METHOD OF AND TOOL FOR THREAD MILL DRILLING

Abstract: A threaded hole (20) in a work piece (22) is formed by rapidly spinning an elongated tool (24) having a drill point (64) at one end, at least one longitudinally extending flute (80) having thread milling faces (84) for milling the thread to be formed, and wherein the tool (24) is first moved axially to carry the drill point (64) against and into the workpiece (22) to form a hole (66), then is displaced radially a distance equal to the thread depth to be formed, then is moved orbitally around the hole (66) to mill threads therein, then is displaced back to the center of the hole (20) and then is retracted from the hole (20). To form a helical thread in the hole (20), the tool (24) is shifted axially one thread pitch during the orbital movement. A thread milling drill head (26) is disclosed for chucking the tool (24) and shifting it through the aforesaid movements. Fluid flow may be induced into the hole (66a) to entrain the waste or fines and carry such out of the hole, and for this purpose a passageway (l00) connects to a source (lll) of fluid pressure with a turbine blade assembly (ll0) mounted on the tool 24(a).

Description: The invention relates to a method of forming a threaded hole in a workpiece comprising the steps of providing an elongated tool having a drilling face at one end and a thread milling profile along the side; rotating said tool about a longitudinal axis while shifting it axially to carry the drilling face against and into a workpiece to form a hole therein and after forming said hole, rotating the tool for milling threads in the wall thereof, as well as to a thread mill drill tool for drilling a hole and milling a thread in the wall thereof. The common approach to providing a threaded hole in a workpiece is to first drill the hole using a drill bit or the like and after removing the bit from the hole, using a tap, to tap the hole to provide threads in the wall thereof. In lieu of a tap, a thread milling tool may be used. Thread milling has only been satsifactory in larger diameter holes such as 5/8 or 3/4, and has been too slow for smaller holes. U.S. Patent 2,813,280 shows a combination drill and tap. Substantial savings in processing costs could be effected if, instead of requiring the use of separate drilling and tapping or milling tools, the same tool could be used for the entire operation because such would avoid the necessity of indexing the workpiece from one forming operation to another, or using a machine tool having separate heads for moving the drilling and tapping or milling tools successively into work performing positions. U.S. Patent 3,334,366 shows a machine tool having a spindle intended to hold either a drill or a tap, but the drill must be removed before the tap may be mounted in the spindle. A common tool for forming a threaded hole comprises a twist drill at the front part of the tool, a peeling tool arranged behind it and a tap thread arranged behind these two single tools. In order to be able to thread into the drilled hole created by the twist drill, the twist drill in this tool must be worked into the tool in a measure which is longer than the thread section so that the common tool can predominantly only be used to create thread sections in bolt holes, however due to this tool structure the workpiece because of the additional drilling length is weakened correspondingly by the projecting twist drill. The structure of this tool requires furthermore that the tool is removed several times from the drill hole. In addition a tool for forming a threaded hole is known (US-PS 2 684 492), in which behind the drilling section a thread section having a larger diameter than the drill section is arranged. When using a tool of this type, in which the threading process is simultaneous with the drilling process, a correct threaded hole cannot be produced. According to my invention, the same tool is used both for drilling the hole in the workpiece and then milling threads in the wall of the hole. Thus, substantial savings in production time are possible and production costs reduced. The tool for performing this operation, which I have termed a thread milling drill, comprises an elongated shank having a drill point or drilling face at one end, and along the side is provided with thread milling surfaces. In operation, the tool is rotated and shifted axially to bring its drill point against the workpiece, and a hole is bored in the workpiece. Then the axis of rotation of the tool is shifted laterally by a distance equal to at least the depth of the thread to be milled to move the thread milling surfaces into contact with the wall of the hole and position the milling surfaces to effect the thread milling operation. The axis of rotation of the tool is then moved orbitally 360 degrees around the wall of the hole while