U.S. patent application number 11/919572 was filed with the patent office on 2010-06-24 for apparatus and method.
Invention is credited to Paul Howard Morantz, Paul Raymond Shore.
Application Number | 20100159803 11/919572 |
Document ID | / |
Family ID | 34674033 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100159803 |
Kind Code |
A1 |
Shore; Paul Raymond ; et
al. |
June 24, 2010 |
Apparatus and method
Abstract
Apparatus comprising a tool (2) for forming an aspherical
surface on a material (14), and a support for supporting the
material (12) for rotation about an axis, the arrangement being
such that the tool (2) is restricted to movement with respect to
the material (14) in two substantially linear axes transverse to
each other. The apparatus may be a high-performance machine
comprising a measuring arrangement (18) mounted so as to extend
substantially across the surface of the material (14) and serving
to measure the distance between the tool (2) and a referencing
region (32) of the measuring arrangement (18) which can be in the
form of a symmetrical metrology device (18), the metrology device
being structurally unloaded. The apparatus is substantially
symmetrical in two substantially vertical planes substantially
perpendicular to and intersecting each other.
Inventors: |
Shore; Paul Raymond;
(Buckinghamshire, GB) ; Morantz; Paul Howard;
(Buckinghamshire, GB) |
Correspondence
Address: |
WILLIAM D. HALL
10850 STANMORE DR
POTOMAC
MD
20854
US
|
Family ID: |
34674033 |
Appl. No.: |
11/919572 |
Filed: |
May 2, 2006 |
PCT Filed: |
May 2, 2006 |
PCT NO: |
PCT/GB2006/001587 |
371 Date: |
January 20, 2010 |
Current U.S.
Class: |
451/5 ; 451/283;
451/361; 451/41; 451/443; 451/56; 451/6 |
Current CPC
Class: |
B24B 13/043
20130101 |
Class at
Publication: |
451/5 ; 451/283;
451/361; 451/443; 451/6; 451/41; 451/56 |
International
Class: |
B24B 49/04 20060101
B24B049/04; B24B 13/04 20060101 B24B013/04; B24B 49/12 20060101
B24B049/12; B24B 53/06 20060101 B24B053/06; B24B 19/00 20060101
B24B019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 29, 2005 |
GB |
0508695.4 |
Claims
1. Apparatus comprising a tool for forming an aspherical surface on
a material, and a support for supporting said material for rotation
about an axis, the arrangement being such that said tool is
restricted to movement with respect to said material in two
substantially linear axes transverse to each other.
2. Apparatus according to claim 1, wherein the two substantially
linear axes are in a substantially vertical plane in which the
rotational axis of the material lies.
3. Apparatus according to claim 2, wherein the two substantially
linear axes are constituted by a substantially vertical axis of
movement in said vertical plane and a substantially horizontal axis
of movement in said vertical plane.
4. Apparatus according to any preceding claim, wherein said
apparatus has a relatively high stiffness loop.
5. Apparatus according to claim 4, wherein the loop of stiffness is
a substantially quadrangular stiffness loop between said tool and
said material.
6. Apparatus according to any preceding claim, wherein said tool is
a grinding machine tool.
7. Apparatus according to claim 6 as appended to claim 4, wherein
both static loop stiffness and dynamic loop stiffness are at a
relatively high level.
8. Apparatus according to any preceding claim, and further
comprising two movement sub-systems by way of which said tool is
moved in said two substantially linear axes.
9. Apparatus according to claim 8 as appended to claim 3, wherein a
first movement sub-system moves said tool vertically and a second
movement sub-system moves said tool horizontally.
10. Apparatus according to claim 8 or 9, wherein the first movement
sub-system is an integral part of the second movement
sub-system.
11. Apparatus according to any one of claims 8 to 10, wherein the
movement sub-systems are removable from said apparatus.
12. Apparatus according to any one of claims 8 to 11, wherein the
movement sub-systems comprise respective pairs of associated linear
motors and linear encoders for movement of said tool along
respective pairs of bearing rails.
13. Apparatus according to any preceding claim wherein said tool is
a high-performance machine tool further comprising a measuring
arrangement mounted so as to extend substantially across a surface
of said material and serving to measure the distance between said
tool and a referencing region of said measuring arrangement.
14. Apparatus according to claim 13, wherein said measuring
arrangement comprises a metrology frame including said referencing
region, and a laser interferometer system mounted on the
high-performance tool.
15. Apparatus according to claim 14, wherein said referencing
region is in the form of a mirror.
16. Apparatus according to claim 15, wherein said mirror is a
low-mass straight-edge mirror and the laser interferometer system
is a small independent laser interferometer mounted on a carriage
unit which carries the tool.
17. Apparatus according to any one of claims 14 to 16, wherein said
laser interferometer is mounted on the upper end of an invar
support beam, at the lower end of which there is an air-bearing
linear variable differential transducer (LVDT) contact probe.
18. Apparatus according to any one of claims 14 to 17, wherein said
metrology frame is a symmetrical metrology frame, and said
apparatus further comprises a support upon which said metrology
frame is mounted, said metrology frame being structurally
unloaded.
19. Apparatus according to claim 18, wherein said symmetrical
metrology frame is a fully symmetrical metrology frame.
20. Apparatus according to any one of claims 14 to 19, the
arrangement being such that the metrology frame is outside of the
working volume.
21. Apparatus according to any preceding claim, wherein said
apparatus is substantially symmetrical in two substantially
vertical planes substantially perpendicular to and intersecting
each other.
22. Apparatus according to claim 21, wherein said apparatus is
substantially box-shaped.
23. Apparatus according to any preceding claim, wherein said tool
is a numerically controlled machine tool having a tool surface of a
pre-determined shape, and a data processing system for generating
geometric information in relation to said tool surface.
24. Apparatus according to claim 23 as appended to claim 6, wherein
said tool surface is an abrasive surface of said grinding tool.
