HK1202041B - Milling method for producing dental prosthesis parts - Google Patents

Description

Milling method for producing a dental prosthesis part

Technical Field

The invention relates to a milling method for producing a dental prosthesis part.

Background

The known dental milling cutter has a ball head section with a curved cutting edge section and with an axial cutting section connected thereto, which has a helical cutting edge section and is adapted in diameter to the dental prosthesis part to be machined from a pre-sintered ceramic blank, in particular a zirconium dioxide blank. That is, the diameter of the bulb used for producing the implant or the like is selected such that, instead of a tooth geometry following a simple basic shape, a tooth geometry provided with three-dimensionally curved surfaces and recesses (einkerburg) can be created and created with the required surface smoothness. But too small a diameter cannot be chosen for strength reasons. Dental ball end mills designed for machining non-fired high performance ceramics, such as zirconia and alumina, are available, for example, from the catalog 2009 by DatronAG, page 70 under the product designation "DATRON VHM-Zirkunoxid-"know.

For producing dental prosthesis parts, such as, for example, implants, sinterable ceramics are used, solid zirconium dioxide ceramics being used nowadays because of their good hygiene and strength properties. Here, the green ceramic blank, the so-called green body, is pre-sintered to a hardness with a certain dimensional stability, i.e. sintered to a so-called white blank (blank) which can still be processed without any problems by means of a dental milling cutter, the shrinkage occurring when sintering to the hardness of the blank being already completed before the final shaping process. The implant blank is then sintered from the blank, which is also completely sintered to form the finished implant, but already in the shape of the finished implant. In addition to implants, bridges and other dental prosthesis parts or dental restorations, in particular braces for tooth crowns, can also be produced in the same way from zirconium oxide, in particular from zirconium dioxide solid ceramics which, in addition to zirconium dioxide in polycrystalline form, also have stabilizing oxides, such as yttrium oxide or magnesium oxide, for example 3Y-TZP, YSZ or TZ-3Y. After shaping by free-form milling on the pre-sintered blank, the pre-sintered dental prosthesis preparation or the dental blank formed is fully sintered to be dense, the sintering shrinkage or volume shrinkage occurring at this point (typically about 50%) having to be taken into account in the shaping carried out beforehand during milling, since the sinter-dense material can no longer be processed or can only be processed within a narrow range without damage to the ceramic structure.

Manual machining methods, such as manual contour milling, are known for milling a dental prosthesis blank.

Here, a plastic or plaster model of the dentition is first produced in a dental laboratory, for example from a dental impression taken by a dentist. In order to separate excess material from softer, but generally adhesive, material during the production of the model, hand-held pressure air turbine mills are used in the dental technology. Here, primarily milling tools are used which have a relatively large right-or left-handed grinding head in the form of a bud, usually with cross-teeth, which has the size of a tooth and is provided with wide and deep chip flutes in order to prevent jamming. The sanding head is here welded on a significantly thinner shank, so that the cutting can be carried out over the entire circumference. Such a tool can be manufactured, for example, by Brasseler GmbH&Product information of co.kg company "SGFA,2007 ".

The model can then be scanned and simultaneously a corresponding blank of the dental prosthesis part can be milled from the presintered zirconium dioxide wafer blank or plate. The dental milling cutters or scanners are clamped parallel to one another on a corresponding profiling milling machine, for example Tizian Mill from schultz GmbH, in which undercuts on the dental prosthesis blank can be produced by the rotatability of the operating table, but tool changes and different forms of clamping fixing of the operating table are necessary for roughing and finishing and for manual reworking.

In the dental field, CNC milling is increasingly used, in which a movement path (Verfahrweg) is generated from CAD/CAM data, on which a machine can be moved in three dimensions on a multi-axis CNC milling machine, wherein most of today's CNC milling machines have two further axes of rotation in addition to three axes of motion, so that undercuts can be formed. The CAD/CAM data are then obtained from the scanned model or, for example, from the dentition scanned by the dentist, so that a computer-aided profiling milling can be used here, wherein the modeling can be dispensed with and the manual work performed in the milling itself is also reduced.

