Note: Descriptions are shown in the official language in which they were submitted.
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A METHOD FOR PRODUCING A DENTAL RESTORATION
TECHNICAL FIELD
The present invention relates to a method for preparing a dental restoration
with at
least one rotating material removing tool.
BACKGROUND
Since the 1980:s developments of automated production of dental restorations
have
been made. Such a production typically include automated acquiring of
topographic
data from a model made from a bite impression from a dental patient, computer
based design of a dental restoration, and automated manufacturing of the
dental res-
toration. For example, CAD/CAM based systems from the design and manufactur-
ing of dental restorations are known from:
- Duret: "Vers unit prothese informatisee" Tonus Dentaire No 73, 1985 pp. 55-
57.
- Duret et al: "CAD-CAM in dentistry", JADA, Vol. 117, November 1988, pp.
715-720.
- Williams: "Dentistry and CAD/CAM: Another French Revolution", Journal of
Dental Practice Administration, January/March 1987.
- Sjolin, Sundh, Bergman: "The Decim System for Production of Dental restora-
tions", International Journal of computerised Dentistry 1999: 3.
In an automated manufacturing of a dental restoration, typically suitable
tools, such
as cutting tools, are used to form the restoration from a blank, the tools
following
paths according to a manufacturing program based on a digital model of the
dental
restoration. Usually, industrial ceramics, such as dense sintered high purity
alumin-
ium or yttrium stabilized zirconium, are used as material for the restoration.
Such
materials present, despite their advantages concerning the esthetical result
of the res-
toration, a number of problems in the manufacturing process. Their hardness
result
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in a high rate of wear on the tools used, which, besides being costly, can
result in
vibrations in the manufacturing process, in turn causing deviations from
tolerance
requirements. Also, the nature of the ceramic materials used is such that they
are
relatively brittle, and therefore caution has to be talcen when the tool paths
are de-
termined, in order not to avoid the risk of failure in the material. Usually,
restriction
on the cutting parameters of the tools during the manufacturing process are
intro-
duced to decrease tool wear and avoid failure in the blank, which in turn
lengthens
the process causing a slow production.
SUMMARY
It is an object of the invention to decrease tool wear in automated
manufacturing of
dental restorations.
It is another object of the invention to decrease, in automated manufacturing
of den-
tal restorations, the risk of a failure in the dental restoration material.
It is another object of the invention to decrease processing time in automated
manu-
facturing of dental restorations.
These objects are reached with a method for preparing a dental restoration
with at
least one rotating material removing tool, comprising the steps of
- providing a dental material piece from which the dental restoration is to be
pre-
pared,
- providing an initial cavity in the material dental piece, and
- removing material outside the initial cavity by moving the tool essentially
in a
plane perpendicular to the rotational axis of the tool.
In brittle dental materials, to reduce risks of material failure, the working
grinding
surface of the tool should have a high speed in relation to the material. The
speed of
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the grinding surface of the tool due to the rotation of the latter, is zero at
the axis of
rotation, which is usually at the tip of the tool. When the tool is moved in a
plane
perpendicular to the rotational axis of the tool, the area on the grinding
surface of
the tool having no or little speed will move parallel to the surface of the
dental mate-
rial piece. Therefore, this area will not be substantially involved in the
material re-
moval process. Instead, areas of the tool further from the axis of rotation,
having a
high speed will be involved in the process. Therefore, not only risks of
material fail-
ure will be reduced, but also, due to the high speed of working grinding
surfaces, the
tool can be moved at a higher speed, which shortens the processing time of the
den-
tal restoration. It is known that dental restoration materials causes a lot of
wear on
material removal tools. An advantage of the invention is that the high speed
of
working surfaces of the tool will reduce wear of the tool itself.
When forming a cavity in a dental material using the method according to the
inven-
tion, movenients of the tool in a direction having a component parallel to the
rota-
tional axis, causing low speed areas of the tool to take part of the material
removing
process, can be limited and concentrated to a step of forming an initial,
central cav-
ity, and a substantial part of the material removal procedure can be performed
by
moving the tool perpendicular to'the rotational axis.
