CA1049301A - Dental bur - Google Patents

Abstract

Abstract A dental bur having a shank provided on one end with a cutting head having a series of similar teeth extending longitudinally and spaced evenly around the circumference thereof; said teeth each having a flat cutting face extending from the cutting edge radially toward the axis of the head to form one side of said tooth and provide a positive cutting rake therefor, a relative deep chip gash adjacent said face, and a convex relief surface extending from the cutting edge toward said gash adjacent the next succeeding tooth to form the opposite side of each tooth, said convex relief surface permitting said chip gashes to be substantially deeper than such gashes in conventional burs without sacrificing strength of said teeth to resist chipping and breakage of said cutting edges, as well as including the benefit of said flat cutting face which extends radially to provide said positive cutting rake.

Description

This invention relates to dental burs and, is particularly applicable to dental burs of the type in which the cutting head is formed fromcarbide blanks which are fixedly connected to one end of stems or shanks formed from conventional steel for attachment to the chuck of a dental hand-piece. The present invention more specifically pertains to the type of dental burs in which the burs are provided with a plurality of cutting teeth that extend longitudinally of the cutting head, each tooth being provided with a cutting edge spaced circumferentially even distances from adjacent cutting edges, said cutting edges being formed by grinding operations. When forming such cutting heads from carbide, diamond grinding wheels are conventionally used to form the edges by grinding operations.
The invention also pertains to dental burs in which the cutting edges may be either of the so-called straight type, in which the cutting edge is within a plane extending through the axis of the bur, or the cutting edges may be of the helical type. In either situation, the dental burs are provided with a chip-receiving gash or groove adjacent one edge of each tooth and the opposite edge of the tooth is formed by a so-called relief surface which provides clearance immediately to the rear of the cutting edge of each tooth and thus, offer no resistance relative to the sidèwail of a cavity~ for example, which is being prepared in a tooth by the use of said bur.
One of the objectives of burs of this type which presently are in use is to provide a chip-receiving gash or groove having a depth adequate to accommodate chips removed from a tooth in which the bur is operating but, in order to provide a gash or groove of substantial depth, it is necessary at present to sacrifice some of the strength of the cutting edge which is afforded by the relief surface that backs up the cutting edge when rotating in operative direction~ This is due to the fact that, in order to grind a relatively deep chip-receiving gash or groove, it is necessary to form a relatively steep relief surface which results in a sharper cutting edge .,.. . . . . . ; - ~