25. Apparatus according to claim 23 or 24, wherein said data
processing system uses Non-Uniform Rational B-Splines (NURBS) to
monitor wear of said tool surface.
26. Apparatus according to any one of claims 23 to 25, and further
comprising a forming device located in one of said substantially
linear axes for forming a desired cross-sectional profile on said
tool surface, a conditioning device having a conditioning surface
for conditioning the formed tool surface, and an inspecting device
for determining a cross-sectional profile of said conditioning
surface.
27. Apparatus according to claim 26, wherein said forming device is
a forming wheel, the conditioning device is a dressing stick and
the inspecting device is a surface-contacting probe which contacts
the conditioning surface of the dressing stick.
28. Apparatus according to claim 26 or 27 wherein said tool
comprises a cup wheel which includes said surface of said tool and
which has a symmetrical toric cross-sectional profile when formed,
such that said measurement of the cross-sectional profile of the
conditioning surface in one direction can be electronically
transposed to give measurements in a direction substantially
perpendicular to that in which the determination is taken.
29. Apparatus according to any one of claims 8 to 28, wherein the
combined mass of said tool and the two movement sub-systems is less
than substantially 750 Kg.
30. Apparatus according to any preceding claim, wherein said
material is a free-form optic.
31. Apparatus according to claim 30 as appended to claim 22,
wherein for a 2 m diameter free-form optic, the box-shape structure
is substantially 3 m in length by substantially 1.5 m in height and
weighs substantially 12 tons.
32. Apparatus according to any preceding claim, and further
comprising a thermal stabilising system for controlling the
temperature of said apparatus.
33. Apparatus according to claim 32, wherein said thermal
stabilising system comprises high diffusivity materials.
34. Apparatus according to claim 32 or 33 as appended to claim 12,
wherein grating scales of the encoders are of low co-efficient of
thermal expansion and are suitably restrained to prevent thermal
creep.
35. A method of forming an asphercial surface on a material,
comprising rotating said material about an axis of rotation, moving
a tool with respect to said surface, and restricting the movement
of said tool to movement in two substantially linear axes
transverse to each other.
36. A method according to claim 35, wherein the two substantially
linear axes of tool movement are in a substantially vertical plane
in which the rotational axis of the material lies.
37. A method according to claim 36, wherein the two substantially
linear axes are constituted by a substantially vertical axis of
movement in said vertical plane and a substantially horizontal axis
of movement in said vertical plane.
38. A method according to any one of claims 35 to 37, wherein said
tool is a grinding machine tool.
39. A method according to any one of claims 35 to 38, wherein said
moving is by way of two movement sub-systems.
40. A method according to claim 37, 38 or 39, wherein a first
movement sub-system moves said tool substantially vertically and a
second movement sub-system moves said tool substantially
horizontally.
41. A method according to claim 40, wherein the first movement
sub-system is an integral part of the second movement
sub-system.
42. A method according to any one of claims 39 to 41, wherein the
two movement sub-systems are removable from said tool.
43. A method according to any one of claims 35 to 42, wherein said
tool is a high-performance machine tool and further comprising a
measuring arrangement mounted so as to extend substantially across
a surface of said material and serving to measure the distance
between said tool and a referencing region of said measuring
arrangement.
44. A method according to any one of claims 35 to 43 and further
comprising providing a pre-determined shape to a surface of said
tool, operating the tool surface against a surface of said material
surface, and generating using a data processing system geometric
information in relation to the tool surface.
45. A method to claim 44, wherein said data processing system uses
Non-Uniform Rational B-Splines (NURBS) for monitoring wear of said
tool surface.
46. A method according to claim 44 or 45, and further comprising
forming with a forming device a desired cross-sectional profile of
said surface of said tool arranged to move in one of said
substantially linear axes, said forming occurring in one of said
substantially linear axes, conditioning said surface of said tool
by a conditioning surface of a conditioning device, and determining
with an inspection device the cross-sectional profile of said
conditioning surface.
47. A method according to claim 46, wherein said forming is by a
forming wheel, the conditioning is by a dressing stick and the
inspecting is by a surface-contacting probe which contacts the
conditioning surface of the dressing stick.
48. A method according to claim 46 or 47, wherein said tool
comprises a cup wheel which includes said surface of said tool and
which has a symmetrical toric cross-sectional profile when formed,
such that said measurement of the cross-sectional profile of the
conditioning surface in one direction can be electronically
transposed to give measurements in a direction substantially
perpendicular to that in which the determination is taken.
49. A method according to any one of claims 35 to 48, and further
comprising a thermally stabilising tool.
50. A high-performance machine comprising a tool for working at a
surface of a material, a support which supports said material, and
a measuring arrangement mounted so as to extend substantially
across said surface and serving to measure the distance between
said tool and a referencing region of said measuring
arrangement.
51. A machine according to claim 50, wherein said measuring
arrangement comprises a metrology frame which includes said
referencing region.
52. A machine according to claim 51, wherein said referencing
region is in the form of a mirror and said machine further
comprises a laser interferometer system mounted on the
high-performance tool.
53. A machine according to claim 52, wherein said mirror is a
low-mass straight-edge mirror and the laser interferometer system
is a small independent laser interferometer mounted on a carriage
unit which carries the tool.
54. A machine according to claim 52 or 53, wherein said laser
interferometer is mounted on the upper end of an invar support
beam, at the lower end of which there is an air-bearing linear
variable differential transducer (LVDT) contact probe.
55. A machine according to any one of claims 50 to 54, wherein said
measuring arrangement is structurally unloaded.
56. A machine according to any one of claims 50 to 55, the machine
being substantially symmetrical in two substantially vertical
planes substantially perpendicular to and intersecting each
other.
57. A machine according to any one of claims 50 to 56, wherein said
machine is a numerically controlled machine having two
substantially linear axes and a rotational axis, a tool surface
having a predetermined shape, and a data processing system for
generating geometric information in relation to said tool
surface.
