MXPA00008347A - Multi-pass grinding method - Google Patents

Abstract

A multi-pass grinding method for fabricating an endodontic instrument. A tapered rod (10) is sucessively advanced past a rotating grinding wheel (12) to remove layers (14, 16, 18, 20) of material from the rod (10) until attaining a desired flute depth. The method is characterized by relatively shallow cutting depths, relatively high feed rates and low grinding wheel wear. The method reduces the process time and operator supervision required for fabricating nickel-titanium endodontic instruments. A further process is disclosed for automatically compensating for wear on the grinding wheel (12) so that periodic recalibration of the grinding wheel (12) may be eliminated or reduced.

Description

METHOD OF RECTIFICATION OF MULTIPLE PASSES FIELD OF THE INVENTION This invention relates generally to an improved method for manufacturing endodontic files and reamers, and, more particularly, to an improved multi-pass rectification method to form channels and associated cutting edges on a tapered rod of Nickel and titanium alloy material.

BACKGROUND OF THE INVENTION As part of a conventional root canal procedure, the deteriorated material is removed from the root canal and the conduits themselves are preformed before a filling material is inserted into the conduit. The crown of the tooth is removed to provide access to the root canal. Specialized endodontic instruments such as files and reamers are used during root canal therapy to clean and form the root canal. These instruments are typically very flexible files or reamers that are rotated and / or manually oscillated by the dentist in the root canal. The dentist begins with very small files and proceeds with increasingly large files until the canal is properly formed and cleaned. After completing this stage of root canal therapy, the tooth is typically filled with an inert filler material, such as gutta-percha, followed by cement. Then a crown can be placed on the tooth. In the past, such endodontic instruments were made with tapered stainless steel rods which were grooved along a working portion thereof to form helical channels, each flanked by one or more cutting edges. Channels on instruments are usually formed in one of two ways. A first method involves twisting a rod of a particular cross section, such as triangular or rectangular, so that the edges of the rod form spirals along the length of the rod. These spiral edges act like the sharp edges of the instrument. Another method is to pass the tapered rod under a grinding wheel and simultaneously rotate and translate the rod to form one or more continuous channels along the length of the rod. The rod can be indexed and rotated and the process repeated to form additional channels separated from each other at a predetermined angle, as desired.

The present invention relates to an improved rectification method particularly suitable for forming channels in tapered rods comprising nickel-titanium alloys (Nitinol ™) and / or the like. These alloys have a superior bending and resistance to fracture when compared to conventional stainless steel alloys and, therefore, have found favorable application in the endodontic field and, particularly, endodontic files and reamers. See, for example, "An Initial Investigation of Bending and File Torsion Properties for the Nitinol Radicular Canal, Journal of Endodontics, Vol. 14, No. 7, at 346-51 (July 1988). Instruments can more easily follow the curved and / or convoluted contours of the tooth root canal system, and are probably less prone than stainless steel to break when placed under stress.These advantages allow faster and more efficient root canal procedures with less chance of damage to the root canal wall than with stainless steel instruments The peculiar properties of the material and the superplastic nature of the nickel-titanium alloys make it a particularly difficult material to machine using conventional grinding techniques. For example, U.S. Patent No. 5,464,362 to Heath et al., Describes some of the difficulties encountered. They are used in the formation of a muted endodontic instrument from nickel-titanium alloys using conventional rectification techniques. To alleviate some of these difficulties, Heath describes a method of rectification which uses a reduced feed rate of approximately 3-8 inches per minute. (7.62-20.32 centimeters per minute) and a reduced grinding wheel speed of no more than 3000 surface feet per minute (914.4 surface meters per minute) to achieve instruments of acceptable quality. However, Heath's method is slow and inefficient and requires turning off and redirecting the grinding wheel at relatively frequent intervals to remove the build-up of nickel-titanium material on the wheel to reform and / or recalibrate the wheel. These operations are slow and labor intensive and, therefore, undesirable for the manufacture of high production nickel-titanium endodontic instruments.

