HK1063144B - Holding device for a ceramic blank - Google Patents

Description

Holding device for ceramic blank

Technical Field

The invention relates to a holding device comprising a cylindrical or prismatic porous ceramic blank and means for clamping the holding device in a machine tool for producing a ceramic workpiece by abrading the blank material. The invention also relates to a method for abrasive machining of a blank material in a holding device and to the use of a holding device.

Background

In the production of ceramic workpieces, enlarged shapes are machined from green or sintered, still porous, pre-fabricated ceramic blanks. And then, carrying out sealed sintering on the enlarged workpiece. During which the workpiece shrinks to a final shape in which the sealed ceramic workpiece conforms to the quality of the pattern or drawing. According to another variant, the ceramic piece is machined from a porous ceramic blank in its original dimensions and the pores are then closed by infiltration.

Since the porous ceramic blank has not yet reached its final hardness, it is easily machined in a material-abrasive manner. In the porous state they are therefore also very sensitive to all mechanical stresses, such as tensile, compressive, bending, torsional, impact and shock stresses. However, the blank must just withstand such stresses during the machining process. These porous ceramic blanks therefore require holding devices specially equipped for their machining, which take into account the sensitivity of the blank in particular during the machining process. The mechanical stresses must be minimized and reliably transferred to the holding device during the machining process.

The holding device is in turn clamped on a rotating shaft of a suitable machine tool to abrade the material of the blank to produce the ceramic workpiece. The ceramic workpiece is machined from the blank using special machining tools. This is done by at least one of a milling, drilling or grinding tool, respectively, for example a milling tool for rough machining and for finishing, respectively. The processing is usually carried out in a programmed manner, but also by means of a replica process in the original or also in an enlarged state by means of a model pattern.

With reference to PCT/CH00/00623 an automatic machine tool is known for the manufacture of dental substitutes, in particular basic frames for crowns and/or bridges of precise three-dimensional shape. In this document, the ceramic workpieces to be machined are referred to as basic frames, which are machined in a machine tool comprising a machine frame or body, a workpiece carrier with a rotary axis, at least one digitizing unit, at least one machining unit with milling cutters and an electronic computing and control unit for all the drives. The holding device for the blanks, which is not shown in detail, is fixed on the rotational axis of a movement unit, wherein the movement unit has three movement axes in the x, y and z directions. The blanks clamped on the same rotating shaft are prepared for model digitization and machining at different times on the same machine tool. Before the machining of the blank, the machining flow of the blank is calculated from the determined and stored digitized data and the scaling factor specific to the preselected material.

DE a 14436231 describes a sheet-like blank for dental crowns which is inserted into a ring and held by the ring in a holding device for machining. The purpose of this device is that, since the diameter of the blank can be reduced, only a very small amount of the very expensive materials used in dental technology remains when machining ceramic workpieces from the blank. Due to this ring, it is not possible to machine a completely freely designed, in particular sheet-like blank, so that even a minimal material consumption is ultimately affected. The retaining device with the ring is also used for making crowns, not suitable for making long bridges.

EP a 20982009 also describes a fixture for manufacturing a blank for a dental replacement part. The blank does not extend beyond the support in any direction, so that the freely accessible surface is small. The clamp is used to protect the blank, particularly when it is handled automatically in a warehouse or the like, which is obviously disadvantageous for free processing.

Disclosure of Invention

It is an object of the invention to provide a holding device of the type mentioned in the opening paragraph which, irrespective of the geometry, provides sufficient protection against excessive mechanical stresses and deformations even for elongated blanks. But also should be freely accessible to as large a portion of the blank surface as possible for material abrasive machining and the machined workpiece should be securely retained.