It is known, for example, from german patent document DE 69625012T 2 to cut out dental molds from a suitable base body using a movement path generated by CAD/CAM and to fill polymerizable acrylate material between a pair of mold parts in order to form artificial teeth with different layers, such as a dental ceramic layer, a cover layer and a backing layer. In particular, ceramic is proposed here as a base body for the mold. The artificial tooth itself is not milled from the base but is molded from a polymerizable acrylate material between the tooth molds.

Another mechanical 3D contour milling method or free contour milling method is used to create a dental ceramic part directly by milling out the dental prosthesis part from a pre-sintered ceramic blank (sintering whitening), wherein the dental prosthesis part is then completely sintered. Such a milling method is known, for example, from WO2004/086999A 1.

For this purpose, shank milling cutters are mostly used, which have a hemispherical rounded ball head section and one to four right-handed chip flutes, which accordingly have one to four cutting edges on the outer edges of the cutting edges arranged between the chip flutes. In order to produce a dental blank, the milling tool is expediently applied from above onto the solid material of the respective pre-sintered ceramic disk blank and then moved stepwise into the solid material.

However, in this case, spalling (abplatzeng) or chipping often occurs on partially sintered and therefore brittle ceramics. Such chipping on the blank is therefore also a criterion for the dental technician to replace the tool at the same time, since it cannot be reliably determined whether the tool has become dull or the chipping has been caused by the forces exerted on the ceramic by the milling process.

Although left-handed milling cutters are also known from the machining of softer materials, such as plastics, wood or plaster as described above, they can achieve the advantage that no tensile forces act on the workpiece during the milling process, which in particular leads to spalling when machining ceramics, since ceramics of the type described above also have a low tensile strength in the sinter-whitened state. Such a left-handed milling cutter for machining plastics, aluminum, brass or copper is known, for example, from the product catalog 2009, page 14 of the Datron AG company under the product designation "Datron VHM-Einschneider, linksspiralechschneider". The milling cutter is designed as a single-edged tool in the form of large chip flute widths and depths that are customary for machining the material. However, such tools can only be used in cases in which the chip removal can take place downwards, i.e. not in machining applications such as 3D free-form milling, in which the milling tool acts on solid material from above, but only in machining applications in which the workpiece is machined on its vertical outer edge and the chips can be removed downwards. Since the machining is advantageously carried out without tension by left-hand turning. But the chips are thereby pushed downward and therefore can lead to jamming of the tool when the chips cannot be directed downward.

Disclosure of Invention

Starting from this, the object of the invention is to improve a milling method of the type mentioned in such a way that the service life of the tool can be achieved and a high process stability during the milling process can be achieved.

This object is achieved by a milling method for producing a dental prosthesis part, in which a dental prosthesis part blank of a finished dental prosthesis part, which is still to be completely sintered, is milled from a pre-sintered ceramic blank by means of a dental milling cutter arranged on a multi-axis CNC milling machine for machining the pre-sintered ceramic blank along a generated path of movement by means of 3D freeform milling, wherein the ceramic blank is plate-shaped or is present as a disk blank, wherein the ceramic blank is moved from above into the solid material of the ceramic blank by means of a dental milling cutter, which is clamped in each case beforehand, and the dental prosthesis part blank is subsequently milled from the ceramic blank by means of a layer-by-layer removal of material along the generated path of movement, wherein the dental milling cutter has a hemispherical rounded ball head section which transitions with its maximum outer diameter of 2 to 3mm into an axial cutting section which extends with this diameter constantly in the axial direction over the outer circumference, a shank section extending in the axial direction with a larger or at least equally large shank diameter is connected to the axial cutting section, wherein the dental milling cutter has three two chip flutes and a corresponding number of cutting wedges, which, starting from the ball head section, are wound along the axial cutting section around a core section composed of solid material and having a circular cross section, and on the outer edge of each cutting wedge facing the chip flutes in the right-hand direction, a cutting edge is provided, which extends arcuately in the axial view in the ball head section and on the radial coordinate of the maximum outer diameter in the axial cutting section, and all chip flutes and cutting wedges are wound with a left-hand pitch, wherein the helix angle is 1 ° to 45 °.

