporting portion 50 by means of a set screw 54 (Fig. 4). The rotary drill shaft 51 passes through the circular hole 53a so that the rotary drill shaft can rotate in the circular hole 53a slides vertically on the inner wall of the circular hole 53a at the cylindrical peripheral surface of the rotary drill shaft 51. Since both the upper end and a portion lower than the upper end of the rotary shaft 51 are supported, the rotary drill shaft 51 is held in parallel with the supporting axis 4 or the drill stand 22 without being displaced laterally even if the rotary drill shaft 51 is very long and/or the cutter 24 performed heavy cutting. This suppresses lateral displacement of the cutter 24 upon drilling, leading to highly accurate drilling and prevention of breakage of the cutter 24.

In Fig. 5, the supporting portion 50 is integral with the disc 3. As shown in Fig. 4, however, the supporting portion 50 can be rigidly connected by fixing means such as bolts 56 (only one being shown in Fig. 4) to an extension 55 integrally formed with the disc 3 and extending radially outward from the outer peripheral wall thereof.

A locking device 30 will be described.

As shown in Fig. 5, the locking device 30 is constructed so that its tightening position D for tightening the disc 3 to the base frame 1 is disposed on a straight line C formed by connecting the center of rotation A of the disc 3 and the center B of the cutter 24 fixed to the lower end of the rotary drill shaft 51 of the electric drill 31.

As shown in Fig. 6, the locking device 30 has a shaft 5 vertical movable with respect to the disc 3.

The lower end 5a of the shaft 5 passes through the disc 3 and an elongate hole 6 formed in the main portion la of the base frame 1. The lower end 5a of the shaft 5 is threadably engaged with a bolt 7 which receives a flat washer 8 abuttable against the undersurface 1g of the main portion la of the base frame 1 and constitutes base frame tightening means. The elongate hole 6 has an arcuate shape having a rotation angle 8 extended at the center of rotation A of the disc 1 as its center of curvature which coincides with the center of rotation of the drill stand 22 and is formed in the outer peripheral portion 1h of the disc 3 at the side of the electric drill 31. The disc 3 can be rotated within the range of the angle 8 with respect to the base frame 1 accordingly.

The locking device 30 has a first inclination block 70 and a second inclination block 80. The second inclination block 20 is formed on the central portion of its upper surface with an inclined surface 20 lowered toward the right side and is provided on the upper surface 3a of the disc 3. The first inclination block 70 is formed on its left side of the undersurface with an inclined surface 70a raised toward the left side. The upper end portion 5b of the shaft 5 is inserted in and is fixed to a hole 70b extending from the bottom to the upper portion of the first inclination block 70. The first inclination block 70 is provided on the second inclination block 80 with the inclined surface 70a of the first inclination block 70 slidably contacted with the inclined surface 20 of the second inclination block 80.

On the left portion of the upper surface of the second inclination block 80 is formed a projecting receiving portion 9 formed with a horizontal hole 9a in which a nut 10 is loosely inserted so that the nut 10 can move vertically with respect to the second inclination block 80. A crank pin 11 passes through the nut 10 so as to be rotatable but is immovable axially with respect to the nut 10. A male screw lla formed on the outer peripheral surface of the crank pin 11 engages a horizontal female screw 70c formed in the upper portion of the first inclination block 70.

A short shaft portion 11b is formed on the left end of the crank pin 11, and a crank lever 12 is irrotationally connected to the left end of the shaft portion 11b by means of a spline connection llc (see Fig. 5). However, the left end portion of the shaft portion 11b may be formed into a polygonal shape such as a hexagonal shape or an octagonal shape in cross section, and a hole having complementary shape to the left end portion of the shaft portion lib for receiving the same may be formed in the right end portion of the crank lever 12. A washer 14 is held between the right end face of the shaft portion 11b and the left end face of the projecting receiving portion 9.

As shown in Fig. 5, a coil spring 13 is provided in a cylindrical hole 12b formed in the rotary shaft 12a of the crank lever 12, between the bottom of the cylindrical hole 12b and the head lle of a rod ild extending from the shaft portion lib toward the left side in the cylindrical hole 12b. The crank lever llb is removed from the shaft portion llb by pulling it toward the left side against the coil spring 13.

After the circumferential direction of the crank lever 11b has been changed, the shaft portion llb is connected to the shaft portion llb again. In this way, the rotational angle of the crank lever 12 which actuates the locking device 30 can be controlled.

The crank pin 11 having the male screw 11a threadably engaging the female screw 70c of the first inclination block 70 is moved leftward or rightward with respect to the first inclination block 70 according to the rotational direction of the crank lever 12.