The invention provides a holding device comprising a cylindrical or prismatic porous ceramic blank and a holder for holding the holding device in a machine tool for producing ceramic workpieces by abrasive machining of the blank, characterized in that a narrow frame is fixed to at least a part of the periphery of the blank by means of an adhesive, wherein the blank is held in a stress-free manner in a planar region containing the longitudinal center axis of the blank, the frame covers only a small part of the surface of the blank, is held detachably and slip-proof in a stable holder by means of a holding joint and is fixed in a machine tool in a non-rotatable and slip-proof manner.

The blank that can be machined in the holding device according to the invention can have any desired geometry and can be in the raw or sintered state. Perpendicular to the longitudinal center axis of the blank, it can be of circular or polygonal design, in the latter case with two respectively parallel sides, in particular square, rectangular, hexagonal or octagonal. In terms of material, the cylindrical or prismatic blank is preferably composed of at least one metal oxide, in particular Al₂O₃，TiO₂，MgO，Y₂O₃And/or mixed crystals of zirconia. In this respect, reference is made to WO 99/47065.

According to the invention, the porous ceramic blank is at least partially surrounded by a narrow frame, which in turn is held in a stable fixture in the machine tool. The narrow frame extends in such a way that the longitudinal center axis of the blank lies in the area of the plane of the frame. The frame thickness measured in the tangential direction of the cylindrical blank is, by way of example, 1 to 8 mm, preferably 3 to 6 mm. The frame thickness is suitably chosen such that at most 25%, in particular at most 15%, of the surface of the blank is covered.

The frame radial width easily allows clamping in the stabilizing fixture for the longitudinal centre axis, in other words preferably in the range of 3 to 10 mm. In most cases, the frame is designed to be circumferentially continuous and regular. But it may be much wider, for example up to 20 mm, at the longitudinal or narrow edges of the blank. In this way, it can be held with a clamp only in this area.

The frame, whether circumferentially continuous or only partially formed, is preferably continuously connected to the blank on all sides facing the surface of the blank. The adhesive is provided such that the blank itself is not subjected to any stress. The inner face of the frame forms a hole which is larger than the blank to be inserted, and they differ by a distance equal to the adhesive gap. An adhesive, in particular a two-component adhesive, is inserted in the narrow gap between the frame or its individual parts. The frame or its individual elements may be roughened on the inside or coated with an adhesive.

According to a preferred variant, the frame is designed to be uniform, i.e. the cross section is constant along the longitudinal axis towards the outer circumference and is designed to be peripherally complete along the entire circumference. However, the frame designed as a single piece may also be U-shaped, the sides of which extend over substantially the entire dimension of the blank. According to this variant, one longitudinal or narrow edge of the blank may be completely free. This is an advantage with regard to the freedom of processing. The disadvantage is that the struts cannot be formed in positions without a frame.

The scope of the invention also includes the case where the frame consists of a number of individual small pieces which are glued to the blank in each case. The individual small pieces can be placed one on top of the other or glued with a small spacing between each other.

The frame is suitably designed to be comprised of a mechanically rigid plastic material or a readily machinable metal, such as aluminum.

Whether the frame size can be varied, for example by means of a manifold inserted one inside the other, or by applying adhesive to the grooves of the blank, or by other variants, is left to the judgment of a person skilled in the art and is particularly from an economic point of view.

The blank with a frame suitable for the manufacturer's application is detachably clamped in a mechanically stable clamp by methods known per se, which has a clamping joint for machine tools, in particular an insert-mounted spindle with an anti-rotation device or a plate mounted by a flange. It is important here that the frame is held such that the blank itself is not subjected to stress. In other words, the holding force of the blank frame in the jig is not directed in the direction of the blank surface. This can be achieved, for example, with a closed or U-shaped clamping frame, or with two clamping jaws when the frame is designed appropriately.

Clamping of the frame in the clamp can be made easier by specially designing the outer face of the frame or parts thereof, on which suitable surfaces for receiving are formed, for example spherical, pyramidal, hemispherical, or dovetail or roof-shaped, which surfaces are also used for determining the position.

