Patent Document

BACKGROUND OF THE INVENTION 
     The present invention generally relates to hand tools and more particularly to an apparatus for preventing disengagement of an axially moveable and rotatable flexible core member of a flexible shaft attachment of the type which has a handpiece on one end, and its other end is coupled to a rotary hand tool. 
     There has been continued innovation and improvement in the design of power tools, particularly rotary hand tools of the type that are used in woodworking, metal working and the like. Examples of such products are those produced under the Dremel brand by the S-B Power Tool Corporation of Chicago, Ill., which also produces many accessory attachments for such rotary hand tools. The rotary hand tools are generally cylindrical in shape and have a motorized drive unit with a rotary output shaft that is adapted to drive the various rotary tools, such as small saw blades, sander discs, grout removal tools and various shaped cutting tool bits. There are also many accessory attachments that can be used in association with the rotary tools, with the accessory attachments being connected to the stationary nose end portion of the rotary tool via a coupling apparatus. Among such accessory attachments is a flexible shaft attachment that conveniently allows the user to operate the various rotary tool bits around corners or in other remote areas of operation. 
     While such flexible shaft attachments have been available for many years, the manner in which the flexible shaft attachments are coupled to the tool has been the subject of continuing efforts to provide a simple and effective mechanism for preventing the disengagement of the flexible core member from the handpiece. Thus, the manner in which flexible shaft attachments are presently coupled to rotary tools, while effective, does not prevent disengagement of the flexible core member from the handpiece end under some operating circumstances. 
     SUMMARY OF THE INVENTION 
     The present invention is related to a particularly simple and convenient stop plug apparatus for preventing disengagement of the axially moveable and rotatable flexible core member of a flexible shaft attachment from the handpiece end during use. The present invention comprises a stop plug apparatus that enables a user to operate a flexible shaft attachment having a handpiece end to be coupled to a rotary tool in any position without risking disengagement of the flexible core member from the handpiece end. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of the assembly in which the stop plug apparatus of the instant invention operates, and illustrates the flexible shaft attachment and rotary tool coupled to one another via the ferrule; 
     FIG. 2 is a top view of the assembly illustrated in FIG.  1 . 
     FIG. 3 is an exploded sectional view of the stop plug apparatus of the instant invention with the ferrule of FIG. 1, taken along the  3 — 3  line. 
     FIG. 4 is a sectional view of the stop plug apparatus of the instant invention assembled within the coupling device of FIG. 1, taken along the  3 — 3  line. 
     FIG. 5 is a perspective view of the coupling device of FIG.  1 . 
     FIG. 6 is a perspective view of the stop plug apparatus of the instant invention. 
     FIG. 7 is an elevational view of the stop plug apparatus of FIG.  6 . 
     FIG. 8 is a bottom view of the stop plug apparatus of FIG.  6 . 
     FIG. 9 is a perspective view of the drive cap of the assembly used in conjunction with the stop plug apparatus. 
     FIG. 10 is a bottom end view of the drive cap of FIG.  8 . 
     FIG. 11 is a section view of the drive cap of FIG. 8 taken along the  10 — 10  line. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The preferred embodiment of the stop plug apparatus includes a generally cylindrical stop plug having an open end and a closed end, and a cylindrical chamber spanning therebetween, where the open end is configured to receive the flexible core member and the closed end prevents undesirable axial movement of the flexible core member of a flexible shaft into the output shaft of the rotary tool, which often results in disengagement of the flexible core member from the handpiece end. 
     Disengagement ordinarily occurs because the output shaft of the motorized drive unit has a predetermined internal depth and circumference, which shaft ordinarily has a collet inserted inside of it and a collet nut threaded on it. However, in applications wherein the output shaft is coupled directly to the flexible shaft attachment, the collet and collet nut are removed and the output shaft and flexible shaft attachment are directly threadedly connected to one another. Therefore, when modest force is applied to the core in the direction of the drive unit, the flexible core member disengages from the handpiece and slides into the output shaft of the rotary tool. 