Preferably, the step of providing an initial cavity includes moving the tool
so that
the direction of the movement of the tool forms an angle to a rotational axis
of the
tool.
At a distance from the center of rotation, the surface of the tool has a
velocity com-
ponent due to the tool rotation in a direction which is tangential to the
local work
piece surface. However, a surface area of the tool close to or at the center
of rotation
has only a small velocity component or no velocity component due to the
rotation of
the tool in the tangential direction of the local surface of the work piece.
By moving
the tool in an angle to the rotational axis of the tool, such an area close to
or at the
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center of rotation will present a velocity component, due to the translational
move-
ment of the tool, in the tangential direction of the local surface of the work
piece.
Thereby, the temperature buildup, and the risk of excessive tool wear and work
piece material failure is substantially decreased. Also, since the temperature
buildup,
and the risk of excessive tool wear and work piece material failure is
decreased, the
tool can be allowed to remove material at a higher rate.
Also, since the tool is moved in an angle to the rotational axis of the tool,
it is as-
sured that an open space will be present close to a tip of the tool, providing
for a
cooling liquid to be distributed to an area close to the effective working
area of the
tool.
Preferably, the angle between tool movement direction and the rotational axis
of the
tool is between 80 and 89.5 degrees. Within this range a high processing speed
is
allowed with risks of material failure kept low. More specifically, while
lnoving the
tool at about 200 mm/min., for relatively hard dental restoration materials a
suitable
value for said angle is around 89 degrees, and said angle can be decreased to
about
85 degree's when working in softer dental restoration materials, giving a high
proc-
essing speed with little risk of material failure, excessive tool wear or tool
failure.
Examples of hard dental restoration materials include aluminium oxides and
fully
sintered yttrium stabilised zirconiumdioxide, and the exceptionally hard, hot
isostatic pressed zirconiumdioxide. Relatively soft dental restoration
materials in-
clude magnesium stabilised zirconiumdioxides.
Preferably, in the step of providing an initial cavity, the tool path, as
projected in a
plane perpendicular to the rotational axis of the tool, forms a closed loop.
Thereby,
the tool path could be helical, or present a screw form having an elliptic,
rectangu-
lar, square, or triangular cross-section. Alternatively, the tool path, as
projected in a
plane perpendicular to the rotational axis of the tool, could present a closed
curve of
any suitable, alternative shape. By letting the tool descend into the work
material
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while the tool movement as projected in a plane perpendicular to the tool
rotational
axis forms a closed loop, the size of the tool surface in contact with the
work piece,
can be controlled so that it does not exceed a desired level, and is kept, at
least sub-
stantially, constant.
5
Preferably, the method comprises
- determining a tool center boundary curve, which represents the outer limit
of the
movements of the rotational axis of the tool, at a plane perpendicular to the
axis
of rotation of the tool, based on
- the intended final cavity surface in a region in the vicinity of an
intersection be-
tween said intended final cavity surface and said plane perpendicular to the
axis
of rotation of the tool, and
- the shape of the tool on at least a part thereof.
When performing the step of removing material outside the initial cavity,
material is
removed essentially until an intended final cavity surface of the dental
material
piece. However, to arrive at the final shape, precision machining has to be
per-
formed to smoothen the surface of the dental material piece. This is a time
consum-
ing stage of the process of obtaining a dental restoration. By considering, in
a pre-
ceding stage, the shape of the tool in relation to the intended final cavity
surface, the
result of such a preceding stage will come closer to the end result. In turn,
less mate-
rial will remain to be removed in a following precision machining stage, and
less
time will be involved in the latter, contributing to shortening the entire
dental resto-
ration production process.
According lo a preferred embodiment, the method comprises
- determining a tool center curve at each level, of a plurality of levels, at,
under
and/or above said plane being perpendicular to the axis of rotation of the
tool, by
offsetting inwards the intersection between the intended final cavity surface
and
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the respective level by an amount corresponding to the radius of the tool at
the
respective level, and
- determining the tool center boundary curve as the most inwardly located of
the
tool center curves.