For material-abrasive machining of the blank in the holding device, the preselected workpiece is continuously machined out of the ceramic blank until only struts are formed, wherein the struts can be freely selected with regard to position and number and directly end up on the remaining blank material in the vicinity of the frame or on the frame.

During the machining process, the connection between the workpiece and the residual material becomes increasingly weaker until, at the end of the machining process, the workpiece is connected to the residual material or to the frame only by means of the struts. The process is not limited by the preselected feed direction and/or the process direction as in the single-sided holding. The workpiece can be firmly held until the machining is finished; there is no risk of the workpiece being released uncontrollably.

Since the frame thickness is small relative to the diameter of the blank, the desired maximum free access to the blank surface is ensured. The type of possible blank cross-section is not limited; an appropriate cross-section of the blank may be selected. The blank length may also accommodate workpieces of longer holding lengths, such as multi-component bridges. The diameter of the blank may also be adapted to accommodate larger or smaller workpieces as appropriate. The requirements for a sharply curved bridge are particularly high, in which connection the retaining device according to the invention has proved satisfactory in tests.

The holding device according to the invention can advantageously be used in particular for producing solid ceramic dental substitutes, in particular crowns and bridges.

The advantages of the present invention are briefly summarized as follows. The perimeter frame covers only a small portion of the blank surface, in other words it provides a working surface with maximum free access. The frame, being at least a part of the perimeter, also provides the possibility of determining the number and position of the struts relative to the frame area. The configuration of the struts can be adapted to the shape of the individual workpiece, for example the shape of a tooth. Therefore, the machining force can be optimally introduced into the jig. Furthermore, the stresses generated by the machining are distributed over the entire range and not concentrated only on a small portion.

Drawings

The invention will be described in more detail hereinafter with reference to an embodiment in the drawings. The figures show schematically:

figure 1 shows a framed ceramic blank in perspective view,

figure 2 according to a variant of figure 1,

figure 3 shows a further variant according to figure 1 in axial cross-section,

figure 4 shows a detail of the frame area in radial section,

figure 5 shows a right parallelepiped blank in perspective view,

figure 6 according to a variant of figure 5,

figure 7 is a plan view of the machined workpiece,

figure 8 is a side view according to figure 7,

figure 9 is according to detail IX of figure 7,

figure 10 shows a variant of the workpiece according to figure 7,

figure 11 is a side view according to figure 10,

figure 12 is a side view of the clip partially cut away,

figure 13 shows the lower part of the clamp according to figure 12,

figure 14 is a plan view of a blank clamped in a fixture,

figure 15 shows a radial section XV-XV according to figure 14,

FIG. 16 is a schematic view of the clamping force transfer.

Detailed Description

Fig. 1 shows a cylindrical porous ceramic blank 10. In this example it consists of zirconia, with a diameter D of 25 mm and a length L of 48 mm. A frame 12 is adhesively closed in the vicinity of the longitudinal central axis a (fig. 3), which is rectangular in cross section and consists of a "Macrolon" solid plastic material. The frame has outer dimensions of 56 x 32 x 5 mm and corresponding inner dimensions of 48.5 x 25.5 x 5 mm. The blank and frame form an average 0.25 mm bond gap 14 (fig. 4) that is filled with a quick-stick adhesive 16, such as an alurt epoxy adhesive, a two-part adhesive from Ciba, Basel. When the frame thickness d is 6 mm, the blank surface coverage is 21%, and when the frame thickness is 3 mm, the coverage is 10%. The frame size can be adjusted accordingly for other blank sizes and shapes, but the adhesive gap remains substantially unchanged.

The variant according to fig. 2 differs from fig. 1 in that the frame 21 with thickness d and width b is not closed but is designed in a U-shape. The two sides of the frame are substantially flush with the circular free end face of the blank 10, but can also be designed to be shorter or longer.

The variant according to fig. 3 has a corresponding frame 12, which is glued to the blank 10 in one piece.