BRIEF DESCRIPTION OF THE INVENTION The present invention is directed to a high speed, multiple pass rectification method for manufacturing files and reamers and / or other instruments from tapered nickel-titanium rods or other material. The preferred method produces instruments of acceptable quality and acrylic efficiency, while reducing manufacturing time, wheel wear, and frequency of redirection of the required wheel. Using the multi-pass rectification method of the present invention, high-quality nickel-titanium endodontic instruments can be manufactured more quickly, with improved manufacturing tolerances and reduced operator supervision and maintenance. The invention also provides, in one embodiment, a method and apparatus for automatically calibrating the grinding wheel as it wears to maintain the desired manufacturing tolerances during extended production periods and without operator intervention. In one embodiment, the present invention provides an improved rectification method in which the channels, and the associated cutting edges, are formed in a tapered nickel-titanium alloy rod and by a multi-pass rectification operation. The tapered rod is subjected to successive passes of a grinding wheel which removes more and more material in each pass, until the final desired channels are formed. Because the depth of the channels typically varies along the length of the tapered instrument, a multi-pass rectification operation allows the forward speed of each pass to be optimized according to the depth of cut for the rectification pass. particular. This is a significant advantage over conventional grinding techniques that use only a single pass, of the grinding wheel at a constant forward speed. This is because in a single-pass system, the feed rate must be slow enough to effectively remove the material in the deeper portions of the channel, even though a higher feed rate could be used to remove material from smaller portions. deep of the channel. The present invention overcomes this limitation by using multiple pitch rectification and variable feed rates. Another advantage of the multi-pass rectification method of the present invention is that the depth of cut and the feed rate for each grinding pass can be optimized to impart minimal wear on the grinding wheel. In contrast, a one-pass rectification method generates wear on the grinding wheel at a relatively higher speed because the depth of the cut is always at a maximum (at least on a portion of the instrument) to remove all the material in a just pass. When the edge of the grinding wheel wears under these conditions, it becomes wider or flatter and, in this way, the channels become wider and shallower. Conventionally, when this occurs an operator must reform or "redirect" the edge of the grinding wheel and calibrate it to maintain manufacturing tolerances. The method of the present invention, however, adjusts the depth of cut for each pass to minimize wheel wear., while maintaining a high production speed. Accordingly, the method of the present invention significantly reduces the amount of downtime and specialized work required to verify and maintain manufacturing operations. In another embodiment, the present invention provides a method and apparatus for automated recalibration of the grinding wheel when it wears. Conventional grinding operations require recalibration and periodic adjustment of the grinding wheel to compensate for the gradual wear of the grinding wheel and the resulting reduction in wheel diameter. This requires the deactivation and temporary interruption of the manufacturing process and increase the costs of verification and supervision of the entire manufacturing process. According to another method of the present invention, however, the detection and / or automatic adjustment of the position of the grinding wheel in relation to the workpiece (ie the tapered rod) compensates for the gradual wear of the grinding wheel. This reduces the amount of manufacturing time and operator supervision required, which is especially advantageous for long production times. According to another embodiment, the present invention provides an adjustable head that holds the grinding wheel, allowing the angle between the axis of the grinding wheel and the shaft of the rod to vary. This allows the additional correction of wear on the grinding wheel and the ability to independently vary the angle of attack, the depth and width of the channels when the grinding wheel moves along the instrument. Those and other features and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiment with reference to the accompanying drawings, the invention is not, however, limited to any embodiment particular preferred.

BRIEF DESCRIPTION OF THE DRAWINGS Figure IA is an elevation view of a rod and a grinding wheel, schematically showing the trajectories and different starting points of the grinding wheel on consecutive passes on the rod; Figure IB is a view similar to that of Figure IA, but illustrating the grinding wheel having a common starting point for each pass; Figure 2A is a front view of a type of file being grinded by a wheel according to the invention; Figure 2B is a top view of the file and the grinding wheel shown in Figure 2A; Figure 3A-1 shows an enlarged view of a cutting edge of a non-worn grinding wheel; Figure 3A schematically illustrates a cross section of the rod formed with the grinding wheel of Figure 3A-1; Figure 3B-1 shows an enlarged view of a cutting edge of a worn grinding wheel; Figure 3B schematically illustrates a cross-section of the rod formed with the grinding wheel of Figure 3B-1; Figure 4 is an elevation view of a rectified file according to the invention; Figure 4A shows a cross section of the distal end of the rectified rod shown in Figure 4A; and Figure 4B shows a cross section of the proximal end of the rectified rod shown in Figure 4.

