Abstract:
A drilling device for producing undercut holes. The drilling device includes a drilling tool having an axis of rotation and a cutting portion arranged such that the cutting portion drills a hole with a longitudinal axis that is radially offset with respect to said axis of rotation and a centering device operable to apply a lateral force to said drilling tool for moving said axis of rotation towards said longitudinal axis to cause said cutting portion to form an undercut in a said hole.

Description:
CROSS REFERENCE TO CO-PENDING APPLICATIONS 
   This invention claims the benefit of German Patent Application No. 103 11 079.8 filed on Mar. 13, 2003, which is hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   The invention relates to a drilling device and method of working concrete, brickwork, stone or the like by means of a drilling operation to produce undercut holes. 
   BACKGROUND OF THE INVENTION 
   The production of cylindrical drill holes in materials such as concrete, brickwork, stone and the like using various techniques generally referred to as “drilling” is common. Often during above-ground, below-ground and street construction, for example, accurately positioned drill holes are needed, for example, for inserting fastening elements. Provided the holes are simple “passage holes”, the process is relatively simple, since there are no special requirements on the shape or geometry of the hole, apart from the nominal diameter. Dowel connections on the other hand impose additional requirements on the shape of the drilled hole. 
   There are already a number of special methods of making so-called undercut holes for receiving heavy-duty dowel connections. The purpose of these methods is to improve the performance of the hole by shaping it so that the force needed to pull the fastening element out of the hole is increased. This applies in general, but in particular to cracked concrete. With cracked concrete, it is necessary to prevent the dowel connection from yielding or subsiding owing to the cracking, which would reduce the load-bearing capacity. A positive connection is clearly an advantage, particularly for filigree elements where there is a limit to the spreading-apart forces which can be absorbed by the component. 
   Methods for making undercut holes, based, for example, on German Utility Model No 8808 256.3 and U.S. Pat. No. 4,993,894, first require the drilling of a standard drill hole with conventional drilling tools and machines. Then, in a second operation, by means of a geometrically not easily definable operation that is frequently called “swivelling or wobbling”, the cylindrical drill hole is widened at its inner end so that the inner end of an inserted dowel connector can become “wedged” in the hole. This enables a relatively defined positive connection to be made with the dowel connector. 
   Alternatively, there are special tools for making radial grooves in a standard drill hole and thus increasing its lateral roughness. These tools are used after drilling in a separate operation requiring a second working step. An example of such tools is disclosed in EP 0 795 677. 
   U.S. Pat. No. 4,989,681 and German Utility Model No 297 15 261 disclose the asymmetrical insertion of cutting edges, on the assumption that the axial force (action of pressure) and the cutting angle will result in an easily reproducible undercut. 
   In addition there are a number of devices on the market for working with movable, controlled cutting edges and blades. The difficulty usually is to remove the drilling dust on site and ensure unrestricted use under the rough conditions found at building sites. These methods and devices, hitherto in hand-guided versions, have not given satisfactory results with regard to the requirements of “widening” and “undercutting”, and therefore have not been commercially successful. 
   SUMMARY OF THE INVENTION 
   In an embodiment, the invention provides a drilling device for producing undercut holes, said drilling device comprising a drilling tool having an axis of rotation and a cutting portion arranged such that it drills a hole with a longitudinal axis that is radially offset with respect to said axis of rotation and a centering device operable to apply a lateral force to said drilling tool for moving said axis of rotation towards said longitudinal axis to cause said cutting portion to form an undercut in a said hole. 
   In another embodiment, the invention provides a method of producing an undercut hole comprising the steps of: drilling a hole with a drilling tool that has a cutting portion arranged to produce a hole having a longitudinal axis that is radially offset with respect to an axis of rotation of said drilling tool; and applying a lateral force to said drilling tool to cause said axis of rotation to move towards said longitudinal axis. 
