Note: Descriptions are shown in the official language in which they were submitted.
.
\
1087389
This invention relates to measuring apparatus. The
apparatus of the invention can be used, for example, to
measure a dimension of an object or a distance by which
an object moves.
The dimension or distance is measured by counting
the number of fringes of an interference pattern that
passes a given point during the movement of a grating with
respect to another grating.
The history of the use of gratings in measuring
apparatus is traced in a review article in the Journal of
Physics E:Scientific Instruments for March 1972 Volume 5
No. 3 pages 193 - 198, publish~d by the Institute of
Physics, London, England and entitled Gratings in MetrGlo~.
Known measuring apparatus-employing interference
patterns are both bulky and expensive.
~ o some extent this is due to the difficulties
experienced in manufacturing and copying accurate gratings
having fine resolution characteristics. The difficulties
experienced in making an accurate master grating, from
which gratings to ~e used in measuring apparatus can be
derived, are explained in the review article r~ferred to
above. Partly as a result of these difficulties, it is the
common practice in such known measuring apparatus to use
gratings having a 10 micron period and to employ analog
electronic division techniques to obtain a measuring
resolution of 1 or 2 microns, as required.
~, One aspect of the present invention is concerned with
the provisions of an improved method for making a master
:,
grating.
- 30 Another aspect of the present invention is concerned
,~ with the provision of a measuring apparatus that in at
least one embodiment is small enough to be held in the
1~8738~
hand, that is self-contained and that can operate without
connection to any additional apparatus or separate power
supplies. It will be appreciated, however, that the
invention is not limited to use with hand held apparatus,
but that it can be applied to other measuring apparatus,
for example bench mounted apparatus.
Accordingly the present invention provides a digilal
read-out measuring instrument including a first measuring
member, a second measuring member, the first measuring
member being slidable relative to the second measuring
member to effect a measuring cperation, a first measure-
ment-providing member of an electromagnetic radiation-
transmissive ma~terial having first and second surface
portions at a given angle relative to one another/ a first
diffraction grating on the first of the surface portions,
the first diffraction grating being coupled through said
first measurement-providing member to and movable in
accordance with movements of the first measuring member,
a second measurement-providing member of an electro-
magnetic radiation-transmissive material having third and
fourth surface portions at said given angle relative to
one another, a second diffraction grating on the third of
the surface portions, the second diffraction grating being
fixed relative to the second measuring member and beinc
so arranged in relation to the first diffraction grating
that interference fringe patterns can be produced by
q~. electromagnetic radiations passed successively thrGugh the
two gratings, a source of electromagnetic radiations, means
; to detect radiations from the source, the source being so
~: 30 arranged that radiations emitted by the source are directed
successively through the gratings and the detector means
being arranged to detect the interference fringe pattern
'~
`'; ~
resulting from the transmission of the radiations through
the gratings and to provide a pulse output according to
changes in the fringe pattern, a pulse counter connected
to the output of the detector means, digital display means
connected to the output of the counter, spacer means in
the form of a non self-supporting coating on one of the
first and third and one of the second and fourth surface
portions to slidably space the first and third surface
portions and the second and fourth surface portions apart
respectively, and means for ccntrolling the speed of
movement of the first measuring member.
A described embodiment has a plurality of features,
apart from the use of an improved method of making a
master grating referred to above, that contribute to a
reduction in the bulk and the cost of the apparatus, thereby
enabling, for example, a self-contained hand held
apparatus to be made.
A further feature of a described embodiment is the
use, in the counting of the fringes of an interference
pattern, of an electrical circuit that is comparatively
simple.
In a preferred embodiment of the invention, a circuit
is used that requires only four pulses per cycle of the
fringes to be generated and these pulses are generated from
the signals obtained from two or more detectors using well
G ~` known electronic techniquesr for example the techniques
- ~ described in the specificiation of United Ringdom Patent
No. 760,321.
In the preferred embodiment of the invention, in order
- 30 to achieve a resolution of 1 micron, the period of the
gratings used to generate the fringes is as small as 4
micro~s.
;\ ~
1~873B9
Yet a further feature of the des~ribed embodiments of
the invention, which feature enables the contrast between
the modulated signal output from the detectors and the
unmodulated background noise to be as high as possible,
is concerned with the provisicn of mounting means which
enable the two gratings, which are to be moved relatively
to one another, to be as close as possible to one another.
_ This requirement is particularly necessary with gratings
having a period as small as 4 microns. In the preferred
embodiment of the invention the spacing between the
gratings is no more than 12 microns, although it is
possible to operate the system with a spacing between
the gratings of up to 20 microns.