Fig. 4 shows the bonding of the frame 12 to the blank 10, the bonding gap 14 being filled with adhesive 16.

In the embodiment according to fig. 5, the blank is designed as a right hexahedron and the frame 12 is designed to widen to a width b at the rear end face so that the jaws can engage when held securely.

The blank 10 according to fig. 6 is designed octagonal in cross-section, as in fig. 5, in each case with two parallel, opposite planes, to which the peripheral frame 12 is glued.

Fig. 7 and 8 show a workpiece 18 machined from the blank 10, in which only certain remaining parts, in the present case a three-component dental bridge basic framework, are visible. The workpiece is held by two end struts 20 and attached to the remaining residual blank 10, 22 adjacent the frame 12. The two end-face remnant discs 22 of the blank 10 may be left as remnant material if the blank is not fully machined with respect to its longitudinal axis. The residual material on the frame 12 protects the processing tools from contact with the frame. If the tool is made of a plastic material rather than metal, additional protection is required to prevent the tool from breaking in contact with the frame.

Fig. 9 shows the area IX in fig. 7 in an enlarged manner, wherein the direct connection of the struts 20 to the frame 12 is shown in dashed lines as an alternative, without the residual disk 22. When the blank is finished, the web 20 is separated from the workpiece 18 and the separation points of the workpiece are ground flat.

However, the workpiece 18 is a four-component bridge according to the plan view of fig. 10 and the sectional view according to fig. 11, which is of more complex design and has a total of four struts 20, two of which are connected to the end faces 22 and two of which are connected in each case to the remaining surface of the blank 10 in the vicinity of the frame 12.

The invention is of importance in that the forces acting on the workpiece 18 are eliminated by the use of the device according to fig. 7 to 11, wherein the struts are bonded to the frame 12 by means of the respective adhesive 16. Thus, the remaining blank or work piece 18 is securely held within the frame 12. This may be accomplished by connecting the struts 20 to the frame 12 directly or indirectly, such as through a residual disc 22. In this respect, the closed frame provides the best solution for the stability and connection possibilities of the strut 20.

As shown in fig. 12 and 13, the clamp 24 for the frame 12 of the blank 10 is substantially formed in two parts, fig. 13 showing a lower clamp portion 26 having a peripheral shoulder 28. The frame 12 of the blank 10 is inserted into a cup-shaped recess formed by a peripheral shoulder 28 of the lower clamp portion 26. The clamp 30 is a flat cover of the same size that, after insertion into the frame 12, presses against the lower clamp portion 26. During processing, it is particularly important that the frame 12 is held only by the application of a clamping force. No clamping force is transmitted to the blank 10. In the present case, the clamping is achieved by means of screws provided at the four corners, corresponding screw holes 32 being drawn in the figure. A clamping sub 34 is secured to the lower clamp portion 26, the sub 34 being retained by a spindle in a machine tool spindle chuck. An anti-rotation device 36 in the form of a flattening portion or a bolt is also provided.

The holding device 42 according to fig. 14 and 15 comprises a clamp 24, wherein the clamp 24 consists of a lower clamp part 26, an upper clamp part 30, a clamping joint 34 and a screw 32, in which the combination of the blank 10 and the frame 12 is inserted. The clamp 24 is stable and may be made of aluminum, aluminum alloy or steel. The connection between the clamp 24 and the frame 12 is detachable so that the frame together with the blank 10 can be simply reinserted or removed. During processing, the fixture 24 does not contact the ceramic blank 12. The frame 12 is inserted into a lower clamp portion 26 having a suitable cup-shaped recess for use with the frame. As already indicated above, the upper clamp part 30 is placed on the lower clamp part 26 and is fixed with screws. During processing, the frame 12 is sandwiched between the lower and upper clamp portions. The dashed lines on the clamp fitting 34 are anti-rotation devices 36. The clamping tabs 34 may be arranged on the longitudinal edges or the narrow edges.