   In yet another embodiment, the invention provides a drilling device for producing undercut holes, said drilling device comprising: a drilling tool comprising a drill shaft that defines an axis of rotation of said drilling tool and a cutting portion carried at an end region of said shaft such that, in use, it drills a hole having a longitudinal axis that is radially offset with respect to said axis of rotation; and a centering device carried by said drilling tool, said centering device comprising an elongate member mounted so as to be slidable alongside said drill shaft into said hole to press against said drill shaft and apply a lateral force to said drill shaft, such that the axis of rotation is moved substantially into line with said longitudinal axis to cause said cutting portion to form an undercut in said hole, and at least one member connected with said elongate member by which a user of the driving device can move said elongate member alongside said drill shaft. 
   In yet another embodiment, the invention provides a method of producing an undercut hole, said method comprising: drilling a hole with a drilling tool that comprises a drill shaft having an axis of rotation and a cutting head that is positioned such that said hole has a longitudinal axis that is radially offset with respect to said axis of rotation; maintaining rotation of said drilling tool with said cutting head at a predetermined location in said hole and sliding an elongate member into said hole alongside between said drill shaft and a wall defining said hole to apply a lateral force to said drill shaft and thereby move said drill shaft to bring said axis of rotation substantially into line with said longitudinal axis causing said cutting head to widen said hole at said predetermined location to form an undercut. 
   In still another embodiment, the invention provides a drill device for producing undercut holes, said drilling device comprising: a drilling tool comprising a drill shaft that defines an axis of rotation of said drilling tool and a cutter carried at an end region of said drill shaft such that, in use, it drills a hole having a longitudinal axis that is radially offset with respect to said axis of rotation; and a centering device operable to apply a lateral force to said drilling tool for moving said axis of rotation towards said longitudinal axis to cause said cutter to form an undercut in a said hole, said centering device comprising: a centering member that is movable from a non-operative position outside of said hole to an operative position inside said hole in which the member is positioned between said drill shaft and a wall defining said hole so as to provide said lateral force; an operating device connected with said centering member by which an operator can move said centering member from said non-operative position to said operative position; a depth stop device by which a drilling depth of said hole is set; and a control device for said operating device, said control device being configured to prevent movement of said operating device by which said centering member is moved from said non-operative position to said operative position until said drilling tool has drilled said hole to said drilling depth set by said depth stop device. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate the presently preferred embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain features of the invention. 
       FIG. 1  shows a drilling tool of an embodiment of a drilling device according to the invention. 
       FIG. 2  is an enlarged view of the drill head of the drilling tool of  FIG. 1 . 
       FIG. 3  shows a drilling device according to a first embodiment of the invention incorporating the drilling tool of  FIG. 1 . 
       FIG. 4  shows the drilling device of  FIG. 3  in a different operating position. 
       FIG. 5  is an enlargement of a portion of  FIG. 3  showing the cutting portion of the drilling tool at the bottom of a drilled hole. 
       FIG. 6  is an enlargement of a portion of  FIG. 4 , similar to  FIG. 5 , but showing a guide tube of the drilling device at the bottom of the drilled hole. 
       FIG. 7  shows an undercut hole produced using the drilling device illustrated by  FIGS. 1 to 6 . 
       FIG. 8  is a view similar to  FIG. 3  showing a second embodiment of a drilling device according to the invention. 
       FIG. 9  is a plan view of a modified guide tube for drilling devices according to the invention. 
       FIG. 10  is a plan view of an alternative modified guide tube for drilling devices according to the invention. 
       FIG. 11  illustrates yet another modification for the guide tube. 
       FIG. 12  shows yet another modified guide tube fitted to a drilling tool. 
       FIG. 13  is a view similar to  FIG. 3  showing a third embodiment of a drilling device according to the invention. 
       FIG. 14  shows the drilling device of  FIG. 13  in a different operating condition. 
       FIG. 15  is a partial view of the third embodiment showing a possible modification. 
       FIG. 16  shows another form of undercut hole that can be produced by drilling devices according to the invention. 