The milling method according to the invention is characterized in that the milling process is carried out with such a left-handed dental milling cutter, wherein the plate-shaped ceramic blank is clamped, the solid material approaching the plate-shaped ceramic blank is then moved from above with the dental milling cutter, the solid material of the plate-shaped ceramic blank is then moved in, and the dental prosthesis component blank is then milled out of the plate-shaped ceramic blank by removing the material layer by layer along a CAD/CAM-generated movement path. The dental milling cutter used according to the invention is characterized in that the left-handed helical geometry, that is to say three or preferably two chip flutes and cutting wedges extending from the ball head section in the axial cutting section, are wound in a left-handed helix, that is to say in the opposite direction of rotation, around the milling cutter core, in particular in a left-handed helical winding of 1 ° to 45 ° or preferably 5 ° to 30 ° relative to the milling cutter axis.

This is based on the surprising recognition that, in particular for sintered ceramics used in the dental field, milling can be achieved with a left-handed construction even when downward chip evacuation cannot be ensured. Since, even when the left-hand dental milling tool is moved from above into the solid material, as is the case when milling out a blank dental prosthesis part from a pre-sintered ceramic wafer blank, the ceramic part, for example in the form of a zirconia blank, is comminuted during the milling operation and therefore does not block the drilled or milled holes. That is to say no chip jamming occurs. On the contrary, the advantageous additional effect is that the clamping of the blank disk is less subject to forces than has hitherto been the case, since no tensile forces, which lift the disk or the plate upwards, occur, but rather compressive forces. The relatively expensive vacuum clamping with suction of the disk blank or plate used up to now can therefore be replaced by a relatively simple clamping.

Due to the left-handed rotation, no further pulling force is exerted on the white blank to be machined by cutting, but only pressure is exerted. It is therefore no longer necessary to tolerate the usual stripping blocks on the white blank and tool changes before the wear limit is reached. Since sinterable dental ceramics, such as zirconium dioxide dental ceramics, have a very high compressive strength, in contrast to their low tensile strength, the occurrence of spalling blocks on the workpiece can be avoided even for very thin geometries. As a result, not only can significantly more elaborate dental prosthesis parts be produced with increased process stability by the free-form milling method, but the service life of the tool is also greatly increased, since replacement is now only necessary when wear actually forms on the tool, and it is no longer necessary to conclude, as hitherto, that the fragments on the workpiece are due to wear of the tool, although in principle they can also occur when the tool is not worn. Meanwhile, the problem of chip blockage does not occur at all due to the dust-like chopping. Due to the high milling precision, the blank of the dental prosthesis part can be completely fired after milling into the finished dental prosthesis part, i.e. no further machining is necessary.

By means of the ball head geometry, the joint or the joint area can be moved/displaced over the entire hemisphere on the free end of the dental mill, wherein a joint width of 0.1 to 0.8 times the maximum outer diameter of the dental mill has been verified in the ball head section. That is to say, advantageously, no complete cutting (Vollschnitt) is carried out, but only partial cutting of 0.1 to 0.8 times the maximum outer diameter is carried out as the joining width, wherein the joining region, i.e. the region in which the cutting edge remains in the material, is moved over the hemisphere spanned by the ball head section and over the cylinder spanned by the axial cutting section connected thereto.

It has proven sufficient here that the length of the sharp cutting edge is a value of 0.5 to 1.5 times the maximum diameter, since higher cutting depths are less likely to occur with 3D contour milling methods when removing material layer by layer.

In order to meet the requirements with regard to the manufacturing accuracy and the strength of the tool when milling ceramic blanks, it has proven to be suitable, in particular when the dental milling cutter is generally produced in one piece from a material, for example hard metal, that is to say when there is no predetermined breaking point in the form of a welded connection, for a maximum outer diameter of the ball head section and thus also for a constant outer diameter of the axial cutting section connected to the ball head section with said outer diameter, a value of approximately 1 to 4mm, preferably 2 to 3 mm. In this case, no additional finishing is required for milling the blank of the dental prosthesis.

It is particularly preferred to provide a clearance (Freischliff) on each cutting edge, the width of which is preferably 0.1mm or less, and particularly preferably has a clearance angle (freiwinnkel) of 12 ° to 25 °. In this way, extremely fine details can be milled into the zirconium dioxide blank over a total maximum cutting length which is preferably less than 0.5 to 1.5 times the maximum outer diameter, in order to achieve a highly accurate reflection of the CAD/CAM data on the prosthesis part blank while simultaneously achieving optimum surface properties, and in this way no further processing is required.