This provides a two dimensional calculation at each level, rendering a 21/2
dimen-
sional calculation for determining the tool center boundary curve, giving a
result,
essentially as accurate as a three dimensional calculation. However, compared
to the
latter, considerably less calculation steps are involved in the preferred
embodiment
of the inventive method. Therefore, the calculation time, and therefore
processing
time of the dental restoration can be kept low, in addition to which the
method can
be performed at a dental technician laboratory having limitations regarding
the
computational capacity of its computer equipment.
Preferably, the location, in a plane perpendicular to the rotational axis of
the tool, of
a center of the initial cavity is deterrnined as the location of the center of
the largest
circle that can be fitted withiii a boundary curve in a plane perpendicular to
the axis
of rotation of the tool. Thereby, the step of removing material outside the
initial cav-
ity is advantageously performed by moving the tool, from the initial cavity
towards
the boundary curve, along circular paths or a spiral shaped path. Determining
the
largest circle, that can be fitted within a boundary curve in a plane
perpendicular to
the axis of rotation of the tool, has the result that the length of circular
paths or a
spiral path inside the largest circle is maximized. In turn, this is
advantageous since
the effective grinding area of the tool can be controlled and variations in
the effec-
? 5 tive grinding area can be kept at a minimum.
Preferably, at least one tool path is determined by creating at least one
offset curve
by offsetting outwards a curve, and trimming the at least one offset curve
against a
tool boundary curve. As will be explained further below, this has the
advantages
0 that the cutting depth of the tool can be controlled, and that it is easy to
check if the
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tool paths result in extraordinary movements that are undesired from a
material
processing point of view, e.g. due to a risk of damaging the material or the
tool.
BRIEF DESCRIPTION OF THE DRAWINGS
Additional advantages of the invention will be presented in the description
below, in
which the invention will be described in detail with the aid of the drawings,
in
which
- fig. 1, 4, and 9-17 show cross-sections of a dental material piece from
which a
dental restoration is to be prepared, in different stages of a method
according to
one embodiment of the invention,
- fig. 2 shows a perspective view depicting a tool and its movement,
- fig. 3 shows a side view of the tool in fig. 2 in action,
- fig. 5 shows a view of a detail of the tool and a detail of the dental
material
piece,
- fig. 6 depicts tool boundary curves,
- fig. 7 shows a sectioned view of the dental material piece, whereby the
section is
oriented perpendicular to a rotational axis of the tool, and
- fig. 8a, 8b, and 8c show tool paths projected in a plane perpendicular to a
rota-
tional axis of the tool.
DETAILED DESCRIPTION
Fig. 1 shows a cross-section of a dental material piece 1 from which the
dental res-
22 5 toration is to be prepared. The dental restoration could be a crown, a
part-crown, an
inlay, an onlay, a bridge, a stump reconstruction, a veneer, also referred to
as a
ligament, a facette, a filling or a connector. The dental restoration could be
formed
according to a digital model, in turn obtained by scanning of a model,
obtained from
a bite impression, and a computer aided design process based on the scanning
data,
~0 known in the art. The dental material piece 1 could be a blank, or the
result of an al-
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ready initiated material removal process on a blank. For example, an exterior
surface
of the dental restoration could be at least partly finished, before commencing
the
steps of the method according to the invention.
The dental material of the piece 1 could be a ceramic material based on
zirconium
oxide, aluminium oxide or any other suitable material. The dental material
piece 1 is
mounted in a machine with at least one holder (not shown in fig. 1).
In the machine any suitable material removing tool 2 can be arranged, such as
a
milling tool or a cutting tool, suitable for working on the material for the
restoration,
whereby the tool is adapted to move automatically in relation to the dental
material
piece 1 according to instructions in a progranl file run in a computer
program. The
rotational axis of the tool 2 is indicated in fig. 1 with a line R. The tool
presents a
cylindrical grinding surface 4, and an essentially flat grinding surface 3' at
the tip
region 3, which flat grinding surface is oriented essentially perpendicular to
the rota-
tional axis R. A radius 5 is provided at the intersection of the flat grinding
surface 3'
and the cylindrical grinding surface 4. Alternatively, the grinding part of
the tool
could have another shape, e.g. of a truncated cone or a sphere.