the tool is simultaneously shifted axially one thread pitch. This serves to mill threads in the wall of the hole. The axis of rotation is then shifted back to the center of the hole and the tool is retracted from the hole. In a preferred embodiment the tool is rotated at a high speed viz., in excess of 15,000 R.P.M. and preferably on the order of 30,000 R.P.M. The drill point and thread milling faces may be formed of various materials. For example, when thread mill drilling plastic compositions such as composites of KEVLAR (TM), glass-fibres, carbon fibres or the like in a binder of epoxy resin or the like, the drilling and milling faces are preferably formed of polycrystalline diamond or polycrystalline cubic boron nitride. For thread mill drilling in ferrous materials, a ceramic surface may be provided, while in thread mill drilling cast iron, silicon nitride or carbide may be used. The thread milling drill tool is mounted in a chuck carried by a high speed spindle which is supported for axial shifting toward and away from a workpiece and for lateral displacement and orbital movement to effect the thread milling. In a production environment the spindle is preferably part of a thread milling drill head which is supported in a machine tool for positioning a relation to a succession of workpieces that are presented to the head for boring and thread milling. Movements of the chuck carried by the thread milling drill head are preferably under computer control. During the thread mill drilling operation the hole in the workpiece is simultaneously purged so that the waste materials being removed do not clog the hole and impede operation of the tool. For this purpose, fluid is introduced into the hole to displace the waste axially along the tool and out of the hole. In the disclosed embodiment, the thread and mill drill is provided with fluid passageway means for delivering fluid under pressure to the hole adjacent the cutting surfaces and as the fluid escapes from the hole it carries the fines or waste with it. The tool may be provided with turbine blade means arranged to closely overlie the opening of the hole around the tool for creating a partial vacuum at the mouth of the hole to induce the fluid pressure flow and remove the fines from the hole. Description of the Drawings Fig. 1 is a schematic vertical sectional view through a thread milling drill head embodying my invention showing a thread milling drill tool therein with a workpiece therebelow which has been provided with a threaded bore through the use of the apparatus; Fig. 2 is a cross sectional view taken on the line 2-2 of Fig. 1; Fig. 3 is a vertical elevation of the thread milling drill tool as it is drilling a hole in a workpiece; Fig. 4 shows the thread milling drill tool retracted slightly from the bottom of the hole preparatory to the thread milling operation; Fig. 5 shows the thread milling drill tool of Figs. 3 and 4 displaced laterally by the thread depth of the thread milling profile preparatory to the thread milling operation; Fig. 6 shows the thread milling drill tool of Figs. 3 and 4 during the thread milling operation; Fig. 7 is a side elevation of a thread milling drill tool usable in the thread milling drill method; Fig. 8 is a cross sectional view taken on a line 8-8 of Fig. 7; Fig. 9 is an end view of the thread milling drill tool of Fig. 7 looking at the drill point in the direction of arrow 9 in Fig. 7; Figs. 10 and 11 are enlarged fragmentary views through the thread milling faces of the thread milling drill tool of Fig. 7 taken substantially in the areas 10 and 11 of Fig. 7; Fig. 12 is a side view of a thread mill drill having a fluid passageway therethrough for introducing fluid pressure into the hole to carry the fines therefrom; Fig. 13 is an end view of the top of the thread mill drill shown in Fig. 12; Fig. 14 is an end view of the bottom of the thread mill drill shown in Fig. 12; Fig. 15 is a cross-sectional view taken on line 15-15 of Fig. 13; Figs. 16-21 show the steps in thread mill drilling utilizing the tool of Fig. 12. Brief Description of Preferred Embodiment As shown in Fig. 1 a threaded hole 20 in a workpiece 22 is formed by a tool 24 which I have termed a thread milling drill. This tool is mounted in a thread milling drill head 26 having the capability of: (1) Rotating the tool at a high frequency such as 15,000 RPM or greater; (2) Shifting the tool axially toward the workpiece 22 to drill a hole therein; (3) Displacing the center of rotation of the tool laterally the depth of the threads to be milled; (4) Moving the tool 24 in an orbital path through 360 degrees while simultaneously moving it axially one-thread pitch thereby milling threads in the hole in the workpiece; and (5) Displacing the center of rotation of the tool back to the center of the hole and retracting the tool therefrom. The aforesaid