Deep-hole drill having back-tapered web

Abstract

A drill including a cylindrical main body and a shank. The cylindrical main body has: (a) flutes providing cutting edges at an axially distal end of the cylindrical main body; (b) lands each provided by a peripheral portion of the cylindrical main body and located between the flutes; and (c) a web provided by a central portion of the cylindrical main body which joins the lands and extends in an axial direction of the cylindrical main body. The web includes a back-tapered portion which is provided by at least an axially distal end portion of the web, such that a thickness of the web in the back-tapered portion is gradually or continuously reduced as viewed in a direction away from the axially distal end of the cylindrical main body toward the shank.

What is claimed is:  
     
         1 . A drill which comprises a cylindrical main body and a shank contiguous to each other, and which is to be rotated about an axis of said cylindrical main body in a predetermined rotating direction, for drilling a hole in a workpiece, wherein said cylindrical main body has: 
 at least one flute each of which is formed in said cylindrical main body and extends from an axially distal end of said cylindrical main body to an axially proximal end of said cylindrical main body, so as to provide a cutting edge at said axially distal end of said cylindrical main body;    at least one land each of which is provided by a peripheral portion of said cylindrical main body not cut away by said at least one flute; and    a web provided by a central portion of said cylindrical main body which extends in an axial direction of said cylindrical main body;    wherein said web includes a back-tapered portion which is provided by at least an axially distal end portion of said web, such that a thickness of said web in said back-tapered portion is continuously reduced as viewed in a direction away from said axially distal end of said cylindrical main body toward said shank.    
     
     
         2 . A drill according to  claim 1 , wherein the thickness of said web in said back-tapered portion is reduced at a constant rate as viewed in the direction away from said axially distal end of said cylindrical main body toward said shank.  
     
     
         3 . A drill according to  claim 1 , wherein said back-tapered portion of said web is provided by not only said axially distal end portion but also axially proximal end and intermediate portions of said web.  
     
     
         4 . A drill according to  claim 1 , 
 wherein said back-tapered portion of said web is provided by only said axially distal end portion of said web which has a predetermined axial length not smaller than a diameter of said drill,    and wherein said web further includes a non-back-tapered portion which is provided by axially proximal end and intermediate portions of said web, such that the thickness of said web in said non-back-tapered portion is constant.    
     
     
         5 . A drill according to  claim 1 , 
 wherein said back-tapered portion of said web is provided by only said axially distal end portion of said web which has a predetermined axial length not smaller than a diameter of said drill,    and wherein said web further includes a non-back-tapered portion which is provided by axially proximal end and intermediate portions of said web, such that the thickness of said web in said non-back-tapered portion is increased as viewed in the direction away from said axially distal end of said cylindrical main body toward said shank.    
     
     
         6 . A drill according to  claim 1 , wherein a relationship between a diameter of said drill and a maximum value of the thickness of said web and a relationship between the diameter of said drill and a minimum value of the thickness of said web are represented by the following expressions:  
       0.2D≦W 1 ≦0.4D; 0.15D≦W 2 ≦0.33D;  where D represents the diameter of said drill; 
 W 1  represent the maximum value of the thickness of said web; and  
 W 2  represent the minimum value of the thickness of said web.  
   
     
     
         7 . A drill according to  claim 1 , wherein a flute-width ratio of a width of each of said at least one flute to a width of each of said at least one land is 0.6-1.5 in a web-thickness maximized portion of said cylindrical main body in which the thickness of said web is maximized while said flute-width ratio is 0.8-1.7 in a web-thickness minimized portion of said cylindrical main body in which the thickness of said web is minimized.  
     
     
         8 . A drill according to  claim 1 , wherein a surface of each of said at least one flute has a roughness curve whose maximum height Ry is not larger than 3 μm.  
     
     
         9 . A drill according to  claim 8 , 
 wherein said cylindrical main body is coated with a hard coating;    and wherein said surface of each of said at least one flute coated with said hard coating is polished so as to have said roughness curve whose maximum height Ry is not larger than 3 μm.    
     
     
         10 . A drill according to  claim 1 , 
 wherein said at least one flute consists of two flutes which are symmetrical with respect to said axis of said cylindrical main body,    wherein said at least one land consists of two lands which are joined by said web and each of which is located between said two flutes,    and wherein a front-side wall of each of said two flutes as viewed in the predetermined rotating direction is concaved in a direction away from a leading edge which is provided by a rear-side one of widthwise opposite edges of each of said two flutes as viewed in the predetermined rotating direction, such that a most concaved portion of said front-side wall is distant from a straight line which passes through said axis and a rear end of a corresponding one of said two lands as viewed in the predetermined rotating direction, by a predetermined distance that is equal to a half of value represented by the following expression:    0.15D≦t≦0.35D;    where D represents a diameter of said drill; and 
 t represents the value that is twice as large as said predetermined distance.  
   
     
     
         11 . A drill according to  claim 1 , wherein said at least one flute has an axial length of at least ten times as large as a diameter of said drill which is not larger than 8 mm.  
     
     
         12 . A drill according to  claim 1 , further comprising an oil hole which is formed through said cylindrical main body and said shank and which opens in an end flank face each located on a rear side of said cutting edge as viewed in the predetermined rotating direction.  
     
     
         13 . A drill according to  claim 1 , wherein said cylindrical main body has a large diameter portion provided by an axially distal end portion thereof, and a small diameter portion provided by a portion thereof adjacent to said shank, said large diameter portion having a diameter which is reduced as viewed in the direction away from said axially distal end of said cylindrical main body toward said shank, said small diameter portion having a diameter which is smaller than said diameter of said large diameter portion and which is constant over an entire length of said small diameter portion.  
     
     
         14 . A process of drilling a hole in a workpiece by using the drill defined in  claim 1 , said process comprising: 
 a step of feeding said drill and said workpiece relative to each other at a feed rate of at least 0.10 mm/rev while rotating said drill and said workpiece relative to each other at a peripheral velocity of at least 80 m/min.