     Alternative methods of preventing disengagement of the core from the handpiece have proven disadvantageous in view of the present invention. For example, lengthening the flexible core to extend between the handpiece and the distal end of the output shaft would necessarily preclude any axial movement between of the flexible core. However, this configuration is disadvantageous for numerous reasons. First, end portions at each end of the flexible core have square cross-sections to facilitate attachment and rotation of the flexible core at its ends. Lengthening the end portion at the driven end would require commensurate lengthening at the handpiece end so that the end portions would be balanced. Lengthening at the handpiece end would result in part of the square end portion at the handpiece end extending past the handpiece, and rotation of the square end portion within the flexible sheath causes undesirable friction. This friction can cause degradation and erosion of the flexible sheath, and unwanted vibrational movement of the square end portion interferes with the rotation of the core. 
     Additionally, the square end portions are formed using a die. The die, while forming a generally square shaped cross section on the end portions of the flexible core, does not form perfectly square shaped cross sections. Thus, there may exist a differential in mass at the corners of the square cross section, causing uneven rotation of the flexible core. This uneven rotation disrupts the operation of the rotary tool unit. Minimizing the length of this square shaped cross section is therefore desirable. 
     The instant invention is directed toward preventing disengagement of the flexible core member from the handpiece by placing the stop plug apparatus in the output shaft to limit axial movement of the flexible core member within the output shaft. The stop plug apparatus has a generally hollow, cylindrical body, an open end and a closed end, and a cylindrical chamber spanning between the ends. The open end of the stop plug preferably nests on an inside surface of a drive cap, which has an aperture at its center for matingly receiving an end of the flexible core member. The inside circumference of the drive cap is threaded to threadedly engage a threaded outside circumference of the output shaft. Thus, the stop plug is disposed within both the drive cap and the output shaft of the rotary tool, with closed end portion of the stop plug projecting into the cavity of the output shaft, while being prevented from sliding into the output shaft by an annular flange around the open end of the drive cap. 
     The flexible core member, which projects through the mating aperture of the drive cap and into the cavity of the stop plug, is therefore prevented from sliding axially into the output shaft by the closed end of the stop plug. In this manner, disengagement of the flexible core member from the handpiece is prevented. 
     Turning now to the drawings, the environment in which the preferred embodiment of the stop plug apparatus operates is illustrated in FIG. 1, where a rotary hand tool, indicated generally at  10 , is shown coupled to a flexible shaft attachment, indicated generally at  12 , via a coupling apparatus or ferrule, indicated generally at  14 . The rotary hand tool  10  has a nose portion indicated generally at  16 , and a rotary output shaft  18 , which is best shown in FIG. 4, and is intended to be attachable to a working tool bit such as a small circular saw blade, a cutting bit, or the like. 
     While it is understood that the instant invention is related to a stop plug apparatus, the particular coupling attachment and rotary hand tool are only illustrated in the drawings to provide a sample environment for operation of the stop plug apparatus. Thus, the instant invention contemplates use with any rotary hand tool of the type having a generally cylindrical, hollow rotary output shaft  18  extending from the rotary tool drive unit  10 , to which working tool bits are ordinarily attached by means of a collet and a collet nut (not shown). The rotary tool drive unit  10  for communicates rotational torque to various working tool bits. The rotary output shaft  18  includes an open end portion  20  having a threaded outer circumference. From its open end portion  20  toward the rotary tool drive unit  10 , the output shaft  18  has a predetermined interior depth and an inner circumference that gradually narrows, and is configured to matingly receive a collet (not shown, but of conventional design), which has a circumference that is slightly larger than the narrowest portion of the inner circumference of the rotary output shaft. In this way, the collet is prevented from sliding into the predetermined interior depth of the rotary output shaft  18  by the narrowing inner circumference of the rotary output shaft. The collet is tightened against a tool bit shank in the understood manner by a threaded collet nut, which threadedly engages the threaded outer circumference of the rotary output shaft  18 . 