movements of the tool 24 as carried by the thread milling drill head 26 are made possible by mounting the tool 24 in a chuck 28 carried by a high-speed spindle 30. Such a spindle is of the type made by Fortuna Werke Maschinenfabrik GmbH, and sold in the U.S.A. by Russell T. Gilman, Inc., 624 Beech Street, Geofton, WI. Other spindle manufacturers are: Bryant Grinder Corporation, Springfield, VT; The Precise Corporation, 3715 Blue River Ave., Racine, WI. The spindle carries the chuck and the tool 24 so that the later projects from an end of the head 26 as shown in Fig. 1. Spindle speed should be at least 15,000 R.P.M. and preferably on the order of 30,000 R.P.M. Various arrangements for supporting the spindle may be provided and I have shown schematically one arrangement that will effect the requisite movements. Other arrange ments will occur to those skilled in the art. In the schematic arrangement shown, the thread milling drill head 26 includes a housing 32 within which the spindle 30 is mounted. The spindle is carried by an outer orbital ring 34 having a cylindrical periphery 35 carried by the inner cylindrical surface 38 of the housing 32. The outer orbital ring has an internal periphery 40 eccentric to its outer periphery 36 as best shown in Fig. 2. A ring gear 42 on the outer orbital ring meshes with a pinion gear 44 on the drive shaft of an electric positioning motor 46 which is secured to the housing 32 whereby operation of the motor will drive the outer orbital ring 34 either clockwise or counter clockwise as desired. The positioning motor 46 may be of the kind manufactured by Portescap U.S., 2550 Eisenhower Avenue, Valley Forge, PA under model NO. P-530. In lieu of meshing ring and pinion gears, timing belt type drives between the motor and ring 34 may be used. Within the eccentric inner periphery 40 of the orbital ring 34 is mounted a spindle displacement ring 48, having a spindle receiving opening 49 which is eccentric to its outer surface 50 and in which is mounted the high-frequency spindle 30. The spindle 30 is mounted in a position eccentric to the outer periphery 50 of the spindle displacement ring so that when he displacement ring is rotated within the orbital ring 34 to the position shown in Fig. 2 the center or axis of rotation 52 of the spindle is coincident with the center or axis of rotation of the orbital ring 34, i.e., coincident with the center of the outer cylindrical surface 36. A second positioning motor 54 is mounted on the spindle displacement ring 48 and is provided with a pinion 56 which meshes with an internal ring gear 58 on the outer orbital ring 34. The positioning motor 54 may be of the same type as the motor 46. Upon operation of motor 54, the spindles displacement ring is rotated within the orbital ring 34 to displace laterally the axis of rotation 52 of the spindle 30. Such displacement serves to shift the tool 24 laterally. It will be appreciated that such displacement may occur while the tool 24 is rotating at the high speed provided by the spindle 30. Thus, the tool 24 may be displaced laterally upon operation of the motor 54 the depth of the thread to be formed. Upon completing the displacement of the tool 24 laterally to the depth of the thread to be formed, energization of the motor 46 will cause rotation of the orbital ring 34 whereby the axis 52 of the spindle and in turn the tool 24 may be rotated through an orbit of 360 degrees to carry the tool 24 around the inside of the bore 20. The radius of such orbit is, of course, the amount of lateral displacement of the axis 52 of the spindle from the axis of the hole. In practice the hole 20 will be drilled with the spindle axis centered on the center of the outer cylindrical periphery 35 of the outer orbital ring, and then the spindle axis will be displace laterally from this position for the thread milling operation. Simultaneously with the orbital movement of the spindle and tool, it is necessary to shift the tool axially one-thread pitch in order to mill helical threads in the bore 20. Such axial movement of the spindle is accomplished by the positioning motor 60 connected to the housing 32 and having a threaded shaft 62 which engages with the spindle as schematically shown in Fig. 1. Upon energization of motor 60 to turn its shaft in one direction the spindle 30 may be shifted axially outwardly of the housing 32, or upon reverse energization of motor 60 the spindle may be retracted away from the workpiece. The housing 32 of the thread milling drill head 26 may be mounted as a unit in a suitable machine tool capable of moving the head into position to operate on a succession of workpieces, such as in a production environment. Such machine tool is not shown as it may be of various configurations which will be apparent to one of ordinary skill in the art. The positioning motors 46, 54 and 60 are