Method and apparatus for making an undercut drilled hole in a fixed base
BACKGROUND OF THE INVENTION 
     The invention relates to a method and apparatus for making an undercut drilled hole in a fixed base structure, for example, concrete, masonry or the like. 
     To reduce stresses in a drill hole provided with an expanding plug, it is known to widen the drill hole at its bottom. However, widening a drill hole at its bottom, known in the art as undercutting, requires special tools which are expensive and complicated to handle. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a method of and an apparatus for making an undercut drilled hole in the base, with the undercut being simple to perform and freely selectable as to depth, shape and length. 
     In the method according to the invention, the milling head, when the retaining device carrying the head is being pushed into the drill hole, works its way into the wall of the cylindrical drilled hole without causing damaging loads on the milling cutter. If the retaining device has been fully advanced into the drill hole, that is, over a predeterminable part of the drill hole depth, the milling head, in accordance with its radial displacement on account of the existing eccentricity, mills the first part of the desired undercut out of the wall of the drilled hole. When the retaining device pushed into the drill hole is rotated about the drill hole axis, the milling head, on account of the eccentric mounting, is guided on a circle about the drill hole axis and the undercut is milled out over 360°. 
     The retaining device is advantageously advanced up close to the milling head so that forces acting radially on the head are diverted directly into the side of the drill hole via the cutter shank and the supporting sleeve. Undercuts of high accuracy can advantageously be made at the end of a blind hole by using the invention. If a combined milling cutter/drill is used, the drill hole can be drilled, the retaining device can be pushed in and the undercut milled in one operating cycle. 
     The apparatus comprises a simple, uncomplicated, rugged tool for making undercut drilled holes in the manner described. By means of a hand wheel, the retaining device can be easily turned without a great expenditure of force. 
     Further features of the invention will be apparent from the description which follows, in particular reference to the application drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view through a drill hole produced with a drill, shown in elevation; 
     FIG. 2 is a sectional view similar to FIG. 1, but showing an inserted milling cutter and a retaining device pushed onto the shank of the cutter; 
     FIG. 3 is a view similar to FIG. 2, but showing the supporting sleeve of the retaining-device pushed into the drill hole: 
     FIG. 4 is a sectional view of the drill hole with a milled-out undercut; 
     FIG. 5 is a side elevational view of the retaining device with a pushed-in milling cutter and a connected drilling machine; 
     FIG. 5a is a sectional view through the supporting sleeve of the retaining device, with a cutter shank being located in the supporting sleeve; 
     FIG. 5b is a sectional view similar to FIG. 5a, with the cutter shank being omitted; 
     FIG. 5c is a side elevational view of the supporting sleeve; 
     FIG. 5d is a side elevational view of the supporting sleeve, taken in the opposite direction of FIG. 5c; 
     FIG. 5e is a side elevational view of a supporting sleeve having an indexing slot pattern; 
     FIG. 5f is an elevational view of the clamping screw; 
     FIGS. 5g, 5h and 5i are bottom plan views of the supporting sleeves shown in FIGS. 5c, 5d and 5e, respectively, with FIG. 5g being taken on line Vc--Vc of FIG. 5c; 
     FIG. 6 a side elevation view of a drill with an integrated milling cutter for tapered undercuts; 
     FIG. 7 is a side elevational view of a milling cutter for tapered undercuts; 
     FIG. 8 is a side elevational view of a modified milling cutter for barrel-like undercuts; and 
     FIG. 9 is a side elevational view of the apparatus according to the invention in use with a drilling machine with a suction system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To produce an undercut drilled hole in a concrete wall 3, a blind hole 1 is first made in the wall 3 of a base structure in a well-known manner with a cylindrical drill 2, reference being made to FIG. 1. The drill 2 is then removed and a milling cutter 11 pushed into the drill hole 1 (FIG. 2). The head 5 of the milling cutter approximately corresponds to the diameter of the drill hole 1 and in the form shown has a lateral cutting edge 6. The shank 7 of the milling cutter 11 is made smaller in diameter than the milling head 5, with the diameter of the cutter shank 7 being preferably only half the diameter of the drill hole 1. The milling cutter 11, via its shank 7, is connected in a known manner to the drive of a drilling machine B, shown schematically in FIG. 5. 
     The cutter shank 7 is positioned in the supporting sleeve 8 of a retaining device 4 which in the exemplary embodiment has a hand wheel 10 which is connected coaxially to the supporting sleeve 8. For this purpose, a clamping screw 13 is screwed radially into the preferably metal hub 12 (see FIG. 5) of the hand wheel 10. The clamping screw 13 provides a nonrotational connection between the hand wheel and the supporting sleeve 8 and also secures the supporting sleeve 8 in its set axial position. When the clamping screw 13 is slackened, the supporting sleeve 8 is axially displaceable, as a result of which the length of the supporting sleeve section (see FIG. 3) extending into the drill hole 1 can be set in accordance with the drilled hole depth to be worked in each case. 