58. A machine according to claim 57, the arrangement being such
that said tool is restricted to movement with respect to said
material in said two substantially linear axes.
59. A machine according to claim 57 or 58, wherein said tool is
substantially linearly movable across said apparatus, and further
comprises a forming device located in the substantially linear path
of said tool for forming a desired cross-sectional profile on said
tool surface, a conditioning device having a conditioning surface
for conditioning the formed tool surface, and an inspecting device
for determining a cross-sectional profile of said conditioning
surface.
60. Apparatus comprising a tool for working at a surface of a
material, a symmetrical metrology device, and a support upon which
said metrology device is mounted, said metrology device being
structurally unloaded and including a single referencing device for
providing positional information of said tool with respect to said
surface.
61. Apparatus according to claim 60, wherein said symmetrical
metrology device is a fully symmetrical metrology frame.
62. Apparatus according to claim 60 or 61, and further comprising a
laser interferometer system mounted on said tool which has only two
axes of tool movement.
63. Apparatus according to claim 62, wherein said two axes of tool
movement are substantially linear axes transverse to each
other.
64. Apparatus according to any one of claims 60 to 63, wherein said
metrology device is outside of the working volume.
65. Apparatus according to any one of claims 60 to 64, said
apparatus being substantially symmetrical in two substantially
vertical planes substantially perpendicular to and intersecting
each other.
66. Apparatus according to any one of claims 63 to 65, wherein said
tool is a numerically controlled tool having said two substantially
linear axes, a tool surface having a pre-determined shape, and a
data processing system for generating geometric information in
relation to said tool surface.
67. Apparatus according to claim 66, wherein said tool surface is
an abrasive surface of a grinding tool.
68. Apparatus according to claim 66 or 67, wherein said data
processing system uses Non-Uniform Rational B-Splines (NURBS) to
monitor wear of said tool surface.
69. Apparatus according to claim 67 or 68, wherein said tool is
substantially linearly movable across said apparatus, and further
comprises a forming device located in the substantially linear path
of said tool for forming a desired cross-sectional profile on said
abrasive surface, a conditioning device having a conditioning
surface for conditioning the formed abrasive surface, and an
inspecting device for determining a cross-sectional profile of said
conditioning surface.
70. Apparatus comprising a tool for working at material, said
apparatus being substantially symmetrical in two substantially
vertical planes substantially perpendicular to and intersecting
each other.
71. Apparatus according to claim 70, wherein said apparatus is
substantially box-shaped.
72. Apparatus according to claim 70 or 71, wherein said tool is for
forming an aspherical surface on said material, and further
comprising a support for supporting said material for rotation
about an axis, the arrangement being such that said tool is
restricted to movement with respect to said material in two
substantially linear axes transverse to each other.
73. Apparatus according to claim 72, and further comprising a
measuring arrangement mounted so as to extend substantially across
said surface and serving to measure the distance between said tool
and a referencing region of said measuring arrangement.
74. Apparatus according to claim 73, wherein said measuring
arrangement is a symmetrical metrology device, said metrology
device being structurally unloaded and including said referencing
region for providing positional information of said tool with
respect to said surface.
75. Apparatus according to any one of claims 72 to 74, wherein said
tool is a numerically controlled tool having said two substantially
linear axes, a tool surface having a pre-determined shape, and a
data processing system for generating geometric information in
relation to said tool surface.
76. Apparatus according to claim 75 wherein said tool is linearly
movable across said apparatus, and further comprises a forming
device located in the substantially linear path of said tool for
forming a desired cross-sectional profile on said tool surface, a
conditioning device having a conditioning surface for conditioning
the formed tool surface, and an inspecting device for determining a
cross-sectional profile of said conditioning surface.
77. A numerically controlled machine comprising a tool and having
two substantially linear axes and a rotational axis, a tool surface
having a predetermined shape, and a data processing system for
generating geometric information in relation to said tool
surface.
78. A machine according to claim 77, wherein said tool surface is
an abrasive surface of a grinding tool.
79. A machine according to claim 77 or 78, wherein said data
processing system uses Non-Uniform Rational B-Splines (NURBS) to
monitor wear of said tool surface.
80. A machine according to any one of claims 77 to 79, wherein said
tool is for forming an aspherical surface on a material, and
further comprises a support for supporting said material for
rotation about an axis, the arrangement being such that said tool
is restricted to movement with respect to said material in said two
substantially linear axes which are transverse to each other.
81. A machine according to claim 80, and further comprising a
measuring arrangement mounted so as to extend substantially across
the surface of said material and serving to measure the distance
between said tool and a referencing region of said measuring
arrangement.
82. A machine according to any one of claims 77 to 81, wherein said
numerically controlled machine is a high-performance numerically
controlled machine.
83. A machine according to claim 81 or 82, wherein said measuring
arrangement comprises a symmetrical metrology device, and a support
upon which said metrology device is mounted, said metrology device
being structurally unloaded and including said referencing region
for providing positional information of said tool with respect to
the surface of said material.
84. A machine according to any one of claims 77 to 83, wherein said
machine is substantially symmetrical in two substantially vertical
planes substantially perpendicular to and intersecting each
other.
85. A machine according to any one of claims 77 to 84, said tool
being substantially linearly movable across said apparatus, and
further comprising a forming device located in the substantially
linear path of said tool for forming a desired cross-sectional
profile on said tool surface, a conditioning device having a
conditioning surface for conditioning the tool surface, and an
inspecting device for determining a cross-sectional profile of said
conditioning surface.
86. A method comprising providing a pre-determined shape to a
surface of a tool, operating the tool surface against a material
surface, and generating using a data processing system geometric
information in relation to the tool surface.
87. A method according to claim 86, wherein said data processing
system uses Non-Uniform Rational B-Splines (NURBS) for monitoring
wear of said tool surface.