desirably under the control of a computer programmed to carry out the motions of the tool 24 as herein described and more particularly as follows. Referring to Figs. 3-6 inclusive, the tool 24 rotating at a high frequency is fed axially along its axis of rotation by motor 60 to bring the drill point end 64 against the workpiece 22 and as the tool continues to move toward the workpiece the drill point will drill a smooth wall cylindrical hole 66 therein whose diameter corresponds substantially to the root diameter of the threads to be formed in the hole. Thus, the first operation performed by tool 24 is to drill the hole 66 in the workpiece. The second step, which may be optional in certain cases, is to retract the tool 24 at least one-thread pitch away from the bottom of the hole and such is shown after the retraction has occurred in Fig. 4 where the point 64 is shown spaced from the bottom 67 of the hole by one-thread pitch. Whether this step is needed may depend upon the speed of rotation of the thread milling drill, the type of material of the workpiece, and other factors dependent upon the particular situation at hand. The third step in the thread mill drilling method is to displace the center of rotation of the tool 24 laterally a distance equal to the depth of the thread to be formed. Such is carried out while the tool 24 is rotating at high speed and following such displacement the position of the tool relative to the bore 66 is as shown in Fig. 5 where the tool has been shown displaced to the left such that the milling surfaces of the thread profile are embedded in the wall of the bore 66 to commence formation of the thread profile in the bore. The next step in the thread mill drilling method is to cause the tool 24 to orbit through 360 degrees around the inside of the bore 66 following a path in which the center of rotation of the tool 24 follows an orbital path whose radius from the center of the hole is equal to the displacement effected in Fig. 5. If it is desired to form helical threads in the bore, it is necessary simultaneously with such orbiting movement to shift the tool 24 axially a distance equal to the thread pitch. Such shifting is carried out at a uniform rate corresponding to the orbital movement of the tool 24 so that the helix angle of the thread is uniform around its periphery. In Fig. 6 I have shown tool 24 having completed substantially 180 degrees of its orbital movement and it have been shifted axially through substantially one-half of a thread pitch toward the bottom 66 of the hole. If it is desired to form annular threads (without a helix angle), the thread milling drill is not shifted axially during its orbital movement. After the tool 24 has completed a 360 degree orbit it is displaced laterally to bring its center of rotation back into coincidence with the center line of the hole in the workpiece and then the tool is retracted from the hole leaving the threads formed therein. It is contemplated that the entire operation of drilling the hole and milling the threads and retracting the tool from the hole may be a matter of but a few seconds, or a fraction thereof. As will be appreciated by those of ordinary skill in the art, the control of the thread mill drill head to carry out the foregoing movements may be readily placed under the control of a programmable computer whereby feed rates, lateral displacement, depth of hole, and the like may be varied in accordance with the size of the screw thread to be formed and the material of the workpiece. The thread milling drill 24 is shown in greater detail in Figs. 7-11. The tool comprises a shank 68 having a chuck engaging portion 70 at one end for reception in the chuck 28 for mounting the tool in the thread milling drill head. At the opposite end of the tool is the drill point 64 having cutting edges 72 and 74 and lower trailing edges 76 and 78. The side of the shank is provided with a thread profile comprising a plurality of flutes arranged around the axis of the shank in a symmetrical configuration. In the embodiment shown two such flutes are provided at 80 and 82. Each of the flutes has a thread profile which may be of any desired thread configuration. In the embodiment shown the profile is that of a thread-locking form described in U.S. Patent No. 4,023,914 and marketed under the trademark SPIRALOCK®. The teaching of Patent No. 4,023,914 is incorporated herein by reference. Each of the flutes is preferably provided with cutting or milling faces 84 and 86 made of a material harder than that of the shank. In a preferred embodiment the surfaces are formed of a composite of natural or synthetic polycrystalline diamond and or polycrystalline cubic boron nitride. Such will be particularly satisfactory for use in boring and thread milling composite workpieces formed of carbon fibres, KEVLAR®, glass fibres and the like in