The tool is to be used in a process of removing material to obtain a cavity of
the
dental restoration. In fig. 1 the dental material piece 1 is shown sectioned
parallel to
the rotational axis R of the tool 2. A contour of the intended cavity is
indicated with
the broken line 6.
Referring to fig. 2, in a step according to a preferred embodiment of the
invention,
an initial cavity is formed in the dental material piece, by moving the tool 2
while in
rotation, wherein the tool follows a helical path. In fig. 2, the helical path
is indi-
cated as a path followed by a center of the too12, and indicated by a curved
arrow P.
Thus, the path P forms an imaginary screw. To avoid material remaining at the
cen-
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ter of the bore formed by the tool, the diameter of this screw is less than
the diame-
ter of the tool itself.
The helical path described results in the effective working surface of the
tool being
substantially constant during this step of the method. However, as an
alternative to
the helical motion described, it is possible to move the tool along another
descend-
ing path with a different shape when projected in a plane perpendicular to the
rota-
tional axis R of the tool. Thus, the shape of the path projected in a plane
perpendicu-
lar to the rotational axis R of the tool could be elliptic, rectangular,
square or trian-
gular. Alternatively, the path P formed in this step of the method is not
closed when
projected in a plane perpendicular to the rotational axis R, whereby it is
simply a
curved or straight declining path.
Referring to fig. 3, during the step described with reference to fig. 2, the
too12 is
moved so that the direction of the movement P forms an angle a to the
rotational
axis R of the tool 2. If the tool is moved at a velocity of about 200 mm/min,
the an-
gle a is suitably about 89 degrees for hard dental restoration materials, and
down to
85 degrees for less hard dental restoration materials. Thereby, the material
removal
rate can be kept relatively high, at the same time avoiding the risk of
material fail-
ure, excessive tool wear or tool failure due to high temperatures in the
effective
grinding region.
Fig. 4 shows the result of the step described above with reference to fig. 2
and 3. An
initial substantially cylindrical central cavity C has been formed with a
diameter es-
:5 sentially equal to the diameter of the screw of the helical path P added to
the tool
diameter. Material has been removed from a first level L 1 of the dental
material
piece 1 to a second level L2 thereof, the first and the second level LI, L2
being
separated by a distance d2 in a direction parallel to the rotational axis of
the tool.
Here the expression "level" means an imaginary flat plane perpendicular to the
rota-
0 tional axis R.
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Referring to fig. 5 and 6, at the second level L2, a tool center boundary
curve TCBC
is determined, which represents the outer limit of the movements of the
rotational
axis R of the tool, at the second level L2. The determination of the tool
center
5 boundary curve TCBC is based on the intended final cavity surface 6 (fig. 4)
in a
region in the vicinity of an intersection between said intended final cavity
surface 6
and the second level L2, and also the shape of the tool.
Referring to fig. 5, more specifically a tool center curve TCi, TCi-1, TCi-2,
TCi-3 is
10 determined at each level, of a plurality of levels, in this example four
levels, i, i-1, i-
2, i-3. The levels, i, i-1, i-2, i-3, which can be of any suitable number, can
be located
at, under and/or above the level L2, but in this example, one level, i, is
identical to
the second level L2, and the remaining levels, i-1, i-2, i-3, is distributed
above the
second level L2. At each level, i, i-1, i-2, i-3, a line fonned by the
intersection be-
tween the intended final cavity surface 6 and the respective level i, i-1, i-
2, i-3 is
offset inwards by an amount corresponding to the radius Ri, Ri-1, Ri-2, Ri=3
of the
tool 2 at the respective level, i, i-1, i-2, i-3, whereby a tool center curve,
TCi, TCi-1,
TCi-2, TCi-3, is determined at each level, i, i-1, i-2, i-3.
Referring to fig. 6, the tool center boundary curve TCBC, indicated in fig. 6
with a
bold line, is determined as the most inwardly located at each segment of the
tool
center curves, TCi, TCi-1, TCi-2, TCi-3.
