Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02356497 2001-08-30
MULTI-AXIS POLISHING MACHINE
This invention relates to an improved method and apparatus forfinishing
the curved surface of lenses. In this context a lens refers to either a
transparent or
a reflective curved surface. In particular, it is intended to produce a more
even
accurate curved surface, especially for aspherical lenses, without the
disadvantages
of the previous techniques.
The apparatus and techniques described herein are applicable to either
grinding or polishing, but especially the latter, and references to polishing
should be
taken to include both.
The development of modern lenses requires not merely the grinding and
polishing of a concave or convex spherical surface but the more difficult
accomplishment of providing extremely even surfaces for quality of light
transmission and clarity of images, as well as the development of aspherical
surfaces
which are not of constant radius.
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Although the term aspherical can refer to a curved surface with different
radii of curvature in different axes or directions, it may also refer to a
curved surface
in which the radius of curvature varies from the centre to the periphery and
it is this
latter concept to which the present invention is chiefly directed, although it
may be
applied to both cases.
Whereas spherical surfaces may be generated by rotating a grinding
or polishing wheel which is moved about a fixed point or axis at a constant
radius,
aspherical surfaces require more complex movement of the grinding or polishing
instrument with changing arcs of curvature.
To accomplish this, polishing devices have been developed which
move under computerized control according to a three-dimensional grid defined
by
coordinates in relation to three-dimensions such as an X, Y, and Z axes. This
sort
of apparatus allows the polishing device to be selectively controlled so that
it can grind
or polish the curved surface selectively and remove more material where there
are
high spots and less where there are areas of lower relief. One traditional
method of
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polishing of lens involved spinning a cylindrical shaped polishing tool with a
circumferential ring about a central axis while the central axis is caused to
swing in a
spherical arc about a fixed point. At the same time the workpiece was rotated
about
an axis coinciding to its axis of symmetry. This technique, however, does not
work
for aspherical lenses where the radius of curvature is constantly changing. It
also
does not work for the high quality technique which requires material to be
ground or
polished from the lens differentially at selective locations.
There also remains the problem of applying the polishing device to the
surface in a manner which is substantially parallel to the surface at the
point of
contact.
In conducting the polishing operation, it is also important that the
polishing device not be applied in a unique directional manner so as to create
a pattern
of rows or furrows corresponding to the direction of travel of the polishing
device.
It is therefore the purpose of this invention to provide apparatus and a
method for grinding and/or polishing lenses which will overcome the problems
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associated with aspherical lenses and at the same time to create a highly
precise and
even curved surface.
These objects and other advantages are sought to be achieved by
means of the present invention which comprises a polishing machine having a
first
carriage movable in a substantially horizontal direction and a second carriage
mounted
on said first carriage and movable in a second substantially horizontal
direction
perpendicular to said first direction and a third carriage mounted on said
second
carriage and movable in a third direction substantially perpendicular to said
first two
directions. A polishing mechanism is mounted on the third carriage and
consists of a
polishing wheel having a polishing surface on its periphery and motor means to
rotate
the wheel about its axis symmetry and a second motor means designed to revolve
the
wheel about an axis perpendicular to its axis of symmetry so that the wheel
will spin
and rotate at the same time. The carriages are controlled by a computer
programmed
to locate the polishing means in any three dimensional position relative to
the three
coordinates of movement of the carriages so that the polishing wheel may be
moved
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about or located at any desirable position on a work piece.
The machine is provided with a work piece positioning means which is
adapted to locate and orient a work piece relative to the polishing wheel.
Preferably
this positioning means is rotatable about at least one and preferably two
perpendicular
axes and is controlled by a computer so that the curved surface of a work
piece such
as a tense may be presented to the polishing wheel in such a way that the
polishing
wheel bears against the surface in a direction substantially normal to the
surface at the
position of contact.
Thus the polishing wheel may be located anywhere along three directions
of movement and the polishing wheel will spin perpendicular to its axis of
symmetry
and rotate in a plane parallel to its axis of symmetry so that the direction
of movement
of the polishing wheel against the work piece surface is constantly changing.
Thus the polishing wheel may move in a series of traverses across the
surface of the work piece, in response to the computer programme, and the
direction
of the polishing movement will be constantly changing so as to avoid creating
furrows
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or patterns corresponding to the travel of the wheel.
The invention may be better understood by a detailed description of one
embodiment thereof with reference to the attached drawings in which:
Figure 1 is a perspective view of one embodiment of the present
invention;
Figure 2 illustrates a typical example of a lens of circular shape having
a concave cross-section with the path of the polishing apparatus
illustrated in broken lines;
Figure 3 is a computer generated graphic display of the irregularities on
the surface of a lens;
Figure 3A is a computer generated graphic display of a cross-section of
the surface of a lens relative to a datum.
