Fastening means for two-piece core bit
United States Patent Meyer FASTENING MEANS FOR TWO-PIECE CORE BIT Apr. 8, 1975 3,388,935 6/1968 Hjalsten et al .6 403/343 Primary Examiner-Werner H. Schroeder Assistant Examiner- Conrad L. Berman [57] ABSTRACT The core bit is rcmovably secured to a power tool shank by threaded fastening means consisting of double sets of helical threads. The threads have an arcuate cross-sectional shape. The helical angle of the threads is at least five degrees.  
 2 Claims, 3 Drawing Figures FASTENING MEANS FOR TWO-PIECE CORE BIT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to core bits, and more particularly to fastening means for socalled two-piece core bits. By the term core bits, reference is made to generally hollow cylindrical devices having an annular cutting formation at one end thereof for drilling holes of substantial diameter, say, on the order of at least two or three inches, in materials, such as concrete and the like. These core bits are provided in various sizes, i.e., with different diameters, to permit the drilling or forming of different sizes of holes in the material being engaged by the core bit. Preferably, these core bits of various sizes are adapted for detachable engagement with a single tool shank of a portable power tool.  
 2. The Prior Art It is known in the prior art to provide a threaded connection between the power tool shank and the various core bits to be driven by such tool shank. As is known to those skilled in the art, core bits of the type under construction are subjected to vibrational and torque forces of very high magnitude. Such forces tend to cause the core bit to become locked to the tool shank, making removal of the core bit from the shank extremely difficult, if not impossible, without the infliction of damage to the core bit.  
 SUMMARY AND OBJECTIVES OF THE INVENTION The present invention relates to new and improved fastening means for securing a core bit to a power tool shank, such fastening means permitting ready attachment of the core bit to the shank as well as ready and easy detachment of the core bit from the shank.  
  A primary object of the present invention is the pro vision of a new and improved threaded fastening means for securing a core bit to a power tool shank.  
  Another object of the present invention is the provision of threaded fastening means for securing a core bit to a tool shank, such fastening means including a first set of double threads on the shank and a second set of complementary double threads on the core bit.  
  Still another object of the present invention is the provisison of threaded fastening means according to the foregoing objects, wherein such threads have a hellcal angle of at least five degrees.  
  Another object of the present invention is the provision of threaded fastening means according to the foregoing objects wherein such threads have an arcuate cross-sectional shape.  
  These and other objects and advantages of the invention will become apparent from the following specification disclosing a preferred embodiment illustrated in the accompanying drawings.  
 DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view of a power tool shank embodying the present invention;  
  F 10. Z is a partial elevational and partial longitudinal section of a core bit embodying the present invention; and  
  FIG. 3 is an end elevation of the core bit as seen looking at the cutting face thereof.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT A tool shank, generally designated 10, includes a hexagonal formation 11 at one end thereof. It will be understood that such hexagonal formation is adapted to be received within the rotary driving member of a power tool so as to transfer rotational movement from such power tool to the tool shank. Of course, other formations, such as a spline drive, may be provided in lieu of the hexagonal formation.  
  The other end of the tool shank is defined by a threaded formation, generally designated 12, which formation is separated from the hexagonal formation II by a cylindrical shank portion 14. The formation 12 is defined by a pair of helical threads l5, 16; in other words, the formation 12 consists of a set of double threads. The pitch of each of the threads 15 and I6 is, of course, the same; the pitch of the thread 15 is designated in FIG. I as P. It should be noted that the crosssectional shape of the threads is smooth and arcuate in form as opposed to being V-shaped. As noted in FIGS. 1 and 2, the crests of the threads adjoin the roots of the adjacent threads in an uninterrupted manner. In other words, the threads do not have sharp edges.  
  The helical angle of the threads is designated in FIG. I and A. This angle is at least five degrees; preferably this angle will be in the range of eight to twelve degrees. In the preferred form of the invention, the minimum helical extent for either one of the threads l5, 16 is in the order of 720&#34;. It should also be mentioned that the threads 15 and 16 are not deeply formed into the body portion of the shank 10. In the embodiment shown for purposes of illustration, the minor diameter of the threads is at least of the major diameter of the threads.  