88. A method according to claim 86 or 87, and further comprising
forming an asphercial surface on said material surface, said
aspherical spherical surface formed by rotating the material about
an axis of rotation, moving said tool with respect to said surface,
and restricting the movement of said tool to movement in two
substantially linear axes transverse to each other.
89. A method according to any one of claims 86 to 88, and further
comprising forming with a forming device a desired cross-sectional
profile of the tool surface, the tool being arranged to move in a
substantially linear path, said forming occurring in said
substantially linear path, conditioning the tool surface by a
conditioning surface of a conditioning device, and determining with
an inspection device the cross-sectional profile of said
conditioning surface.
90. Apparatus comprising a tool having a material-contacting
surface, said tool being substantially linearly movable across said
apparatus, a forming device located in the substantially linear
path of said tool for forming a desired cross-sectional profile on
said material-contacting surface, a conditioning device having a
conditioning surface for conditioning the formed
material-contacting surface, and an inspecting device for
determining a cross-sectional profile of said conditioning
surface.
91. Apparatus according to claim 90, wherein said forming device is
a forming wheel, the conditioning device is a dressing stick and
the inspecting device is a surface-contacting probe which contacts
the conditioning surface of the dressing stick.
92. Apparatus according to claim 90 or 91, wherein said
material-contacting surface is an abrasive surface of a grinding
tool having a cup wheel which has a symmetrical toric
cross-sectional profile when formed.
93. Apparatus according to any one of claims 90 to 92, wherein said
determining of the cross-sectional profile of the conditioning
surface in one direction can be electronically transposed to give
measurements in a direction substantially perpendicular to that in
which the determination is taken.
94. Apparatus according to claim 92 or 93, wherein said grinding
tool is for forming an aspherical surface on a material, and
further comprises a support for supporting said material for
rotation about an axis, the arrangement being such that said
grinding tool is restricted to movement with respect to said
material in two substantially linear axes transverse to each
other.
95. Apparatus according to claim 94, and further comprising a
measuring arrangement mounted so as to extend substantially across
said material and serving to measure the distance between said
grinding tool and a referencing region of said measuring
arrangement.
96. Apparatus according to any one of claims 92 to 95, wherein said
grinding tool is a high-performance grinding tool.
97. Apparatus according to claim 95 or 96, wherein said measuring
arrangement comprises a symmetrical metrology device, and a support
upon which said metrology device is mounted, said metrology device
being structurally unloaded and including said referencing region
for providing positional information of said grinding tool with
respect to the material surface.
98. Apparatus according to any one of claims 90 to 97, wherein said
apparatus is substantially symmetrical in two substantially
vertical planes substantially perpendicular to and intersecting
each other.
99. Apparatus according to any one of claims 90 to 98, wherein said
tool is a numerically controlled tool, said material-contacting
surface having a predetermined shape, and a data processing system
for generating geometric information in relation to said
material-contacting surface.
100. Apparatus according to claim 99, wherein said data processing
system uses Non-Uniform Rational B-Splines (NURBS) to monitor wear
of said material-contacting surface.
101. A method comprising forming with a forming device a desired
cross-sectional profile of a material-contacting surface of a tool
arranged to move in a substantially linear path, said forming
occurring in said substantially linear path, conditioning said
material-contacting surface by a conditioning surface of a
conditioning device, and determining with an inspection device the
cross-sectional profile of said conditioning surface.
102. Apparatus according to claim 101, wherein said tool is a
grinding tool having a cup wheel which has a symmetrical toric
cross-sectional profile when formed, said method further comprising
electronically transposing the determination of the cross-sectional
profile of the conditioning surface taken in one direction to give
measurements in a direction substantially perpendicular to that in
which the determination is taken.
103. A method according to claim 101 or 102, and further comprising
forming an asphercial surface on a material, including rotating
said material about an axis of rotation, moving said tool with
respect to said surface, and restricting the movement of said tool
to movement in two substantially linear axes transverse to each
other.
104. A method according to any one of claims 101 to 103, and
further comprising providing a pre-determined shape to said
material-contacting surface, operating said material-contacting
surface against the material surface, and generating using a data
processing system geometric information in relation to the tool
surface.
105. A method according to claim 104, wherein said data processing
system uses Non-Uniform Rational B-Splines (NURBS) for monitoring
wear of said material-contacting surface.
Description
[0001] This invention relates to machine tools, and in particular,
grinding machine tools.
[0002] A known use of grinding machine tools is for the production
of mirror segments needed to produce ground-based telescopes or
extra large telescopes (ELT's). The proposed next generation of
ground based telescopes or ELT's will bring about an unprecedented
demand for hundreds of large off-axis mirror segments each having a
diameter in the range of 1 to 2 metres. Such mirror segments will
be made from glass or ceramic material and have a hexagonal shape
as used, for instance, in the Hobby-Eberly telescope. At present,
the manufacturing technologies for producing ultra-precise mirrors
having a diameter of 1 to 2 metres are associated with processing
times of hundreds of hours. Consequently, the time to manufacture
hundreds, even thousands, of such mirrors for an ELT would involve
many years of production.
[0003] In the late 1970's, high-precision diamond turning machines
were devised to produce large optics in the 1 to 2 meter diameter
range. However, these machines and subsequent machines tend to be
of a very large size and weight (many tonnes).
[0004] According to a first aspect of the present invention, there
is provided apparatus comprising a tool for forming an aspherical
surface on a material, and a support for supporting said material
for rotation about an axis, the arrangement being such that said
tool is restricted to movement with respect to said material in two
substantially linear axes transverse to each other.
[0005] According to a second aspect of the present invention, there
is provided a method of forming an asphercial surface on a
material, comprising rotating said material about an axis of
rotation, moving a tool with respect to said surface, and
restricting the movement of said tool to movement in two
substantially linear axes transverse to each other.
