MICRO DRILL

Abstract: A micro twist drill (11) having a pair of oppositely disposed flutes (13) which helically extend from the drill tip (22) along the body (15). The flutes (13) define a pair of oppositely disposed wings (17), each of which has a leading surface (45) (relative to the direction of rotation of the drill) which is undercut to provide a peripheral positive rake cutting edge which makes an angle (A) of from about 5<o> to about 30<o> to a radius of the drill body (15) which extends to the cutting edge. The drill (11) has a traverse cross section in which the leading surface (45) of each wing is provided with a curved channel (51) which trails a line between the cutting edges (47) of the two wings (17). The trailing surface (53) of each of the wings (17) defines a curve which substantially continuously recedes from the path of the cutting edge (47) to the point where it meets the leading surface (45) of the other wing (17).

Description: MICRO DRILL TECHNICAL FIELD The present invention relates to twist drills and more particularly to so-called micro twist drills which are adapted for drilling holes for a diameter of 0.125 inches and usually le than 0.050 inches. BACKGROUND ART Micro drills range in sizes from approximately 0.125 inches down to about 0.002 inches in diameter and are employed for many precision purposes. They are used to provide precision holes in plastic, metal and the like, in the manufacture of such products as printed circuit boards, carburetors, watches and the like. Such drills often have a relatively short life and th other problems which they present relate to the removal of chips and the precision of the hole which is formed by the drill Because of the small sizes of the drills the chips generated are extremely small, but at the same time, there is not a sufficient large passageway in the flutes to carry away the chips. Also, w normal drill constructions the chips are worked against the side walls of the hole being drilled by the rotation of the drill and tend to cause galling on the sides of the holes. This is parti¬ cularly troublesome when the drills are provided with the usual back taper, i.e. a small amount of taper is provided between the shank and the drill tip so that the tip is of somewhat larger diameter than the shank. Under these conditions, the tendency of the chips to be caught between the peripheral surfaces of the drill and the wall of the hole being drilled is increased and thereby increases the presence of the imperfections in the hole, the generation of heat, as well as stresses on the drill. Also, because of the back taper, there is a tendency for the drill to wobble because of the fact that the peripheral areas of the drill are not in contact with the sides of the hole in the area remote from the tip. This results in a hole which at times is not true and, again, causes stresses in the drill which tend to shorten its life. Another problem that is presented with normal drill constructions in the small sizes is that a land is provided which extends beyond the body of the drill and, in the small sizes, S fTBAtT O £ - material tends to "hang up" between the body of the drill and th side of the hole especially in the area adjacent the step which occurs between the land and the drill body. DISCLOSURE OF INVENTION Accordingly, it is a principal object of the invention to provide an improved construction for micro drills. It is another object of the invention to provide a drill construction for micro drills which minimizes chip retention and provides passageways which are capable of conducting the chips out of the drill hole. It is a further object of the invention to provide a drill which can be fabricated without the necessity of a back taper. These objects are accomplished by providing a micro twist drill having a rigid cylindrical body with a drill point t one end and having two oppositely disposed flutes helically extending along said body from said drill point to define a pair of oppositely disposed helically extending wings. Each wing has a leading surface in relation to the direction of rotation of the drill and the leading surface is undercut to provide a peripheral, positive rake, cutting edge which makes an angle of from about 5° to about 30° to a radius of said cylindrical body which extends to said cutting edge. The wings are symetrically disposed relative to one another to provide a drill body with a traverse cross section of a generally S-shape wherein the leadin surface of each wing is provided with a curved channel trailing (relative to the direction of rotation of the drill) a line bet¬ ween the cutting edges of said two wings and wherein the trailin surfaces of each of said wings defines a curve which substantial continuously recedes from the path of the cutting edge to the point at which it meets the leading surface of the other wing. Other objects and advantages of the invention will become known by reference to the following description and following drawings in which: BRIEF DESCRIPTION OF INVENTION; FIGURE 1 is a fragmentary side view of a drill embodyin the construction which is ordinarily employed in the prior art; FIGURE 2 is an end view of the