In a particular embodiment of the invention to be
described below in more detail, the invention is employed
with a hand held micrometer and the embodiment to be
described has the particular advantage over the conventional
screw threaded instrument that measurements can be taken
much more quickly. One reason for this is that the device
to be described employs an instantaneous digital readout
of the relative positions of the spindle of the instrument
with reference to an anvil and has a spindle which can be
slid directly to the position required, whereas it can take
, .
as long as 20 seconds to move the spindle of a conventional
instrument employing a screw to the fully open position.
A further feature of a described embodiment is the
; ~ use of a system which exerts a substantially constant
- force to urge the spindle against the anvil, thereby
ensuring that a force of the same value is consistently
exerted upon an object being measured between the anvil
and the spindle.
Yet another feature of an embodiment of the invention
~8`7~
is the provision of a damper system that controls the
speed at which the spindle slides and thus the rate at
which pulse signals are applied to a pulse counting circuit.
Embodiments of the invention will now be described,
by way of example, with reference to the accompanying
drawings wherein like parts are indicated by the same
reference numerals and in which:
Fig. lA, Fig. lB and Fig. lC are a partly cut-away
plan view, a longitudinal section on the line B-B of Fig.
lA, and a cross section on the line A-A of Fig. lB
respectively of a hand-held mi_rometer gauge,
~ig. 2A, Fig. 2B and Fig. 2C' are a side elevation, an
end elevation and an exploded perspective view of a glass
measu~ing block arrangement,
Fig. 3 is a longitudinal section through a coupling
unit used in the gauge,
Fig. 4A and Fig. 4B are end and plan views respectively
- of the glass measuring block arrangement showing an
associated radiation emitter and receiver arrangment,
Fig. 5 is a block schematic circuit diagram,
Fig. 6A and Fig. 6B are a longitudinal section and a
cross section on the line A-A of Fig. 6A of a grating and
damper assembly,
Fig. 7A, Fig. 7B and Fig. 7C are a diagrammatic cross-
.:; -
sectional view of the arrangement shown in Fig. 6A and 6B,
t together with a mask system, a view on the line A-A of
Fig. 7A and a view in the direction of the arrow B of the
arrangement shown in Fig. 7A respectively and
Fig. 8 is a flow chart illustrating the steps in the
making of a master grating.
Referring now to the drawings, there is shown in Figs.
lA, lB and lC a hand-held measuring gauge having a spindle
1 (which constitutes the first of two "measuring members")
which is arranged to slide towards and away from an anvil
2 (which constitutes the second of the two "measuring
members") located on a jaw frame 3. The anvil 2 and the
adjacent end of the spindle 1 form jaws, in a similar
manner to the jaws of known micrometer screw gauges.
~ - The gauge has a main frame 4 from which a bearing
holde: 5 extends around the ~pindle 1. The jaw frame 3,
which extends around the bearing holder 5 is attached to
the f~ame 4 by means of glue and a key 6. The bearing
holde~ 5 carries a bearing 7 which provides a bearing
surface for the spindle.l. A second bearing surface 8 for
the spindle 1 is located in the main frame 4. A first part
of an arm 9 is connected between the spindle 1 and a
sliding button 10. m e said first part of the arm 9
extends through a slot 11 in a case 12. A second part of
the arm 9 is connected between the spindle 1 and a damper
assembly 13. A "flexator" coil spring 14 is connected
between a location pin 15 on the spindle 1 and a location
pin 16 on the frame 4. There may be more than one such
,
. "flexator" spring. A "flexator" spring is a tension coil
spring arranged so that it operates in a flexing mode. In
; this manner a substantially constant force is exerted
between the pins 15 and 16 in ~rder to urge the spi.ndle 1
,:~
~, towards the anvil 2. The force will be exerted upon the
spindle 1 even when the spindle 1 is positioned against
the arlvil 2. Other means can, of course, be used to urge
the spindle 1 towards the anvil 2.
The damper 13 includes a piston 17 within a body 18.
The damper body 18 is attached to the arm 9 and the piston
17 is coupled, via a flexible rod 19, to an end cap 20.
m e end cap 20 is attached by a screw 21 to the main frame
10~738~ `
4. The damper assembly further includes sealed bellows 22
and is filled with a. viscous fluid.
