Patent Publication Number: US-2002004362-A1

Title: Countersink bit for glass

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a countersink bit. More particularly this invention concerns such a bit used to form a countersink at a bore in a glass workpiece.  
       BACKGROUND OF THE INVENTION  
       [0002] In order to mount a piece of glass flush it is standard to form the glass piece with a plurality of normally cylindrical bores and then cut a countersink in the outer face of the glass at each bore. This way a fitting of the Multipoint™ or Multiplex™ type can be secured to the glass which will not project appreciably beyond the outer face of the glass.  
       [0003] The standard countersink bit comprises a steel shaft extending along and centered on an axis and, at one end, a head formed unitarily with the shaft and having a conically or frustoconically tapered surface also centered on the axis. The tapered surface is provided with an abrasive layer, for instance of carbide or diamond particles. Such a tool is held in a chuck and rotated at high speed about its axis while being pushed axially into the bore, thereby forming the desired frustoconical countersink. This type of tool is distinguished from a simpler deburring tool which is used merely to ease the outer edge of a bore, not to form a relatively deep countersink intended to receive a fitting head.  
       [0004] The problem with forming a countersink in a glass workpiece is that, due to the extreme hardness of the workpiece, it is difficult to prevent the workpiece from jumping about somewhat in the bore, a phenomenon known as chatter. If chatter is excessive, the workpiece is chipped or cracked. When several holes have to be bores in an expensive workpiece, for instance a shaped piece of laminated glass intended for use as a motor-vehicle sun roof, such damage at any of the holes to be countersunk renders the entire workpiece unusable.  
       [0005] There are several likely causes for this chatter effect:  
       [0006] the relatively great axial distance between the chuck and the machining surface of the bit head,  
       [0007] the considerable radial spacing between the shaft axis and the machining surface,  
       [0008] torsional deformations of the shaft,  
       [0009] flexibility of the frame on which the chuck is mounted, and  
       [0010] instability of the support holding the workpiece.  
       [0011] These all can lead to imperfect alignment of the tool axis with the bore axis and offcenter point stresses that can lead to undesired chipping or cracking. Furthermore the equipment holding the chuck and rotating the bit all have a certain amount of elasticity that can result in vibration when the rotating tool engages the workpiece. When the vibration is at or near a resonant frequency of the structure, the resultant chatter can rapidly destroy the workpiece and bit.  
       OBJECTS OF THE INVENTION  
       [0012] It is therefore an object of the present invention to provide an improved countersink bit for glass.  
       [0013] Another object is the provision of such an improved countersink bit for glass which overcomes the above-given disadvantages, that is which forms a relatively deep frustoconical countersink without any significant likelihood of chipping or cracking the workpiece.  
       SUMMARY OF THE INVENTION  
       [0014] A countersink bit for glass has according to the invention a shaft extending along and rotatable about an axis, a head fixed to the shaft and having a frustoconical surface centered on the axis, a layer of grinding material on the surface, and an axially relatively incompressible plastic body capable of transmitting torque between the surface and the shaft. The plastic body has good damping capabilities so that any tendency of the head to vibrate or chatter is largely eliminated. Thus the vibrations that inevitably develop in the equipment not transmitted to the grinding surface. Such a bit can therefore be used in a standard countersinking system without any modifications to the equipment.  
       [0015] The advantages of the system of this invention are chiefly due to how the vibrations in the drive and mount for the drill bit are eliminated and damped out by short-term elastic deformations of the plastic body. Vibrations will therefore go away rather than get worse, being in effect absorbed in the plastic body so the grinding surface stays true and does not chatter on the workpiece. Since the deformations of the plastic body are easily confined to below its elastic limit, they do not permanently deform or damage it.  
       [0016] The plastic body can be an artificial rubber of high Shore hardness, a polyamide, polytetrafluorethylene, or the like. The vibrations it absorbs are converted, at worst, to some heat that is easily dissipated. The equipment normally has a certain amount of elasticity due to the mounting of the chuck holding the bit and the drive chain, and this elasticity encourages the development of vibrations that are damped out and eliminated by the plastic body of this invention. In fact the damping body normally makes the diamond grit stay sharp for the full service life of the tool, as peak loads are reduced.  
       [0017] The body according to the invention forms the frustoconical surface and the grinding-material layer is fixed directly to the body. In addition the body and shaft are formed with passages opening at the surface and through which a coolant liquid is forced. The passages include a central axially extending feed passage in the shaft and a plurality of smaller branch passages extending from the central passage to the surface. The surface is formed with a plurality of radially open pockets into which the branch passages open. These pockets are radially extending and outwardly open grooves. In order to distribute the coolant, the branch passages open into the respective grooves at different spacings from the axis. This coolant also serves to flush out particles freed from the surface by the diamond grit, preventing them from clogging the grit and reducing abrasiveness.  
       [0018] The frustoconical surface carrying the grit layer has according to the invention an apex angle of more than 0° and less than 180°, more particularly between 110° and 130°.  
       [0019] In a particularly simple embodiment of the invention the body forms the entire head. Alternately the head and shaft have axially interfitting and complementary formations and can therefore be made of different materials. The formations include an axial projection of predetermined axial length, an axially open seat of predetermined axial depth shorter than the length and having a rim, and a shoulder fixed relative to the projection and bearing axially on the rim. The head and shaft form at a base of the seat and end of the projection a chamber. The shaft is formed of metal with an axial large-diameter feed passage opening into the chamber and the head is formed with a plurality of smaller-diameter branch passages extending from the chamber to the surface. As described above, a coolant is forced through the passages to the surface.  
       [0020] In the system described immediately above an adhesive between the rim and the shoulder serves to secure the head on the shaft. In addition a plurality of set screws engaged radially through the seat into the projection further lock it in place.  
       [0021] The shaft in accordance with the invention can be further unitarily formed centered on the axis with a boring tool projecting axially past the head. Furthermore the layer of grinding material is adhesively secured to the surface and the layer of grinding material is sintered.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0022] The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:  
     [0023]FIG. 1 is a partly diagrammatic and axially sectional view of a countersinking system according to the invention;  
     [0024]FIG. 1 a  is a diagram illustrating the instant invention;  
     [0025]FIG. 2 is a large-scale end view of the countersink bit in accordance with the invention;  
     [0026]FIG. 3 is a partly sectional and diagrammatic side view of another countersink took according to the invention; and  
     [0027]FIG. 4 is a view like FIG. 1 of yet another countersinking system in accordance with the invention.  
    