       FIG. 17  shows yet another form of undercut hole that can be produced by drilling devices according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     FIG. 1  shows a drilling tool  2  comprising a drill shaft having means  3  for locating the drilling tool in a drive device, such as in a chuck of a hammer, drill and a conveying screw  4 , which, in use, conveys debris upwardly and out of a hole during a drilling operation. The drilling tool  2  further comprises a drill head  5 , and a cutting plate, or blade  6  mounted in the drill head. The cutting blade  6  will be made of a hard metal or other abrasion-resistant cutting material. Such materials will be well-known to those skilled in the art and will not, therefore, be described in detail herein. 
   The drilling tool  2  differs from standard drilling tools in that the cutting blade  6  is not symmetrically radially fixed in the drill head  5 . Instead, as shown in  FIG. 2 , the cutting blade  6  is radially offset by an amount “e” with respect to the axis of symmetry, or rotation A, of the drilling tool  2 . 
   Referring to  FIGS. 3 and 4 , a drilling device  7  incorporating the drilling tool  2  includes a centering device in the form of a guide tube  9  fitted on the drill shaft. The guide tube  9  is arranged to be axially slidable on the drill shaft and is connected to a laterally projecting handle  10  by which the guide tube can be moved by an operator. The handle  10  projects from a longitudinally extending slot  18  provided in an outer tube, or sleeve,  20  that surrounds the drilling tool  2  and guide tube  9 . The slot constrains the movement of the handle  10  so that the guide tube  9  can only move axially with respect to the drill shaft and for practical purposes cannot rotate. The outer tube  20  is fitted to a chuck  21  of a drive device, such as, for example, a hammer drill. The chuck  21  illustrated is an SDS chuck and projects from a body portion  19  of the hammer drill in a conventional manner. As is known to those skilled in the art, the outer portion of an SDS chuck does not rotate (the drilling tool  2  is rotated by an unillustrated internal part of the chuck. Thus, the outer sleeve  20  does not rotate when the drilling tool  2  rotates. A spring  11  is connected to both the handle  10  and the outer tube  20  and biases the guide tube  9  to the retracted, non-use, position shown in  FIG. 3 . 
   It will be understood that although illustrated in use with an SDS chuck, the drilling device  7  may alternatively be used with drilling machines fitted with conventional chucks. In this case, the mounting of the outer tube  20  will be such that the chuck can rotate freely and the outer tube  20  does not rotate. It will be appreciated that the outer tube  20  can be fitted to a hammer drill (or any other suitable drive machine that may be used to drive the drilling device  7 ) in any convenient way that provides for non-rotation of the outer tube  20  while the drilling tool  2  is freely rotatable. 
   As shown in  FIG. 3 , in use, the drilling tool  2  is used to drill into a body, or component, such as a wall  14 . As shown in  FIGS. 3 and 5 , the result is the production of a conventional plain cylindrical drilled hole  1 . As a consequence of the asymmetrical positioning of the cutting blade  6 , the operator will experience unsteady movement of the hammer drill as drilling is commenced. With reference to  FIG. 5 , it will be seen that because the cutting blade  6  is radially offset by the amount “e” with respect to the axis of rotation A of the drill shaft, the centre line B of the hole  1  is offset by the distance “e” with respect to the axis of rotation A. 
   Once the hole  1  has been drilled to its full depth, as determined by the leading end of the outer sleeve engaging the wall  14 , in an immediately following step, without switching the hammer drill off or over, the operator grips the handle  10  and uses it to move the guide tube  9  down the drill shaft into the drilled hole  1 . This is illustrated by  FIG. 4 . In  FIG. 4 , the guide tube  9  is shown at its fully extended position at the bottom of the drilled hole. The guide tube  9  is moved to this position by operating the handle  10  and while rotation of the drilling tool  2  is maintained. With the guide tube  9  in this position, radial widening of the bottom of the hole  1  occurs to produce an undercut  12 . 