Fig. 16 shows the clamping force F with arrows 38, 40; they act tangentially with respect to the blank. In other words, the direction of action of the clamping force F is not directed towards the blank 10. The gripper 24 together with the combination of the frame 12 and the blank 10 is rotated by means of a machine-side gripper joint 34 into a predetermined position in the machine tool, in which case the blank can be machined there. The predetermined position is fixed by an anti-rotation device 36 (fig. 12, 13).

According to a variant not shown, the clamp 24 extends along only a portion of the frame 12, for example when the frame is U-shaped. Also, when the frame 12 is widened at the end face, only two jaws may be formed.

Claims (18)

1. Holding device (42) comprising a cylindrical or prismatic porous ceramic blank (10) and a fixture for clamping the holding device in a machine tool for producing a ceramic workpiece (18) by material abrasive machining of the blank (10), characterized in that a narrow frame (12) is fixed with an adhesive to at least a part of the periphery of the blank (10), wherein the blank is clamped stress-free in a planar area containing the longitudinal central axis (a) of the blank (10), the frame (12) covers only a small part of the surface of the blank (10), is held detachably and non-slip fixedly in the stable fixture (24) with a clamping joint (34) and is non-rotatably and non-slip fixed in the machine tool.

2. Holding device (42) according to claim 1, characterized in that the blank (10) is designed in the direction perpendicular to the longitudinal center axis (a) as a circle or as a polygon such as a square, a rectangle, a hexagon or an octagon with two parallel sides.

3. The holding device (42) according to claim 1, wherein the raw or sintered blank (10) consists of one or more metal oxides.

4. Holding device (42) according to claim 3, characterized in that the blank (10) is made of Al₂O₃，TiO₂，MgO，Y₂O₃And/or mixed crystals of zirconia.

5. The holding device (42) according to claim 1, wherein the frame (12) covers at most 25% of the surface of the blank (10) to be machined.

6. Holding device (42) according to claim 5, characterized in that said frame (12) covers at most 15% of the surface of the blank (10) to be worked.

7. The holding device (42) according to claim 5, characterized in that the thickness (d) of the frame is 1 to 8 mm.

8. The holding device (42) according to claim 7, characterized in that the thickness (d) of the frame is 3 to 6 mm.

9. The holding device (42) according to claim 1, wherein the width (b) of the frame (12) is 3 to 10 mm.

10. Holding device (42) according to claim 9, characterized in that the width (b) of the frame (12) is even up to 20 mm at the longitudinal or narrow sides, but is otherwise uniformly formed.

11. Holding device (42) according to claim 1, characterized in that the frame (12) is designed in a closed or U-shape, the sides of which extend over the entire length of the blank (10).

12. Holding device (42) according to claim 1, characterized in that the frame (12) consists of several parts glued to the blank (10) and connected to each other, close to each other or at a small distance from each other.

13. The holding device (42) according to any one of claims 1 to 7, characterised in that the narrow gap (14) between the frame (12) or its individual parts and the blank (10) is filled with adhesive (16).

14. The holding device (42) according to claim 1, characterized in that on the exposed part of the frame (12) a spherical, pyramidal, hemispherical or dovetail and/or roof-shaped suitable surface is formed for receiving the clamp (24).

15. The holding device (42) according to claim 1, characterized in that visually readable or machine readable information is provided on the exposed part of the frame (12).

16. The holding device (42) according to claim 1, characterized in that the frame (12) is held in the mechanically stable clamp (24) with a clamping force that does not act on the blank (10).

17. Holding device (42) according to claim 16, characterized in that the clamping nipple (34) is a plate with an anti-rotation device (36) inserted into the mounting spindle or mounted by flanges.

18. Holding device (42) according to claim 16, wherein the clamp (24) comprises a closed frame with a lower holding part (26) and an upper holding part (30) or two clamping jaws for receiving the frame or parts thereof.