Cutting by use of rotating axially moving tool – Tool or tool with support – Having peripherally spaced cutting edges
Reexamination Certificate
2000-03-06
2001-02-20
Bishop, Steven C. (Department: 3722)
Cutting by use of rotating axially moving tool
Tool or tool with support
Having peripherally spaced cutting edges
C076S108600, C408S224000, C408S229000, C408S226000, C451S048000
Reexamination Certificate
active
06190097
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to twist drill bits having flutes that extend from a tip of the drill bit to a termination point on the shank. The drill bit flutes define a web thickness which increases along the length of the flute from the tip to the termination point. The invention uses a much steeper web taper rate than is used in the industry. It has been discovered that bits employing the present invention experience significantly enhanced toughness and resistance to breakage in metal, yet still perform well in wood. This enhanced effect is particularly pronounced when the drill bit is driven by a portable drill.
BACKGROUND OF THE INVENTION
Conventional wisdom in the art holds that it is advisable to use a relatively shallow web taper rate in twist drills so that the flute depth along the length of the flute is as great as practicable. This should provide the maximum amount of volume to convey chips, swarf or sawdust back from the tip and out of the hole being drilled. This convention is embodied in two Standards: The American Society of Mechanical Engineers (ASME B94.11M-1993), and The National Aerospace Standard of the Aerospace Industries Association of America, Inc., (NAS 907) (hereinafter collectively referred to as “Standards”), both of which Standards are specifically incorporated in their entireties by reference herein. The Assignee of the present invention has been selling conventional Jobber-length, straight-shank drill bits which have parameters that track the Standards, and which have conventional web thickness taper rates between 0.024 inches to 0.030 inches. Also, it has been selling a self-centering drill bit with pilot tip under the BULLET® trademark having a web taper rate of about 0.027 inches per inch of flute length. This self-centering drill bit was designed with the end user who uses a portable drill in mind, particularly one who desires a bit which is optimized to drill in both metal and wood. The first versions ofthese bits are disclosed in U.S. Pat. No. 4,968,193, issued Nov. 6, 1990, to Chaconas et al., and U.S. Pat. No. 5,288,183, issued Feb. 22, 1994, to Chaconas et al., each of which is assigned to the same Assignee of the present invention. The entire disclosures of each of these two patents are also specifically incorporated by reference herein.
One of the needs of an operator who uses the bit in a hand-held power drill is increased toughness, or resistance to breaking (typically in the flute portion of the drill bit). The increased robustness is needed because in metal drilling the portable power drill user puts considerable side stresses on the drill bit, not having the stability that a drill press provides. However, the BULLET® drill bit, which was optimized to drill well both in metal and in wood, not only needs strength while drilling metal, it also must drill holes through wood with a minimum of “woodpeckering”, that is, repetitively retracting and reinserting the drill bit to clear the flutes of sawdust. There is also a need to enhance the BULLET® drill bit's longevity if it should be subjected to an overspeed condition in metal, as well as the strength in its tip portion. Finally, there is a need to reduce its tendency to produce a disc in laminated materials.
SUMMARY OF THE INVENTION
It has been discovered that by defying convention, and instead, using a radically-increased web taper rate, a much more robust drill bit will be created. In addition, it has been discovered that a particular range of increased web taper rates will not only provide sufficiently enhanced robustness or strength when drilling in metal, it simultaneously provides a bit which minimizes the amount of woodpeckering required when drilling wood. Accordingly, it is an object of the present invention to manufacture a twist drill bit having a web thickness taper rate over the length of the flute which is about twice as great as the web taper rate of conventional drill bits, thereby providing significantly increased resistance to breakage while drilling in metal, yet, for drill bits having nominal diameters of 0.250 inch or greater, enabling the user to drill holes in inch and one-half thick wood in one pass.
It is a particular object of the preferred embodiments of the invention to provide a twist drill bit in which the web thickness increases from the tip to the shank portion at a uniform taper rate along the length of the flute in the range from about 0.050 inch to about 0.071 inch per inch of flute length. It is yet another particular object of the preferred embodiments of the invention to provide a bit marking zone on the shank portion of the drill bit which is outside of the area of the shank typically gripped by the jaws of a chuck in which the drill bit is inserted.
A feature by which the above objects may be attained is by forming the flutes with the web taper rate of the present invention along a flute length which is less than the flute length specified for a predetermined bit nominal diameter by the ASME B94. 11-M-1993 Standard, and specifically by reducing the flute length by about 0.3 inch to about 0.7 inch. Optimally, the flute length is reduced by about one-half inch, for all nominal diameters of the drill bits. A preferred feature is to form indicia such as the nominal size ofthe bit, in the marking zone, so that in the engagement by the chuck of the bit shank does not erase the indicia.
Another feature by which the above objects can be attained is in the manufacturing process: causing relative axial movement between a grinding wheel and drill bit stock while rotating the drill bit stock about its axis, grinding the flute having the desired flute length with the web thickness taper rate of the present invention, then retracting the grinding wheel from the drill bit stock upon reaching a point which is a predetermined distance before the point at which the flute would terminate at the shank if the taper rate were to be maintained.
It is yet another object of the present invention to enhance the robustness of self-centering drill bits with pilot tip of the type sold under the trademark BULLET®. One feature by which the above object can be attained is by optionally providing the drill bit tip portion with a fishtail angle of less than 180°. Yet another preferred feature is to form a chamfer adjacent the fishtail such that a cutting edge on the chamfer connects a cutting lip on the fishtail with the outer diameter of the drill bit. Still another preferred feature is to provide the pilot portion with a back taper in the axial direction. Another preferred feature is to provide a back taper which is at least 1° negative. And yet another feature is to size the width or diameter of the pilot portion to be about one-half the nominal bit diameter.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.
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Bishop Steven C.
Black & Decker Inc.
Harness & Dickey & Pierce P.L.C.
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