   In more detail, when the guide tube  9  is moved in the direction of advance  17  ( FIG. 4 ) by use of the handle  10 , the asymmetrically fixed cutting blade  6 , is gradually forced into a central position thereby driving its radially outermost edge  16  ( FIG. 6 ) into the wall  14  of the previously drilled hole  1 . Since the edge  16  rotates continuously during this movement, it will remove further material from the wall  14  so that a radial widening is automatically obtained to produce an undercut  12  at the end of the hole  1 .  FIG. 7  shows the undercut  12  after the drilling tool  2  has been removed. 
     FIG. 8  shows a second embodiment of the drilling device  7 . For ease of reference, like reference numerals are used to indicate similar or identical parts. In this embodiment, the outer tube  20  is replaced by a generally L-shaped support  20  that cooperates with an adjustable depth stop  8 . The handle  10  connects with the guide tube  9  via a slot  31  in a limb of the support  20  that extends parallel to the drilling tool  2  and sleeve  9 . The slot allows the handle  10  to move axially, but substantially prevents rotational movement thereof. Accordingly, the operator can move the guide tube  9  axially on the drill shaft, but rotation of the guide tube  9  is substantially prevented. 
   A marking device  30  is carried by the handle  10  and is arranged to mark the part  14  being drilled at a position  32  when the undercut  12  is made. The marking device  30  will only mark the part  14  when the guide tube  9  is fully inserted in the hole to produce a fully formed undercut  12 . The distance moved by the handle  10  to fully insert the guide tube  9  is indicated at  35 . 
   The second embodiment of the drilling device  7  is operated in much the same way as the first embodiment. That is, a hole  1  is first drilled to a depth determined by the setting of the depth stop  8 . The guide tube  9  is then slid down the drill shaft while rotation of the drilling tool is maintained. As before, the insertion of the guide tube centres the drill shaft in the hole, thereby driving the radially outermost edge  16  of the cutting blade into the wall of the drilled hole to produce an undercut, as illustrated by  FIGS. 6 and 7 . Once the operation is complete, the guide tube  9  is retracted and then the drilling tool  2  is removed from the hole, which has been marked as completed by the marking device  30 . 
   The guide tube  9  is a relatively inexpensive part, which in view of the abrasion caused by the drilling dust in the inner wall, will wear. However, by simply releasing the handle  10 , it can be quickly replaced, without the need for auxiliary or additional tools. 
   As illustrated by  FIGS. 9 to 12 , the external surface of the guide tube may be relieved to reduce the area of its surface that will contact the wall  14  of the hole  1 .  FIG. 9  shows a modified guide tube  9  with a recesses  29  defined by a radius  25 . At the outer diameter  23  of the guide tube, the extent of the recess  29  is indicated as  26 . As shown in  FIG. 10 , the guide tube  9  may be provided with a plurality of such recesses. 
   In its most basic form, the guide tube  9  is a plain cylinder with a wall thickness  22 , outer diameter  23  and inner diameter  24 . The provision of one or more recesses  29  has the advantage of reducing the area of its surface that contacts the wall  14  of the hole  1  and, in particular, in the case of brick or stone, provides the advantage of reducing the force used to insert the guide tube into the hole. The recesses  29  may be disposed symmetrically (as shown) or asymmetrically about the periphery of the guide tube  9 , as desired. 
   As illustrated by  FIG. 11 , the recess  29  may be twisted, or helical, with a pitch angle  28 . More than one slanted recess  29  could be provided if desired. 
   Referring to  FIG. 12 , the guide tube  9  may be formed by rolling a sheet to form a split sleeve having a longitudinally extending slit  30 . Preferably, the slit  30  is twisted at a pitch angle so that the cutting blade  6  will not jam in the slit during the undercutting operation. 
   The guide tube shown in  FIG. 12  can be used with drilling tools  2  where the diameter of the conveying screw part  4  is less than the diameter of the locating means  3  such that the sleeve cannot be slid onto the drill shaft over the locating-means  3 . Although it is preferable that the slit  30  does not run parallel to the axis A of the drill shaft, the slit may be so-aligned. 
