ENDODONTIC INSTRUMENT AND METHOD OF FABRICATING SAME USING COMBINATION OF COLD FORGING AND MACHINING OPERATIONS

Abstract: A method of fabricating an endodontic instrument (10), wherein a cylindrical metal blank (B) is first cold forged in a rotary swaging machine (26) to form a tapered end portion (35). Thereafter, the tapered end portion (35) is machined to form at least one helical flute (21, 22) along its length, with the helical flute (21, 22) defining a cutting edge (24) along each side edge thereof. The combination of the cold forging and machining operations has been found to produce instruments which are stronger than instruments produced by conventional practices. Also, the instruments may be produced more quickly and thus at less expense, and they may be produced without loss of raw material.

Description: ENDODONTIC INSTRUMENT AND METHOD OF FABRICATING SAME USING COMBINATION OF COLD FORGING AND MACHINING OPERATIONS Background of the Invention The present invention relates to a method of fabricating an endodontic instrument adapted for use in performing root canal therapy on teeth. Root canal therapy is a well-known procedure wherein the crown of a diseased tooth is opened so as to permit the canal to be cleaned and then filled. More particularly, a series of very delicate, flexible, finger-held instruments or files are used to clean out and shape the root canal, and each file is manually rotated and reciprocated in the canal by the dentist. Files of increasingly larger diameter are used in sequence, to achieve the desired cleaning and shaping. When the canal is thus prepared, it is solidly filled with a filling material, which typically comprises a waxy, rubbery compound known as gutta percha. In one procedure, the gutta percha is positioned on an instrument called a compactor, and the coated compactor is inserted into the prepared canal and rotated and reciprocated to compact the gutta percha therein. The dentist thereafter fills the tooth above the gutta percha with a protective cement, and lastly, a crown is fitted to the tooth. Endodontic instruments of the described type were originally fabricated by permanently twisting a stainless steel rod of triangular or square cross section. The apices of the triangular or square cross section thus formed cutting edges which spiral along the length of the instrument. More recently, such instruments have been produced by a machining process, and wherein a cylindrical rod of stainless steel or nickel titanium alloy is cut into blanks of about two inches in length, and one end portion of each blank is tapered by machining the blank in a centerless grinding machine. Helical flutes are then machined on the tapered end portion, by moving the blank past a rotating grinding wheel and while the blank is slowly rotated to impart the desired helical configuration to the flutes. A cutting edge is thus formed along each side edge of each flute, and a helical land is preferably formed between the spiral flutes, as illustrated in U.S. Patent No. 4,871,312 to Heath. A machining process as described above and which is particularly suitable for machining nickel titanium alloy is further described in U.S. Patent Nos . 5,464,362 and 5,527,205 to Heath, et al . , the disclosures of which are incorporated herein by reference . Summary of the Invention The above described machining processes for forming both the tapered end portion of the blank and the flutes, produce very satisfactory instruments, which have enjoyed a high degree of commercial acceptance. However, the novel manufacturing process of the present invention has been found to produce several significant and unexpected improvements in both the process and the resulting product. In particular, the manufacturing process of the present invention involves the steps of providing a cylindrical rod- like blank of metallic material and which has a diameter less than about 0.1 inches, and cold forging a conical taper on one end portion of the blank, with the taper defining an included angle of between about 1/2 and 8°. Further, at least one helical flute is then machined on the tapered end portion so as to extend along the length thereof, and such that the helical flute defines a cutting edge along each side edge thereof. In the preferred embodiment, the cold forging step includes positioning the one end portion of the blank coaxially within a plurality of circumferentially arranged forging dies of a rotary swaging machine, then rapidly reciprocating the dies radially against the one end portion of the blank, while rotating the reciprocating dies with respect to the one end portion of the blank. Also, the machining of the flutes includes forming the flutes so as to have a curved concave bottom wall when viewed in transverse cross section, and wherein a helical land is positioned between axially adjacent flute segments. The method of the present invention also preferably includes cold rolling a plurality of axially spaced apart depth indicating calibrations on the blank at a location spaced from the tapered end portion with the calibrations thus being in the form of annular grooves. Further, a handle is mounted on the end of the blank so as to be engageable between the fingers of the user or by a machine driven handpiece. The step of cold forging the tapered end portion on the blank has been found to condense the grain of the metal, rather than stretching out the grain as results in a machining operation. The condensed grain in turn provides an increase