Preferably, a step of removing material outside the initial cavity C by moving
the
too12 essentially in a plane perpendicular to the rotational axis R of the
tool 2, in-
cludes moving the tool 2 along concentric circular paths. To provide for
obtaining a
maximum length of such circular paths, a center H (see fig. 4) of the initial
cavity C
described above with reference to fig. 2 and 3 is determined in the following
way:
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Referring to fig. 6, the center H of the initial cavity C is determined as the
location
of the center of the largest circle C9 that can be fitted within a the tool
center
boundaiy curve TCBC. Alternatively, the center H of the initial cavity C can
be de-
termined as the location of the center of the largest circle that can be
fitted within
some other boundary curve, for example, the intersection between the second
level
L2 and the intended cavity surface 6, (see fig. 4).
Thus, the center of this circle C9 is the lateral position of the center of
the screw
formed by the helical path P described above with reference to fig. 2.
Accordingly,
preferably, the tool center boundary curve TCBC at the second level L2 is
deter-
mined before creating the initial cavity C.
Following the step of providing an initial cavity C, material is removed
between the
first and the second level L 1, L2 by moving the too12 while in rotation
essentially in
a plane perpendicular to the rotational axis R of the too12. Material is to be
removed
approximately until the intended cavity surface at the second level L2.
Fig. 7 shows, in a cross-section perpendicular to the rotational axis of the
tool, a step
following the step of providing an initial cavity. The movements of a center
position
of the tool 2 at the rotational axis R thereof, are indicated with lines with
arrows.
The movements have directions essentially perpendicular to the rotational axis
R of
the too12. The movements follow circular tracks 11 essentially centered on the
cen-
ter H of the initial cavity C, and presenting suitable differences in
radiuses, whereby
an orbit of the center position of the too12 following one circular track is
followed
by a step 12 outwards to a larger circular track.
In the step described in with reference to fig. 7, at each orbit of the tool
2, material
is removed mainly by the grinding surface 4, (see fig. 1). Depending on the
size and
rotational speed of the too12, and the type of dental restoration material
used, a suit-
able amount of material is removed at each orbit of the tool 2.
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Of course, regarding the movements of the tool there are a number of
alternatives to
the circular tracks separated with radial steps, described with reference to
fig. 7. For
example, while moving the tool 2 essentially in a plane perpendicular to the
rota-
tional axis R, the tool could, at least at an early phase of the step of
removing mate-
rial between the first and the second level L1, L2, follow a track shaped as a
spiral,
at which the tool is gradually moved outwards from the starting point, so that
a suit-
able amount of material is removed at each orbit of the tool.
The movements of the tool 2 essentially in a plane perpendicular to the
rotational
axis R has the following advantage: Since the grinding surface 4 of the tool 2
is at a
radial distance from the rotational axis R, and since the grinding surface 4,
due to
the lateral movement of the tool, takes part in the material removing process,
it is
accomplished that essentially all of the working grinding surface of the tool
2 has a
high velocity. This results in a high material removal rate. The nature of
dental res-
toration materials, i.e. dental ceramic materials, includes a relatively small
elastic
deformation and essentially no plastic deformation before a breaking stress of
the
material is reached. As a result, if some working surfaces of the tool is
moving due
to the rotation with a relatively slow velocity, the translational movement of
the tool
combined with a relatively small material removal rate, could cause
deformations in
the dental restoration material followed by a failure when the ultimate stress
has
been reached. The high velocity of the working grinding surface of the too12
ac-
complished by the invention will drastically reduce the risk of failure in the
dental
restoration material.
In general, the method according to the invention drastically reduces the risk
of
damages on materials or tools by making it possible to prevent the cutting
depths
from becoming too large, (discussed closer below), to avoid or minimise
movements
mainly in the axial direction of the tool, and to prevent a contact surface
between the
tool and the material from becoming too large, (discussed closer below).
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Referring to fig. 7, the tool center boundary curve TCBC has an irregular
shape. Re-
ferring to fig. 8a, the movements of the tool is limited outwards by the TCBC,
and
tool paths in the area enclosed by the tool center boundary curve TCBC are
deter-
mined in the following way: A curve, in this example the circle C9, is offset
out-
wards to form tool paths C10, C11 outside this curve C9, which paths has
shapes
corresponding to the shape of said curve C9. Instead of offsetting from a
circle C9,
the outwards offsetting could be made from any suitable curve, with any shape.