than otherwise and, particularly wher, the bur is formed from carbide, which is quite brittle, the sharper cutting edge is more readily subject to chipping when encountering hard substances than if the cutting edge were less sharp and a larger angle extended between the opposite faces of the cutting edge.
The foregoing phenomenon is particularly prevelant when carbide burs having helical teeth thereon are ground by means of formed or shaped grinding wheels operating, for example, about a fixed axis while the bur is rotated about its axis when being fed forwardly against the periphery of the grinding wheel suring such axial rotation of the bur to provide the helical outline of the cutting teeth by simultaneously forming the cutting face and relief surface with a single cutter during a single pass along the blank to form each tooth. Under such circumstances, the grinding of cutting teeth on carbide burs has, historically, been achieved by the use of profile diamond grinding wheels of commercial type.
Also in regard to the grinding of carbide burs of the type referred to by the use of profile grinding wheels, especially in regard to making burs having helical teeth extending generally in axial direction of the bur, a certain amount of generation of the curved surface which forms both sides of each tooth is necessary and this generally results in the formation of a cutting face having a negative cutting rake on said surface as distinguished from a positive cutting rake which is more efficient than a negative rake, as well as a relief surface which has a lesser angle between the clear~nce surface and a plane perpendicular to the cutting face at the cutting edge of each tooth.
According to the present invention there is provided a dental bur comprising a shank and a head fixed to one end thereof, said head being provided with a plurality of similar teeth extending longitudinally there-along and having a sturdy sharp cutting edge outermost thereon, each of said teeth having a flat cutting face extending from the cutting edge radially toward the axis of the head, a relief surface which is convex in cross-section extending from said cutting edge partially toward the cutting face of the next succeeding tooth, and a chip-receiving gash having a concave surface in cross-section which extends between said relief surface and said flat cutting face, said relief surface and adjacent edge of the gash adja-cent each tooth meeting along a bluntly rounded ridge extending longitudin-ally of said head circumferentially midway between each pair of adjacent cutting edges of the teeth and radially at a level farther from the root of the g~sh than from the cutting edge of each tooth, thereby to provide teeth on a dental bur which have a positive cutting rake afforded by said flat cutting face and a cutting edge rendered strong by said convex relief surface, in combination with a relatively deep chip gash adjacent said flat cutting face without sacrificing strength for the cutting edge of each tooth.
This arrangement may be employed on straight or tapered tissue burs and on inverted cone burs.
When the teeth are of substantially straight type, i.e., having the cutting edges within a plane extending through the axis of the bur, each tooth may be formed by a profile grinding wheel of the diamond type, for example, or, when the teeth are of the helical type, the chip clearance gash 2Q or groove is formed by a grinding wheel as a first operation, and the relief surface is formed by a second grinding wheel in a second grinding operation, both of said surfaces being generated by the grinding wheel when forming such helical teeth.
It is preferred that the edge of the gash adjacent each flat cutting face engages the cutting face at a position spaced from the cutting edge a distance which is between 20% and 30% of the radial dimension of the head, the optimum percentage being substantially 25% of said radial "'r ~ ' ~ 3 -.^i,~,,~ .
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~04C~391 dimension .
Still further the convex relief surface of each tooth maybe defined in cross-section by means of a radius extending substantially from the root of the chip clearance gash adjacent the diametrically opposite tooth of the cutting head.
Ancillary to the foregoing, the convex relief surface may extend at an angle from each cutting edge wherein the angle between the plane per-pendicular to the flat cutting face of each tooth and a plane tangent to said relief surface at the cutting edge of each tooth is not substantially in excess of 30.
In embodiments in which helical cutting teeth are provided, the helix angle of the teeth relative to the axis of the bur may be substantially between 10and 30, with an op~imum helix angle of substantially 12.
According to another embodiment, the cutting faces of the head, are provided at the ends thereof opposite the shank end, with cutting points defined on one side by end grooves having concave root surfaces and extending radially toward the axis of the head ir alignment with the ends of the chip-receiving gashes between said teeth and said end grooves extending at an acute angle to the cutting edges of the teeth, one sidewall of the end groove between each successive pair of teeth being nearly parallel to an axial plane extending along the axis Qf t~e head and intersecting the flat radial cutting face of each tooth to define a substantially stubby cutting tip at the intersection of said radial cutting face and relief surface of each tooth with said one side-wall of each end groove, and the other sidewall of said end grooves sloping gradually toward the cutting point of the next tooth to provide relief for said cutting points while providing substantial material mass circumferentially rearward of said cutting points relative to the direction of rotation of said bur.

In the accompanying drawings which illustrate an exemplary , :

embodiment of the invention:
Figures 1 ~nd 2 respectively show an outer end elevation and a side elevation of one type of dental bur of the prior art in which the teeth are of a helical type;
Figure 3 is an outer end view of a dental bur comprising the pre-ferred embodiment of the present invention, said view showing the cutting tips at the outer end of the bur, as well as the outline of the cutting teeth which extend longitudinally of the bur;
Figure 4 is a fragmentary elevation comprising a side view of the dental bur illustrated in Figure 3, and in which the longitudinal teeth are of the helical type;
Figure 5 is a transverse sectional view of the embodiment of improved dental bur illustrated in Figures 3 and 4 as seen on the line 5-5 of Figure 4, said sectional view being on a substantially larger scale than employed in Figures 3 and 4 for purposes of showing numerous details of the shape of the cutting teeth of the bur and the surfaces which define the same;
Figure 6 is a fragmentary end elevation of a portion of the outer end of the dental bur shown in Figure 3, as seen on the line 6-6 thereof for purposes of illustrating the details of the cutting tips at the outer end of each cutting blade, the scale employed in Figure 6 being substantial-ly larger than that in Figure 3; and Figure 7 is a view similar to Figure 5 but being diagrammatic for purposes of illustrating details of the manner in which the preferred embodi-ment of the dental bur shown in Figures 3-6 is formed by the preferred processes described hereinafter for grinding said bur from a bur blank.
A typical type of conventional helical dental bur of the carbide type is illustrated in end view, in Figure 1, and in fragmentary side elevation in Figure 2, for purposes of contrasting certain characteristics thereof with the preferred construction of the dental bur comprising the subject matter Of the~prese~ invention, a preferred embodiment of which is illustrated in Figures 3-7. As indicated above, in regard to grinding the helical flutes, grooves or chip-receiving gashes 10 to form the helical cutting edges 12, it is conventional to form said flutes, grooves or gashes by means of using a contoured diamond wheel which is operable about a sub-stantially fixed axis extending transverse to the axis of the bur 14, and the bur is advanced to the grinding wheel while being rotated about the axis of the bur in order to form the helical cutting edges 12. As a result, the substantially concave flutes, grooves or gashes are substantially ogee in end view as is clearly evident from Figure 1, due to said surface being generated by said profiled wheel. As a result, a number of deficiencies are produced, as follows:
Among the foregoing of such deficiencies and objectionable features are the facts that the cutting face 16, which extends along each tooth has a negative rake as distinguished from a positive rake as is clearly apparent from Figure 1. Another deficiency is that the chip-receiving gashes 10 are only of limited depth because, if the same were to be provided with a greater depth, the relief surface 18 which defines the-opposite side of each cutting edge 12 from the cutting face 16 would be at a sharper angle with respect to said cutting faces and thereby weaken the tooth and render the same more prone to chipping. As long as the compound chip-receiving gashes 10 and relief surfaces 18 are generated by a single grinding wheel when making a helical type bur, it is necessary to sacrifice strength in favor of depth of chip-receiving gash or vice versa.
~ I contr~st to the foregoing, Figures 3 to 7 illustrate a bur 20 which, particularly by reference to Figure 5, will be seen to have a chip-recei~ing groove or gash 22 which is of a much greater depth in a radial direction than in the conventional bur shown in Figure 1 and is defined along " .