     When a rotary hand tool is coupled to an accessory attachment, such as a flexible shaft attachment  12 , a coupling device  14  is generally used to couple the rotary tool drive unit  10  to the attachment. In doing so, the collet and collet nut are removed so that the output shaft  18  can be mechanically coupled to the flexible shaft attachment  12 , as will be described. The present invention contemplates use with any number of coupling devices, and should not be construed to be limited to the coupling device shown and described. 
     One such coupling device is illustrated in FIGS. 1,  3 ,  4  and  5 . The coupling device  14  includes a mounting portion or ferrule having a generally hollow cylindrical body with an internal surface and an external surface. An open mounting end portion  22  has a circumferential opening in the ferrule and has a predetermined diameter configured to receive the nose portion  16  and rotary output shaft  18  of the rotary tool drive unit  10 . Opposite the open mounting end portion  22  is a smaller open end portion  24  to which the attachment  12  is mounted, where the smaller open end portion has a smaller diameter than the diameter of the open mounting end portion. Separating the two open end portions is a conical transition portion  26 , which is a sloped, funnel-shaped portion of the ferrule  14  that gradually narrows the diameter of the ferrule, and terminates in the generally cylindrical smaller open end portion  24 . 
     When the nose portion  16  and rotary output shaft  18  are properly inserted into the open mounting end portion  22 , opposing latch members  28  disposed on the outer circumference of the ferrule clamp down on and engage both the nose portion of the rotary tool to releasably secure the rotary tool drive unit  10  to the ferrule  14 . When in the fully locked position, the latch members  28  also engage a shelf-like locking flange  30  of the ferrule, to releasably lock the latch members into the locked position. In this way, the coupling device is securely attached to the rotary tool. 
     To couple the flexible shaft attachment  12  to the ferrule  14 , a lower portion of the cylindrical smaller open end portion  24  of the ferrule slidably engages a generally cylindrical mounting portion, designated generally at  32 , of the accessory attachment  12 . As illustrated in FIG. 1, the mounting portion of the accessory attachment  32 , which includes a coiled spring  34 , couples a hollow flexible rubber shaft  36 , which houses a flexible core member  38 , to the ferrule  14 . The coiled spring  34  has an inner circumference that is slightly smaller than the outer circumference of the smaller open end portion  24  of the ferrule  14 . Therefore, when the coiled spring  34  is mounted around the outer circumference of the smaller open portion  24 , the resulting force fit maintains frictional engagement of the coiled spring  34  with the smaller open end portion  24  of the ferrule  14 . The flexible core member  38  housed within the flexible rubber shaft  36  is consequently coupled to the ferrule  14  as well. 
     The flexible shaft  36  of the flexible shaft attachment  12  is generally hollow, and houses the generally cylindrical flexible core member  38 , which terminates at a handpiece  40  of the flexible shaft attachment and at its driven end, within the rotary output shaft  18  in the ferrule. At its driven end, the flexible core member  38  contains an engagement portion  42 , which includes a generally square-shaped, rather than cylindrical, circumference. Within the ferrule  14 , disposed on the bottom of the internal surface, is a drive cap  44  having an axial aperture  46  therethrough for receiving the engagement portion  42  of the flexible core member  38 . The aperture  46  of the drive cap is aligned with the opening of the smaller open end portion  24 . The drive cap  44  includes a main portion  48 , which is a generally cylindrical body having a uniform circumference, and a nut portion  50  that is unitary with the main portion. The transition between the main and nut portions  48 ,  50  forms an annular shelf or shoulder  52  on the inside of the drive cap  44 . The aperture  46  of the drive cap  44  extends through both of the main and nut portions  48 ,  50 . The nut portion  50  of the drive cap  44  abuts the bottom internal surface of the ferrule  14 . Because the engagement portion  42  of the flexible core member  38  is square, the aperture  46  of the drive cap  44  is similarly sized and configured to have a square-shaped circumference that is slightly larger than that of the flexible core member  38 . This mechanical configuration ensures that when the drive cap  44  is rotated, the flexible core member  38  inserted into the aperture  46  of the drive cap is consequently rotated. 