[0006] Owing to these two aspects of the invention, it is possible
to provide a high level of loop stiffness between a tool and the
material to be worked.
[0007] Advantageously, the two substantially linear axes of tool
movement are in a substantially vertical plane in which the
rotational axis of the material lies.
[0008] In this way a machine tool such as a grinding machine or a
diamond turning machine can have its tool limited to motions in
only two axes, namely substantially vertical movement in a vertical
plane and substantially horizontal movement in the vertical plane.
Thus, the amount of moving parts in the machine is reduced, thereby
enabling the machine to be relatively stiff.
[0009] Preferably, the loop of stiffness is a substantially
quadrangular stiffness loop between the tool and the material.
[0010] Having a high level of loop stiffness in a grinding machine
is extremely important in grinding ceramics and glasses rapidly
whilst maintaining good form accuracy. In order to ensure that
subsequent polishing operations are effective, the output quality
from a fixed abrasive grinding operation must have both good form
accuracy and minimal sub-surface damage, which may be caused by the
abrasive grain penetration depth of the grinding tool. In order to
control the abrasive penetration depth it is necessary to have
control of the relative motion of the abrasive surface of the
grinding tool with respect to the material, or workpiece,
surface.
[0011] Loop stiffness can be divided into two categories, namely
static loop stiffness and dynamic loop stiffness, both of which
are, preferably, at a relatively high level. Low levels of static
loop stiffness result in edge "roll-off" errors produced when the
grinding wheel of the grinding tool moves out of full contact with
the workpiece surface. High levels of dynamic loop stiffness are
also critical to permit the control of the abrasive penetration at
sufficiently high force levels to provide effective material
removal rates.
[0012] An advantage of having a relatively high level of loop
stiffness is that, per unit of time, there is a high output of
finished product which is of good quality.
[0013] According to a third aspect of the present invention, there
is provided a high-performance machine comprising a tool for
working at a surface of a material, a support which supports said
material, and a measuring arrangement mounted so as to extend
substantially across said surface and serving to measure the
distance between said tool and a referencing region of said
measuring arrangement.
[0014] Owing to this aspect of the invention, a measuring
arrangement can be provided on a high-performance machine tool for
referencing machine motions against the measuring arrangement.
[0015] A high-performance machine tool offers a machining
capability approaching a relative accuracy level of 5 parts in one
million, i.e. 5 microns for a 1 metre workpiece diameter. The
measuring arrangement allows the accuracy to be improved to around
a relative accuracy level of 1 part in one million, i.e. 1 micron
for a 1 metre workpiece.
[0016] Advantageously, the machine tool is a high-performance
grinding or diamond turning machine and the measuring arrangement
comprises a metrology frame which has a referencing region in the
form of a mirror and which is mounted on the material support of
the machine, and a laser interferometer system mounted on the
high-performance tool.
[0017] In this way, the tool, such as a grinding tool or a diamond
turning tool, can be moved with great accuracy without a
deterioration in performance.
[0018] Preferably, the referencing mirror of the metrology frame is
a low-mass straight-edge mirror and the laser interferometer system
is a small independent laser interferometer mounted on a carriage
unit which carries the tool. Advantageously, the laser
interferometer is mounted on the upper end of an invar support
beam, at the lower end of which there is an air-bearing linear
variable differential transducer (LVDT) contact probe. Such an
arrangement helps to compensate for any errors in the tool
motion.
[0019] According to a fourth aspect of the present invention, there
is provided apparatus comprising a tool for working at a surface of
a material, a symmetrical metrology device, and a support upon
which said metrology device is mounted, said metrology device being
structurally unloaded and including a single referencing device for
providing positional information of said tool with respect to said
surface.
[0020] Owing to this aspect of the invention, a symmetrical
metrology device with a single referencing device can be provided
on a machine tool and not have any load bearing parts of the
machine attached to it.
[0021] Advantageously, the metrology device is a fully symmetrical
metrology frame associated with a laser interferometer system
mounted on the tool which has only two axes of tool movement.
[0022] Thus, a high-accuracy feedback-controlled machine tool can
be obtained in which the position of the tool relative to the
material can be monitored without the need for a multi-axis
interferometer system or the need for the metrology frame to
protrude into the working volume.
[0023] According to a fifth aspect of the present invention, there
is provided apparatus comprising a tool for working at material,
said apparatus being substantially symmetrical in two substantially
vertical planes substantially perpendicular to and intersecting
each other.
[0024] Owing to this aspect of the invention, it is possible to
provide a fully symmetrical machine tool which is structurally
stable.
[0025] Not only is the machine tool symmetrical in a right-to-left
direction but also in a front-to-back direction, which gives the
machine tool a box-shape appearance. Such a machine tool is
relatively thermally more stable and suffers less from tilt errors
caused by thermal gradients when the machine is in operation.
[0026] According to a sixth aspect of the present invention, there
is provided a numerically controlled machine comprising a tool and
having two substantially linear axes and a rotational axis, a tool
surface having a pre-determined shape, and a data processing system
for generating geometric information in relation to said tool
surface.
[0027] According to a seventh aspect of the present invention,
there is provided a method comprising providing a predetermined
shape to a surface of a tool, operating the tool surface against a
material surface, and generating using a data processing system
geometric information in relation to the tool surface.
[0028] Owing to these two aspects of the invention, it is possible
to generate geometric information in relation to change of the
shape of the tool surface.
[0029] The tool surface can be an abrasive surface of a grinding
tool. The surface of the material to be shaped may be
non-symmetrical such as that for a free-form optical element, such
that, during the grinding operation in which the grinding surface
wears in such a manner as to depart from the pre-determined shape,
the contact zone between the abrasive surface of the grinding tool
and the surface of the material changes to tend to produce a
non-optimal contact zone. The shape of the abrasive surface is
determined by a data processing system such that any change
required or any error to be compensated for can be dealt with.