When the body 18 moves relatively to the piston 17,
which slides within it, a re.straining or drag force is
exerl:ed between the piston 17 and the b.ody 18 due to the
presence of the viscous fluid, thereby limiting the speed
at which the spindle 1 is able to slide in the bearings 7
and 8. me spindle 1 is urged to move towards the anvi.l 2
by the "flexator" spring 14, which has a rate which is
apprcximately zero. To open the jaws, the spindle 1 is
caused to slide in the bearings 7 and 8 by an operator
sliding the button 10, which i.s coupled via the first part
of the arm 9 to the spindle 1, along the casing 12.
As may also be seen more clearly with reference to
., Figs. 2A, 2B and 2C, the gauge includes a glass block 23
- which carries an optical grating 24 on a surface 25,
(constituting a..first surface portion), the surface 25
being adjacent to a surface 26 (constituting a third
surface portion) of a second glass block 27 which is fixed
to the frame 4. The surface 26 carries a second optical
grating 28 and the block 23 is arranged to slide adjacent
'~ the block 27, the block 23 being coupled via a coupling
unit 29 to the spindle 1. The sliding block 23 has a
', . bevel:Led face via which the block is urged against the back
or reference -block 27 and a glass base block 30 by a leaf
spring 31 (Fig. lC), which is retained by two location pins
: (not shown) against an arm 32. This ensures that intimate
- contact is maintained between the block 23 (which constitutes
the first of two measurement-providing members) and the
.~ 30 blocks 27 and 30 (which together constitute the second ofthe two measurement-providing members). I~e arm 32 transfers
the reaction force of the spring 31 via a ball arrangement
~08~
33 to a strip ~f low friction material 34, which is
attached to an extension of the main frame 4. The arm 32
is connected to the spindle 1 in such a way that it is
free to rotate about the spinc.le with a minimum amount of
orthcgonal movement. This is achieved, as may more
clearly be seen by reference to Fig. 3, by employing a
bearing member 35 which passes through a hole in the arm
32, is attached to the end of the spindle 1, and provides
one end of the coupling unit 29.
The coupling unit 29, as may be more clearly seen
from Fig. 3, includes a pin 36 having a conical portion
at one end which is inserted into a conical socket 37 in
a member 38, the member 38 being coupled, as indicated~ to
the sliding glass block 23. At its other end, the pin 36
has a conical portion which locates in a conical socket 39
` in the bearing member 35. The conical sockets 37 and 39
define included angles which are greater than those of the
conical end portions of the pin 36. The members 35 and 38
' u'
are coupled together flexibly by means of a coil spring 40
which is under tension and which holds the pin 36 in place
between the sockets 37 and 39. It will be seen that the
coupling unit 29 is so designed that, while it constrains
the sliding block 23 to move directly in accordance with
` the movement of the spindle 1 along its longitudinal axis,
it allows a degree of linear and rotational movement of the
block 23 in other directions, thereby enabling wear and
~ ._ .
variations due to manufactuxing tolerances to be taken up.
The coupling unit 29 is thus a universal coupling which
permits 5 degrees of freedom.
The construction and disposition of the glass blocks
23, 27 and 30 will now be described in more detail, par-
ticularly with reference to Figs. 2A, 2B and 2C. It will
~ f ~ :
be appreciated that, although transparent glass blocks
are used, other combinations of mechanically stable
transparent materials, or transparent and reflective
materials could be used.
The material for the blocks 23, 27 and 30 is first
selected and then machined and/or polished to provide
smooth rectangular surfaces. The gratings 24 and 28 are
line and space gratings and are printed on the surfaces 25
and 26 using standard photo-mechanical techniques and
emploxing optically opaque thin films. In the preferred
embodiment the period of the gratings is chosen to be 4
microns. Other periods could, of course, be used. me
;~ gratings 24 and 28 are produced by coating the surfaces 25
~. ' .
and 26 of the blocks 23 and 27 in a vacuum with a chromium
fil~. A film of photo-resist material is then applied to
the c~lromium film. The film of photo-resist material is
...
then exposed by means of ultra-violet light to an image of
a master grating. The parts of the resist material which
have been exposed to ultra-violet light are more soluble
-~ 20 in a "ldeveloper" than are the unexposed parts, with the
, .
~,; result that, u~on the development of the coating, the areas
of the layers of chromium that are not required are exposed.
e exposed areas of chromium are then etched away and an
image of the original master srating is obtained. Similar
techniques are well known in the manufacture of printed
circuits.
A method of making a master grating will be described
below with reference to Fig. 8.
m e lines of the gratings 24 and 28 are produced on
the surfaces 25 and 26 in such a way that they are substan-
tially perpendicular to the longitudinal edges of the
surfaces 25 and 26 respectively.