DESCRIPTION OF THE PREFERRED MODALITIES Figures IA, 2A and 2B illustrate a multi-pass rectification method according to the present invention. A rod 10 is passed over a grinding wheel 12 to remove a first layer 14, a second layer 16, a third layer 18 and a fourth layer 20, as shown in Figure 1. The grinding wheel is shown in its second pass where it will remove the second layer 16, forming a channel. The rod 10 generally comprises a body 22, a distal end 24, and a working portion 26. The working portion 26 extends from a proximal end 28 adjacent the base of the body 22 to the distal end 24. The rod 10 can rotated along a longitudinal axis 30. The body 22 may also contain calibrated depth marks 32, a handle 34, grooves or slots for a tool sleeve (not shown) to accommodate for manual handling or use with a handpiece motorized, as desired. The rod 10 may be of any cross-sectional shape, although a circular cross section is preferred. The rod 10 may or may not be tapered, although a taper 40 is preferred along the work portion 26, as shown in Figure IA in exaggerated form. The rod 10 may be composed of titanium alloy or stainless steel alloy. Such titanium alloys typically have a titanium content of at least 40%. Nickel-titanium alloys, which are preferred for endodontic work, typically consist of at least 40% titanium and at least 50% nickel. In a preferred embodiment, the alloy consists of 44% titanium and 56% nickel and no appreciable amounts of other materials that could adversely affect the purity required for the endodontic instruments. The dimensions of the different rods formed in endodontic instruments can be conventional. The grinding wheel 12 generally comprises a first side 50, a second side 52, and a beveled side 54. The second side 52 and the beveled side 54 form a corner 56 with an inclined angle of about 32 ° in a preferred embodiment. A cutting edge 58 is formed at the tip of the corner 56. The second side 52 of the grinding wheel 12 and the longitudinal axis 30 of the rod 10 form an upper angle 70. The size of the grain on the grinding wheel 12 can fluctuate from about 200 to about 800, more preferably from about 250 to about 550, and most preferably about 400. The surface of the grinding wheel may be composed of a cubic boron nitride (CBN) material, distributed and manufactured by Norton Superabrasives. A grinding wheel 12 composed of diamond ore has also been shown to be satisfactory. The speed of the grinding wheel 12 can range from about 2000 rpm to about 10,000 rpm and more preferably from about 5,000 rpm to about 8500 rpm. Currently, the preferred speed is 5730 rpm for a six-inch wheel (15.2 centimeters). High-speed rectification has been achieved with favorable results using a Rollomatic 600x 6-axis machine. The Rollomatic 24F3 3-axis machine has also shown satisfactory results, but can not be used with the variation of the upper angle, discussed in more detail below. In a multi-pass rectification method according to the present invention, the grinding wheel 12 first comes into contact with the rod 10 at an entry point of the first pass 60, somewhere between the distal end 24 and the proximal end 28 The path of the cutting edge 58 of the grinding wheel 12 forms a cutting angle 72 with the longitudinal axis 30 of the rod 10. The cutting angle 72 is preferably greater than or equal to about 0 °, to provide a cutting edge with a cutting edge. Neutral or positive attack angle, although the method is not specific for a particular instrument geometry. The depth of the first pass or "cut" 14 is preferably less than the total maximum channel depth, which includes the first cut 14, the second cut 16, the third cut 18, and the fourth cut 20. Although the depth of either of the cuts is not critical, the depth of the cut or final rectification pass (the fourth cut 20 in Figure IA) is, preferably, about 5 microns up to 30 microns deep, and more preferably about 10 microns. microns of depth, to provide a fine surface finish quality of the channels and associated cutting edges. The depth of the other cuts can range from just a few microns more to a depth of approximately 100 microns or more, with approximately 40 microns per cut being the most satisfactory. Figure IA describes a multi-pass rectification method using four passes. The system can also operate successfully with as few as two passes, and as many as ten or more passes. More preferably, between about three to five grinding passes will be most suitable for most applications. Shaped rod shaped instruments of smaller diameters will generally require fewer grinding passes than those formed from rods of larger diameters. As it moves between the distal end 24 and the entry point of the first pass 60, the grinding wheel 12 can move at a fast forward speed, such as 1800 mm / min (approximately 72"/ min). contacting the rod 10 at the entry point of the first pass 60, the advance speed preferably decreases at a suitable speed to cut the material.The speed of advancement of the rectification, which fluctuates from about .5"/ min ( 12.7 mm / min) to