[0030] Advantageously, the data processing system uses Non-Uniform
Rational B-Splines (NURBS) to monitor wear of the tool surface.
[0031] According to an eighth aspect of the present invention,
there is provided apparatus comprising a tool having a
material-contacting surface, said tool being substantially linearly
movable across said apparatus, a forming device located in the
substantially linear path of said tool for forming a desired
cross-sectional profile on said material-contacting surface, a
conditioning device having a conditioning surface for conditioning
the formed material-contacting surface, and an inspecting device
for determining a cross-sectional profile of said conditioning
surface.
[0032] According to a ninth aspect of the present invention, there
is provided a method comprising forming with a forming device a
desired cross-sectional profile of a material-contacting surface of
a tool arranged to move in a substantially linear path, said
forming occurring in said substantially linear path, conditioning
said material-contacting surface by a conditioning surface of a
conditioning device, and determining with an inspection device the
cross-sectional profile of said conditioning surface.
[0033] Owing to these two aspects of the invention, it is possible
to determine the cross-sectional profile of the material-contacting
surface by determining the cross-sectional profile of the
conditioning surface.
[0034] Advantageously, the forming device is a forming wheel, the
conditioning device is a dressing stick and the inspecting device
is a surface-contacting probe which contacts the conditioning
surface of the dressing stick. Preferably, the material-contacting
surface is an abrasive surface of a grinding tool having a cup
wheel which has a symmetrical toric cross-sectional profile when
formed, such that the measurement of the cross-sectional profile of
the conditioning surface in the one direction can be electronically
transposed to give measurements in a direction substantially
perpendicular to that in which the determination is taken. This has
the advantage that no movement of the tool is needed in the
direction perpendicular to that in which the determination is
taken. This arrangement enables a machine tool which requires
forming and dressing of a tool surface to have a relatively high
degree of stiffness.
[0035] In order that the invention can be clearly and completely
disclosed, reference will now be made, by way of example, to the
following drawings in which:--
[0036] FIG. 1 is a cross-sectional perspective view of a grinding
machine,
[0037] FIG. 2 is a perspective view from above and one end of the
grinding machine,
[0038] FIG. 3 is a cross-section of the grinding machine in a plane
at substantially a right-angle to that of FIG. 1, and
[0039] FIG. 4 is a perspective view from above and the opposite end
to that of FIG. 2.
[0040] Referring to FIG. 1, a grinding machine tool 2 comprises a
grinding tool piece 4 which includes a tool spindle portion 6 and a
grinding cup wheel 8. The machine 2 further comprises two movement
sub-systems by way of which the tool 4 is moved and a
material-supporting table 12 for supporting material, or a
workpiece 14, to be acted upon by the tool 4. There is a work
station 16 at which an operator of the machine 2 can control the
machine. The machine 2 also comprises a metrology frame unit 18
mounted from a base portion 3 of the machine 2.
[0041] In order to minimise the moving masses, the machine motions
are limited to three axes, namely two stacked linear axes which
carry the grinding spindle 6 over a single rotary axis of the
workpiece 14 supported on the table 12. A tool carriage unit 10 in
which the grinding spindle 6, which is preferably a hydrostatic oil
bearing spindle, is mounted, is preferably of aluminium
construction. The carriage unit 10 is itself further mounted on
tube-type hydrostatic oil linear bearing rails 20 and movement of
the carriage unit 10 along the rails 20 (i.e. in to and out of the
page of FIG. 1) is driven by a pair of linear motors 22 mounted
either side of the hydrostatic bearing rails 20. Two high
performance linear encoders are employed closely positioned to the
bearing rails 20. An advantage of this slideway sub-system of the
carriage 10 and bearing rails 20 is that the carriage 10 can be
removed and replaced with other slideway tool sub-systems, for
example, a diamond turning unit. Movement along the bearing rails
20 enables the tool 4 to be moved substantially horizontally in a
substantially linear path across the workpiece 14 in a
substantially vertical plane of the machine 2, and in relation to a
three-dimensional positioning system such movement is in the X-axis
direction. The tool 4 is also movable in a substantially vertical
direction in the substantially vertical plane of the machine 2,
which in the three-dimensional positioning system would be movement
in the Z-axis direction. Movement in the Z-axis is achieved by way
of a further pair of motors 22' (shown also in FIG. 3). To ensure
safe operation of the Z-axis, and to reduce motor loading, a
double-acting seal-less air counter-balance cylinder 24 is used in
association with Z-axis bearing rails 20' (see also FIG. 3) and
positioned such that it acts close to the centre of gravity of the
moving Z-axis mass. The movement permitted in the Z-axis is much
shorter than that permitted in the X-axis. The Z-axis sub-system
forms an integral part of the longer X-axis slideway sub-system
which also includes an X-axis carriage unit 21 in order to minimise
any cantilevers. The hydrostatic bearing rails 20 are, preferably,
rectangular in cross-section, as shown, and are directly mounted
onto the upper portion 23 of the main machine structure. As is the
case with the X-axis sub-system, two linear motors and two encoders
are employed using a symmetrical design with minimal parallax
errors, i.e. Abbe Offset Error.
[0042] The substantially vertical rotary axis of the table 12 which
carries the workpiece 14 is driven by a direct drive hydrostatic
oil bearing unit 28 which is of a low moving mass. This direct
drive hydrostatic oil bearing unit 28 has a small depth to diameter
ratio to ensure the distance from the motor and an associated
rotary encoder to the workpiece surface is minimised.
[0043] The grinding spindle 6 is inclined relative to the vertical
and is fixed firmly in position via the carriage unit 10. The
grinding wheel 8 is therefore also inclined to the same degree and
uses a toric-shaped cup wheel. The cup wheel has an external
diameter of approximately 325 mm and provides a grinding speed in
the range of 25 to 35 m/s (25 to 35 Hz). The combined mass of the
tool 4 and the Z-axis sub-system embedded within the X-axis
sub-system is minimised to less than 750 Kg.