Figure 4 illustrates the position of the polishing wheel of the present
invention at the centre of a lens;
Figure 5 illustrates the relative position of the polishing wheel near the
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periphery of the lens;
Figure 6 illustrates the details of the movement controlling the grinding
wheel which is part of the apparatus shown in Figure 1;
Figure 7 is a perspective view showing the workpiece holding apparatus
partially shown in Figure 1; and
Figure 8 is a vertical cross-section showing the relative position of the
polishing apparatus in relation to the workpiece.
In the perspective drawing of Figure 1, the apparatus of the present
invention is shown generally supported on a foundation 2 and having a first
carriage
4 movable along a first horizontal axial direction by means of parallel tracks
6. The
accordion pleated cover 8 covers a helical gear mechanism (although a system
of
cables and pulleys or rack and pinion could also be used) which under control
of the
central computer (not shown), is capable of positioning the polishing device
with
reference to defined coordinates along a first axis, which may be referred to
as the X
axis.
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Mounted on the first carriage 4 is a second carriage 10 designed to
move along a second axial direction perpendicular to the first axis along
similar
tracks (not shown) covered by the accordion pleated cover 12. In a similar
manner
a helical gear mechanism (or equivalent device) moves under control of the
computer
to locate the second carriage 10 in relation to the coordinates of a second
axis which
maybe referred to as the "Y" axis..
The second carriage 10 supports a vertical track and gear mechanism
11 which is designed to support and move the tower 14 in a vertical direction
in
response to computer controls, so as to move the polishing apparatus in the
vertical
direction in relation to coordinates of a third axis "Z".
Thus, by means of computer controls, the tower 14 can move in any
combination of the three-dimensional axes X, Y and Z and is capable of
positioning
the polishing mechanism, shown generally in Fig.6, in any selected position.
As can be seen more clearly in Figure 6, the column 14 has mounted
thereon a dove-tailed guide 16 to which is mated a vertical arm 18 of the
support 20,
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which also has a plate 15 extending outwardly from the column to support the
mechanism of the polishing wheel described hereafter. The dove-tailed guide 16
and
the mating arm 18 allow the support to move vertically under the influence of
gravity
and the opposing force of the cable 22 which passes over the pulleys 24 from
the top
of the arm 18 to the counterweight 26 which carries a series of weights 28 of
various
sizes.
In operation, the weights 28 are chosen so that the counterweight is
slightly less than the weight of the polishing mechanism on the other end of
the
cable 22 and the difference in weight represents the amount of pressure which
the
polishing wheel will exert on the surface of the workpiece during the
polishing
operation to be described later.
On the horizontal portion 15 an electric motor 30 is positioned to drive
the horizontal gear wheels 32 and 34 which in turn drive the worm gear 36 to
rotate the
axle 40 and the polishing wheel 38. The polishing surface of the polishing
wheel is
the periphery 42 which contacts the workpiece on the surface to be polished.
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In addition, a second electric motor 44 drives a pair of gears 46 and 48
and causes the bracket 50 to revolve about a vertical axis (which coincides
with the
centre of the polishing wheel) so that the polishing wheel 38 supported on the
bracket
is constantly rotating in a different axial plane or direction relative to the
workpiece
surface, as illustrated in Figure 7.
Figure 7 also illustrates the means by which the workpiece is supported
and positioned relative to the polishing wheel. In Figure 7 the workpiece 60
is shown
resting on a bed 62 supported by a bridge 64. The bridge 64 is capable of
rotation
about a horizontal axis 66 which is positioned above the level of the
workpiece at a
distance which represents approximately the radius of curvature of the lens
(in this
case concave).
The bridge 64 and its pivot points 68 are supported by a platform 70
which is in turn supported at pivot points 72 so that the platform 70 is
rotatable
about a second horizontal axis 74 perpendicular to the other axis 66.
This gimbal arrangement allows the bed 62 of the workpiece to be
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oriented in a manner that presents the surface of the workpiece substantially
perpendicular to the downward pressure of the polishing wheel 38 at which ever
position on the surface is being polished.
In the illustrated embodiment the workpiece is concave and situated
beneath the axes 66 and 74. However, it is also possible to position a
workpiece
with a convex surface by positioning it above the level of the axes so as to
achieve a
configuration in which the polishing wheel is directed normal to the surface
during the
polishing operation.
Figure 8 illustrates in vertical cross-section how the support 20 is
movable in a vertical direction under the influence of the computer controlled
tower 14
so that the computer causes the polishing wheel 38 to contact the surface for
whichever configuration of the workpiece is selected during the grinding or
polishing
operation. The counterweight 26 will modify the weight of the polishing
mechanism so
as to determine the pressure to be applied by the polishing wheel to the
surface.
Figure 8 also illustrates how the working surface 42 of the polishing
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wheel 38 rotates about its own horizontal axis while revolving about a
vertical axis
perpendicular to the rotational axis and substantially perpendicular to the
peripheral
surface of the polishing wheel.