In order to guide the cutting forces with the correct trajectory and magnitude in the application shown here schematically, it has proven sufficient for the depth of the chip flutes to take into account the dust-like, chopped, pre-sintered, whitened ceramic material, and it is advantageous in terms of tool strength for the core diameter in the axial cutting section to be about 40 to 65%, preferably 50 to 65%, or even 55 to 65% of the maximum outer diameter, that is to say the diameter of the outer periphery of the round tool core section which is not touched by the chip flutes is about 40 to 65%, preferably 50 to 65%, or even 55 to 65% of the diameter of the outer periphery in the axial cutting section of the tool and at the transition to the ball head section. The dental milling cutter thus acquires a rigidity, wherein, due to the pre-sintered ceramic material being comminuted to dust form, a sufficient "chip removal" or material removal is possible despite the small depth of the chip flutes.

In connection with the design of the chip flute, in terms of high tool stability and low chip volume requirements, due to the comminution of the pre-sintered ceramic material to dust form, it is advantageous here, in particular in dental milling cutters designed as double-edged cutters, that the transition from the outer diameter at the cutting edge to the rear side of the core diameter in the chip flute is realized at least in the axial cutting section by a transition region, which may be designed, in particular, in the form of an arc segment, wherein the outer diameter at the transition region, which is offset by 90 ° in the circumferential direction from the largest outer diameter at the cutting edge, amounts to 65% to 85%, in particular approximately 75%, of the largest outer diameter, so that the tool is additionally strengthened. When the zirconium dioxide dental ceramic is subjected to free-form milling, the tool can work at the rotating speed of 50000 r/min.

It has also been shown that it may be advantageous for certain applications for a dental milling cutter to have small transverse cutting edges. Since this facilitates the penetration into the material and the pressure is slightly reduced during the penetration. This is verified especially in the case of a particularly deep Z-feed. Without the transverse cutting edges, in the experiments, zirconium dust was deposited centrally on the milling cutter tip and resulted in surface deterioration. However, with a suitable CAM strategy (e.g. circumferential cutting), said problems can also be avoided, for example by providing the dental milling cutter with transverse cutting edges.

Drawings

Further advantageous developments of the invention are described with reference to the drawing, which shows an advantageous embodiment of the invention.

Fig. 1 shows a side view of a dental milling cutter for use according to an advantageous embodiment of the invention; and

fig. 2 shows an enlarged view of the end face of the dental milling cutter shown in fig. 1, with the milling cutter shank omitted.

Detailed Description

The dental milling cutter shown in the figures has a ball head section 1, an axial cutting section 2 and a shank section 3. The shank section 3 has a diameter Ds which is larger than the constant outer diameter Dk in the axial cutting section, i.e. the diameter Dk which the dental mill has on the outer circumference in the axial cutting section 2. The ball-end section 1 of the dental milling cutter is here rounded hemispherical and transitions into the axial cutting section 2 with its largest outer diameter which at the same time corresponds to the diameter Dk of the axial cutting section 2.

Starting from the free end of the dental milling tool, on the hemispherical rounded ball head section 1, the dental milling tool has two helically extending chip flutes 4 and two cutting wedges 5 separated by the chip flutes, which are left-handed at a helix angle b of 25 ° in the illustrated embodiment, i.e. opposite to the direction of rotation set for the dental milling tool. The cutting edge 6 extends on the outer edge of the cutting wedge 5 facing the chip flute 4 disposed upstream in the direction of right rotation. At the free end of the dental milling cutter, the two cutting edges 6 are connected by a short transverse cutting edge 10.

If, in the context of the present invention, a hemispherical rounded ball segment is mentioned, this means that the rounding extends along the cutting edge 6 of the dental milling cutter (which is double-edged in the exemplary embodiment shown) or, in other words, the dental milling cutter has a substantially semicircular contour in the ball segment in a side view and in a corresponding radial positioning of the two edges. The cutting edge 6 extending in the transverse direction at the free end of the dental milling cutter transitions here with a radius approximately corresponding to half the outer diameter Dk of the dental milling cutter in the axial cutting section 2 into a cutting edge section extending here helically around the axial direction.