in the strength of the product. In addition, the forging operation tends to elongate the rod rather than remove metal, and thus there is a significant savings in raw material cost. Still further, the forging process of the present invention is faster than machining. The step of cold rolling the depth indicating calibrations, as opposed to the conventional grinding of the calibrations in the blank, is also advantageous in that the conventional grinding process weakens the blank and results in a fracture point, whereas the cold rolling process is believed to avoid any weakening of the blank. Brief Description of the Drawings Some of the objects and advantages of the present invention having been stated, others will appear as the description proceeds, when taken in conjunction with the accompanying drawings, in which: Figure 1 is a side elevation view of an endodontic instrument manufactured in accordance with the present invention; Figure 2 is a schematic representation of the working length of the instrument before and after the cold forging operation of the present invention; Figure 3 is a perspective view of a rotary swaging or cold forging machine suitable for performing the cold forging operation of the present invention; Figure 4 is a perspective view of one of the dies of the rotary swaging machine shown in Figure 3 ; Figure 5 is a schematic side elevation view of a machining apparatus suitable for forming the flutes in the instrument in accordance with the present invention; Figure 6 is an enlarged side elevation view of the lower end portion of the instrument shown in Figure 1 ; Figure 7 is a transverse sectional view taken substantially along the line 7-7 of Figure 6; Figure 8 is a fragmentary perspective view of the portion of the instrument which includes the depth indicating calibrations in accordance with the present invention; Figure 9 is a fragmentary sectional view of one of the calibrations and taken along the line 9-9 in Figure 8 ; Figure 10 is a schematic view of an apparatus for cold rolling the calibrations in the instruments; and Figure 11 is a fragmentary sectional view taken along the line 11-11 of Figure 10. Detailed Description of the Preferred Embodiments Referring more particularly to Figures 1 and 6, an endodontic instrument 10 is illustrated which comprises a shank 12 which is preferably composed of stainless steel or a nickel titanium alloy as further described below. The shank 12 typically has a length of about 30 mm (1.2 inches), and it includes an outer or proximate end which mounts a conventional handle 14. The illustrated handle is of a configuration which is intended to be gripped between the fingers of the user, but the handle may alternatively be configured for engagement by a machine driven handpiece as known in the art . The portion of the shank immediately below the handle is cylindrical and has a diameter of between about 0.5 and 1.6 mm (0.02 and 0.1 inches), and this shank portion includes depth indicating calibrations 15 as further described below. The shank further includes an opposite distal or pilot end 16, and a working length 18 is defined adjacent the pilot end 16. The working length is slightly tapered toward the pilot end 16 at an included angle of between about one half and eight degrees, preferably about one degree. The working length 18 may have a length of about 2 mm (0.08 inches) up to the full length of the shank 12, i.e. about 30 mm (1.2 inches) . However, the working length 18 preferably has a length sufficient to extend substantially the full depth of a tooth root canal, which typically is about 16 mm (0.63 inches). The peripheral surface of the working length 18 includes two continuous helical flutes 21, 22 formed therein. The flutes have an arcuate curvature as best seen in Figure 7, and so as to define a curved concave bottom wall 23 and a cutting edge 24 along each side edge thereof. Also, the flutes have a pitch so as to define helical lands 25 on the outer periphery of the instrument and between axially adjacent flute segments. .An instrument of this general construction is further described in U.S. Patent No. 4,871,312 to Heath, and U.S. Patent No. 5,106,298. The novel method of the present invention for producing the endodontic instrument as described above will now be described. The method starts with a length of drawn wire of suitable metallic material, such as stainless steel or a nickel titanium alloy. A particularly suitable alloy comprises at least about 40% titanium and at least about 50% nickel and as further described in Patent No. 5,464,362. The wire has a diameter of about 0.1 inches or less, and it is cut into blanks of about two inches in length by a conventional cutting operation. A conical taper is then cold forged on one end portion of the blank to define an included angle of between about 1/2 and 8°, as described above . A rotary swaging machine suitable for cold forging the taper on the blanks is generally illustrated at 26 in Figures 3 and 4. In particular, the machine 26 includes an outer tubular frame 27, and a rotatable spindle 28 which is coaxially disposed within the tubular frame . The end of the spindle is slotted so as to accommodate four radially extending sets of a die 29, a shim 30, and a hammer 31. A roll cage, which includes a plurality of roller bearings 32, surrounds the spindle 28. The opposite end of the spindle is rotated by an electric motor (not shown) , and as the spindle rotates, centrifugal force throws the dies 29, shims 