   In a further, non-illustrated, variation, the guide tube  9  could be surface relieved by perforating the guide tube. Advantageously, the guide tube could be made from a perforated material. 
     FIG. 13  shows a third embodiment of the drilling device  7  in which, in contrast to the embodiment shown in  FIGS. 3 and 4 , the depth stop system is provided inside telescopic outer tubes  42  and  43 , which replace the outer tube  20 . 
     FIG. 13  shows the third embodiment in position at the commencement of a drilling operation. For ease of reference, like or similar parts will be referenced by the same reference numerals as in the previously described embodiments. In this embodiment the previously described outer tube  20  is replaced by a telescopic tube arrangement comprising at least two tubes  42 ,  43 . The upper telescopic tube  42  is fastened to the hammer drill  19  and the lower telescopic tube  43  is connected to the tube  42 . The lower telescopic tube  43  is secured to the upper telescopic tube  42  by means of a screw  46  that penetrates an elongate slot  45  provided in the upper telescopic tube. This slot  45  is sufficiently long to permit the tubes  42 ,  43  to telescope as the drilling tool  2  moves to a predetermined depth defined by stops  47 ,  48 . The telescoping movement of the tubes  42 ,  43  is made against a spring  44  that biases the lower telescopic tube  43  to its fully extended position. 
   The drilling depth is determined by an upper stop  47  and a lower stop  48  and by a support ring  38  fixed on the drilling tool  2  in conjunction with a second lower stop  49 . The supporting ring  38  will typically be secured to the drill shaft by means of a screw or by welding, although other means of securement could be used. As shown in  FIG. 14 , the drilling tool can be axially fixed at the drilling depth position by an optional releasable locking means  53 . It will be appreciated that in alternative arrangements, the drilling depth may be determined simply by the stops  47 ,  48  or the support ring  38  and stop  49  alone. 
   After the drilling depth has been reached, the undercut is made as follows. The handle  10 , previously held in the position shown in  FIG. 13  by a spring securing device  11  is moved downwards in the slot  18 , which in this embodiment is in the lower telescopic tube  43 , pushing the guide tube  9  into the drilled hole. When the guide tube  9  reaches the fully inserted position shown in  FIG. 14 , the undercutting operation is complete and the volume of structural material removed by the undercutting process can be seen through at least one inspection window  50  provided in the lower telescopic tube  43 . The opening  50  also provides a means of discharging the volume of drilled material to ensure a fault-free drilling process, even when the dimensions are larger. It will be appreciated that in an analogous manner, the outer tube  20  shown in  FIGS. 3 and 4  may be provided with one or more apertures. 
   This manner of operation, shown in  FIGS. 13 and 14 , avoids numerous causes of faults during rough operation in a building site environment associated with a number of components, such as the depth stop, since all of the critical functional elements that have a strong influence on the overall process are in a protected position inside the two telescopic tubes  42  and  43  and need not be adjusted relative to one another. This is another advantage that increases reliability of the drilling device. 
     FIG. 15  shows a modification of the third embodiment, by which the operator of the drilling device  7  is prevented from moving the guide tube  9  into the hole  1  until the drilling tool  2  has drilled the hole to its full depth, as determined by the depth stops  47 ,  48  and/or  38 / 39 . 
   Referring to  FIG. 15 , the upper, or outer, telescopic tube  42  has a guide member  70  for the handle  10  fitted to it. The guide member  70  defines a slot, in which a narrow portion  72  of the handle  10  is received (the larger diameter portion of the handle, as shown in  FIGS. 13 and 14 , has been omitted for clarity). The slot has a first elongate portion  74  that extends parallel to the axis of rotation A of the drilling tool  2  (not shown in  FIG. 15 ) and a second elongate portion  76  that is spaced from and extends parallel to the first elongate portion  74 . The two elongate portions are connected by a transverse extending connecting portion  78 , which in this example extends perpendicular to the axis of rotation A. The arrangement of the slot portions  74 ,  76 ,  78  is such that the slot is generally U-shaped. Although not shown, the corners of the slot are preferably curved to make movement between the portion  74 ,  76 ,  78  smoother. 