  A core bit, generally designated 18, is seen to include a hollow cylindrical body portion 19 having an integral helical rib 20 over a portion of the outer surface thereof. The body portion 19 includes an annular sur face 21 at one end thereof, which surface includes a plurality of notches or recesses 22 in which carbide drilling tips 23 are mounted. Although only four of such tips are illustrated in FIGS. 2 and 3, it will be understood that a greater number of tips are provided in actual practice; for example, a core bit having an outside diameter of three inches may be provided with fourteen drilling tips. Of course, the diameter of the body portion 19 will vary to provide core bits of different sizes for drilling holes of various diameters.  
  The body portion 19 of the core bit includes a reduced-in-diameter portion 25 at the end thereof remote from the annular cutting face of the bit. When the core bit has a body portion diameter smaller than the diameter of the core bit illustrated, the portion 25 may have an outside diameter the same as the outside diameter of the portion 19. The reduced-in-diameter portion 25 includes a central portion 26 communicating with the interior of the body portion 19 of the core bit. The bore 26 includes a threaded formation, generally designated 28, complementary with the threaded formation 12 on the tool shank for threading interengagement therewith. Since the threaded formation 28 is complementary in all respects with the threaded form ation 12, the former need not be described in detail. It is sufficient to say that the threaded formation 28 in cludes a double set of helical threads 29, 30 having the same cross-sectional shape, helical angle and helical extent as the complementary threads l5, 16.  
  It is understood, of course, that the core bit 18 is detachably secured to the tool shank by interengagement of the threaded formation 12 and 28. During tightening of the core bit to the tool shank. relative rotation between these two members is preferably stopped by abutting engagement of an annular surface 26a on the core bit with an annular flange 14a on the tool shank.  
  It has been found. by the use of tools embodying the present invention. that it is virtually impossible for the core bit to become locked to the tool shank. notwithstanding the fact that the core bit is subjected to rotational and vibrational forces of very high magnitude. Locking of the core bit to the tool shank is prevented to a large degree by the provision of the double threads which, in effect, establishes a pitch double the pitch that would he achieved by using a single thread. This increase in pitch lessens the tendency of the threads to lock up with each other.  
  Another feature contributing to the prevention of this lockup is the rather low helical angle A for the threads maintaining such angle at least five degrees prevents the threads from interlocking with each other. The provision of rounded or arcuate threads eliminates sharp surfaces or edges, thereby reducing stress concentrations and their consequent fatique breaks or fractures.  
  Since the tool shank l0 and the core bit 18 do not become locked together, these parts may tend to separate prematurely and inadvertently upon their removal from the work as the operator will normally rotate the shank in the opposite direction to facilitate such removal. In such an event. the tool shank will tend to rotate relative to the core bit as the latter will resist rotation to some degree due to its frictional engagement with the work. To prevent such unintended separation. the tool shank 18 is preferably provided with an annular recess 32 mounting a high temperature resistent. flexible O-ring 33. When the tool shank is threaded with the core bit 18, this O-ring will be received within the bore 26 and squeezed into engagement with the wall of the latter The O-ring 33 provides sufficient frictional engagement with the tool skank and the core bit to prevent their inadvertent separation during removal from the work. However, the frictional forces developed by the O-ring are not so great as to prevent the ready and easy manual separation of the tool shank and core bit.  
 I claim:  
  1. A drilling bit assembly including a cylindrical core bit and a power tool shank element detachably secured together by fastening means, said fastening means being defined by a first set of double threads on said shank element and a second set of double threads on said core bit, said first and second sets of threads being of complimentary external and internal configuration for threading interengagement thereby to fasten said core bit to said shank element, the diameter of the one of said sets of threads, which is of external configuration, being slightly less than the diameter of the other of said sets of threads, which is of internal configura&#39; tion, thereby to facilitate their interengagement, said first and second sets of threads being otherwise identical oppositehand versions of each other in all respects, said first and second sets of threads being in substantial co-extensive surface engagement with each other throughout substantially all of their respective helical extents thereby securely but detachably securing the core bit to the shank element. the cross-sectional shape of said threads being smooth and arcuate with the arcuate crests of the male threads adjoining the arcuate roots of the female threads in an uninterrupted manner, the portions of the surfaces of said threads between the zenith of the crests thereof and the nadir of the roots thereof being contained in planes defining a substantial angle with the co-axial longitudinal central axes of said bit and said shank. the minor diameter of said threads being at least percent of the major diameter thereof. the helix defined by each of said threads having an extent of at least 720 degrees, the helical angle of each of said first and second sets of threads being at least five degrees.  
  2. The improvement according to claim 1 wherein said first set of threads is formed externally of said shank at one end thereof and wherein said second set of threads is formed internally on said core bit at one end thereof.