one side by a radially extending flat cutting face 24 which defines one side of the cutting edge 26 of each tooth 28. The opposite surface of each cutting edge 26 is defined by a relief surface 30 which is convex in cross-section and merges with the adjacent side surface 32 of the chip-receiving gash 22 along a blunt, rounded ridge 34 which extends longitudinally along the head of the bur 20. The preferred location of the ridge 34 is approximately midway between the successive cutting edges 26 but other locations thereof within reasonable limits are possible. Further, it will be seen that the edge of the concave groove or gash 22 which merges with the flat cutting face 24 in no way interferes with the radial dispositi~ of the face 24 of substantial ra-dial dimension which provides a positive cutting rake, as clearly illustrat-ed by the dotted line 36 comprising projections of said cut~ing faces 24 radially inward toward the axis of the head of the bur. -Under circumstances where the present invention is to be applied to a so-called straight bur in which the cut*ing edges 26 of each tooth are disposed within a plane substantially containing the axis of the bur, it is conceivable that the chip-receiving grooves or gashes 22 and the relief surfaces 30 may be formed by a contoured diamond wheel. However, forming the compound surface of 22 and 30 which meet along the blunt rounded ridge 20 34 is best achieved in connection with the forming of helical teeth having helical cutting edges 26. This is due to the fact that the full contour of each tooth 28 readily is formed by generation resulting from the use of a diamond wheel 36, a fragmentary portion of the edge of which is illustrated in Figure 5 relative to the gash 22.
It will be seen that the preferred type of diamond wheel 36 has a flat face 38 and a beveled edge 40 resulting in a sharp peripheral edge 42.
The wheel 36 rotates about an axis extending transverse to the axis of the bur 20 and is fixed relative to the machine which supports and operates it.
In the preferred operation of the machine in which the burs 20 are formed, the blank burs have a cylindrical outer surface 44 on the head thereof, as shown in outline in Figures 5 and 7. In making the initial grinding cut for each tooth, the flat cutting face 24 thereof is continuously formed along one side of each tooth by the sharp peripheral edge 42 of the diamond wheel 36 and the flat face 38 thereof. Rotation of the bur blank about its axis and axial feed is effected by the machine on which it is formed. The helix angle of the teeth with respect to the axis of the bur, as illustrated in Figure 4, is approximately 12 , which is the preferred optimum angle. However, said helix angle may vary within a range of between 10 and 30 within the spirit of the present invention.
As the bur blank is rotated about its axis and axially fed, the concave root surface of each groove or gash 22, as clearly shown best in Figures 5 and 7 in enlarged detail, is generated and said initial cut to form said chip-receiving grooves or gashes 22 is defined on one side by the cutting fac~ 24, shown in Figure 7, and on the opposite side by the slightly convex outline 46, which is best shown in Figure 7. The bur blank is indexed one aliquote space after each initial cut has been made and the next successive initial cut is formed in the blank until all of the initial c~ts have been made by the grinding action of the diamond wheel 36 to form the gashes 22 The bur then preferably is advanced to another station on the grinding machine where another diamond wheel, not shown, but preferably similar to the wheel 36, progressively forms the relief surfaces 30 by re-moving additional amounts of material 48 from the bur blanks, as shown by the cross-hatched illustration thereof in Figure 7. The removal of the additional amounts of material 48 by said diamond wheel preferably is accomplished by the wheel rotating in a direction by which the grinding is accomplished from the cutting edges 26 toward the gashes 22. As a result, this is a sharpening cut and no final or additional finishing operations for the cutting edges 26 are required, thereby minimizing the cost of production of such burs. Such final grind or cut is made preferably by a finer grit diamond grinding wheel to insure against any ragged edge being formed since smooth cutting edges afford the longest life in carbide burs.
The relief surfaces 30 described above comprise an arc generated by a radius extending substantially from the root of the gash of the diametrical-ly opposite tooth on the bur, as indicated by the dotted radius line 50. As also described above, this relief surface joins the side edge of the gash 22, which is opposite the flat cutting face 24, along a blunt, rounded ridge 34.
This provides adequate relief rearward of the cutting edges 26 of the teeth without sacrificing strength, while at the same time providing a chip-receiving gash 22 of substantial depth and, in general, having a much greater radial dimension than any chip-receiving gashes in conventional dental burs.
In conventional burs, and e~pecially straight burs, the clearance surface rearward of the cutting edges is coincident with a straight edge ex-tending between adjacent cutting teeth. From Figure 5, it will be seen that the tangent line 52 is above the next adjacent cutting edge 26, thus provid-ing a stronger relief surface 30 to permit the cutting edge to resist chipp-ing or other damage.
Further, the angle between the tangent line 52 and line 54 which is perpendicular to the flat cutting face 24, at the cutting edge 26 of the tooth, is approximately 24 but this is stated for purposes of being exemplary rather than restrictive. Further, the radial dimension of the blunt rounded ridge 34 relative to the cutting edge 26 of each tooth should always be less than the radial dimension between the root of the gash 22 and the blunt rounded ridge 34. In Figure 5, the latter dimension is indicated by a bracket labeled "a" and the former is indicated by a bracket labeled "b".
Another preferred characteristic of the chip-receiving groove or gash 22 is that as illustrated near the right-hand side of Figure 5, the 10~9301 additional radial dimension of ~he actual bottom of the gash 22 beyond the inner edge of the flat cutting face 24 preferably is between 40% and 60%
of the radial dimension of the flat cutting face. Said additional radial dimension of the innermost surface of gash 22 is indicated ~e" and the radial dimension of the flat cutting face is indicated by "f'~. In accordance with the present invention, it is preferred that the radial dimension of the flat cutting face "f" should be within a range between 209'0and 30~of the radius "g" of the cutting head of the bur, all of which dimensions are illustrated in Figure 5.

