Drill

0001The present invention relates to drills comprising the features of the preamble of claim 1 (cf. transactions of the ASME, February 1957, page 227). Such drills are suitable for deep hole drilling and have helical or straight flutes formed in the shank for discharging chips therethrough.

0002Generally, drills are formed with helical flutes in the shank for discharging chips. The chips are curled while moving through the flutes and are discharged in the form of conical helical chips. Such discharged chips revolve with the rotation of the drill, wind around neighboring portions of the drilling apparatus and therefore cause trouble when the apparatus is operated automatically continuously in the absence of the operator. Attempts are also made to cause drills to produce chips in the form of small fragments instead of forming continuous chips. Further to discharge chips smoothly, conventional drills are formed with enlarged flutes for curled chips to pass therethrough easily.

0003In the case where chips are thus discharged as curled, there arises the problem that the chips are cumbersome to remove. Especially, deep hole drilling encounters the problem that chips clog up the flute without being discharged smoothly to make it impossible to conduct the drilling operation.

0004Further when large flutes are formed to discharge chips smoothly, there arises the problem that the shank has reduced torsional rigidity, failing to drill holes efficiently.

0005Deep hole drilling, which involves the necessity of supplying oil to the cutting portion, encounters another problem. When the oil is supplied through the flute from outside, the chip moves up while curling to the shape of a funnel to scoop up the oil supplied to the flute, with the result that the oil fails to reach the cutting portion. To overcome this problem, conventional twist drills are produced by a complex method, ie. by forming an axial bore in a shank, then twisting the shank and forming oil supply ports.

0006A drill according to the present invention comprises a shank with helical or straight flutes for discharging chips therethrough, each said flute having walls which are respectively positioned forward and rearward with respect to the direction of rotation of the drill, a pair of cutting edges formed at one end of the shank each extending from the centre of the drill to its periphery, each said cutting edge being formed with a rake face over the length of said edge, the flutes having a depth 0.25 to 0.40 times the diameter of the drill and being approximately L-shaped in cross-section with their forward and rearward walls at an angle of 70 to 120° relative to each other, the rearward walls being arranged to be engaged by a portion of the chip formed by an outer peripheral portion of the cutting edge, and the forward walls of the flutes being arranged to limit the curling of the chips.

0007By providing a drill having flutes approximately L-shaped in cross-section, it is possible to arrange for the chips to be discharged smoothly while being prevented from continuously curling, without the necessity of forming the flutes to a greater depth, even for use in deep hole drilling. By limiting the curling of the chips, they are encouraged to break spontaneously when emerging from the hole formed by the drill or when leaving the drill.

0008The forward wall of each flute may be recessed so that the peripheral side of the flute is enlarged forwards with respect to the direction of rotation.

0009A form of drill proposed in the article "Some Experiments in the Influence of Various Factors on Drill Performance" (Transactions of the ASME, February 1957, page 191) has flutes with a depth of some 0.3 times the diameter of the drill and a convexly-curved approximately U-shape cross-section. Central cutting edge portions extend inwards from outer cutting edge portions at the tip of the drill, in place of the conventional chisel edge, to give a cutting edge extending from the centre of the drill to its periphery and formed with a rake face over its length. While this special tip formation of the drill enhances its cutting ability, the drill is otherwise conventional in the chips it produces will take a continuous curled form.

0010In the use of a drill embodying the present invention, a chip formed by the drill is prevented from curling by the flute-defining walls. The portion of the chip toward the center is plastically deformed by being stretched by the outer portion of the chip, and the chip is continuously discharged in the form of a flat plate from the hole or bore formed. The chip is fractured by being bent by centrifugal force and the upper end of the fluted portion. The chip is in the form of a continuous piece while passing through the flute and is therefore smoothly dischargeable without clogging the flute even when the drill is used for deep hole drilling.

0011Since the chip does not curl, oil can be readily supplied to the cutting portion through the flutes.

0012Preferred embodiments of the invention will be described by way of example with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Fig. 1 is a side elevation partly broken away and showing a drill embodying the invention;</li><li>Fig. 2 is a bottom view of the same;</li><li>Fig. 3 is a view in section taken along the line III-III in Fig. 1;</li><li>Figs. 4 and 5 are views corresponding to Fig. 3 and showing further embodiments of the invention; and</li><li>Fig. 6 is a perspective view of a chip formed by the drill.</li></ul>

0013With reference to Figs. 1 to 3, the shank 10 of a drill has helical flutes 3 for discharging chips therethrough and a conical end, which is formed with a pair of cutting edges 2 arranged in point symmetry and extending from the center of rotation, O, of the drill to the outer periphery of the shank 10. Each of the cutting edges 2 comprises an edge portion 21 providing a chisel portion and an edge portion 22 extending from the portion 21 toward the periphery. These edge portions 21 and 22 are generally straight and in a V-shaped arrangement with an obtuse angle formed therebetween at the junction when seen in a bottom view. The edge portions 21,22 may be curved forward or rearward with respect to the direction of rotation of the drill.

0014The end face of the shank 10 comprises a flank 13 having the cutting edge 2, and a land portion 20 in the form of a slanting surface and positioned in the rear of the flank 13 with respect to the direction of rotation.

0015Each of the flutes 3 is approximately L-shaped in cross section and defined by a wall 31 positioned forward with respect to the direction of rotation and a wall 32 positioned rearward with respect to the direction at an angle of 90 degrees with the wall 31. This angle is variable within the range of 70 to 120 degrees, preferably within the range of 85 to 100 degrees. If the angle is greater than this range, the chip will not be prevented from curling by being pressed against the flute defining wall as will be described later, whereas if the angle is smaller than the range, it is difficult to form the flute.