[0044] The whole machine structure is based around a substantially
symmetrical box-shape. It is substantially symmetrical not only
from side-to-side but also front-to-back and simple shaped castings
are used to support the main active Z- and X-axis movements. In
order to produce a 2 m diameter free-form optic for a large
telescope, the box-shape structure is substantially 3 m in length
by substantially 1.5 m in height and would weigh around 12 tons
which is 10% of the total mass of some existing machines.
Obviously, for the production of smaller optics a smaller box-shape
structure can be used.
[0045] By having the absolute minimum number of active motions of
minimal mass, namely the tool being only movable in a single
substantially vertical plane of the machine 2, and the use of high
stiffness bearings allows the machine 2 to have relatively high
dynamic and relatively high static loop stiffness. The loop of
stiffness is substantially quadrangular in form with operational
forces of the grinding tool 4 being transferred upwardly and
outwardly through the periphery of the machine 2 and subsequently
downwardly and inwardly to beneath the workpiece 14. Having such a
relatively high degree of static and dynamic loop stiffness, a
relatively high output of finished work pieces having good quality
can be achieved.
[0046] In having the machine 2 of relatively low mass and of a
compact modular design, thermal stabilising systems have been
incorporated to control the temperature of the hydrostatic bearing
fluids. Motor cooling systems have been incorporated for the linear
and rotary motors and, in addition, temperature control of the
machine structure itself and the grinding fluid are also present.
High diffusivity materials have been employed to reduce the effects
of the main slow moving heat sources, i.e. the X-axis motors 22.
Furthermore, the top structure 23 of the machine 2 which mounts the
X-axis encoders is thermally monitored in order to independently
validate thermal stability. The grating encoder scales which are
measuring devices which measure the position on the linear X- and
Z-axes are of low co-efficient of thermal expansion and are
suitably restrained to prevent thermal creep. These grating scales
are positioned symmetrically either side of the moving carriages
for both the X- and Z-axes.
[0047] Referring to FIG. 2, the metrology frame unit 18 is non-load
bearing and is mounted from the base portion 3 of the machine 2 and
includes, on an upper substantially horizontal beam 30, a
referencing region in the form of a single low mass straight edge
mirror 32. Only a single mirror 32 need be used owing to the
restricted movement of the tool 4 in the Z- and X-axes.
[0048] Referring to FIG. 3, the Z-axis carriage unit 10 is provided
with an independent laser interferometer 34 mounted on an upper end
of an invar support beam 36 which is thermally stable. The laser
interferometer 34 has a short measurement path in order to minimise
ambient effects. At the lower end of the invar support beam 36, a
first air bearing linear variable differential transducer (LVDT)
probe 38 is present. The laser interferometer 34 measures the
distance to the straight edge mirror 32 when the LVDT probe 38 is
brought into contact with the surface of the workpiece 14. The
metrology frame unit 18 and the laser interferometer 34 therefore
provide the ability of in-situ post-process measurement, owing to
the measured distance being progressively across the surface of the
workpiece 14 along the substantially linear path of the X-axis
movement and thereby providing a profile of that surface. In a
similar way to the structure of the machine 2, the metrology frame
unit 18 is substantially fully symmetrical both from side-to-side
and from front-to-back.
[0049] Referring to FIGS. 3 and 4, after the grinding wheel 8 is
fitted onto the inclined grinding spindle 6, the abrasive surface
40 of the wheel 8 needs to be shaped into the correct
cross-sectional form which is a toric cross-sectional shape.
Therefore, it is necessary to machine this desired cross-sectional
shape onto the abrasive surface 40. In order to impart the correct
toric cross-sectional shape to the abrasive surface 40, the
abrasive surface 40 is formed, or trued, against a forming or
truing wheel 42. The truing wheel 42 which has an axis of rotation
substantially perpendicular to the rotary axis of the workpiece 14
is shaped such that the rim of the wheel, which is preferably of
diamond coated steel, is shaped to have the inverse cross-sectional
shape as that to be imparted to the abrasive surface 40. In order
to true the grinding wheel abrasive surface 40, the tool 4 is moved
in the X-axis direction in the vertical plane to above the rim of
the truing wheel 42 which is located in a position towards one end
of the machine 2, as shown, and lies in the vertical plane through
which the tool 4 is moved. After the truing operation, the abrasive
surface 40 has the correct cross-sectional shape, but requires a
further operation to condition the abrasive surface 40 such that
the diamond abrasives thereon protrude beyond the bond matrix of
that surface. This is to ensure that the diamond abrasives cut
effectively during the grinding process. This conditioning process
is conventionally known as dressing and is carried out by plunging
the abrasive surface 40 of the grinding wheel 8 into a fixed
dressing stick 44 which is located proximally and to one side of
the truing wheel 42, and is preferably made of an abrasive ceramic
compound. The tool 4 is moved in the X-axis direction after the
truing operation to bring the abrasive surface 40 into contact with
the top conditioning surface 46 of the dressing stick 44.
Consequently, the conditioning surface 46 is shaped using the
abrasive surface 40 of the grinding wheel 8 which, thus, imparts
the cross-sectional shape of the abrasive surface 40 into the
conditioning surface 46. Since the dressing stick 44 is located to
one side of the truing wheel 42, the truing and dressing operations
occur at different positions on the inclined toric-shaped abrasive
surface. Once the dressing operation has been completed, the tool 4
can be used for grinding the surface of the material 14. The width
of the rim of the truing wheel 42 and the width of the dressing
stick 44 are the same as or greater than the width of the abrasive
surface 40 of the grinding cup wheel 8.
[0050] After grinding for some time, the abrasive surface 40 wears,
such that the correct toric cross-sectional shape may wear away.