   The slot  18  in the lower, or inner, telescopic tube  43  is aligned with the second elongate portion  76  and includes a transverse extension  80  that extends from the upper end of the slot  18  and is positioned behind the guide member  70 . The transverse extension  80  of the slot  18  extends sufficiently for the narrow portion  72  of the handle to be received in the first elongate portion  74  while the second elongate portion  76  is aligned with the slot  18  in the lower telescopic tube  43 . 
     FIG. 15  shows the modified third embodiment in the drilling position illustrated by  FIG. 13 . In this position, the narrow portion  72  of the handle is positioned in the slot extension  80  and at the lower end of the first elongate portion  74 . When drilling of the hole  1  commences, the upper telescopic tube  42  moves downwardly relative to the lower telescopic tube  43  as indicated by the arrow  82 . As this happens, the guide member  70  is also moved downwardly relative to the handle  10  so that in effect, the narrow portion  72  of the handle approaches the top of the first elongate portion  74 , although, of course, the handle does not move. The handle is at this stage prevented from moving in the slot  18 ,  80 . 
   When the drilling operation is complete and the full hole depth is reached, the narrow portion  72  of the handle is positioned at the top of the first elongate portion  74 . The operator can then move the handle across with the narrow portion  72  of the handle simultaneously moving along the connecting portion  78  and the slot extension  80 . This brings the narrow portion  72  of the handle into the upper end of the second elongate portion  76 , which is aligned with the slot  18 . The handle  10  can then be pushed downwards forcing the guide tube  9  into the hole  1  and allowing it to be moved to the position shown in  FIG. 14  to form an undercut, as previously described. 
   It will be appreciated that the length of the first elongate portion  74  is such that the narrow portion  72  of the handle is held in the first elongate portion until the full hole depth (as determined by the stops) is drilled and only then is the handle able to move transversely to bring it into a position at which downward movement of the guide tube  9  can commence. Similarly, the length of the second elongate portion  76  is such that the handle can move the distance necessary to allow the guide tube  9  to be inserted to the desired depth in the drilled hole  1 . Preferably, this brings the leading end of the guide tube  9  to a position just behind, or adjacent to, the cutting blade  6 . 
   It will be understood that further modifications to the principle illustrated by  FIG. 15  are possible. For example, the shape of the slot defined by the guide member  70  could be altered as desired. All that is required is that the arrangement of the slot in the guide member  70  and the slot  18  in the lower telescopic tube is such that the handle  10  cannot be operated to move the guide tube  9  into the hole  1  until the full depth has been drilled. As another example, it is not necessary for the lower end of the first elongate portion  74  to be open as shown. This is merely convenient. 
   It will be understood that the arrangement shown in  FIG. 15  might be modified to allow for more than one depth setting. In this case, a second transverse connecting portion (not shown) could branch from the first elongate portion (for example in the opposite direction to the connecting portion  78 ) to a third elongate portion (not shown). Similarly, the extension  80  of the slot  18  could then extend further in the same direction to a second slot in the telescopic tube, which would be aligned with the third elongate portion. This could of course be matched to a desired drilling depth with a second drill stop setting. Further modifications would be possible to provide an arrangement similar to the gate of an automobile gearbox, so that multiple depth stops could be provided for. 
     FIG. 16  shows a variation of the radial widening or undercut that can be obtained with a suitably modified cutting blade  6 . Similarly,  FIG. 17  shows another variant of the radial widening or undercut in detail. 
   The drilling devices illustrated by the embodiments are technically simple, but efficient and potentially completely reliable. 