various resin binders such as epoxy or the like resins, or in any other workpiece material which is particularly abrasive. Polycrystalline cutting surfaces suitable for this purpose may be of the type described in U.S. Patents 4,241,135; 4,242,106; and 4,247,304 which are incorporated herein by reference, but the composite is not limited to these teachings. For drilling and thread milling cast iron, the work surfaces of the thread milling drill are preferably faced with a silicon nitride or carbide, and when drilling and thread forming in ferrous materials such as steel, a ceramic surface may be used. Polycrystalline diamond and or cubic boron nitride composite is shown in Fig. 8, 10, and 11 at 88 and may be secured to the flutes in any suitable fashion. For example, but without limitation, the composite may be provided with a metallic backing 90 by which the composite may be secured through a brazing layer 92 to the flute 80. The adhering layer is generally indicated in Fig. 8 at 94. The center of mass of the thread milling drill should lie substantially on its intended axis of rotation to minimize dynamic unbalance at high rotational speeds. It will be understood that whether the thread formed in the workpiece is left-hand or right-hand will depend upon the direction of orbital movement of the thread milling drill and the direction of its axial shifting. For example, a clockwise orbital movement will give a left-hand thread if the thread milling drill is simultaneously shifted axially in one direction, but a right-hand thread if the axial shifting is in the opposite direction. In Figs. 12-15 there is shown a modified form of the thread mill drill designed to induce a fluid flow into the hole within which the tool is working which will entrain the waste or fines created by the tool and carry such out of the mouth of the hole around the tool. To create the fluid flow a longitudinal fluid pressure passageway 100 extends coaxially along the tool from its upper end 102 toward the lower drill point end 64a. Just short of the drill point end passageway 100 divides into branches 104 and 106 which open through the drill point end behind the lower trailing edges 76a and 78a as shown in Figs. 12, 14 and 15. Fluid under pressure is introduced into the passageway 100 at the upper end of the tool and flows down through it to the cutting end 64a and thence passes back up the hole and out of the mouth thereof around the cutter. This fluid flow, in the embodiment shown, is induced by a source of fluid pressure 111 schematically shown in Fig. 12 which is connected by a fluid pressure live 113 connected to the upper end of the passageway 100. A the turbine 110 mounted on the shank portion 68a adjacent the upper end of the cutting portion and just below the chuck engaging portion comprises a series of radially extending blades 112. The blades may be formed on a annulus 114 which is slipped down over the shank portions 68a and 70a and thereafter locked upon the shank of the cutter. The inclination of the blades is such as to induce a partial vacuum at the mouth of the hole within which the cutter is operating and in the embodiment shown the blades are designed to induce a partial vacuum upon clockwise or right hand rotation of the tool. In some instances it may be desirable to completely eliminate the turbine 110 and rely solely upon the introduction of fluid pressure at the upper end of passageway 100, from the source 111 or the turbine 110 may be used in association therewith. Figs. 16-21 show the tool 24a in various stages of movement to effect the thread mill drilling operation. In Fig. 16 the tool is poised above a work piece 22a. The thread milling drill head is not shown in Figs. 16-21 but it will be understood that the tool 24a is carried thereby. In Figs. 17 the tool 24a has been moved axially into the work piece to drill the hole 66a therein. As the turbine 110 approaches the upper surface 116 of the work piece 22a, the air space "S" between turbine and the surface 116 decreases until the turbine blade are disposed closely adjacent the surface and at such point cooperate therewith to effectively form a vacuum chamber adjacent the mouth of the bore 66a whereby a vacuum is created within the bore assisting to induce the fluid flow through the passageway 100, as previously described in conjunction with Figs. 12-15, whereby the fines or waste created by the cutter as it moves into the work piece are carried out of the hole through the mouth thereof around the tool. In Fig. 18, the tool has been retracted one thread pitch. In Fig, 19, the tool has been displaced radially one thread diameter in preparation for its orbital movement around the hole. Following the thread milling the tool is moved back to the center of the hole and withdrawn as shown in Fig. 21 until it reaches its starting position of Fig. 16.