1~8'7;38~ `
In assembling the blocks, it is important that the
base block 30 be fixed accurately to the back or reference
block 27 and that its face 41 and the face 26 of the
reference block 27 should both be flat. It is also
important that, when the glass blocks 23, 27 and 30 are
assembled together as a unit, the slider block 23 should
fit accurately into the angle between the base block 30
and the reference block 27. This is most conveniently
achieved by ensuring that the face 42 (constituting a
second surface portion~ of the sli~ing block 23 and the
. .
face ~3 of the base block 30 to which it is adjacent are
perpendicular to the surface 25 of the sliding block 2:
which carries the grating 24. The surface 25 of the
sliding block 23 is, of course, arranged to be flat.
It is not necessary for the gratings 24 and 28 to
:-l extend to the longitudinal edges of the blocks 23 and 27
. r
and, in order to prov~de low friction sliding surfaces and
to space the gratings thereby reducing the possibility of
the gratings becoming wor~ during use, the surfaces 25 and
26 of the blocks 23 and 27 are provided with spacer rails
44 and 45 respectively along the edges of the blocks. The
rails 44 and 45 are suitably between 1 and 10 microns thick.
In the preferred embodiment the rails are 4 microns thick.
Thin spacer layers 46 (constit~ting a fourth surface portion)
and 47 on the surfaces 43 and 42 of the blocks 30 and 23
respectively space the blocks 30 and 23 and provide low
friction sliding surfaces. In the particular embodiment,
the spacer rails 44 and 45 and the surfaces 46 and 47 are of
PTFE applied by a spraying process. Other solid lubricant
materials, for example molybdenum disulphide, tungsten
diselenide or carbon may be used. These materials can be
applied either by spraying or by a vacuum deposition process.
~87389
In order to improve the adhesion of the sprayed on
rails 44 and 45 and the layers 46 and 47, it is advanta-
geous to roughen the surfaces of the glass blocks locally,
for example by etching or shot blasting, before the
lubricant material is applied
Other methods of spacing and lubricating the blocks
- can be used. For example an oil film can be used between
the surfaces 25 and 26 to provide lubrication and the
spacing between the blocks can be obtained by using vacuum
deposited metal spacer rails. Alternatively the block 23
can be maintained between two films of oil of approximately
. . ,
~` equal thickness which are constrained by the block 27 and
~ an additional similar block or the other side of the block
f:,', 23.
To assemble the blocks 23, 27 and 30 as a single unit
they are arranged in a jig in ~hich they are aligned as
~', required. m e angle between the gratings 24 and 28 is
adjusted by tilting the base block 30 until fringes of the
re~uired period are generated between the two gratings.
In the preferred embodiment a fringe period of about 12
millimetres is chosen. An anaerobic or ultra-violet light
curing cement is then introduced by capillary or other
action between the surface 26 of the back or reference
block 27 and the face 41 of the base block 30 which is
` in contact with a part of the surface 26 and the cement is
cured, thereby holding the two blocks together at the
co~rect angle. It will be appreciated that other types
of cement or other methods of securing the blocks together
can be used.
The arrangement of the glass blocks 23, 27 and 30 and
radiation transmitting and detecting devices will now be
described, particularly with reference to Figs. 4A and 4B
11 -
1~8~7389
which show a source o radiations, in the form of a lamp
48, arranged to direct a beam of radiations via a lens
49, the sliding block 23, the gratings 24 and 28, the
reference block 27 and a lens arrangement 50, to an a^ray
of ph~to-sensitive devices 51 and 52. The photo-sensitive
devices 51 and 52 are spaced ~n a perpendicular direction
to the fringe pattern generated by the passage of
radiations from the source 48 through the gratings 24 and
28. lrhe devices 51 and 52 det:ect movements in the fringe
pattexn due to relative longitudinal movement between t-he
~locks 23 and 27 and the consequent movement between tke
; gratings 24 and 28. In the particular embodiment beiny
described the devices 51 and 52 are silicon photo transistors
and are spaced apart by ~ne quarter of the period of the
fringe pattern, so that on relative movement between the
gratings 24 and 28, output signals are obtained from the
photo-transistors 51 and 52 which are approximately sinu-
soidal and have a xelative phase difference of approximately
90 . In the particular embodiment being described, the
lamp 48 is an infra-red light emitting diode, and the
beam of radiation from the lamp 48 is deflected by means of
mirrors 53 and 54 in order to enable the width of the
instrument to be reduced while maintaining the path of the
radiations. It is, of course, possible to use other sources
of radiation and other radiation detectors. A further photo-
sensitive device 55, which in the particular embodiment is
a photo-transistor, i5 used to monitor the output from the
lamp 48 and thus give a signal which is used to adjust a
circuit and compensate for variations in voltage from the
power supply or due to ageing of the lamp.