30, and hammers 31 outward against the roll cage. Each time the hammers pass directly under a roller 32, they are driven inward, forcing the dies 29 to close and apply a forging stroke upon the blank B, which is placed coaxially inside the dies in the manner illustrated. As the hammers 31 pass out from under the rollers, the dies are again thrown open, ready for the next forging stroke. The forging strokes may be applied simultaneously by all of the hammers, or they may be applied alternately, depending upon the configuration of the rollers 32 in the roll cage. In the case of blanks formed of nickel titanium alloy, the spindle is preferably rotated at a speed sufficient to impart about 3500 forging strokes from each of the dies per minute, and it typically takes less than a second for the forging operation to be completed. Figure 4 illustrates in more detail the configuration of one of the forging dies 29 of the machine shown in Figure 3. As illustrated, the operative upper surface of the die includes a U-shaped channel 33 along its length, with the channel being of decreasing depth toward the inner end of the die. A rotary swaging machine 26 as illustrated and described above with reference to Figures 3 and 4 is conventional, and a suitable machine is manufactured by Fenn Manufacturing of Newington, Connecticut, as model NF. Figure 2 schematically illustrates the original outline of the end portion of the cylindrical blank which is subjected to the forging operation in dashed lines at 34, and the resulting tapered outline in solid lines at 35. As will be apparent, the forging operation results in an elongation of the blank, indicated by the distance E, and no material is removed and lost as is the case when the taper is formed by the conventional machining operation. Upon completion of the cold forging operation, the tapered end portion of each blank is subjected to a machining operation which forms at least one, and preferably two or more helical flutes 21, 22 along the length thereof. More particularly, each blank B is mounted in a collet 40 at the forward end of an indexing block 42 of a conventional centerless grinding machine, with a work holding fixture 44 positioned to support the forward end of the blank adjacent the periphery of a rotating grinding wheel 46. The block 42 is then advanced so that the blank is axially moved past the rotating grinding wheel 46, while the blank is slowly rotated about its axis. When the blank has advanced past the rotating wheel 46 a distance sufficient to form the first flute 21 along the desired working length on the instrument, the table 48 supporting the indexing block 42, and the fixture 44 is moved laterally, then axially rearwardly, and then laterally back to its original position. Concurrently, the blank iε rotatably indexed about its axis. The angular extent of this blank indexing will depend upon the number of flutes desired on the finished instrument, and where three flutes are to be formed, the rod is indexed 120°. The blank is then again axially advanced while being slowly rotated, and so as to form the second flute 22. The table 48 is then again moved laterally and rearwardly in the manner described above, and the blank is rotatably indexed another 120° . The grinding process is then repeated to form the third flute of the instrument. Where the instrument has only two flutes as illustrated, the rod is indexed 180° between the two machining operations. The outer periphery of the grinding wheel 46 is preferably curved in cross section, as opposed to being flat, and as a result the flutes have a curved concave bottom wall 23 when viewed in transverse cross section, and as seen in Figure 7. Also, the grinding operation results in a sharp cutting edge 24 being formed along each side of the flute, and the helix angle imparted to the flutes is sufficient to form the helical land 25 between the axially adjacent flute segments . A more detailed description of the machining process, and which is particularly suitable for machining nickel -titanium instruments, is disclosed in the above referenced U.S. Patent No. 5,464,362. At the conclusion of the above operations, the calibrated depth markings 15 are formed on each blank, and the handle 14 is attached to the end of the blank which is opposite the working length. Preferably, the calibrations 15 are formed by a cold rolling operation as schematically illustrated in Figures 10 and 11. A modified conventional thread rolling machine 50 may be employed for this operation, and which comprises a rotary cylinder 52 and an arcuate die plate 54 which is fixed so as to underlie a portion of the lower periphery of the cylinder 52. The die plate 54 includes a number of ridges or projections 56 which are perpendicular to the axis of the cylinder, and which form the calibrations 15 as the blanks are fed through the arcuate slot between the cylinder 52 and the die 54 by the rotation of the cylinder 52. The operation is extremely fast, and results in the calibrations being cold rolled in the form of annular grooves of arcuate con iguration as seen in Figure 9. These cold rolled calibrations do not appreciably weaken the instrument, and the formation of fracture points is thereby avoided. In the drawings and specification, there has been set forth preferred embodiments of the invention, and although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims: THAT WHICH IS CLAIMED IS: 1. A method of f bricating an endodontic instrument suitable for extirpating and shaping a root canal during root canal therapy, and comprising the steps of providing a cylindrical rod- like blank of metallic material and which has a diameter less than about 0.1 inches, cold forging a conical taper on one end portion of the blank, with the taper defining an included angle of between about 1/2 and 8°, and machining at least one helical flute so as to extend along the length of the tapered end portion of the blank, and such that the helical flute defines a cutting edge along each side edge thereof. 