Process and device for manufacturing a structural part, especially of a ceramic tooth restoration, and a process of making sonotrode crowns

In a process and apparatus for the overall machining of a ceramic tooth restoration and producing of appropriate sonotrode crowns, these sonotrode crowns act on a workpiece one after the other, however from the same direction in space and are activated by the same ultrasonic transmitter. For the production of said sonotrode crowns, a geometrical model corresponding to the ultrasonic machining apparatus is used.

BACKGROUND OF THE INVENTION 
The present invention relates to ultrasonic working of workpieces for 
making structural parts, especially for producing ceramic tooth crowns, 
as, for example, dental veneers, inlays, crowns or bridges from dental 
ceramic material. Due to their favourable biological-chemical properties, 
their high tissue-tolerance and their low tendency for 
plaque-accumulation, ceramic tooth replacement materials are generally 
discussed as being material systems of top quality. 
More specifically, the invention relates to a process of producing a 
ceramic tooth restoration by ultrasonic machining apparatus in which a 
receiver for a profiled sonotrode crown is positioned on each side of the 
workpiece to be worked, the receiver being in opposing relation to one 
another, and wherein in a first machining step, the workpiece is held by a 
workpiece holder, a first sonotrode crown is activated and one of the 
workpiece halves is machined to the desired shape by bringing the first 
said sonotrode crown into form-fitting engagement with said workpiece, and 
wherein in a second machining step the workpiece is held by the sonotrode 
crown by the form-fitting engagement, and a second sonotrode crown is 
activated for machining the remaining half of the workpiece to the desired 
shape. The invention also relates to an ultrasonic machining apparatus for 
carrying out the process. 
The process as above described and the ultrasonic machining device for 
carrying out the process are known from DE 39 28 684 C2 
(HAHN)(corresponding to U.S. application Ser. No. 07/678,367). Particular 
reference is additionally made to this patent publication and the prior 
art named therein. 
Further, the publication "Keramikbearbeitung" (Working of Ceramics), Carl 
Hanser Verlag, Munchen 1989, pages 423-443 discloses a process and a 
device for working brittle materials, e.g. ceramic, glass, glass-ceramics 
etc., with the aid of ultrasound. The mechanical energy of the oscillating 
tool is transferred to a lapping mixture in a working gap between the 
sonotrode crown and the tool, which leads to a formation of chip and 
finally to a projection of the sonotrode crown in the workpiece. However, 
these methods are restricted to the forming of one workpiece surface. The 
necessary shaping tools are made by conventional metal-cutting methods, 
e.g. turning on a lathe, milling, drilling, etc., or by electrical 
discharge machining. Moreover, geometrically complex shapes of small 
dimensions frequently cannot be made from conventional workpiece 
materials. 
DE 40 29 285 A1 (SIEMENS) describes a sonotrode having its shaping part, 
i.e. its sonotrode crown, made at least partially from silicon. This 
choice of material serves to optimise the wear behavior. It furthermore 
facilitates the use of the so-called microstructure technique 
(Mikrostrukturtechnik) for forming the working surface of the sonotrode 
crown. The microstructure technique is known from the field of manufacture 
of semiconductor elements and comprises, for example, photolithography and 
isotropic and anisotropic etching methods. 
The above-mentioned electrical discharge machining process for making 
structural parts for the working of dental prostheses is known, for 
example, from DE 37 35 558 C2 (HERAEUS) and DE 35 44 123 C2 (WALTER). The 
disadvantages thereof have been discussed in the above-mentioned DE 39 28 
648 C2 (HAHN). 
Due to the unavoidable and partly significant wear and the short tool life 
of the sonotrode crowns resulting therefrom, conventional precise 
mechanical or microelectronic manufacturing processes or manufacturing 
processes by electrical discharge machining are not economical for the 
making of sonotrode crowns. 
Finally, DE 36 06 305 A1 (HANSEN) describes an ultrasonic machining tool 
comprising an ultrasonic generator (ultrasonic converter including 
amplifier) and a sonotrode. The sonotrode is fixedly clamped to the 
ultrasonic generator by a differential screw and centered by cylindrical 
shoulders. The sonotrode can thereby easily be substituted without 
necessitating a new adjustment of the workpiece. 