The shape of the cutting teeth 28, as illustrated in Figures 3-7 and as described above is such as to provide ample relief clearance rear-wardly of the cutting edges 26 of the teeth while, nevertheless, affording adequate backup mass of the material from which the head of the bur is formed so as to resist undue wear and especially the possibility of chipping the cutting edge due to the stubby nature of the cutting teeth in cross-section adjacent the cutting edge and this stubby configuration is present for substantially half of the radial width of the teeth between successive cutting edges before the adjacent sides of the chip-receiving grooves or gashes 22 commence to extend radially inward at a greater rate than the clearance surfaces 30 in order to provide chip-receiving grooves or gashes 22 of very substantial depth and fully adequate to accommodate the chips resulting from operation of the burs in the preparation of tooth structure for restorative purposes. Such grooves or gashes 22 also are provided in a manner which in no way interferes with the flat radial cutting faces 24 which provide positive cutting rakes for each tooth without sacrificing strength thereof, while at the same time providing very substantial resistance to chipping of the cutting edges 26.
The illustrated embodiment also includes improvements for the outer ends of the bur, especially the cutting tips 56 which respectively are ~049301 formed at the outer end of each cutting edge 26, as best shown in Figure 6.
Referring also to Figure 3, it will be seen that each cutting tip 56 is form-ed by grinding a radial groove 58, shown in end view in Figure 3, but also illustrated in side elevation in Figure 6 with respect to a single tooth 28.
The radial groove also extends from the outer ends of the chip-receiving gashes 22, inward toward the axis of the bur at an acute angle to the cutting edges 26 of the teeth. Said grooves preferably are formed by a cutting wheel of the diamond type similar to wheel 36 which operates to generate the groove 58, as the bur is rotated slowly about its axis and, by the time the cutting wheel reaches the portion of each tooth on which the relief surface 30 occurs, a more gradually sloping outer end surface 60 is formed by the cutting wheel. Such configuration results in backing up the cutting tips 56 for strength and resistance to chipping, while at the same time, providing adequate clearance surfaces for chips.