     Turning now to FIGS. 3,  5 ,  6  and  7 , the stop plug apparatus embodying the instant invention is designated generally as  54 . In the preferred embodiment, the stop plug apparatus  54  nests at least partially within the drive cap  44 . Generally, a closed end portion  56  of the stop plug apparatus  54  protrudes into at least a portion of the output shaft  18  of the rotary tool drive unit  10 , and an open end portion  58  abuts the internal shoulder  52  the drive cap  44 . The distance from the open end portion  58  to said closed end portion  56  is less than the predetermined interior depth of the output shaft  18 , and the stop plug apparatus  54  thereby limits the depth of penetration of the flexible core member  38  in the output shaft. The internal circumference of the drive cap  44  is threaded to threadedly engage the threaded outer circumference of the output shaft  18 , which has already been decoupled from the collet nut used in other applications. Thus, when the rotary tool drive unit  10  and flexible shaft attachment  12  are coupled to one another via the ferrule  14 , the rotary output shaft  18  is threadedly coupled to the drive cap  44  within the ferrule  14 . In turn, the aperture  46  of the drive cap  44 , which is aligned with the open end portion  58  of the stop plug apparatus  54 , matingly receives the flexible core member  38  of the flexible shaft attachment  12 . The flexible core member  38  is enclosed within the closed end portion  56 , which protrudes into the output shaft  18  of the rotary tool drive unit  10 . 
     However, because the flexible core member  38  must be free to rotate within the flexible shaft  36 , this limits the manner in which the flexible core member  38  can be mounted and retained at its handpiece end. Consequently, the flexible core member  38  is only loosely mounted at its handpiece end, and when the handpiece end is raised above the horizontal plane at which the ferrule is located, gravity causes the flexible core member to disengage from the handpiece  40 . The diameter of the flexible core member  38  is smaller than even the narrowest portion of the inner circumference of the rotary output shaft  18 , and by force of gravity, slides or slips into the output shaft of the rotary tool drive unit  10 . 
     Accordingly, the stop plug apparatus  54  is placed within the output shaft  18  of the rotary tool drive unit  10  to restrict or eliminate axial movement of the flexible core member in the direction of the rotary tool. The stop plug apparatus is preferably a plastic device, such as nylon filled glass, and includes a generally cylindrical body, the open end portion  58  and the closed end portion  56 , and a cylindrical chamber extending therebetween. The closed end portion  56  has a first, relatively constant circumference, and the open end portion  58  has a second relatively constant circumference, where the first circumference is smaller than the second circumference. A sloped transition portion  60  separates the open end portion  58  from the closed end portion  56 , which, as it slopes from the closed end portion to the open end portion, has a gradually increasing circumference. Thus, in the preferred embodiment of the instant invention, the transition portion  60  gradually and smoothly bridges the differential between the open end portion  58  and closed end portion  56 . Among other things, this sloped transition portion  60  facilitates insertion of the stop plug apparatus  54  into the output shaft  18 . 
     The closed end portion  56  of the stop plug apparatus, which has a smaller external circumference than the internal circumference of the output shaft  18 , extends into the predetermined internal depth of the rotary output shaft. However, the outer circumference of the open end portion  58  is only slightly smaller than the internal circumference of the output shaft  18 , and is thereby configured to frictionally engage the internal circumference of the output shaft. Further, because the internal circumference of the rotary output shaft  18  narrows along its depth, the fit between the outer circumference of the open end portion  58  and the inner circumference of the output shaft  18  becomes gradually tighter, increasing the frictional engagement of the stop plug apparatus  54  with the output shaft. 