However, owing to the arrangement of the truing wheel 42 and the
dressing stick 44 on the machine 2, the machine 2 has a system
whereby wear of the abrasive surface 40 is determined by inspecting
the cross-sectional shape of the conditioning surface 46. As is the
case with the truing wheel 42, the conditioning surface 46 has a
cross-sectional shape which is the inverse of the cross-sectional
shape of the abrasive surface 40 and, thus, corresponds to the
cross-sectional shape of the truing wheel 42, but transposed
through substantially 90.degree., which is achieved by the dressing
stick 44 being located to one side of the truing wheel 42. This
inspection system also comprises a second air-bearing contact probe
48, shown in FIG. 3, located on the Z-axis carriage unit 10 on the
opposite side of the carriage unit 10 to the first probe 38.
[0051] The dressing stick probe 48 is moved across and contacts the
conditioning surface 46 by movement of the tool in the X- and
Z-axes which results in measurements which give the profile of the
cross-sectional shape of the abrasive surface 40. Probing of the
conditioning surface 46 occurs only in the X-axis direction along a
path which is substantially parallel to the linear path of the tool
piece 4. Owing to the symmetrical nature of the abrasive surface
40, the measurements from the probing of the conditioning surface
46 in the X-axis direction can then be electronically transposed to
measure the cross-sectional shape of the abrasive surface 40 in the
Y-axis direction indicated by the arrow 50 in FIG. 4. By measuring
only in the X-axis direction, there is no need for the addition of
other linear motion axes to the machine 2 which can be expensive
and reduces the overall stiffness of the machine 2. In conventional
grinding machines, there are typically four or five motion axes and
one of these is dedicated to permit the truing and dressing
operations. In the machine 2, there is no dedicated axis for truing
nor dressing. Thus, the machine 2 not only has a relatively high
degree of stiffness but it is also of simpler construction and is
therefore relatively less expensive to produce. Furthermore, the
construction of the machine 2 allows it to be of relatively low
mass.
[0052] The relatively low mass of the machine 2 enables an increase
in the frequency of production of good quality finished workpieces
14.
[0053] The use of the rotary axis about which the workpiece 14
turns in combination with the linear motion of the tool 4 to define
a surface on the workpiece 14 requires a control system and
associated computer software to deal with a change in shape of the
contact zone between the abrasive surface 40 and the workpiece 14
owing to wearing away of the abrasive surface 40.
[0054] Non-Uniform Rational B-Splines, commonly referred as NURBS,
have become the industry standard for the representation and
design, and data exchange of geometric information processed by
computers. NURBS provides a unified mathematical basis for
representing both analytic shapes, such as conic sections and
quadric surfaces, as well as free-form entities, such as the
surfaces of optical elements. A NURBS curve is defined by
C ( t ) = i = 0 n N i , k ( t ) w i P i i = 0 n N i , k ( t ) w i
##EQU00001##
where k is the order of basis functions, N.sub.i,k are the B-spline
basis functions, P.sub.i are control points, and the weight w.sub.i
of P.sub.i is the last ordinate of the homogeneous point
P.sub.i.sup.w.
[0055] One of the key characteristics of NURBS curves is that their
shapes are determined by the positions of control points. The basis
functions determine how strongly control points influence the
curve. A series of points, called knots vector, are used in the
basis functions to partition the time into non-uniform intervals so
that some control points affect the shape of the curve more
strongly than others.
[0056] With the machine 2 having its toroidal shape grinding wheel,
the center of curvature of the wheel is not in the wheel's
rotational axis. This makes the tool path over the surface of the
workpiece 14 more complex. As already mentioned, the diamond
grinding wheel abrasive surface 40 will experience a substantial
wear in the grinding process. Wear will induce changes of grind
wheel shape and result in significant form errors on the surface of
the workpiece 14, which could be an optical surface.
[0057] By using a NURBS algorithm, compensation for wheel wear can
be provided. The toroidal grinding wheel shape is defined by a
NURBS representation which has several control points. Changes of
wheel shape due to wear of the abrasive surface 40 can be modelled
by NURBS interpolation which is achieved by adjusting the control
points used for defining the toroidal grinding wheel shape.
Therefore the complex shape changes of the grinding wheel are
presented by using relatively little data. Owing to the wheel shape
changes, the tool path across the workpiece 14 will also be
adjusted by NURBS interpolation to compensate for the grinding
wheel wear. The NURBS data will help to maintain the motion
smoothness and achieve optical surfaces with high form
accuracy.
[0058] The advantages of the NURBS grinding wheel wear compensation
system are that NURBS offers a way to represent complex toroidal
wheel shapes while maintaining mathematical exactness and
resolution independence, NURBS gives accurate control over the
changes of wheel shapes (the set of control points and knots which
guide the wheel shape, can be directly manipulated to control its
smoothes and curvature), the grinding wheel wear compensation is
numerically stable as NURBS curves and surfaces are invariant under
common geometric transformations, such as translation, rotation and
perspective projection, and the grinding wheel wear compensation
process is fast as relatively little data is needed to represent
complex wheel shape before and after wear occurs.
[0059] The machine 2 is able to grind surfaces of the workpiece 14
such as optical surfaces to a precision of 1 .mu.m over a 1 m
diameter surface, the finished surfaces having minimal sub-surface
damage at depths of 2 to 5 .mu.m. This high precision accuracy
capability is the result of the relatively high motional
repeatability of machine motions through thermal control, fluid
film bearings, machine symmetry, minimised parallax errors, and
error compensation and correction via the in-situ metrology frame
unit and its associated post-process measuring system.
[0060] Plans for an ELT to be built are in place which has a 100 m
diameter and will need 2000 ultra-precisely machined optical
segments of 2 m diameter. With conventional grinding machines, for
the production of the required free-form optics, each such segment
will take around 280 hours to produce. The machine 2 is capable of
producing such segments in around just 20 hours.
* * * * *