This ensures that the dental milling tool can be used at any desired angle relative to the workpiece in the region of the joint that moves on the ball head section 1. The sharp cutting edge 6 extends here over a region of the axial cutting section which is smaller than the length L of the axial cutting section shown in fig. 1, which is equal to the length of the chip flute plus the length of the run-out which extends up to the transition with the shank section 3.

The sharp cutting edge 6 extends in the illustrated embodiment over a length equal to three times the maximum outer diameter Dk of the ball head section 1 or the diameter Dk of the axial cutting section 2, so that milling can be carried out over a greater length on the section of the cutting edge 6 extending in a left-handed spiral of the chip flute 4. As shown in fig. 2, a clearance 7 is provided on the cutting edge 6 on the rear side, which clearance has a clearance angle a of 20 ° in the exemplary embodiment shown. The clearance 7 also ensures a high surface quality over a large length of the sharp cutting edge 6 on the ceramic blank to be machined, wherein an arc-segment-shaped outwardly protruding transition region, which connects back to the frei surface 8 on the clearance 7 and in turn to the frei surface, has proven to be advantageous for achieving low vibrations and high rigidity of the tool, by which transition into the respective chip flute 4 is achieved.

The space provided in the chip flute 4 as chip volume is here smaller. However, since the presintered ceramic material to be processed, in particular zirconium dioxide, can be comminuted to a dust-like state, this is considered to be advantageous for the rigidity of the tool described above. If the core diameter Dk of the milling cutter core 9, i.e. the diameter of the dental milling cutter at the deepest position of the chip flute 4, is compared with the outer diameter Dk of the axial cutting section, the maximum depth of the chip flute 4 is here also smaller, where said proportional relationship is about 55% in the shown embodiment. This also contributes to the service life of the dental milling cutter. It is noted that the inner circle is drawn in fig. 2 only to show the core diameter dk and does not represent a feature that actually exists.

For the intended purpose of use in the dental field, i.e. the machining of a zirconium dioxide blank, it has proven advantageous here to design the dental milling cutter as a double-edged tool with low vibrations, although a three-edged variant is also conceivable.

The left-handed construction of the chip flutes 4 or of the cutting edges 6 on the cutting wedges 5 avoids tensile loads on the ceramic blank to be machined, as a result of which not only a significantly better surface quality can be achieved, but also a higher chip volume per unit time than with the right-handed drills which are customary up to now in the dental field.

The figures are not drawn to scale. For example, the shank milling cutter shown has an outer diameter Dk of 2mm in the axial cutting section 2 or at the end of the ball head section 1. For the application of 3D free-form milling of white-sintered zirconium dioxide ceramics, values of 1 to 4mm, preferably 2 to 3mm, for example 2mm, have proven advantageous here for the outer diameter Dk in order to achieve the necessary surface quality and dimensional accuracy for dental prosthesis parts, such as implants, dental bridges or the like, while achieving a high chip volume per unit time.

All working steps can be carried out here with the tool shown, that is to say after the material has been removed layer by layer with a dental milling cutter acting on the ceramic blank from above, no additional finishing is required. In this way, the dental prosthesis part can be produced without tool changes and with less production time, wherein in particular the left-handed construction and the resulting nonexistent tensile load result in a lower tendency to peel off and thus in a higher surface quality. If a shank milling cutter is used on a corresponding, for example five-axis, CNC milling machine which allows the dental milling cutter to be tilted relative to the workpiece during machining, undercuts can be formed even here on the dental prosthesis part to be completed by means of the ball head section 1 with the cutting section extending arcuately here.

Since the ceramic blank for a dental milling cutter is comminuted in the form of dust, even when solid material is moved in from above and the downward removal of chips is not possible at this time, here, despite the left-handed construction, no jamming of the chip flutes 4 occurs.

Changes and modifications may be made to the illustrated embodiments without departing from the spirit of the invention.

In this way, it is conceivable, for example, for the diameter of the shank section 3 to be selected to be the same as the maximum outer diameter Dk of the ball head section 1, and for the shank milling cutter to be produced with an outer diameter which is virtually identical over the entire length. But the diameter should not be chosen small so as not to impair the stability of the tool. Particularly good machining results have been achieved in experiments on zirconium dioxide disc blanks with a helix angle b of 25 ° and in the range of 5 ° to 30 ° around said angle, it also being conceivable for said helix angle to vary within wide limits, as long as the left-hand rotation is maintained with respect to the right-hand cutting direction of the dental mill and thus the entry of the dental mill into solid material can take place from above when a pressure load is exerted on the pre-sintered ceramic disc blank to be machined.