SUMMARY OF THE INVENTION 
The present invention provides a new process of ultrasonically making a 
structural part, particularly of a ceramic tooth restoration, and a 
corresponding ultrasonic machining apparatus; further, a process and an 
apparatus suitable therefor for making sonotrode crowns for use in the 
above process and for use in the above ultrasonic machining apparatus. 
These objects are met in accordance with the first embodiment of the 
invention by, between the two machining steps, exchanging the first 
sonotrode crown with the second sonotrode crown attached to the opposing 
receiver and aligning both sonotrode crowns with respect to one another 
for continuing the machining of said workpiece, with the workpiece being 
held in form-fitting engagement or again being brought into form-fitting 
engagement with the first sonotrode crown. 
In accordance with a modified form by the invention, a disposable model of 
the structural part is produced, a plastically deformable and hardening or 
curable material is deposited on the disposable model, and the sonotrode 
crowns are produced by directly molding them from the disposable model. In 
this method, the body formed by curing of the curable substance is 
separated along the equatorial line of the disposable model and the two 
body parts thus obtained are each worked to form complete sonotrode 
crowns. 
The subject matter according to the invention improves the manufacture of 
small-lot production series significantly. Particularly, it enables an 
especially economical, less cost- and time-intensive manufacture of this 
kind of series. Only a single ultrasonic sound producing unit is needed 
for overall working of the workpiece without necessitating a re-adjustment 
in connection with any necessary change of the sonotrode crown. The 
modified form of the invention is based on the principles of the model and 
articulation technique and enables the production of exactly fitting 
complementary sonotrode crowns for overall machining of dental ceramic 
blanks. 
A further feature of the invention is that the free edges of the sonotrode 
crowns correspond to the equatorial line of the tooth crown. This has a 
advantage of the particularly simple exact adjustment of the two 
sonotrodes with respect to one another, namely by detecting a form-fitting 
between the two sonotrode crown edges. 
The further of the invention resides in providing two equal, preferably 
identical, devices for holding the sonotrode which, significantly 
facilitates interchangeability without needing a re-adjustment. 
The variants of the mechanical partition the ultrasonic generator and 
sonotrode crown-holding device lead to a further simplification of the 
fitting interchangeablility of the sonotrode crown holding device. The 
term "ultrasonic generator" in this context refers to the actual 
generator, including any necessary amplifiers. This does not exclude the 
possibility that further amplifiers may be provided in the sonotrode 
itself. 
The coupling of the sonotrode with the ultrasonic amplifier is per se known 
from DE 36 06 305 (HANSEN) or the corresponding U.S. Pat. No. 4,751,916. 
The additional threaded connection includes the threaded connection 
described in these two documents. 
Means are provided at the end of the device adjacent to the ultrasonic 
generator for receiving and adjusting of the means supporting the 
sonotrode crowns, enabling the supporting engagement of the first 
sonotrode crown with the workpiece during a second working step. 
Adjustment means are provided permitting a convenient and exact 
positioning of the two sonotrode crowns with respect to one another. 
The method includes the per se known functional reconstruction of the tool 
restoration to be formed (e.g. inlay, veneer, partial crown, crown, bridge 
etc.) from a thermoplastic modelling material, e.g. moulding wax. 
The curable substance specified in claim 14 can be a known moulding wax or 
a polymer material. 
The equatorial line defines the so-called model equator, for instance the 
tooth crown equator measured by the largest diameter of the model, related 
to the model longitudinal axis or the virtual working axis. This 
limitation guarantees that so-called "working shadows" are not formed 
during the cutting of the sonotrode crown into the workpiece. The model 
equator defines the plane, oriented essentially traverse to the 
longitudinal axis of the model, from which the outer surfaces of the tooth 
restoration are surrounded by monotonically or strictly monotonically 
tapered sections of the restoration. Therefore, the body formed by curing 
of the moulding substance is separated along the equatorial line of the 
disposable model. 
A further feature of the invention is the provision of a key-lock which 
serves a later positionally exact adjustment of the two sonotrodes with 
respect to one another. 
According to a further aspect of the invention a so-called "wax-up" may 
stay on the model base normally provided by a dental laboratory and will 
be adjusted releasably on one model sonotrode. Imaginarily, the "wax-up", 
in other words the model form, is separated into two "halves" along the 
prosthetical equator. Initially, the "occlusal" half of the restoration is 
covered with a low shrinking, fast curing polymer up to the equator and is 
fixedly aligned in the restorational axis to a prefabricated secondary 
sonotrode. After termination of the process steps, the wax model is 
carefully taken out of the sonotrode crown forms achieved by moulding. 