drill shown in FIGURE 1; FIGURE 3 is a side view of a drill embodying various features of the invention; FIGURE 4 is an end view of a drill shown in FIGURE 3; FIGURE 4A is a fragmentary enlargement of a portion of FIGURE 4; FIGURE 5 is a side view of another embodiment of a dril embodying various features of the invention; and FIGURE 6 is an end view of the drill shown in FIGURE 5. BEST MODES FOR CARRYING OUT THE INVENTION By way of explanation, FIGURES 1 and 2 illustrate a drill 11 having the usual prior art configuration. The drill 11 is made from a length of cylindrical stock which is provided wit a pair of opposed helical flutes 13 which provides a drill body or web 15 having a pair of opposed wings 17. Each of the wings 17 includes a face 19 which is parallel to a radius 21 of the drill and is positioned ahead of the radius 21 based upon the direction of rotation of the drill. The cutting action of the drill is accomplished by the drill tip 22 and by a so-called chisel edge 23 at the peripheral edge of the face 19 insofar as concerns the outer diameter of the hole being drilled. Rear- wardly (with reference to the direction of rotation of the drill from the chisel edge 23, the wing 17 is provided with a land 25 which provides a circumferential arcuate area which determines the diameter of the drill. The land 25 follows the helical path of each of the flutes 13 up to the shank of the drill. At the rearward end of the land (with reference to the direction of the rotation of the drill) an inwardly directed ste 27 is provided and the remainder or heel 29 of the wing 17 follo the curvature of the path 30 of the chisel edge 23 but is inwardl spaced therefrom as illustrated. The distance from the chisel edge 23 to the end 31 of the heel 29 is normally of the order of about 90° of arc or more. From the end 31 of the- heel 29, the wing is defined by a concave surface 33, which merges in to the face 19 of the opposite wing. The tip 22 of the drill is sharpened in the usual manner with a tip angle of from about 90° to 120°, this angle varying in the manner known to the art depending upon the material being drilled. As pointed out above, the usual drill is provided with a back taper so that the diameter across the lands 25 at the tip of the drill is larger than the diameter across the lands adjacen the shank. This is known as "back taper" and is employed to reduce the friction between the lands 25 and the wings 17 against the sides of the hole at points remote from the tip 22. Such a construction is satisfactory for drills of larger sizes. However, in smaller sizes, such as the microdrills contemplated by this invention, i.e. drills ranging in diameter from 0.125 to 0.002 inches, such a construction causes a number o problems. For example, the face 19 of the wing 17 forms an acute angle with the wall of the hole indicated by the dotted line 30 which is the path of the chisel edge 23 in FIGURE 2, so that as the drill turns and the chips are forced out of the hole being drilled, they are constantly wedged between the wall of the hole and the land 25 causing galling of the wall. In the small drill sizes, galling is extremely undesirable because it can destroy tolerances. The galling problem is aggravated at positions remote from the tip because of the back taper. Also, it has been found that with small diameter drills the fine drill particles tend to build up on the face of the heel 29 of the wing 17, especially in the area of the step 27 causing additional friction between the drill wing 17 and the side of the hole. All of this results in additional friction which increases the amount of torque required to turn the drill as well as in increased heat both of which shortens drill life and increases the tendency for the drill to break. Also, any uneven distribution of chips in the relieved area between the heels 29 and the innersurface of the hole may cause the drill to wobble and may result in a hole which is not true as does the passage of the particles between the chisel edge and the wall of the hole. The back taper of the normal drill construction is also a problem in the small sizes. As pointed out above, the drill has a larger diameter at the tip than adjacent the shank. As a result, the hole does not fully support the sides of the drill at a point remote from the tip. Thus, the walls of the hole whic has been drilled does not aid in preventing bowing of the drill which is always a problem in the smaller sizes. FIGURES 3 and 6 illustrate various features of the present invention. In general, our improved micro twist drill comprises a rigid cylindrical body having two oppositely disposed flutes helically extending from the drill point along the cylind cal body towards the shank which defines a pair of oppositely disposed helically extending wings. Each of the wings has a lea surface in relation to the direction of rotation of the drill, the leading surface being undercut to provide a peripheral, positive rake cutting edge which makes an angle of from about 5° to about 30° to a radius of the cylindrical body which extends t the cutting edge. The wings are symmetrically disposed relative to one another to provide a drill body or web