    
     SPECIFIC DESCRIPTION  
     [0028] As seen in FIGS. 1 and 2 a countersink bit  1  according to the invention is centered on an axis A and has a cylindrical steel shaft or spindle  6  whose rear end  5  is secured in a chuck illustrated schematically at  4  for rotation by a motor  3  about the axis A and advance by a drive  2  along the axis A. A glass workpiece  10  has a cylindrical bore  12  that is coaxial with the tool  1  and that is to be formed with a frustoconical countersink  13  also centered on the axis A. This workpiece  10  is supported on a two-part conveyor  11 .  
     [0029] The tool  1  has a head  7  here having a body  14  formed of a material that is relatively incompressible but deformable, such as a rubber of high Shore hardness, a polyamide, or polytetrafluorethylene with good vibration-damping characteristics. The head  7  has a frustoconical outer face with an apex angle  8  centered on a point  15  and equal as shown in FIG. 1 a  to more than 0° and less than 180°, here about 120°. This end surface is provided with a layer  9  of carbide grit and is formed with four angularly equispaced outwardly open grooves  19  serving for conducting a coolant and flushing liquid to the interface between the grit layer  9  and the countersink  13  being formed.  
     [0030] The shaft  6  has a small-diameter forward projection  20  of cylindrical shape having an axial dimension  23  and the head  7  is formed with a rearwardly directed cylindrical collar  29  that forms a cylindrical recess or seat  21  that fits snugly around the projection  20  and that has a depth  24  equal to slightly more than the projection length  23 . A shoulder of the shaft  6  thus bears via a layer of adhesive  28  on the rear end of the collar  20  to solidly support the head  7  on the shaft  6 . In addition a plurality of angularly spaced set screws  27  extend radially through the collar  29  and have inner ends that engage in the projection  20  to lock the head  7  on the shaft  6 . All torque from the motor  4  and shaft  6  is thus transmitted via the damping body  14  to the grit layer  9 .  
     [0031] The interior of the shaft  6  is formed with a large-diameter axially throughgoing passage  17  that opens into a distributing chamber  18  formed between the floor of the seat  21  and the end of the projection  20 . The head body  14  is formed with a central axial blind bore open toward and aligned with the passage  17 , and with four smaller-diameter branch passages  16   a  through  16   d  communicating between this bore  22  or the chamber  18  and the respective grooves  19 . The rear end of the passage  17  is connected to a supply  26  of a coolant liquid, normally water for glass. As shown in FIG. 2, the outer ends of the passages  16   a  and  16   b  open generally equidistant between the ends of the diametrally opposite grooves  19  they feed, while the passages  16   c  and  16   d  open into the radial inner ends of the respective grooves  19 .  
     [0032]FIG. 3 shows a tool  1 ′ where the head  7  and shaft  6  are both formed of the relatively incompressible plastic, unitarily with each other. This eliminates the need for the interfitting formations  20  and  21  and the set screws  27 .  
     [0033] In FIG. 4 the shaft  6  of the tool  1 ″ is formed centered on the axis A with a cylindrical forward extension  26  having a grit bead  30  on its front end so that this part  25  can itself cut the bore  12 . Thus the same tool forms both the cylindrical bore  12  and the frustoconical countersink  13 . In this case the body  14 ′ forming the head  7  is a ring surrounding the shaft  6 .