The step of casting of the crown hollow form with liquid metal serves to 
optimise wear resistance of the sonotrode crowns by transforming the 
polymer sonotrode crowns into metal. The transforming occurs according to 
a casting process known in the dental field. Manual finishing of the 
sonotrode crowns is not necessary 
In order to avoid the forming of burrs during subsequent ultrasonic 
machining with the sonotrode crowns, the manufacturing process may be 
carried out so that their free edge zones overlap in a scissor-fashion. 
According to a further advantageous embodiment, the sonotrode crown is 
soldered, welded or glued to the adjacent sonotrode part. 
The geometrical model in accordance with the invention ensures a mutual 
fitting adjustment of the sonotrode crown at maximum degrees of freedom 
for aligning of the sonotrode parts adjacent to the sonotrode crowns and 
further ensures sufficient working space for manual activity during the 
manufacture of the sonotrode crowns. In the following, the invention will 
be further described with reference to embodiments shown schematically in 
the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The embodiment shown in FIG. 1 comprises an ultrasonic generator 2, which 
is mechanically coupled to a primary sonotrode 8 by means of a fixing 
screw 4 and a centering cone 6 with a shoulder, the sonotrode 8 including 
a conical flange 9 corresponding to the centering cone 6. In the 
following, the ultrasonic generator 2 and the primary sonotrode 8 are 
referred to as "ultrasonic transmitting device" 10. The ultrasonic 
transmitting device 10 is advanced in a known manner using an advancing 
control 12 toward the workpiece 14 to be machined, which in the present 
case is a dental ceramic workpiece 14, such that the machining gap between 
the active sonotrode crown 16 and the workpiece surface has the desired 
dimensions. A secondary sonotrode 18 serves as a device for supporting the 
sonotrode crown 16, is formed to be conical at its end adjacent to the 
primary sonotrode 8 and is centered by a corresponding centering cone at 
the end of the primary sonotrode 8 facing the workpiece. 
The secondary sonotrode 18 serves to support the sonotrode crown 16. The 
sonotrode crown 16 can be pressed, soldered, welded and/or glued to it, or 
it can be attached by any other known joining means. 
The dental ceramic workpiece 14 is held in form-fitting engagement with a 
further passive sonotrode crown 20 at its side opposing the active 
sonotrode crown 16. FIG. 1 therefore illustrates the second working step 
mentioned in the introductory part of the description. 
The passive sonotrode crown 20 is also positioned on a secondary sonotrode 
22, which is formed like the opposing secondary sonotrode 18. 
Specifically, it has the same conical centering flange which is inserted 
in a corresponding centering cone of a receiving member 23. The identical 
form of the devices for supporting the sonotrode crowns 16, 20, (in the 
present case the secondary sonotrodes 18, 22) allows a simple exchange of 
the two secondary sonotrodes 18, 22 in the primary sonotrode 8. 
The correct alignment of the two sonotrode crowns 16, 20 is achieved by a 
key, i.e. by at least one form-fitting location at their free edges. This 
form fit can be controlled by taking the workpiece 14 out of its working 
position, bringing said sonotrode crowns 16, 20 into form-fitting relation 
with one another and inserting them into the centering cone 6 into this 
alignment. Subsequently, the workpiece 14 is brought into form-fitting 
relation with the passive sonotrode crown 20 again, and the active 
sonotrode crown 16 is advanced in the forward direction towards said 
workpiece 14. 
Alternatively, a position-locking key of the sonotrode crowns can be 
provided by a groove and tongue arrangement in said centering cone 6 and 
the conical flanges for the secondary sonotrode(s) 18, 22. 
The receiving member 23 for the passive sonotrode crown 20 is supported by 
two adjusting shafts or rods 24 which are aligned parallel to the 
advancing direction, i.e. the machining axis 26. If the workpiece 14 and 
the sonotrode crowns 16, 20 are arranged in their machining position, the 
receiving member 23 is fixed by a fixing screw 28. 
