Patent Publication Number: US-2010124466-A1

Title: Trepanning Drill

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of trepanning drills and more specifically to the field of trepanning drills for drilling into composite materials. 
     BACKGROUND OF THE INVENTION 
     The use of composite materials has been widely spread these days, especially in the aerospace industry. The composite materials typically comprise of several layers having different structure from one another. 
     When drilling into a composite material, a main drawback is the dust produced in the drilling process. The negative effects of this dust on the human body are known and, therefore, there is a need to reduce to a minimum the amount of dust produced during a drilling process. 
     Another problem that arises when drilling into composite materials is that during the drilling process the drill applies axial forces on the material. When drilling a through bore, the axial forces tend to cause delamination of the drilled material in the periphery of the drill bore. Such a delamination is an undesired outcome of the drilling process and should be mitigated. 
     At some drilling processes a trepanning drill is used. However, a drawback of such a use is the time required for dismantling the drill in order to remove the core produced after each or several drilling processes. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a trepanning drill comprising: 
     a drill body having a drill axis of rotation (A) and comprising a drill bore extending rearwardly from a front portion of the drill body; 
     the drill body having at least two cutting edges formed in the front portion of the drill body, each of the cutting edges having a cutting external edge and a cutting internal edge with the cutting external edge being radially outward of the cutting internal edge; 
     a circle passing through the cutting external edge of each of the at least two cutting edges forms a first circle having a first diameter (D 1 ) and a first center (A 1 ); 
     a circle passing through the cutting internal edge of each of the at least two cutting edges forms a second circle having a second diameter (D 2 ) and a second center (A 2 ); wherein: 
     in an end view of the trepanning drill, the first center (A 1 ) coincides with the drill axis of rotation (A), and, the second center (A 2 ) is offset from the drill axis of rotation (A). 
     Typically, the offset between the second center (A 2 ) and the drill axis of rotation (A) is between 0.01 mm to 1 mm. 
     Further typically, the offset between the second center (A 2 ) and the drill axis of rotation (A) is between 0.05 mm to 0.5 mm. 
     If desired, the drill bore is a through bore. 
     Advantageously, the cutting internal edge of each of the at least two cutting edges lies on a guiding rib that extends rearwardly therefrom. 
     If desired, each of the guiding ribs extends rearwardly along an entire length of the drill bore. 
     Further if desired, each of the guiding ribs extends linearly rearwardly. 
     Still further if desired, an entire length of each of the guiding ribs extends parallel to the drill axis of rotation (A). 
     Typically, a rib intermediate space that extends between two consecutive cutting internal edges lies on a third circle having a third diameter (D 3 ), and the third diameter (D 3 ) is larger than the second diameter (D 2 ). 
     Further typically, the cutting external edge of each of the at least two cutting edges merges with a wiper surface extending rearwardly therefrom. 
     Still further typically, the drill body comprises chip evacuation flutes that extend between each two consecutive wiper surfaces. 
     In some embodiments, the trepanning drill has a unitary construction. 
     In some embodiments, the trepanning drill comprises a cutting head in the front portion thereof and a shank portion connected to a rear portion of the cutting head. 
     Typically, the cutting head is made from a material having a first hardness; the shank portion is made from a material having a second hardness; and the first hardness is greater than the second hardness. 
     Further typically, the cutting head is made of cemented carbide; and the shank portion is made of steel. 
     If desired, the cutting head is brazed to the shank portion. 
     Further if desired, the cutting head is interchangeably connected to the shank portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention and to show how the same may be carried out in practice, reference will now be made to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a trepanning drill in accordance with the present invention; 
         FIG. 2  is an enlarged perspective view of a front portion of the trepanning drill of  FIG. 1 ; 
         FIG. 3  is an end view of the trepanning drill of  FIG. 1 ; and 
         FIG. 4  is a cross-sectional view of the trepanning drill taken along line IV-IV in  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Attention is drawn to  FIGS. 1 to 4  showing a trepanning drill  10  in accordance with the present invention. The trepanning drill  10  comprises a drill body  12  and a drill axis of rotation A defining a front to rear direction  14  of the trepanning drill  10 . The drill body  12  comprises a cutting portion  16  of a first length L 1 , in a front portion  18  of the drill body  12 , and a shank portion  20 , in a rear portion  22  of the drill body  12 . The shank portion  20  may be cylindrical and serves as a holding surface for a machine (not shown) that holds the trepanning drill  10 . The trepanning drill  10  may be formed from cemented carbide or other hard metal by any known technique. 