which is in trans- verse cross section of generally "S" shape. The leading surface of each wing is provided with a channel, a substantial portion o which trails (relative to the direction of rotation of the drill a line between the cutting edges of the two wings, so as to pro¬ vide a channel inwardly of the cutting edge along which chips wi be transported out of the hole being drilled. The trailing surf of each of the wings is defined by a surface which substantially continuously recedes from the path of the cutting edge to the point at which it meets the leading surface of the other wing. This construction provides passageways for chips in the flutes which are of large cross section and which have substantially no corners or grooves in which chips or other debris can be lodged. Preferably, the micro drill includes a circumferential land between the cutting edge of each wing and the point where the trailing surface of the wing recedes from the path of the cutting edge. Preferably, the land on each wing is of a width which subtends an arc of less than about 12°. The above construction permits the minimization or even the elimination, of back taper with its attendant disadvant¬ ages in connection with the possible interference of chips and the lack of support by the hole being drilled of peripheral surfaces of the drill at points remote from the tip. Now referring to FIGURES 3 and 4, which illustrate one embodiment of a drill embodying features of the invention, the drill is fabricated from a rigid cylindrical body 37 from suitable materials known to the drill art. The drill material will be dependent upon the material being drilled and the life required for the drill, all as known in the art. As Illustrated, two oppositely disposed flutes 39 are provided which helically extend from the drill point 40 along the body 37 towards shank 41 of the drill. The flutes 39 define a pair of oppositely disposed helically extending wings 43. The helix angle is consta in the illustrated drill. However, depending upon the action requred in removing chips it can be varied along the length of th drill to accelerate the removal of chips after they leave the are adjacent the tip of the drill by decreasing the helix angle in th area remote from the tip 40. Each of the wings 43 has a leading surface 45 in relati to the direction of rotation of the drill, the surface 45 being undercut adjacent its periphery to provide a positive rake cuttin edge 47. It has been found that a tangent to the face 45 at the cutting edge should make an angle of from about 5° to about 30° t a radius 49 of the cylindrical body which extends to the cutting edge 47, angle "A" in FIGURE 4. As illustrated, the wings 43 are symmetrically disposed relative to one another to provide a drill web which is generally of "S" shape. In order to cause the chips to be directed inward¬ ly away from the cutting edge the leading surface of each wing is forme< in a concave shape to provide a channel or trough 51, a substantial portion of which trails the radius 49 which extends to the cutting edge 47. Thus, in operation the chips formed by the drilling operation will be moved inwardly by the positive rake cutting edge away from the sides of the hole being drilled. In the channel 51, the rotation of the drill causes the chips to be moved out of the drill hole in the flutes 39. Each of the wings 43 is provided with a trailing surface 53 which begins at the periphery of the drill body and substantially continuously recedes from the periphery of the drill body. Preferably, as illustrated, both the leading surfaces 45 and the trailing surfaces 53 are smooth, concave and convex curves, respectively, with a smooth transition therebetween. This results in a structure in which clearance opening between trailing surface 53 and the periphery of the hole being drilled constantly increases along the trailing surface 53 so that there is no area in which chips can be lodged or wedged and the smooth curved surface per¬ mits the chips to fall freely against the leading surface 45 of the next following wing along which they are carried out of the drill hole. For some materials being drilled, particularly those_____ ( G-.TΓ which are relatively soft, it is possible to have the trailing edge 53 begin at the cutting edge 47. However in most instances it is desirable to have a land area 55 which enhances the streng of the cutting edge 47. As the land 55 has a curvature which corresponds to the curve transversed by the cutting edge 47 and extends from the cutting edge rearwardly (based upon the directi of rotation of the drill). It has been determined that the land 55 should preferably subtend an arc "B" of less than about 12°. (See FIGURE 4A). With the described construction of helical flutes and wings the drill point 40 may be ground on the end of the drill remote from the shank in the usual manner. As in conventional drills, the point will be ground to provide a tip angle at the point of from approximately 90° to approximately 120°. The exact angle depends upon the material being drilled, all as know in the art. With the above described drill construction it has been found that it is usually unnecessary