It can also be seen that the working surface is substantially parallel to the
curved surface 80 of the workpiece 60 so that the grinding and polishing
effect is
directed in a vertical direction which is substantially perpendicular to the
surface of
workpiece by virtue of the tilting of the workpiece on the bridge 64 about the
axis 68.
What is not shown in Figure 8 is a comparable rotation of the platform
70 about the other axis of support 72, referred to and shown in Figure 7.
Rotation
about both axes can achieve the appropriate orientation of the workpiece in
any
location of the polishing wheel on the surface.
It should also be understood that the rotation of the work piece about the
axes 66 and 74 of the platform 70 and bridge 64 is controlled by computerized
mechanism which coordinates with the computers controlling the location of the
polishing wheel 38.
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Therefore, for any given position on the surface of the workpiece, support
is capable of positioning the workpiece in the correct orientation and the
polishing
mechanism is capable of positioning the polishing wheel in the correct
location.
As previously mentioned, early lenses which were spherical and
rotationally symmetrical could be polished by moving a polishing device
designed to
spin about an axis perpendicular to the working surface, and move it from side
to side
(or periphery to periphery), while the workpiece was being rotated about a
vertical
axis, or its axis of symmetry normal to the surface. This technique, however,
does
not apply to aspherical surfaces and does not apply to polishing techniques
which
are designed to improve the perfection of the surface of a lens by selectively
removing
surface irregularity such a high spots. In order to achieve the latter purpose
it is
necessary for the polishing device to dwell longer on those portions of the
surface
which have high spots and to spend less time on portions of the surface which
are of
low relief. In order to do this techniques have been developed to identify by
optical
means and computer data a profile of the surface of the lens to be treated.
Figure 3
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illustrates a computer generated graphic illustration of the shape of the
surface of a
typical lens identifying high spots to be removed. Figure 3A shows a cross-
section
identifying a profile through a portion of a lens on which the surface is
related to a
common datum line or level.
In order to do the grinding or polishing necessary to remove these
irregularities at specific locations, the wheel 38 must work on identifiable
portions of
the surface of the lens and must therefore move across the surface of the lens
in a
series of traverses, as illustrated in Figure 2, which are controlled by and
identified
to the computer and enable the computer program to relate the position of the
grinding
wheel relative to the location of the high spots on the surface of the lens.
The
computer can, by appropriate programming, cause the polishing wheel to dwell
longer
on the high spots, travel more quickly across areas of low relief thereby
achieve a
more even surface closer to an ideal contour.
It will be recalled that the polishing wheel is capable of such movement
by virtue of its support on the column 14 which is movable in the first and
second axes
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by the first carriage 4 and second carriage 10. It will also be apparent from
Figure
1 and Figure 8 that polishing mechanism is controlled in a vertical direction
and the
balance between the weight of the polishing mechanism and the counterweight 26
will
establish the downward force applied to the polishing operation.
Figure 4 illustrates in simplified terms the location of the polishing wheel
relative to the surface of the lens in a generally central location. It should
be noted that
the polishing surface 42 of the wheel 38 has a degree of curvature
perpendicular to
the plane of the wheel which is substantially similar to the radius of
curvature of the
wheel in the plane of rotation.
Figure 5 illustrates how the surface of the polishing wheel engages the
surface of the lens when the lens is rotated about an axis remote from the
axis of the
wheel at a distance which is determined by the specification of the lens.
Although the arrow 43 represents the distance of the working surface of
the workpiece from the axis 66 or 74, it should be remembered that the
curvature of
an asymmetrical lens changes from the centre to the periphery and therefore
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CA 02356497 2001-08-30
does not represent a rotation of curvature for the entire lens.
With reference to Figure 2, it should be appreciated that it is undesirable
that the polishing wheel travelling along one of the illustrated lines of
traverse should
create a furrow or groove relative to the direction of movement. For this
reason, the
present invention provides a polishing wheel which not only rotates but
revolves so
that the direction of rotation constantly changes and the wheel is always
polishing in
a different direction relative to the direction of movement across the surface
of the
lens. This is, of course, accomplished by means of the motor and gear
mechanism
which causes the bracket 50 to rotate. In this arrangement it is also
advantageous
that the optimum point of contact (or the point of convergence between the
curvature
of the wheel and curvature of the lens) is also a point at which there is
substantial
relative movement between the wheel and the lens surface so as to achieve the
desired amount of grinding or polishing. This is not possible for a device
which spins
about its own axis perpendicular to the work surface.
Thus, by means of the illustrated apparatus and the method described
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above, it is possible to grind and/or polish aspherical lenses and to achieve
a high
degree of precision in the surface thereof.
While the illustrated embodiment has been described in terms of
horizontal and vertical relative directions, these may be altered to other
orientations
without changing the nature of the invention.
It will, of course, be realized that numerous modifications and variations
of the illustrated embodiments and methods described herein may be employed
without departing from the inventive concept herein.
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