     A drill bore  24  extends rearwardly from a front end  26  of the drill body  12  to a rear end  28  of the drill body  12 . The present invention is not limited to having a drill bore  24  that extends to the rear end  28  of the drill body  12 . In other embodiments, the drill bore may extend only partially along the drill body. Advantageously, the drill bore extends rearwardly from the front end of the drill body and has a bore length (not shown) that is not shorter than the first length L 1  of the cutting portion  16 . Furthermore, in a case where the drill bore does not extend to the rear end of the drill body, the drill body  12  may be provided with a cooling bore (not shown) for the passage of any cooling medium that is required, for example; oil, emulsion, MQL, air, or the like. In such a case, the cooling bore extends rearwardly from a rear end of the drill bore to the rear end  28  of the drill body  12 . 
     According to one embodiment, the drill body  12  has five cutting edges  30  formed in the front end  26  of the drill body  12 . The present invention is not limited to five cutting edges, and any number of cutting edges is equally applicable according to the present invention. Preferably, the number of cutting edges should be greater than one in order to better employ the benefits of the present invention. The cutting edges  30  may be equally or unequally peripherally spaced and their number may be odd or even. 
     Each of the cutting edges  30  has a cutting external edge  32 , in a radially outward end  34  of the cutting edge  30 , and a cutting internal edge  36 , in a radially inward end  38  of the cutting edge  30 . The cutting edge  30  may comprise a first cutting edge portion  40  adjacent the drill bore  24 , and a second cutting edge portion  42  extending radially outwardly from the first cutting edge portion  40 . 
     In the embodiment shown in the figures, each of the cutting edges  30  is provided with a rake surface  44  and a relief surface  46 . In a case where the cutting edge  30  comprises a first cutting edge portion  40  and a second cutting edge portion  42 , the relief surface  46  is divided into a first relief surface portion  52  and a second relief surface portion  54 . 
     The construction of the cutting edge varies according to design needs and is not limited to a specific embodiment. Hence, the cutting edge may comprise a single cutting edge portion or a plurality of cutting edge portions. The cutting edge may comprise a chamfer for strengthening the forward end of the cutting edge or it may be formed without a chamfer. The cutting edge may be generally inwardly or outwardly directed, i.e., the forward end of the cutting edge may be adjacent the radially externally end of the cutting edge or adjacent the radially internally end of the cutting edge. The cutting edge portions may be straight or curved. 
     The cutting external edge  32  of each cutting edge  30  merges with a wiper surface  56  that extends rearwardly from the cutting external edge  32 . The wiper surface  56  extends axially rearwardly and tangentially rearwardly in a common manner known in the art. 
     According to a specific embodiment of the present invention, the wiper surfaces  56  extend rearwardly in a spiral manner. However, the wiper surfaces may be differently rearwardly extended. Furthermore, the wiper surfaces may be provided with a small back taper angle as known in the art in order to provide clearance from the wall of the workpiece being drilled. 
     The drill body  12  comprises chip evacuation flutes  58  that extend between each two consecutive wiper surfaces  56 . A front portion  60  of each chip evacuation flute  58  merges with a chip recess  62  that extends from a leading surface  64  of the associated cutting edge  30  and opens to the drill bore  24 . 
     A circle that passes through the cutting external edge  32  of each of the cutting edges  30  forms a first circle  66  having a first diameter D 1  and a first center A 1 , as seen in an end view of the trepanning drill  10 . 
     A circle that passes through the cutting internal edge  36  of each of the cutting edges  30  forms a second circle  68  having a second diameter D 2  and a second center A 2 . 
     The cutting internal edge  36  of each of the cutting edges  30  lies on a guiding rib  70  that extends rearwardly therefrom. In some embodiments, the guiding ribs  70  extend along the entire length of the drill bore  24 , from the front end  26  of the drill body to the rear end  28  of the drill body. However, in other embodiments, the guiding ribs may extend only partially along the drill bore. 
     As seen in the figures, each of the guiding ribs  70  extends linearly rearwardly. However, in other embodiments, each of the guiding ribs may extend rearwardly in a different manner, for example, in a spiral manner. 