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Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A dental bur comprising a shank and a head fixed to one end there-of, said head being provided with a plurality of similar teeth extending longitudinally therealong and having a sturdy sharp cutting edge outermost thereon, each of said teeth having a flat cutting face extending from the cutting edge radially toward the axis of the head, a relief surface which is convex in cross-section extending from said cutting edge partially toward the cutting face of the next succeeding tooth, and a chip-receiving gash having a concave surface in cross-section which extends between said relief surface and said flat cutting face, said relief surface and adjacent edge of the gash adjacent each tooth meeting along a bluntly rounded ridge extend-ing longitudinally of said head circumferentially midway between each pair of adjacent cutting edges of the teeth and radially at a level farther from the root of the gash than from the cutting edge of each tooth, thereby to provide teeth on a dental bur which have a positive cutting rake afforded by said flat cutting face and a cutting edge rendered strong by said convex relief surface, in combination with a relatively deep chip gash adjacent said flat cutting face without sacrificing strength for the cutting edge of each tooth.

2. The dental bur according to claim 1 in which said relief surface and gash provide substantially an ogee surface in transverse cross-section, one edge of such surface abutting said flat radial face in radially spaced relationship with respect to the cutting edge of each tooth.

3. The dental bur according to claim 1 in which the edge of the gash adjacent each flat cutting face engages said face within a range between 20% and 30% of the radius of said head, measured from the circumference.

4. The dental bur according to claim 1 in which the edge of the gash adjacent the flat cutting face of each tooth engages the same at approximately 25% of the radius of the head, measured from the circumference.

5. The dental bur according to claim 3 in which the root surface of each gash extends radially a distance within the range between 40% and 60%
of the radial length of the flat cutting face of each tooth.

6. The dental bur according to claim 3 in which the convex relief sur-face of each tooth in cross-section has a radius extending substantially from the root of the gash of the diametrically opposite tooth to the cutting edge of each tooth.

7. The dental bur according to claim 6 in which the angle between a plane perpendicular to the flat cutting face of each tooth and a plane tan-gent to the relief surface at the cutting edge of each tooth is not sub-stantially in excess of 30°.

8. The dental bur according to claim 1 in which said cutting teeth are helical and the convex relief surfaces thereof are generated by a grind-ing wheel having edges acutely angular in cross-section, whereby said convex generation results from the rotation and axial feed of said bur relative to a grinding wheel rotating about a fixed axis extending transversely to the axis of said bur.

9. The dental bur according to claim 8 in which the helix angle of said teeth relative to the axis of said bur in between substantially 10° and 30°%.

10. The dental bur according to claim 8 in which said helix angle between the teeth and the axis of the bur is substantially 12°.

11. The dental bur according to claim 8 in which the gash adjacent each tooth has a root which is concave in cross-section and generated by a grinding wheel having an edge acutely angular in cross-section and said con-cave generation resulting from the axial feed and rotation of the bur about its axis relative to the grinding wheel rotating about a fixed axis transverse to the axis of the bur.

12. The dental bur according to claim 8 in which the wall of each gash opposite the radial cutting face of each tooth is slightly convex in cross-section, and said convex relief surface extending from the cutting edge of each tooth is produced by a second grinding operation which produces said bluntly rounded ridge which extends longitudinally between adjacent teeth.

13. The dental bur according to claim 1 in which the ends of the teeth opposite the shank end of the head terminate in cutting points res-pectively defined on one side by end grooves formed in the outer end of said head and having concave root surfaces extending radially toward the axis of said head in alignment with the ends of said chip-receiving gashes extend-ing along one side of each tooth, one sidewall of said end grooves extending nearly parallel to an axial plane intersecting in parallel to the axis of the head and within which the flat radial cutting face of each tooth extends to define a substantially stubby cutting tip at the intersection of said radial cutting face and relief surface of each tooth with said one sidewall of each end groove, and the outer sidewall of said end groove sloping gradually to-ward the cutting point of the next tooth to provide a relief surface for said cutting point while providing substantial material mass circumferentially rearward of said cutting point relative to the direction of rotation of said bur.