Claims: The method of forming a threaded hole (20) in a workpiece (22) comprising the steps of providing an elongated tool (24) having a drilling face at one end and a thread milling profile along the side; rotating said tool about a longitudinal axis while shifting it axially to carry the drilling face against and into a workpiece to form a hole (20) therein and after forming said hole, rotating the tool for milling threads in the wall (66) thereof, characterised in that after forming the hole, the thread milling profile is within the hole and it is not in cutting engagement with the wall (66) of the hole, and that while rotating the tool (24), the axis of rotation is radially displaced a distance equal to the thread depth to be formed and, while rotating the tool (24), its axis of rotation is orbitally moved around the hole to mill threads in the wall (66) thereof whereupon the axis of rotation is radially displaced back to the center of the hole and the tool is retracted from the hole. The method defined by claim 1 wherein during orbital movement of the axis of rotation of the tool it is shifted axially at least one thread pitch to mill helical threads in the wall (66) of the hole. The method defined by claim 2 including the step of retracting the tool (24) from the bottom (67) of the hole at least one thread pitch after forming the hole and before radially displacing the axis of rotation. The method defined by claims 1 to 3 wherein the tool (24) is displaced radially to displace its axis of rotation. The method defined by anyone of the preceeding claims wherein the axis of rotation of the tool (24) is moved orbitally through 360 degrees around the hole. The method defined by claim 1 including the step of introducing a fluid under pressure into the hole adjacent the bottom (67) thereof and causing the fluid to flow along the tool (24) toward the mouth of the hole to scavenge waste material from the hole. The method defined by claim 7 wherein said fluid under pressure is introduced into the hole through the tool (24). The method defined by claim 7 including the step of creating a partial vacuum at the mouth of the hole around the tool (24) to draw fluid pressure introduced at the bottom (67) of the hole outwardly thereof. The method defined by claims 1 to 8 wherein subsequent to forming the hole but prior to displacing the tool (24) radially a distance equal to the thread depth to be formed, the tool is retracted from the bottom (67) of the hole a distance equal to at least one-thread pitch. The method defined by claims 1 to 8 wherein during said orbital movement of the tool (24) it is axially shifted in a direction toward the bottom (67) of the hole one-thread pitch. The method defined by claims 1 to 8 wherein during said orbital movement of the tool (24) it is axially shifted in a direction out of the hole one-thread pitch. A thread mill drill tool comprising an elongated shank (68) adapted to be engaged at one end for rotation and manipulation and having hole making means (64) at the other end for drilling a hole (20) as the tool (24) is rotated and shifted longitudinally into a workpiece (22), at least one longitudinally extending flute (80,82) on the shank, and thread milling faces (84, 86) on the flute having a thread profile for milling the thread to be formed in the workpiece characterized in that the maximum diameter of the thread milling faces (84, 86) on the elongated shank is no greater than the maximum diameter of the hole making means (64) associated therewith so that the tool having both the thread milling means and the hole making means may be moved into the workpiece to drill the hole and that the thread milling means can enter the hole while the hole is being drilled without being in cutting engagement with the wall of the hole. The tool defined by claim 12 wherein the thread milling faces (84,86) and drill point (64) are provided with a surface including materials selected from the group consisting of polycrystalline diamond, polycrystalline cubic boron nitride, ceramics and silicon nitride. The tool defined by claim 12 wherein means are provided defining a fluid passageway (100) extending axially of the elongated shank (68) for communication at said one end with a source of fluid pressure and opening outwardly of the shank (68) at a point to deliver fluid pressure to the workpiece (22) to scavenge waste material from a hole (20) within which the tool (24) is operating. The tool defined by claim 14 wherein said passageway means (100) opens through said other end of the shank (68) for delivering fluid pressure adjacent the drill point (64) into a hole in a workpiece. The tool defined by claim 14 or 15 wherein blade means (110,112) are provided on the shank for creating a partial vacuum at the mouth of a hole within which the tool is spinning. The tool defined by claim 14 or claim 15 wherein a plurality of blades (112) extend radially from the shank between said one end and said flute and inclined to the axis of the shank to create a vacuum at the mouth of a hole within which the tool is spinning.

Inventors: PALM, EDWARD B.
Applicants: TURCHAN, MANUEL, C.
Application year: 1987
Main classification: B23G5
IPC codes: B23Q11, B23Q1, B23G1, B23G5