     Referring to FIGS. 5-5i, the supporting sleeve 8 arranged coaxially to the hand wheel 10 is produced from a solid material, preferably steel, and is formed with an axially extending receptacle groove 14 for the cutter shank 7. The receptacle groove 14 is open toward the periphery of the supporting sleeve 8 and has an axis 22 (see FIG. 5b) which lies parallel to the supporting sleeve axis 23 at a distance &#34;e&#34; from the latter. Thus, the receptacle groove 14 is located eccentrically in the supporting sleeve 8 so that, when the hand wheel 10 is turned about the supporting sleeve axis 23 and thus about the axis of the hand wheel 10 or about the axis 1&#39; of the drill hole 1 (see FIG. 4), the axis 22 of the receptacle groove 14 or the axis 24 of the cutter shank 7 located therein is moved on a circle of radius e about the supporting sleeve axis 23 so that the milling cutter 11 together with the head 5 moves eccentrically about the supporting sleeve axis 23 when the hand wheel 10 is turned through 360°. 
     The longitudinal receptacle groove 14, as a receptacle for the cutter shank 7, is not only a simple and adequate guide for the cutter shank, but it also ensures removal of the accumulating drilling dust between the side of the drill hole and the cutter shank 7 rotating in the longitudinal groove 14. The cutter shank 7 when inserted in the receptacle groove 14 also forms an outer jacket or continuing peripheral surface of the supporting sleeve 8, as shown in FIG. 5a, thus giving the supporting sleeve 8 additional-stability. In the exemplary embodiment, the eccentricity &#34;e&#34; is equal to the radius of the cutter shank 7, but it can be selected as desired. 
     In the method according to the invention, the rotating milling cutter 11 is advanced into the drill hole 1 to the bottom of the bore, and, with the milling cutter 11 rotating, the supporting sleeve 8 is pushed into the drill hole 1 (see FIG. 3). The diameter of the supporting sleeve 8 generally corresponds to the drill hole diameter. As a result of the supporting sleeve 8 being pushed into the drill hole and the existing eccentricity &#34;e&#34; between the axes of the receptacle groove 14 and the supporting sleeve 8, the rotating milling cutter is radially displaced in the drill hole and cuts into the cylindrical drill hole wall with its head, as shown in FIG. 3. 
     The supporting sleeve 8 is advanced until its free end lies closely behind the milling head 5 and supports the latter at the side of the drill hole against radial forces. The length of the supporting sleeve section required for this is set beforehand by adjustable axial displacement of the supporting sleeve in the hand wheel 10. 
     When work is being carried out with the device, the hub 12 of the hand wheel 10 is supported against the wall 3 by three circumferentially spaced pins 9 (see FIG. 3) which extend outwardly from the adjacent surface of its hub. The pins 9 thus space the hub from the wall to provide a free space between the hub 12 and the wall 3 for drilling dust to come out unimpeded. Instead of the pins 9, other supporting means can also be provided. Moreover, an embodiment with only one pin is possible. The quicker the guide sleeve 8 is inserted into the drill hole 1, the quicker the milling head 5 with the cutting edge 6 obtains adequately stable guidance in the drill hole so that said head can absorb corresponding milling pressures and make accurate undercuts even in hard rock or if the milling head 5 meets harder resistances, for example, iron parts or the like. At the same time, the forward feed speed of the milling head 5 can be easily adapted to the particular resistance during drilling by correspondingly turning the hand wheel slowly. 
     The supporting sleeve 8 designed with a longitudinal groove open toward its periphery has the advantage that drilling dust can escape by the shortest route over the entire length of the supporting sleeve so that the risk of the cutter shank 7 seizing in the supporting sleeve 8 is avoided. 
     After the supporting sleeve 8 has been advanced into the drill hole 1 until the pins 9 abut, the hand wheel 10 is turned, the milling head moving on a circle of radius e about the coaxial bore axis 1&#39; and supporting sleeve axis 23, thereby milling out the undercut 16. Once the hand wheel 10 has been turned through 360°, the undercut 16 is finished as shown in FIG. 4. 
     Indexing slots 15 are formed in the periphery of the supporting sleeve 8 and are advantageously arranged in such a way that they are adapted to the axial working depth of the milling head 5. 
     Once the desired undercut has been produced, first the supporting sleeve 8 and then the milling cutter 11 are removed from the drill hole 1. Rather than turning the supporting sleeve 8 by hand wheel 10, other means could also be provided, for example, a lever connected non-rotationally to the supporting sleeve 8. 
     The combined drill/milling cutter according to FIG. 6 is suitable both for drilling the cylindrical drill hole 1 and for producing a sharp-tapered undercut. A changing operation between the drill and the milling cutter is therefore not necessary so that the operating cycle for making an undercut drilled hole is advantageously shortened. 
     Rectangular undercuts can be produced with a milling cutter according to FIG. 7, and spoon-like or barrel-like undercuts can be produced with the milling cutter according to FIG. 8. The milling cutter can also be of cylindrical design or have any other desired shape. With the method according to the invention, varying shape undercuts can therefore be made in a simple manner. 
     In a further development of the invention shown in FIG. 9, provision is made for drawing off the drilling dust produced in particular during the milling of the undercut 16. For this purpose, the drilling machine 20 has a suction connection 17 which is connected via a suction tube 18 to a collecting housing 19 arranged between the hand wheel 10 and the wall 3. The drilling dust is drawn out of the collecting housing 19 via the suction tube 18 into a collecting bag 21 as shown. The shank 7 of the milling cutter extends from the supporting sleeve secured in position in the hand wheel 10 and is clamped into the drill chuck B of a drilling machine 20.