   It will be appreciated that the embodiments provide a drilling device that can produce an undercut hole using a single drilling tool by means of two steps taken in direct succession, without needing to remove the drilling tool from the basic hole and without using an additional “special tool”. Therefore, it is not necessary to perform the drilling and undercutting operation as two quite distinct steps involving the changing of tools or resetting of the machine/drilling device. This avoids the need for an additional drilling device and/or an additional machine operator and ensures, during rough operation on site, that “not a single hole is forgotten” during “undercutting/widening”, which can be the case where a second distinct operation is required. 
   Furthermore, usually the basic drilling operation is accelerated by so-called hammer drilling, i.e. the tool is axially accelerated in rotation by striking mechanisms. With existing systems, hammer drilling during the undercutting operation must be avoided, and may in practice result in occasional faulty operation and failure of the entire system, for example, in a useless hole at a necessarily predetermined position. This may spoil an entire job. The drilling devices of the embodiments can reliably produce a satisfactory undercut while still hammer drilling. 
   In the embodiments, the centering device takes the form of a guide tube  9  that is mounted on and is slidable relative to the drilling tool  2 . It will, however, be understood that this is not essential. The centering device could take the form of one or more elongate members (like fingers) arranged to be insertable into the hole to provide a laterally acting force on the drilling tool  2  that will move the axis of rotation A towards the hole centreline B. Advantageously, three equispaced fingers could be supported on a collar that could then be connected with a handle in the same way as the guide tube  9 . 
   An advantage of the drilling devices of the embodiments is that the operation of radially undercutting a drilled hole to provide a positive connection for a fixing element (such as a dowel), is performed in a staggered operating sequence in two successive steps with a single drilling tool. During this process, the cutting portion of the drilling tool does not leave the drill hole, which is advantageous with regard to the process and organisation. While the hole is initially drilled, the centering device remains inoperative at the upper end of the drilling tool. 
   By means of the centering device, which is moved axially downwards along the drilling tool and can be in contact with or partially spaced from the drilling tool, the offset cutting blade is deflected by a predetermined amount to effect the desired undercut. This co-operation of the centering device and drilling tool results in the advantage for the user of obtaining defined changes in shape or undercut in a single operation that is reproducible, reliable and practical. 
   It will be appreciated that by suitably shaping the outermost edge  16  of the cutting blade  6 , it is possible to produce undercuts of varying shape. As shown in  FIGS. 7 ,  15  and  16 , the undercut may be plain cylindrical (generally circular in cross-section) or it may be conical. A conical undercut can have sides that taper inwardly towards the bottom of the hole ( FIG. 15 ) or taper inwardly towards the top of the hole ( FIG. 16 ) depending on the shape of the cutting blade. Of course, other shapes are possible. 
   It will be understood that while the preferred embodiments have been described as separate and distinct embodiments, features of the individual embodiments can readily be applied to the others to provide yet more embodiments. For example, it would be possible to provide a marking device, for example a colour marking device, with any of the embodiments. 
   In the embodiments, the guide tube  9  is shown fitted with a handle  10  by which the guide tube is moved manually by the drill operator. However, it will be understood that a mechanism could be provided by which the guide tube would be non-rotationally driven by take-off from the chuck drive. 
   While the invention is not limited to this, with presently preferred embodiments, the diameter of the drilled hole relative to the widening produced by the undercut is substantially in the range 1:1.05 to 1:1.35. Alternatively, a ratio of 1.025 to 1.175 is preferred. 
   In the preceding description, reference has been made to “upper” and “lower” and “upward” and “downward” movement. It is to be understood that this should not be taken as limiting and this language is used merely for convenience of description in conjunction with the arrangement as illustrated. It will be appreciated that the drilling device can readily be used to drill: upwards, for example into a ceiling; transversely, for example into a wall; and downwardly, for example into a floor. 
   While the invention has been disclosed with reference to certain preferred embodiments, numerous modifications, alterations, and changes to the described embodiments are possible without departing from the sphere and scope of the invention, as defined in the appended claims and their equivalents thereof. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.