As a
further alternative, the tool paths can be determined as outwardly offsetting
curves
of a predetermined shape, for example circles or circle segments, from a point
with
a suitable location.
The distance between the offset curves C 10, C 11 corresponds to a suitable
radial
cutting depth of the tool 2.
Curves created by outwards offsetting can intersect the tool center boundary
curve
TCBC. Additional outer tool paths are created by outwards offsetting, until
created
curves do not intersect the tool center boundary curve TCBC, i.e. are located
outside
the latter.
Where needed, the offset curves are trimmed against the tool center boundary
curve
TCBC, removing curve parts outside the latter, so that segments C 10, C 11 of
closed
curves or circles are created. Such segments, or clusters of segments, form
sections
S1, S2, S3 of the processing region, which sections are formed in pockets
inside the
tool center boundary curve TCBC, where the latter presents a more abrupt
curvature
than the curves C10, C11 created by outwards offsetting. Each section, (for
example
S1 in fig. 8a), can present subsections S1-1, S1-2 which are smaller sections
or
pockets, each with their own curve segments.
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Preferably, in each section S1, S2, S3, (pocket inside the TCBC), and in each
sub-
section SI-l, S1-2, the curve segments C10, C11 are interconnected to foml a
con-
tinuous tool path. Preferably, the interconnection between the segments are
formed
by interconnecting segments of the TCBC, or by linear segments taking into
account
a suitable clearance towards the TCBC. Thus, when removing material, the tool
paths within a section S 1, S2, S3, or a subsection, S 1- l, S 1-2, are
followed succes-
sively to minimise the number of tool lifting measures between different
sections or
pockets S1, S2, S3. This will reduce the processing time.
A precision cut following the tool center boundary curve TCBC is made to clean
the
contour.
An advantage with the technique of determining tool paths by offsetting
outwards a
curve, and trimming offset curves against a boundary curve TCBC is that the
cutting
depth of the tool can be controlled.
Another advantage is that it is easy to check if the tool paths result in
extraordinary
movements that are undesired from a material processing point of view, e.g.
due to a
risk of damaging the material or the tool. For example, referring to fig. 8b
and 8c,
such a case can arise when a tool path stretches into a "shaded" area, e.g.
behind a
"peninsula" 21 or an island 22 formed by the tool center boundary curve TCBC.
Such a shaded tool path is marked with "SX" in fig. 8b and 8c. Since material
has
not been removed inside of the shaded tool path the contact surface of the
material
and the tool becomes very large. The appearance of the shaded path as such is
easy
to detect, when using the technique of offsetting a curve outwards.
Preferably, if a tool path is found to be undesired according to predetermined
re-
quirements, e.g. regarding the size of the contact surface of the material
aild the tool,
the tool path is rejected. Preferably, a region 23 is defined including an
area covered
by the rejected tool path SX, and a set of curved, preferably part-circular,
tool paths
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24 are defined with a suitable center of curvature and radiuses.
Alternatively, such
tool paths 24 can be straight.
Fig. 9 shows, in a view of the dental material piece I sectioned as in fig. 1
and 4, a
5 result of the step described above, to remove material between the first and
the sec-
ond level Ll, L2, in the form of a cavity 15. It can be seen that material has
been
removed approximately up to the contour 6 of the intended final cavity, at the
sec-
ond level L2. It can be seen that a portion 16 of the dental material piece 1,
outside
the cavity 15, and between the cavity 15 and the contour 6 remains to be
removed.
10 Preferably, this is done by introducing a number of sublevels between the
first and
second levels L1, L2, and, starting from the lowest sublevel and raising the
tool in a
stepwise manner, removing material at each sublevel. Similar to what was
described
above with reference to fig. 5 and 6, at each sublevel, i-1, i-2, i-3, a tool
center
boundary curve, TCBCi-1, TCBCi-2, TCBCi-3, is determined. At each sublevel,
for
15 example on sublevel i-2, tool paths are created by offsetting outwards the
tool center
boundary curve TCBCi-1 from the sublevel below, i-1, towards the tool center
boundary curve TCBCi-2 at the sublevel i-2. The result is shown in fig. 10.