As the crank pin 11 is moved rightward, the second inclination block 80 is also moved rightward and the inclined surface 20 of the second inclination block 80 engages the inclined surface 70a of the first inclination block 70 more. The first inclination block 70 is moved upward as shown by the upward direction of the arrow Y to tighten the locking device 30 as will be described later. When, on the other hand, the crank pin 11 is moved leftward, the second inclination block 80 is also moved leftward, and the inclined surface 20 less engages the inclined surface 70b. Thus, the first inclination block 80 is moved downward as shown by the downward direction of the arrow Y to loosen the locking device 30.

As shown in Fig. 6, a hole 80a whose cross section extends in the X directions and through which the shaft 5 passes is formed in the central portion of the second inclination block 80. A space is left between the left side wall portion of the hole 80a and the left side wall portion of the shaft 5 so that the second inclination block 80 can be moved toward the first inclination block 70 until the first inclination block 70 is lifted to the highest level.

In this arrangement, the projecting receiving portion 9, the nut 10, the crank pin 11, the male screw lla, the crank lever 12, the washer 14, the first inclination block 70, the female screw 70c and the second inclination block 80 constitute base tightening means, and the projecting receiving portion 9, the nut 10, the crank pin 11, the male screw lla, the crank lever 12, the washer 14 and the female screw 70c constitute moving means.

A understood from the structure of the locking device 30, the center of the shaft 5 coincides with a tightening position D of the disc 3 to the base frame 1. As shown in Fig. 5, the tightening position D is disposed on the straight line C formed by connecting the center of rotation A of the disc 3 to the center B of the blades of the cutter 24.

The operation of the drilling machine according to the present invention will be described with reference to Figs. 4 and 6 and 8A to 8C.

First, the holding plates 2a and 2b are opened more widely than the width of the workpiece 21 to loosen the locking device 30. The drilling machine is placed on a portion of the workpiece 21 which is close to the position to be drilled, and the workpiece 21 is lightly held by the holding plates 2a and 2b by operating the holding lever 111. When the locking device 30 is in this condition, the second inclination block 80 is at the extremely left position, as shown in Fig. 8A. The first inclination block 70 is lowered to the lowest level and the flat washer 8 is separated from the undersurface lg of the base frame 1. In this state, the disc 3 can be rotated freely with respect to the base frame 1.

Next, the electric drill 31 is rotated around the supporting axis 4 of the disc 3 and the.position of the drilling machine on the workpiece 21 is finely adjusted. The center B of the tip 25 of the cutter 24 is aligned with the punch mark on the intersection of the marking lines at which a hole is to be formed.

Then, the workpiece 21 is firmly held by the holding plates 2a and 2b by operating the locking lever 111 again.

Thereafter, as the crank lever 12 is rotated rightward in Fig. 4, the second inclination block 80 in the blocking device 30 is moved rightward as shown by an arrow X1 by the crank pin 11 from the extreme left side position as shown in Fig. 8A in which the clamping device 30 is in a loosened state. The inclined surface 20 of the second inclination block 80 is moved rightward with respect to the inclined surface 70a of the first inclination block 70, and the first inclination block 70 is raised as shown by an arrow Y1.

As the crank lever 12 is rotated rightward further in Fig. 4, the distance between the left side wall portion of the hole 80b and the left side wall portion of the shank 5 is made smaller, and the second inclination block 80 is moved rightward and finally takes a locking state as shown in Fig. 8C via an intermediate state as shown in Fig, 8B. In this locking state, the second inclination block 80 arrives at the extreme right side position, and the inclined surface 20 is at the extreme right side position on the inclined surface 70a. Both the first inclination block 70 and the shaft 4 are raised to the uppermost level.

The flat washer 8 is pressed against the undersurface lg of the base frame 1, and the undersurface of the second inclination block 80 is pressed against the upper surface 3a of the disc 3. As a result, the disc 3 is fixed to the base frame 1 in a state in which the tightening position D is disposed on the straight line C formed by connecting the center of rotation A of the disc 3 to the center B of the cutter 24.

With the clamping device 30 tightened as shown in Fig. 8C, the drilling operation is performed. The drilling machine according to the present invention has the following technical advantages.

First, even if the rotary drill shaft 51 is long and/or heavy cutting is carried out, the cutter 24 is not displaced laterally because the rotary drill shaft 51 is supported at its upper end by the electric drill 31 and at a portion lower than the upper end by the supporting portion 50 formed on the disc 3.

Secondly, the disc 3 is prevented from being lifted from the base frame 1, and the drill stand 22 does not resonate when heavy cutting is performed, because the tightening position D of the disc 3 to the base frame 1 is disposed on the straight line C formed by connecting the center of rotation A of the disc 3 to the center B of the cutter 24. In this arrangement, the drilling machine does not slide on the workpiece 21 during the drilling operation.