Claims (11)

1. Milling method for producing a dental prosthesis part, in which a dental prosthesis part blank of a finished dental prosthesis part, which is still to be completely sintered, is milled from a pre-sintered ceramic blank by means of a 3D free-form milling on a multi-axis CNC milling machine, which is arranged on the CNC milling machine for machining the pre-sintered ceramic blank, along a generated path of movement, wherein the ceramic blank is plate-shaped or is present as a disk blank, wherein the ceramic blank is moved from above into the solid material of the ceramic blank by means of a dental milling cutter, said ceramic blank being clamped in advance in each case, and the dental prosthesis part blank is subsequently milled from the ceramic blank by removing material layer by layer along the generated path of movement, characterized in that the dental milling cutter has a hemispherical rounded ball head section (1) which transitions with its maximum outer diameter (Dk) of 2-3mm to an axial cutting section (2) which extends axially constantly with this maximum outer diameter (Dk) over the outer circumference Wherein a shank section (3) extending in the axial direction with a shank diameter (Ds) of greater or at least the same size is connected to the axial cutting section, wherein the dental milling cutter has three or two chip flutes (4) and a corresponding number of cutting wedges (5), the chip flutes and the cutting wedges are wound from the ball head section (1) along the axial cutting section (2) around a core section (9) of solid material with a circular cross section, and a cutting edge (6) is arranged on the outer edge of each cutting wedge (5) facing the chip cutting groove (4) along the right-turning direction, said cutting edge extending arcuately in the axial view in the ball head section (1) and on a radial coordinate of the maximum outer diameter (Dk) in the axial cutting section (2), and all chip flutes (4) and cutting wedges (5) are wound with a left-hand twist, wherein the helix angle (b) is 1 ° to 45 °.

2. Milling method according to claim 1, characterized in that a tooth replacement part blank of the finished tooth replacement part, which is still completely sintered, is milled with a single dental milling cutter.

3. The milling method according to claim 2, characterized in that the milled dental prosthesis part blank is thereafter completely sintered into a finished dental prosthesis part.

4. Milling method according to claim 3, characterized in that the ball head section (1), the axial cutting section (2) and the shank section (3) of the dental milling cutter are generally integrally formed of one material.

5. Milling method according to claim 4, characterized in that there is a clearance (7) at each cutting edge (6) of the dental milling cutter.

6. Milling method according to claim 5, characterized in that a cutting angle of 8-25 ° is provided on each cutting edge (6) at least in the axial cutting section (2) of the dental milling cutter.

7. Milling method according to one of the preceding claims, characterised in that the core section of the dental milling cutter has an outer circumference with a diameter, i.e. a core diameter, of 40 to 65% of the outer circumference of the dental milling cutter in the axial cutting section and 40 to 65% of the outer circumference on the transition of the axial cutting section to the ball head section.

8. Milling method according to claim 5, characterized in that each cutting edge (6) of the dental milling cutter transitions into a chip flute (4) arranged opposite to the right-turn direction at the back face by a back face (8) directly adjoining at the back side or a back face (8) adjoining at a clearance (7).

9. Milling method according to claim 5, characterized in that the dental milling cutter is configured as a double-edged tool and in the axial cutting section (2) the transition from the maximum outer diameter (Dk) on the cutting edge (6) to the rear side of the core diameter (Dk) in the chip flute (4) is realized by an arc-segment-shaped transition region which connects on the relief clearance or relief face (8), wherein the outer diameter on the transition region, which is offset by 90 ° in the circumferential direction from the maximum outer diameter (Dk) on the cutting edge (6), is 65% -85% of the maximum outer diameter (Dk).

10. Milling method according to claim 7, characterized in that the length (L) of the cutting edge (6) of the dental milling cutter in the axial direction corresponds to at least 100-150% of the maximum outer diameter (Dk).

11. Milling method according to claim 7, characterized in that a wear protection layer consisting of diamond or cubic boron nitride is provided at least in the region of the ball head section of the dental milling cutter.