In this example, the processing of the dental restoration continues with
similar steps
as those described above. Referring to fig. 11, in a step corresponding to the
step de-
scribed above with reference to fig. 2, 3, and 4, material is removed from the
dental
material piece 1 from a first level L 1 of the dental material piece 1 to a
second level
L2 thereof, the first and the second level Ll, L2 being separated by a
distance d2 in
a direction parallel to the rotational axis of the tool. In this example, the
first level
L1 in the step described with reference to fig. 11, is the same as the second
level L2
in the step described with reference to fig. 4.
According to the invention, in a subsequent step, material is removed between
the
first and the second level L1, L2 by moving the tool 2 while in rotation
essentially in
a plane perpendicular to the rotational axis R of the tool 2, the result of
which is
CA 02633248 2008-06-13
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16
shown in fig. 12. This is done in the same manner as described above with
reference
to fig. 7 and 8a. Similar to what has been described with reference to fig. 9,
it can be
seen that a portion 16 of the dental material piece 1, outside the cavity 15.,
and be-
tween the cavity 15 and the contour 6 remains to be removed. In the same
manner as
described above with reference to fig. 9 and 10, this is done by introducing a
num-
ber of sublevels between the first and second levels LI, L2, and, starting
from the
lowest sublevel and raising the tool in a stepwise manner, removing material
at each
sublevel. The result is shown in fig. 13.
Referring to fig. 14, continuing the processing of the dental restoration with
similar
steps as those described above, in a step according to the invention, material
is re-
moved from the dental material piece 1 from a first level L1 of the dental
material
piece 1 to a second level L2 thereof. In this example, the first level L1 in
the step
described with reference to fig. 14, is the same as the second level L2 in the
step de-
scribed with reference to fig. 11.
Similar as described above with reference to fig. 7 and 8a, in a subsequent
step, ma-
terial is removed between the first and the second level L1, L2 by moving the
tool
while in rotation essentially in a plane perpendicular to the rotational axis
R of the
tool 2, the result of which is shown in fig. 15. Fig. 16 shows the result of
removing a
portion 16, shown in fig. 15, outside the cavity 15, and between the cavity 15
and
the contour 6.
Preferably, the lowest level for using the tool 2, used in the steps described
above, is
a level that permits creating an initial cavity C of a predetermined minimum
diame-
ter, so that it is ensured, during the creation of the initial cavity C, that
the direction
of the movement of the tool 2 forms an angle a to a rotational axis R of the
tool.
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17
Fig. 17 shows the dental material piece 1 after removing further material in a
similar
manner to what has been described above, whereby a cavity 15 is obtained. A
por-
tion 17 at the bottom of the cavity 15 can be removed by a suitable tool.
Levels processed by the relatively large tool 2 are analysed regarding areas
not
processed due to the size of the tool 2. Preferably, this analysis is
performed from
the bottom and up. At each level an inner curve is determined based on the
proc-
essed area. The inner curve is expanded outwards similarly to what has been de-
scribed above with reference to fig. 8a, to create tool paths to remove
remaining ar-
eas. This is done with a suitable tool with smaller dimensions.
Above, the cavity in the dental material piece 1 has been described as being
created
by two steps being repeted alternately, namely: providing an initial cavity C
in the
material dental piece 1, and removing material outside the initial cavity C.
It should
be noted that these steps can be carried out using the same or different
tools.
Alternatively, a step of providing an initial cavity C in the material dental
piece~ 1
can be followed by repeated steps of removing material outside the initial
cavity C,
whereby the initial cavity C is relatively deep and material is removed
outside of the
initial cavity at a plurality of levels. Thereby, initial cavities or pre-
cavities, can be
pre-made in dental restoration blanks or work pieces. In such a case, the
initial cav-
ity C can advantageously be formed before sintering of the material, or, when
com-
pression moulding the blanks, the initial cavity C can be formed by providing
a pro-
truding part in the mould.