Reference will now be made to Fig. 5, which shows the
output 'rom ~he photo-transistors 51 and 52 applied to
- ~2 -
"
10~37389
respective amplifiers 56 and 57 whose outp4ts are then
squared during amplification in further respective
circuits 58 and 59. The outputs from the circuits 58 and
59 are maintained symmetrical about a voltage level which
is adjusted in accordance with changes in a d.c. output
signal from the reference photo-detector 55. Ideally, the
outputs from the amplifiers 5~ and 59 are square waves in
quadrature. The outputs from the amplifiers 58 and 59 are
both applied to a detector 60, which provides a pulse
- 10 output for every amplitude transition of both square waves,
and a phase detector 61. A pulse is obtained from the
detector 60 for every quarter of a cycle of the fringe
~, pattern. In the preferred em~iodiment being described with
;. i
a 4 micron period grating, this corresponds to a spindle
movement of 1 micron. The movement would be 2 microns for
an 8 micron period grating. The pulse output from the
detector 60 is applied to an up-down counter 62, to which
the output from the phase detector 61 is also applied.
The phase detector 61 determines the direction of movement
of the fringe pattern from the inputs applied to it and sets
the up-down count mode of the counter 62 accordingly. The
~ output from the counter 62 is fed to a digital display 63
from which the exact position of the end of the spindle 1
relative to the anvil 2 can b~ read out.
The housing 12 includes a power pack 64, which may
suitably be rechargeable batteries, electric circuit
elements 65, a switch 66, which controls the supply of
power to the circuit arrangement, and the digital display
device 63.
In operation, the electric circuits are energised
and the counter 62 is set to zero upon the operation of
the switch 65. The spindle 1 is then withdrawn from the
- 13
1087389
anvil 2 by sliding the thumb operated button 10 along the
slot 11 in the case 12 against the force exerted by the
;' approximately zero rate, spring 14 and the damper 13.
An object to be measured is introduced between the
anvil 2 and the spindle 1, the sliding button 10 is
released, and the spindle 1 is allowed to return under the
influ~nce of the spring 14, towards the anvil 2 until it
meets the object which is located against the anvil 2.
The spring 14 then exerts a c~osure force, which is
approximately constant over the full operating range of
the instrument, due to the zero rate spring 14. The
speed of return of the spindle 1 is controlled by the
damper 13. The movement of the spindle 1 is indicated
during i,ts movement continuously on the display 63 as a
result of the interrogation of the fringe pattern produced
by radiations from the source 37 passing through the
gratings 24 and 28 by the photo-transistors 51 and 52, in
the way described above; the counter 62 counting up when
, the movement of the spindle is in one direction and down
when it is in the other direction. The display can easily
be read when the object is between the anvil 2 and the
spindle 1, or the display can be set to zero by operating
the switch 66 when the spindle 1 is on the object to be
measured, and then by removing the o~ject and allowing the
counter to count back until the closed position of the
spindle is reached, a negative indication of the object
size can be read from the display. There is thus provided
a facility that enables measurement to be made when the
display cannot conveniently be read in situ.
In another embodiment in which oil or other fluid is
used to space the gratings and provide damping it is
particularly convenient to use a circular section grating
1~ -
1~87389
assembly.
A particular ~orm of such a circular section assembly
will now be described with reerence to Figs. 6A, 6B, 7A,
7B an~ 7C in which there is shown a rod 67 of transparent
material, for example glass, which slides, with a small
clearance, in a tube 68 of transparent material. When the
arranc~ement is assembled in a gauge, the rod 67 is attached
at one end 69 via a coupling joint (not shown) such as that
shown at 29 in Fig. lB to a spindle of a measuring instru-
ment and the tube 68 is attached to the main frame of the
instrl1ment. A transparent viscous damping fluid is
conta ned between t~e rod 67 and the tube 68 and within a
bellows unit 70, which serves to seal the assembly. The
viscous fluid operates as a damping medium, as the rod 67
moves through the tube 68 in accordance with the movement
of the spindle of the measuring instrument. Gratings 71
and 72 are printed on the outside of the rod 67 and the
inside of the tube 68 respectively. During the printing
of the gratings, the relative angles between the two
gratings are so controlled that, on assembly, the fringe
pattern between the *wo gratings has a required period.