     The open end portion  56  of the stop plug includes an aperture having an annular flange  62  surrounding its circumference, and the underside of the annular flange is a generally flat, planar surface. In operation, this planar surface abuts the internal shoulder  52  on the inside of the drive cap  44 , and accordingly, has an outer circumference that is slightly smaller than the internal circumference of the drive cap, so that the stop plug apparatus  54  is configured to nest within the drive cap. Moreover, the annular flange  62  has an outer circumference that is slightly smaller than, or roughly equal to, the outer circumference of the output shaft  18 , so that the annular flange  62  will not fit within the internal circumference of the output shaft, but will not interfere with the threading of the internal circumference of the drive cap  44  to the outer circumference of the drive shaft. 
     When inserted into the output shaft  18  of the rotary tool drive unit  10 , a majority of the cylindrical chamber protrudes into the internal depth of the output shaft. Accordingly, the stop plug apparatus  54  is prevented from entirely slipping or sliding into the output shaft  18  in two ways. First, the frictional engagement of the open end portion  58  with the narrowing internal circumference forms a tight fit between the two, and second, the annular flange  62  surrounding the outer circumference of the open end portion is sized and configured to prevent its passing through the opening of the output shaft  18 . 
     The annular flange further  62  includes an annular groove  64  around its circumference in the preferred embodiment, although a less than circumferential annular groove is also contemplated by the instant invention. The annular groove  64  is provided so that when the user wants to remove the stop plug apparatus  54 , the surface of the flange  62  will be uneven enough to allow the user to grab the flange and apply a moderate amount of force to remove the plug. The annular groove  64  contours the surface so that a fingernail or a tool can engage the surface to remove the plug. However, when the friction fit between the stop plug apparatus  54  and the output shaft  18  cannot be overcome by pulling on the annular flange  62 , the stop plug apparatus is further provided with at least one and preferably two opposed, internally protruding ribs  66  (best shown in FIG.  8 ). The ribs  66  extend internally into the cylindrical chamber of the stop plug apparatus  54  at a predetermined distance that is large enough to allow pliers or other suitable tool to engage the rib, and allow the user to pull the plug from the output shaft  18 , but are also small enough so as not to interfere with insertion of the flexible core  38  into the stop plug apparatus. The ribs  66  extend in the axial direction within the cylindrical chamber from the annular flange to the transition portion of the stop plug apparatus  54 . 
     Once the stop plug apparatus  54  is inserted into the output shaft  18 , the drive cap  44  can be threaded to the output shaft of the rotary tool drive unit  10 . Alternatively, the stop plug apparatus  54  can first be nested within the drive cap  44 , and the assembly of the stop plug apparatus and drive cap can be subsequently threaded to the output shaft  18 . 
     When the flexible shaft attachment has been secured to the smaller end portion  24  of the ferrule  14 , the engagement portion  42  of the flexible core member  38  protrudes into the cavity of the ferrule. When the drive cap  44  and stop plug apparatus  54  are in place, the flexible core member  38  protrudes through the corresponding aperture  46  in the drive cap, into the cylindrical chamber of the stop plug apparatus. The sloped nature of the transition portion  60  helps to guide the tip of the flexible core member  38  into the narrower closed end portion  56 , where it is unable to extend past the closed end. Thus, once the entire assembly is inserted into the output shaft  18  of the rotary tool drive unit  10 , axial movement of the flexible core member into the output shaft  18  is prevented insofar as the closed end portion  56  restricts such axial movement. The predetermined length of the stop plug apparatus  54  in nesting engagement with the drive cap  44  therefore corresponds to the length of the flexible core member  38  that may extend into the ferrule  14 . 
     While a particular embodiment of the present stop plug apparatus has been described herein, it will be appreciated by those skilled in the art that changes and modifications may be made thereto without departing from the invention in its broader aspects and as set forth in the following claims.

Technology Category: b