2. The method as defined in Claim 1 wherein the machining step includes forming the one flute so as to have a curved concave bottom wall when viewed in transverse cross section, and wherein a helical land is positioned between axially adjacent flute segments. 3. The method as defined in Claim 2 wherein the blank is composed of an alloy comprising at least about 40% titanium and at least about 50% nickel. 4. The method as defined in Claim 3 wherein the blank has a length of about two inches . 5. The method as defined in Claim 1 wherein the cold forging step includes positioning the one end portion of the blank coaxially within a plurality of circumferentially arranged forging dies, then rapidly reciprocating the dies radially against the one end portion of the blank, while rotating the reciprocating dies with respect to the one end portion of the blank. 6. The method as defined in Claim 5 wherein the machining step includes axially moving the blank past a rotating grinding wheel, while rotating the blank about its axis, to thereby form the one helical flute on the blank. 7. The method as defined in Claim 6 wherein the machining step includes the further subsequent step of rotatably indexing the blank about a rotational axis of not more than 180° and then repeating the steps of axially moving the blank past a rotating grinding wheel, while rotating the blank about its axis, to thereby form a second helical flute on the blank. 8. The method as defined in Claim 6 wherein the grinding wheel is configured and positioned so that said helical flute defines a curved concave bottom wall when viewed in transverse cross section. 9. The method as defined in Claim 8 wherein the blank is axially moved past the rotating grinding wheel at a speed so as to form a helical land between axially adjacent flute segments. 10. The method as defined in Claim 1 comprising the further steps of forming a plurality of axially spaced apart depth indicating calibrations on the blank at a location spaced from said one end portion . 11. The method as defined in Claim 10 wherein the step of forming depth indicating calibrations on the blank includes forming each of the calibrations by cold rolling an annular groove about the blank. 12. An endodontic instrument suitable for extirpating and shaping a root canal during root canal therapy and which is produced in accordance with the method defined in Claim 1. 13. A method of fabricating an endodontic instrument suitable for extirpating and shaping a root canal during root canal therapy, and comprising the steps of providing a cylindrical rod- like blank which has a length of about two inches and which is composed of an alloy comprising at least about 40% titanium and at least about 50% nickel, and which has a diameter less than about 0.1", cold forging a conical taper on one end portion of the blank, with the taper having a length of about 0.08 inches and defining an included angle of between about 1/2 and 8°, said cold forging step including positioning the one end portion of the blank coaxially within a plurality of circumferentially arranged forging dies, then rapidly reciprocating the dies radially against the one end portion of the blank, and while rotating the reciprocating dies with respect to the one end portion of the blank, and machining at least one helical flute so as to extend along the length of the tapered end portion of the blank, and such that the helical flute defines a cutting edge along each side edge thereof, said machining step including forming the one flute so as to have a curved concave bottom wall when viewed in transverse cross section, and wherein a helical land is positioned between axially adjacent flute segments. 14. The method as defined in Claim 13 wherein the machining step includes axially moving the blank past a rotating grinding wheel, while rotating the blank about its axis, to thereby form the one flute on the blank. 15. The method as defined in Claim 14 comprising the further step of applying a handle to the end of the blank opposite the one end portion, with the handle being configured to be engageable between the fingers of the user or by a machine driven handpiece . 16. The method as defined in Claim 15 comprising the further step of forming a plurality of axially spaced apart depth indicating calibrations on the blank so as to be positioned between the handle and the one end portion of the blank. 17. The method as defined in Claim 16 wherein the step of forming depth indicating calibrations on the blank includes forming each of the calibrations by cold rolling an annular groove about the blank. 18. An endodontic instrument suitable for extirpating and shaping a root canal during root canal therapy and which is produced in accordance with the method defined in Claim 17.

Inventors: HEATH DEREK E, HEATH, DEREK, E.
Applicants: DENTSPLY INT INC, HEATH DEREK E, DENTSPLY INTERNATIONAL, INC., HEATH, DEREK, E.
Application year: 1997
Main classification: A61C5
IPC codes: A61C5, B21K5