about 35"/ min (889 mm / min), and more preferably from about 10" / min (254 mm / min) to about 15"/ min (381 mm / min), and from more preferably about 12"/ min (approximately 300 mm / min) provides satisfactory results. This advancement of the grinding may vary while the grinding wheel 12 is removing material along a pass or from a pass to another pass, but more preferably it remains constant while the grinding wheel 12 is removing the material from the rod. 10. After the first cut 14, the grinding wheel 12 can now move at a faster forward speed towards the entry point of the second cut 62 and begin to remove the second layer of material 16 at a selected grinding feed rate. The process is repeated until the depth of the desired channel is reached. A third entry point of the third layer 64 and an entry point of the fourth layer 64 are located at the distal end 24. Employing this multiple-pass system, significant savings in time and materials are achieved. This is possible because the single pass system must operate at a grinding feed rate from the distal end 24 to the proximal end 28, since that speed must be relatively slow because the single pass system removes a maximum amount of material at a proximal end 28. A multi-pass grinding method can use faster grinding feed rates because less material is removed per pass. It can also use even faster forward speeds when it is not removing material (or removing only a minimal amount of material) from the rod, such as after the initial pass and before the second pass. A four pass system with a grinding feed rate of 12"/ min (304 mm / min) has approximately a 20% saving in time over a single pass system operating at 3" / min (0.76 mm) / min). Further, when the final layer in a multiple pass system, the fourth layer 20 in Figure 1A, remains relatively thin, an improved surface finish is obtained. Referring now to Figure IB, the multiple pass method of this invention can also be be practiced starting from roughly common starting points at the distal end 24. In Figure IB, similar numbers refer to similar structures between Figures IA and IB, and numbers that have raw marks (') refer to modified elements of agreement. to this alternative principle. The method will be essentially the same as described above, except that the layers 14 ', 16', 18 'and 20' will each be rectified with the wheel 12 starting at points 60 ', 62', 64, 66. It will be appreciated that the points 60 'and 62' have been moved distally from the respective points 60, 62 shown in Figure IA. Those points can vary in depth. Also in accordance with this invention, the grinding passes may vary in the current forward speed through their entire length. For example, the forward speed at the start of a rectification pass of a channel may be 10"per minute (254 mm per minute) or greater and the forward speed through the remainder of the rectification pass may be progressively greater, often exceeding 12"per minute (304 mm per minute) but preferably not exceeding 30" per minute (762 mm per minute). l Referring now to Figures 3A, 3A-1, 3B and 3B-1, the multi-pass system not only saves manufacturing time and costs, but also reduces wear on the grinding wheel 12. The non-worn cutting edges 58 of the grinding wheel 12 produces a channel 100 on the rod rod 10, as illustrated in Figure 3A. Channel 100 must be maintained within specifications to produce instruments of acceptable quality and clinical efficiency. The width "a" of the channel and the depth "b" of the channel, among other physical properties, can be measured to appreciate the acceptance of the shapes and dimensions of the channels. Figure 3B-1 describes a worn cutting edge 58 'and a cross section of its corresponding rod 10 and channel 100'. As illustrated, a worn cutting edge 58 'will produce a shallower and more flat channel, so that a' is greater than a and b 'is smaller than b. Eventually, the worn edge 58 'will prevent the channels 100' from satisfying the specifications. The manufacturing process will then have to be interrupted, so that an operator can reform the cutting edge 58 and recalibrate the grinding wheel 12 by moving the cutting edge 58 'a distance "c" in the direction of the arrow 59 in Figure 3B- 1, so as to produce a channel 100 of appropriate dimensions. The multiple pass system does not need this recalibration because the grinding wheel 12 wears more slowly in a multiple pass system as compared to a single pass system. According to what has been described, the need to stop the manufacturing process for the recalibration of the wheel increases the process time and the need for operator supervision. Therefore, any increase in the period of time between recalibrations increases the efficiency of the process. This is still the case if a single pass or multiple passes system is used. The present invention, in one embodiment, increases the time between the recalibrations required significantly by automatically feeding the grinding wheel 12 and its cutting edge 58 towards the rod 10 as the edge 58 wears. Following the wear of the