     The guiding ribs  70  according to one embodiment of the present invention extend parallel to the drill axis of rotation A along the entire length of the guiding ribs  70 . In a differently expressed manner, the guiding ribs  70  have a uniform thickness T along the entire length thereof. Alternatively, the guiding ribs may have a varying thickness along their length. 
     A rib intermediate space  72  that extends between two consecutive cutting internal edges  36  lies on a third circle  74  having a third diameter D 3  such that the third diameter D 3  is larger than the second diameter D 2 . 
     In an end view of the trepanning drill  10 , the first center A 1  coincides with the drill axis of rotation A, and, the second center A 2  is offset from the drill axis of rotation A. Thus, while the radially outermost portions, e.g., the cutting external edges  32  of the cutting edges  30  are at equal radial distances from the drill axis of rotation A, the radially innermost portions, e.g., the cutting internal edges  36  of the cutting edges  30  are not at equal radial distances from the drill axis of rotation A. 
     In a case when the second center A 2  coincides with the drill axis of rotation A, as in prior art trepanning drills, then, when drilling through a workpiece W (not shown), the diameter of the core of the drilled workpiece is exactly like the diameter of the circle that passes through the cutting internal edge of each of the cutting edges. As a consequence, the core fits tightly within the drill bore and needs to be dismantled in a time consuming task as described with respect to the prior art. 
     On the other hand, according to the present invention, the second center A 2  is offset from the drill axis of rotation A. Such an offset causes the internal edges of the cutting edges to cut a bore of an internal diameter that is slightly larger than the diameter of the core of the drilled workpiece. In this manner, when the drilling of the workpiece W is through, the core may be easily removed. 
     The clearance between the core and the internal diameter of the surrounding bore depends, among other things, on the diameter and length of the drill, the workpiece material, the required ease of removing the core, and the spatial orientation of the drill. The spatial orientation means that the trepanning drill may be directed horizontally, vertically upwardly, vertically downwardly, slanted upwardly or slanted downwardly. 
     In practical use, the offset between the second center axis A 2  and the drill axis of rotation A is between few hundredths of a millimeter to few tenths of a millimeter and even more. In one embodiment, the offset between the second center axis A 2  and the drill axis of rotation A is about 0.13 mm. 
     Thus, upon finishing the drilling process, a clean drilled bore is produced and the core of the workpiece may be easily removed from the drill bore  24 . The core may fall independently out of the drill when the drill is directed vertically downwardly. In other spatial orientations, a short blast of compressed air or fluid may be sufficient for removing the core out of the drill bore  24 . In such a case, the compressed air or fluid is driven through the cooling bore, from the rear end  28  of the drill body  12  towards the front end  26  of the drill body  12 . 
     The trepanning drill  10  described above is particularly useful for use in composite materials, since it produces less dust comparing to center drills that drill the center of the produced bores. Furthermore, since the cutting edges of the trepanning drill  10  are located near the periphery of the drill, the entire cutting edges sense a positive cutting speed, in contrast to zero cutting speed in the center of center drills, thereby the axial cutting forces applied on the workpiece are reduced to minimum and delamination of the composite material may be mitigated. 
     The trepanning drill  10  described above is not limited to use for one kind of workpiece material and it may be used to drill through a variety of materials, such as; composite materials, steel, alloys, stone, plastic, wood, etc. 
     Although the present invention has been described to a certain degree of particularity, it should be understood that various alterations and modifications could be made without departing from the spirit or scope of the invention as hereinafter claimed. 
     For example, the trepanning drill does not have to have a unitary construction made from a single material. Thus, the trepanning drill may comprise a cutting head, made from a material having a first hardness, like cemented carbide, and, a shank portion that is connected to a rear portion of the cutting head and made from a material having a second hardness, like steel, complex material, or the like. Typically in such a kind of construction, the first hardness is greater than the second hardness. 
     The chip evacuation flutes may be formed in the cutting head only, or, may extend also to the shank portion. 
     The cutting head may be connected to the shank portion by brazing. Alternatively, the cutting head may be connected interchangeably to the shank portion by a suitable mechanical connection such as a bayonet connection or the like. 
     The trepanning drill is not limited for being used in drilling through bores and it may be used for drilling blind bores.