The dental ceramic tooth restoration is finished as soon as the free edges 
of the sonotrode crowns 16, 20 meet one another. The tooth restoration is 
punched by the sonotrode crowns 16, 20 by machining from both sides in two 
subsequent machining steps. The tooth restoration is always machined from 
one and the same direction in space, namely from the direction in which 
the ultrasonic transmitting device 10 is arranged. 
In order to prevent the formation of burrs, the free ends of the edges of 
the sonotrode crowns 16, 20 overlap in a scissor-fashion. 
FIG. 2 shows the primary sonotrode 8 in enlarged perspective. It is 
characterised by a centering conical flange 9 facing the generator and a 
conical flange 31 facing the workpiece. Preferably, the primary sonotrode 
8 has a regular cylindrical outer wall with a diameter which is equal to 
the diameter of the receiving member 23, and a position key with the 
centering cone 6 of the ultrasonic generator 2, especially of the groove 
and tongue key type. 
According to a further embodiment not shown in the drawings, it is the 
primary sonotrode 8 which serves as an exchangeable device for supporting 
the sonotrode crown 16. The secondary sonotrode 18 is in this case not 
necessary. Instead, the receiving member supporting the passive sonotrode 
crown 20 is also shaped as primary sonotrode 8. 
The geometrical model of an ultrasonic machining apparatus shown in FIG. 3 
serves to make two sonotrode crowns which are complementary to one another 
for overall machining of a ceramic tooth restoration, in the present case a 
crown. The geometrical model comprises a base plate 32, shown in a 
longitudinal sectional view in FIG. 3, i.e. in a vertical sectional view, 
the sectional plane of which contains the machining longitudinal axis 26. 
The cross section of the base plate 32 corresponds to the section shown in 
FIG. 5 by another embodiment of a geometrical model. As is shown in these 
sectional views, two intersecting trough-shaped recesses 34 and 37 are 
formed in the base plate 32 such that column-shaped protrusions 36 
protrude at the four corners of the base plate 32 in an upward direction. 
The column-shaped protrusions 36 have a rectangular cross-sectional shape 
in a horizontal sectional view. 
A guiding rod 38 is provided on each column-shaped protrusion 36 which 
projects beyond said column-shaped protrusion 36 on both sides. 
Altogether, four guiding rods 38 are provided. All guiding rods 38 are 
aligned in parallel to the machining axis 26. They support, in pairs, a 
receiving member 40, 41. Each receiving member 40, 41 is shiftable on the 
guiding rods 38 in the direction of the machining axis 26 and can be 
positioned in any position on the guiding rods 38 by fixing screws 28 or 
by tension brackets or other means. 
The receiving member 40 shown in the left part of FIG. 3 comprises a 
section 42 facing the workpiece which can be shifted in a plane which is 
orthogonal to the machining axis 26. This section 42 is therefore 
shiftable in all three space dimensions; thus, it is shiftable to a 
maximum extent. It can be connected to the section 43 being fixable on the 
guiding rods 38 by a cross support. In the present embodiment it is fixed 
by magnetic supports 44 and is therefore moveable in the orthogonal plane 
particularly easily. A magnet may be provided such as a permanent magnet 
or--to allow changing the magnetic force--an electromagnet with a 
controllable current supply. The shiftable section 42 projects over the 
trough-shaped groove or recess 34 and is therefore easily accessible. The 
wax model ("wax-up") 46 supplied from the dental laboratory together with 
a model base 48 is fixed by putty material in a recess 47 of the shiftable 
receiving member 42. The putty 49 facilitates the alignment of the wax 
model 46--its prosthetical equatorial plane must be aligned with the 
machining longitudinal axis 26. The prosthetical equator is preferably 
determined in the dental laboratory and drawn on the "wax-up" 46. Its 
positional alignment is thereby facilitated. 
After aligning the wax model 46 in the machining position, i.e. in the 
position in which the tooth restoration is, so to speak, "punched" by the 
sonotrode crowns at a later stage, the occlusal surface is coated with a 
low-shrinking, fast-curing polymer up to the equatorial line, in other 
words, it is directly moulded. The polymer layer is then aggregated until 
it reaches a crown-facing cone 50 and is fixed therewith. The crown-facing 
section of the secondary sonotrode 22, called the sonotrode head, may have 
a thread instead of the cone 50. The secondary sonotrode 22 may further 
have a different form of retention for the sonotrode crown. 
