Abstract:
Method and apparatus for improving the efficiency of the rotating cutter of a tool by converting at least part of the rotational torque applied to the cutter to repeated rotational impact forces.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of my U.S. patent application Ser. No. 10/134,865, filed Apr. 29, 2002 now abandoned, which is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to a driver for a rotary tool. More particularly, it relates to a driver for a rotary tool which imparts repeated rotational impact forces to the tool. 
   BACKGROUND OF THE INVENTION 
   Various tools have rotating cutters which modify the shape of a work piece. An example of such a tool is a tapper which cuts the bore of a work piece to provide internal threads. Another example is a milling machine, in which a rotating cutter removes metal from the surface of a work piece to provide a desired shape. 
   In such tools, the cutter material is harder than the work piece in order to effect cutting of the latter. As a general rule, the harder the cutter material is, the more brittle is the cutter, and the cutter becomes prone to fracture. The problem can be alleviated by hardening only the outer surface of the cutter leaving a core which is softer but tougher. However, it is difficult to harden only the cutting surface in small diameter taps. Moreover, the cutter is prone to fracture if too much work piece material is sought to be removed in a single pass, no matter how hard or soft the work piece material is. 
   Taps employed to cut internal threads in a work piece are particularly vulnerable to breakage, for a variety of reasons. If, for example, the bore in the work piece is slightly undersized, torsional stress is increased, frequently resulting in a broken tap. The machine must be shut down, the broken tap removed, and a new one inserted before operations can resume. Oftentimes, multiple tapping heads are used at a single station in high speed production lines, increasing the frequency of tap breakage. 
   Self-tapping screws for insertion in metallic or non-metallic work pieces, such as concrete, have much the same properties as taps for cutting threads in metallic work pieces. They are hard and brittle, and easily broken. 
   SUMMARY OF THE INVENTION 
   It is a principal object of the present invention to provide a method and apparatus for improving the efficiency of tools having a rotating cutter. It is a more particular object of the present invention to provide a method and apparatus for improving the efficiency of rotating cutting tools by imparting repeated rotational impact forces to the cutter. Yet another object of the present invention is to provide a method and apparatus for improving the efficiency of a taps and self-tapping screws by imparting repeated rotational impact forces to the tap or self-tapping screw. 
   In accordance with the present invention, these and other objects are achieved by the provision of a driver for a rotating cutter through which the torque applied to the cutter is converted from a constant torque to an intermittent torque, thereby imparting repeated rotational impact forces to the cutter. Although the teachings of the present invention may well find application in various rotary tools, they are particularly applicable to taps and self-tapping screws. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects of the present invention will be apparent upon consideration of the ensuing detailed description, taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  is a cross-sectional view of a driver constructed in accordance with the present invention in which the driver is disengaged; 
       FIG. 2  is a cross-sectional view of the driver of  FIG. 1  taken along lines  2 — 2 ; 
       FIG. 3  is a cross-sectional view of the driver of  FIG. 1  in which the driver is engaged; and 
       FIG. 4  is a cross-sectional view of the driver of  FIG. 1  in which the rotating cutter is a self-tapping concrete screw. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4  shows the driver chuck  23  receiving a hexagonal socket  39  mating with the head of a self-tapping concrete screw  41 . 
   As shown in  FIG. 1 , there is provided a driver  11  comprising a driver rotor  13  and a driver cup  15 . The driver rotor  13  includes a driver shank  17 , adapted to be rotatably driven as by an electric, pneumatic or hydraulic motor (not illustrated). 
   The driver rotor  13  is rotatably supported within the driver cup  15  by an inboard bearing  19  and an outboard bearing  21 . 
   Integral with the driver cup  15  is a chuck  23  in which a tap  25  is secured by one or more set screws  27 . 
   As best shown in  FIG. 2 , a longitudinal slot  29  is formed in the driver rotor  13  extending diametrically therethrough, in which are disposed rollers  31  and  31 ′, separated by a spring  33 . The spring  33  biases the rollers  31 ,  31 ′ against the inner wall  35  of the driver cup  15 . 
   A plurality of arcuately curved recesses  37 ,  37 ′ and  37 ″ are formed at regular intervals around the circumference of the inner wall  35  of the driver cup  15 , formed so as to mate with the rollers  31 ,  31 ′. 
   In  FIGS. 1 and 2 , the driver rotor  13  is shown out of engagement with the driver cup  15 , the rollers  31 ,  31 ′ not being received in the recesses  37 ,  37 ′ or  37 ″. In operation, when the driver  11  is rotated by the shank  17  against the resistance of the tap  25  within a bore to be threaded, the driver rotor  13  advances to an engaged position, as shown in  FIG. 3 , wherein the rollers  31 ,  31 ′ are received in the recesses  37 ,  37 ′ or  37 ″, causing the tap to rotate and begin its advance into the bore. 
   At a preselected torque determined by the constant of the spring  33 , the rollers  31 ,  31 ′ climb out of the recesses in which they are initially received, and the driver rotor  13  slips within the driver cup  15  until the rollers  31 ,  31 ′ drop into the succeeding recesses  37 ,  37 ′ or  37 ″. It will be understood that the rollers  31 ,  31 ′ rotate freely within the slot  29  and against the spring  33 , hindered only by frictional resistance. 
   Once the preselected torque is reached and the driver rotor  13  slips circumferentially within the driver cup  15 , the torque applied to the driver shank  17  is converted into an intermittent torque, exerting repeated rotational impact forces on the tap  25 . 
   It has been found that the repeated rotational impact torque forces exerted on the tap  25  or screw  41  provide a surprisingly improved effectiveness in cutting, i.e., threading, the bore of the work piece. Moreover, in accordance with the present invention the life of the tap is substantially improved, inasmuch as the torsional stress exerted on the tap is limited by the torque which can be applied to the tap before the rotor slips within the cup. 
   It is well known that an impact wrench enables a nut to be tightened to a greater extent than is possible with the application of constant torque and, similarly, that loosening a nut is accomplished much more effectively with an impact wrench than if a constant torque is applied. It is not believed, however, that the advantages of rotational impact have been utilized to a significant extent in tools of the type described herein. 
   An important feature of the present invention is the provision of impact forces which are exclusively rotational forces. Substantially no longitudinal impact forces are transmitted to the cutter. 
   The drawings and the foregoing descriptions are not intended to represent the only forms of the invention in regard to the details of its construction and manner of operation. Changes in form and in the proportion of parts, as well as the substitution of equivalents, are contemplated as circumstances may suggest or render expedient; and although specific terms have been employed, they are intended in a generic and descriptive sense only and not for the purposes of limitation, the scope of the invention being delineated by the following claims.