to provide a back taper on the drill. However, in the event that the material to be drille provides an unusual amount of friction, a slight back taper of t order of a few ten/thousandths of an inch may be provided. In operation, the drill is rotated in the material to be drilled and the chips generated by the tip will move inwardly of the periphery of the hole along the channels provide in the leading edge of each of the wings. The cutting edge and its associated land move along the walls of the hole being drill and have the function of scouring the walls and collecting any chips which might adhere to the walls and move them into the channel 51. Any chips which are not collected by the cutting • edge and pass between the land and the walls of the hole being drilled have little tendency to become lodged on the smooth conv surface 53 of the trailing edge of the wing but instead are collected and transported out of the hole by the next succeeding wing. As pointed out above, the cutting edge makes an angle " with the radius 49 in the range of from about 5° to about 30°. Preferably, the angle is maintained between about 5° and 15°, and most preferably about 10°. Drills, as described above, with an appropriate point have been found to drill materials of all types with a minimi¬ zation of galling and a minimization of wobbling along the drill length as a result of the effective removal of chips. FIGURES 5 and 6 illustrate a drill of the type illust- rated in FIGURES 3 and 4, the drill being provided with a so- called split point. This enhances the speed of drilling and aids in drilling an accurate hole. Split points are known in the art and are formed by grinding away a pair of triangular areas as illustrated at 57 and 59 in FIGURES 5 and 6. Various of the features of the invention believed to be new are set forth in the appended claims.

Claims: THE CLAIMS 1. A micro twist drill comprising a rigid cylindrical body with a drill point at one end and having two oppositely dis¬ posed flutes helically extending from said drill point along said body, to define a pair of oppositely disposed helically extending wings, each wing having a leading surface in relation to the direction of rotation of the drill, said leading surface being undercut to provide a peripheral, positive rake, cutting edge which makes an angle of from about 5° to about 30° to a radius of said cylindrical body which extends to said cutting edge, the wings being symmetrically disposed relative to one another to provide a drill body with a traverse cross section of generall S-shape wherein the leading surface of each wing is provided with a curved channel trailing (relative to the direction of rotation of said drill) a line between the cutting edges of said two wings and wherein the trailing surfaces of each of said wings defines a curve which substantially continuously recedes from the path of the cutting edge to the point at which it meets the leading sur¬ face of the other wing. 2. The micro drill of Claim 1 wherein a circumferential land is provided between the cutting edge of each wing and the point where the trailing surface recedes from the path of said cutting edge, each said circumferential land subtending an arc of less than about 12°. 3. The micro drill of Claim 2 wherein the peripheral, positive rake, cutting edge makes an angle of from 5° to 15° to a radius of said cylindrical body which extends to said cutting edge. 4. The micro drill of Claim 3 wherein the angle is about 10°. 5. The micro drill of Claim 3 wherein the diameter of the drill at the lands is uniform along the length of said drill. 6. A micro twist drill comprising a rigid cylindrical body having a drill point at one end and having two oppositely disposed flutes helically extending from said drill point along said body to form two oppositely disposed helically extending wings, each of said wings having a cutting edge at the peripheral surface of said body, the leading surface of each wing (relative V VIFO to the direction of rotation of said drill) being generally conc and the cutting edge being defined by the intersection of said leading surface with the peripheral surface of said body and hav a rake angle of from 85° to 60°, the trailing surface of each of said wings being generally continuously convex and extending fro a point in the periphery on said cylindrical body adjacent said circumferential cutting edge to the leading surface of the other web, and a circumferential land between the cutting edge and the convex trailing surface of its associated wing encompassing an a of less than about 12°. 7. The micro drill of Claim 6 wherein the cutting edge lie on diametrically opposed points on said generally cylindrica body and the concave leading surfaces provide a concave channel which trails a radius to the cutting edge over a substantial portion of the radial dimension of the drill. 8. The micro drill of Claim 7 wherein the diameter of the drill across the cutting edges is uniform along the length of said drill.

Inventors: ARPAIO JR JERRY, HEATH DEREK E, ARPAIO, JR., JERRY, HEATH, DEREK, E.
Applicants: MICRO INTERNATIONAL
Application year: 1982
Main classification: B23B51
IPC codes: B23B51