cutting edge 58, it is possible adjust the location of the cutting edge 58 to take the wear into account. By repeatedly adjusting the location of the cutting edge 58 as it wears out, the channel 100 remains within these specifications for a much longer period of time. Even after making 50 parts, the grinding wheel 12 used according to the invention will produce a channel 100 within the specifications. The advancement is achieved by programming a computer (not shown) that will move the grinding wheel 12 along an axis parallel to its second side 52, so that the upper angle 70 remains constant. The present invention in another embodiment is also capable of adjusting the upper angle 70 shown, for example, in Figure IA. This process is similar to the progress process discussed above. Except that in this case the upper angle 70 is altered to correct the wear on the cutting edge 58, instead of the distance between the axes of the grinding wheel 12 and the rod 10. The resulting decrease in time due to angle correction The upper one keeps the channels 100 more symmetrical for a longer period of time. Referring now to the Figures? 4, 4A and 4B, the upper angle adjustment 70 can also be used to improve and control other variables of the channels 100. As one skilled in the art will understand, material can be removed from the rod 10 to produce one or a plurality of channels 100. Those channels 100 are helical, and form leading edges 102 that cut the damaged material when the rods 10 are maneuvered in the root canal. Channels 100 also form rear edges 104. As Figures 4A and 4B illustrate, the leading edges 102 form angles of attack which may be 0 °, positive, or negative, as desired. As can be seen from these two figures, the cross section at the distal end 24 has an angle of attack of about 0 ° and the angle of attack at the proximal end 28 is of the order of about 20 degrees. The variation of the angle of attack of the distal end 24 and the proximal end 28 is caused by the fixed upper angle employed in the present systems. The fixed upper angle produces different angles of attack depending on the depth of the cut. By varying the upper angle, the angle of attack can be kept constant if desired, or varied at a controlled rate. The same is true for the thickness of the network. The top angle can be adjusted with a programmable computer or other devices, as will be readily apparent to those skilled in the art. Although the present invention has been described in the context of certain preferred embodiments, it will be understood by those skilled in the art that the present invention extends beyond the specifically described embodiments to other equivalent and obvious alternative embodiments. In this way, it is intended that the scope of the present invention described herein is not limited by the modalities particularly described herein, but is defined solely by the faithful reading of the following claims, including the full range of equivalents that may be provided by the law. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (10)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A multi-pass rectification method for manufacturing an endodontic instrument used during root canal therapy, the method is characterized in that it comprises: axially moving a metal rod in relation to and against a rotating grinding wheel to rectify a quantity of metallic material on the surface externally of the rod along a path and during a first pass, and axially moving the metal rod in relation to and against the rotating grinding wheel during at least one additional pass through at least a portion of the path to rectify an additional amount of metallic material from the outer surface of the rod to a desired depth.

2. The method according to claim 1, characterized in that the metallic material includes at least about 40% titanium.

3. The method according to claim 1, characterized in that the number of passes made by the grinding wheel is between 3 and 10.

The method according to claim 1, characterized in that the rod moves in relation to the grinding wheel during each pass at an axial feed rate for approximately 10 to 30 inches per minute (254 and 762 millimeters per minute).

5. The method according to claim 1, characterized in that the amount of material removed in each pass is between about 10 microns and 100 microns.

6. The method according to claim 1, characterized in that the amount of material removed in each pass is approximately 40 microns.

The method according to claim 1, characterized in that the speed of the grinding wheel is between approximately 5000 rpm to approximately 8500 rpm.

The method according to claim 1, characterized in that a channel of the desired depth and with a spiral cutting edge is formed along the path.

9. The method according to claim 1, characterized in that each pass starts from approximately the same initial point along the length of the rod. The method according to claim 1, characterized in that the first pass and the additional pass initiate from different points along the length of the rod.