Thirdly, it is unnecessary that the clamping lever 12 of the clamping device 30 be provided in a narrow space between the drill stand 22 and the electric drill 31 in which the clamping device 30 would be difficult to be operated if it were disposed therein, but the clamping lever 12 is disposed in a wide space which is at the side of the members 22 and 31 and in which the clamp lever 12 is easy to operate although the clamping device 30 is provided in this narrow space. Further, the crank lever 12 has an advantage that it does not require a wide horizontal area as is different from that of the conventional drilling machine, because the lever crank 12 is operated vertically.

As a result, the drilling accuracy can be enhanced and the cutter can be prevented from being broken in the drilling machine according to the present invention.

After the drilling operation has been finished, the operation of the electric drill 31 and the rotary drill shaft 51 is stopped, and the clamping lever 12 is rotated leftward in Fig. 4. The clamping device 30 changes its state from the tightened state as shown in Fig. 8C to the loosened state as shown in Fig. 8A via the intermediate state as shown in Fig. 8B. The flat washer 8 is disengaged from the undersurface 1g of the base frame 1 to release the disc 3 from the base frame 1. When the next drilling position is selected not by moving the drilling machine on the workpiece 21 but only by rotating the electric drill 31, the electric drill 31 is rotated to align the point 25 of the cutter 24 with the new position to be drilled on the workpiece 21 and the drilling operation is carried out.

Thereafter, the holding plates 2a and 2b are

Claims

What is claimed is:

1. A drilling machine comprising: a base frame (1) fixed to a workpiece (21); a disc (3) having a center of rotation (A) and provided on said base frame (1) rotatable around said center of rotation (A) within a range of a predetermined angle (S); a drill stand (22) vertically provided on said disc (3); an electric drill (31) provided on said drill stand (22) and movable in parallel with said electric drill (31); a rotary drill shaft (51) having one end supported by said electric drill (31) and the other end fixed by a cutter (24) having a center (B); a locking device (30) for detachably connecting said disc (3) to said base frame (1); and a supporting portion (50) formed on said disc (3), for rotatably and axially slidably supporting said rotary drill shaft (51).

2. A drilling machine according to claim 1, wherein said supporting portion (50) comprises a platelike portion extending radially outward from said disc (3) and having a circular hole (52), and a bush (53) fitted in said circular hole (52) of said supporting portion (50), for rotatably and axially slidably holding said rotary drill shaft (51).

3. A drilling machine according to claim 1 or 2, wherein said locking device (30) has a tightening position (D) disposed on a straight line (C) formed by connecting said center of rotation (A) of said disc (3) to said center (B) of said cutter (24).

4. A drilling machine according to 3, wherein said base (1) has a peripheral portion adjacent to said electric drill (23) and an elongate arcuate hole (6) having a rotational angle (8) extended at said center of rotation (A) of said disc (3) as a center of curvature of said arcuate hole (6), and said locking device (30) has a shaft (5) passing through said elongate arcuate hole (6) and said disc (3) at said tightening position (D).

5. A drilling apparatus according to claim 4, wherein said locking device (30) comprises base frame tightening means (8) engageable with and disengageable from the undersurface (lg) of said base frame (1), and disc tightening means (9, 10, 11, lla, 12, 14, 70, 80) which cooperates with said shaft (5) and which is pressed against the upper surface (3a) of said disc (3) when said base frame tightening means (8) engages said undersurface (lug) of said base frame (1) and is separated from said undersurface (lug) of said base frame (1) when said base frame tightening means (8) disengages from said undersurface (lg) of said base frame (1).

6. A drilling machine according to claim 5, wherein said disc tightening means comprises a first inclination block (70) having an undersurface formed with an inclined surface (70a) and fixed by the upper end (5b) of said shaft (5), a second inclination block (80) provided between said first inclination block (70) and said disc (3) and having an upper surface formed with an inclined surface (20) slidably engaging said inclined surface (70a) of said first inclination block (70), and moving means (9, 10, 11, lla, 12, 14, 70b) for causing said second inclination block (80) to slide on said upper surface (3a) of said disc (3) along said inclined surfaces (70a, 20) of said first and second inclination blocks (70, 80).

7. A drilling machine according to claim 6, wherein said moving means comprises a crank pin (11) passing through said first and second inclination blocks (70, 80) along said inclined surfaces (70a, 20) of said first and second inclination blocks (70, 80) and in parallel with said upper surface (3a) of said disc (3) so as to be rotatable but immovable axially, a male screw (lea) formed on the outer peripheral surface of said crank pin (1), and a female screw (70c) formed in said first inclination block (70), for threadably engaging said male screw (lea).

8. A drilling machine according to claim 7, wherein said moving means includes a crank lever (12) provided on one end of said crank pin (11).

9. A drilling apparatus according to any one of claims 5 to 8, wherein said base frame tightening means comprises a flat washer (8) held by the lower end (5a) of said shaft (5).