` ~ It would, of course, be possible to print the grating 72
on the outside of the tube 68, although such an arrangement
would not be preferred.
As shown particularly in Fig. 6A, light from a lamp
73 is collimated by a lens 74 and passed through one side
of the tube 68 and the two gratings 71 and 72. The
transparent rod is so chosen that it acts as a lens, which
then focusses the light, modulated by the interference
fringe pattern resulting from the effect of the gratings
71 and 72, on to a detector array indicated at 75. The
array 75 includes a pair of photo-transistors 76 and 77,
-- 15 --
1(~87389
.
which are indieated more clearly in Fig. 7B.
In order to obtain quadrature signals from the photo-
transistors 76 and 77, it is convenient to use a mask 78,
as shown in Fig. 7. The rod ~7 acts as a cylindrical lens
and the mask 78 is required in order to select the fraction
of th2 fringe pattern which is to be focussed on to the
respective photo-transistor. Thus an aperture 79 in the
mask 78 defines the light that is to be focussed on to the
detector 76, whilst an apertur~e 80 in the mask 78 defines
the light that is to be focus~ed onto the other detector 77.
A method of making a master grating will now be
described with reference to Fig. 8. ~he method employs
adaptations of techniques used in the manufacture of solid
state semiconductor eircuits. A description of the kncwn
techniques is given on pages 193 - 205 of a book entitled
"Dividing, Ruling and Mask-Making" by D.F. Horne, published
by Adam Hilger, London 1974.
The first step in the method, indieated by block 81
in Fig. 8, is the making of a master. In the particular
method, a Rubylith master is employed. A Rubylith master
is a plasties laminate material eonstituted by a base of
translueent white material and a strippable coating of
ruby coloured material. The master represents a small
section of a grating which is 100 times the size of the
required grating. In the particular method a pattern is
eut in the Rubylith material by means of a programme
eontrolled co-ordinate eontrolled eutting machine, generally
known as a co-ordinatograph. The master comprises a sheet
of Rubylith material which is 35 cm long and 4 cm wide.
100 lines, each extending in the direction of the length
of the material, are eut aeross its width with a 400 micron
spaeing between the lines.
1087389
The master is then photographed with a high degree of
accuracy and reduced in size by a factor of 10, as
indicated by block 82. The negative which is so produced
is col~nonly referred to as a reticle plate.
'~e reticle plate is then placed, as indicated by
block 83, in a known step and repeat camera, for example
of the type described on pages 200 - 205 of the book by
D.F. E[orne referred to ~bove, and photographically reduced
in size by a further factor of 10 on to a part of a final
master plate, which may be a photographic plate or a
photo-resist coated chromium plate. The relative positions
of the reticle plate and the final master plate in the
s~ep and repeat camera are then changed so that an image
of the reticle plate is obtained at a very accurate
interval along the master plate from the first image and an
im~ge of the reticle plate is again photographically
produ~ed on an adjacent part of the final master plate
reduced by a factor of 10. The process is repeated until
a photographic image is produced on the final master plate
of a grating of the required length. The image is then
developed, as indicated by block 84, and the grating on the
final master grating can then be used in the making of line
and space gratings on glass blocks, as described with
refer~n~e to Figs. 2A, 2B and 2C.
Although the invention has been described with
reference to particular embodiments, it will be understood
that variations and modifications can be made within the
scope of the invention claimed.
For example the sliding spindle 1 could be caused to
slide in the bearings 7 and 8 by the rotation of an
a.ssociated knob or wheel, even though the spindle itself
does not rotate.
,''
:
1087389
It will also be appreciated that other faces of the
glass block 23, for example,the bevelled'surface, than
those referred to can have a coating of lubricating material
applied to it.
It will also be understocd that the spacer rails 44
and 45 on the surfaces 25 and 26 of the blocks 23 and 27
could be replaced by a continuous rail on one of the
surfaces and a series of separate regions of lubricant
material, for example dots, on the other surface. Further-
more, the lubricant material on one of the surfaces need
not be the same as the lubricant material with which it
co-operates on the other surface.
In order to give good adhesion between the surfaces 25,
26 and the lubricant material, the lubricant material is
applied in particulate form, for example by spraying or
vacuum,deposition and not by causing a preformed body to
' adhere to either of the surfaces.
:
, 20
.
.
~ 30
,
':
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- 18 -