0016The forward wall 31 is recessed (by being cut away) so that the peripheral side of the flute 3 is enlaged forward with respect to the direction of rotation. More specifically, the forward wall 31 has a projection 33, and beyond this position toward the periphery, the wall 31 is recessed to provide a wall 35. Instead of forming the wall 35, a slanting wall 39 may be formed so as to increase the width of the flute toward the periphery. As shown in Fig. 1, the projection 33 continuously extends approximately centrally of the flute 3.

0017Preferably, the flutes 3 have a depth A 0.25 to 0.40 times the diameter D of the shank 10. If the depth A is smaller than this range, difficulty is encountered in discharging chips, whereas deeper flutes impair the rigidity of the shank 10. The depth A of the illustrated flutes is 0.25 times the diameter D of the drill. The peripheral enlarged portion of the flute 3 has a dimension B which is about 0.15 D and a dimension C which is about 0.1D.

0018Fig. 4 shows another embodiment of the invention which has the same construction as above in that the flute 3 thereof is defined by walls 31 and 32 intersecting each other at a right angle but differs from the first embodiment in that the wall 31 has no projection 33 and extends to the periphery of the shank.

0019The walls 31 and 32 defining the flute 3 are each straight in cross section and make an angle of 90 degrees therebetween. However, these walls can be modified variously as shown in Figs. 5 to 10.

0020With reference to Fig. 5, a wall 61 forward with respect to the direction of rotation of the drill is at an angle of 120 degrees with a rearward wall 62. The ealls 61, 62 are each straight in cross section.

0021When the drill is used for forming a hole or bore, chips 4 as shown in Fig. 6 are produced. The chip 4 comprises small curl portions 41 formed by the portion of the cutting edge 2 close to the center of the drill, and a flat plate portion 40 formed by the other portion of the edge 2 toward the drill periphery. The curl portions 41 extend from one side of the plate portion integrally therewith and are arranged discretely longitudinally of the plate portion 40, with cracks 44 formed in the side part.

0022The chip 4 thus shaped is produced in the following manner. With reference to Fig. 4, the flat plate portion 40 formed by the outer portion (edge portion 22) of the cutting edge 2 and forced into the flute 3 has its one side pressed against the base portion of the wall 31, with the planar part of the plate portion 40 pressed against the wall 32, whereby the chip 4 is prevented from curling in its entirety. (The chip is subjected to plastic deformation by the cutting action and further plastically deformed by being stretched by its outer portion. Consequently, the chip in the form of a flat strip with cracks 44 formed in the side part thereof is forced upward through the flute 3. The chip thus moving upward along the gentle helical shape of the flute 3 egresses from the drilled bore and fractured at the cracked portions 44 by a centrifugal force. When the bore drilled is deep, the chip is bent outward by a curved fluted portion 38 at the upper end of the flute as seen in Fig. 1, with the result that the chip 4 made brittle by cracking and marked plastic deformation is broken into fragments.

0023The chip 4 formed by the drill of the construction shown in Figs. 1 to 3 is forced upward into the flute 3 with the flat plate portion 40 initially pressed by the rake face 32a and with the side part pressed against the wall 31. The side outer portion of the chip 4 then moves to the enlarged flute portion defined by the wall 35 as seen in Fig. 3. This assures smooth discharge of the chip 4. While the wall 31 acts to prevent the chip from curling by pressing contact with the side part of the flat plate portion 40, the enlarged flute portion provided by the wall 35 positioned outward from the wall 31 thus facilitates passage of the chip. Especially when the drill is used for deep hole drilling, the drill exhibits the advantage of smoothly discharging chips 4.

0024With the structure shown in Fig. 5, the chip 4 has its flat plate portion 40 pressed against the wall 62, with one edge thereof in contact with the base portion of the wall 61, whereby the chip 4 in the flute 3 is prevented from curling in its entirety before discharge.

0025According to the embodiments of the invention described above, the chip 4 can be discharged in the form of a flat strip without curling in its entirety within the flute 3, so that the flute 3 need not have a large depth. This increases the core thickness (the dimension between the deepest portions of the flutes) α (see Fig. 4) of the shank 10 to thereby give improved rigidity to the shank, consequently precluding the axis of the drill from deflecting during drilling.

0026The embodiments described above are free of the likelihood that the chip to be discharged will not scoop up the oil supplied to the flute. This makes it possible to supply oil through the flute from outside and therefore results in the advantage that an oil supply opening need not be formed in the drill.

0027Although the flutes 3 formed in the shanks of the foregoing embodiments are all helical, such flutes need not always be helical but can be straight.

0028With the embodiments of the present invention described above, the chip is prevented from curling and plastically deformed by the flute defining walls, continuously discharged in the form of a flat plate and fractured by a centrifugal force when projecting from the drilled hole by a predetermined amount. Further when the drilled bore is deep, the chip is bent outward at the upper end of the flute and thereby fractured. The chip extends continuously through the flute and can therefore be discharged smoothly without clogging the flute even during deep hole drilling.

0029The reduced depth of the flute gives improved rigidity to the shank, consequently rendering the drill operable with a high efficiency and high accuracy even for deep hole drilling and easily usable at a high feed speed. Since oil can be supplied through the flute from outside, the drill has the advantage that there is no need to form an oil supply opening in the drill.