A sonotrode crown 54 manufactured in this way usually cannot yet be used 
for ultrasonic machining of a brittle hard material. In this case, it is 
transferred to a metal casting mould. Up to the transfer into the metal 
mould, it constitutes a presonotrode 54.--The secondary sonotrode 22 may 
also be referred to as a "semifinished secondary sonotrode", whereby the 
term "secondary sonotrode" refers to covering all of the semifinished 
secondary sonotrodes and secondary sonotrode crowns. 
After the presonotrode 54 has been attached to the secondary sonotrode 22, 
the wax model 46 adheres exactly to the cured sonotrode crown surface. 
The free edge surfaces of the presonotrode crown 54 ending at the 
equatorial line are now isolated and the model base 48 is carefully 
removed. Now a second "cervical" sonotrode crown is built up instead of 
the model base 48 in generally the same manner as the occlusal 
presonotrode crown 54. For this, a second secondary sonotrode is arranged 
in the shiftable receiving section 42 of the left receiving member 40, 
analogously to the first secondary sonotrode 22. 
The cervical presonotrode crown is attached to the corresponding secondary 
sonotrode in the same manner as the occlusal presonotrode crown 54. 
Preferably, the same materials are used for making the second presonotrode 
crown as for the first presonotrode crown 54. The presonotrode crowns may 
be fixed to their corresponding secondary sonotrode heads at the same time 
as the moulding process or afterwards. 
Subsequently, the "wax-up" 46 is carefully removed, for example by infusing 
in hot water--which may be done before or after the separation of the 
presonotrodes along the isolated equatorial line. 
For optimising the wear behaviour of the sonotrode crowns, the sonotrodes 
including the attached sonotrode crowns are embedded in a refractory 
mould. The crowns are then removed, for example by burning out. The cavity 
thus formed is then poured out with metal. In this way, wear-resistant 
sonotrode crowns are directly cast with the head of the secondary 
sonotrodes. The sonotrode head carrying the sonotrode crown is retentively 
shaped in a suitable manner, for example by forming a reversed cone or by 
the above-mentioned thread. The joining may be optimised by additional 
soldering, welding or gluing of the joints. 
A further possibility consists in the conical forming of the 
already-mentioned secondary sonotrode heat, which enables a removal of the 
sonotrode crown after curing. The sonotrode crowns are exactly 
repositionable through the conical head of the secondary sonotrode and can 
now be pressed, glued, soldered and/or welded by conventional bonding 
methods. 
As already mentioned, the subsequent ultrasonic machining is carried out in 
two successive steps. First of all, the first sonotrode crown is completely 
moulded using a suitable lapping suspension in a blank made of dental 
ceramics. Afterwards, the moulded blank is held by the first sonotrode 
crown and is machined by the second sonotrode crown from the contralateral 
side. The intrusion depth is limited by contact of the corresponding 
sonotrode crowns. A so-called "ultrasonic punching" of the desired 
structured part is achieved, wherein scissor-like overlapping edge regions 
of the sonotrode crowns inhibits the forming of burrs. 
A further embodiment of an apparatus for making of two complementary 
sonotrode crowns 54 according to the invention is shown in FIG. 4--however 
in a different state of machining as the embodiment of FIG. 3. 
The two sonotrode crowns 54 are already fixed on the corresponding 
secondary sonotrodes or semifinished sonotrodes 18, 22. Only the wax model 
46 must be removed and the two sonotrode crowns 54 must be separated from 
one another. The step of casting-on of the sonotrode crowns 54 in the 
mentioned refractory mould is still to follow. In this embodiment, in a 
first process step the wax model 46 has been releasably fixed directly by 
the putty material 49 to the secondary sonotrode 18. 
The vertical cross sectioned view in FIG. 5 along the line A--A of FIG. 4 
shows a regular cylindrical receiving member 40 which lies on the likewise 
circular cylindrical guiding rods 38 under line contact. 
FIG. 6 finally shows a further embodiment of a geometrical model of an 
ultrasonic machining device for making sonotrodes which are complementary 
to each other. 
In this embodiment, receiving members 40, 41 are attached in exactly the 
same way to two fixed supports 58 facing each other, just as the primary 
sonotrode 8 in the ultrasonic generator 2 of FIG. 1. 
A primary sonotrode 8 may also be used directly as a receiving member 40 in 
the geometrical model.