U.S. patent number 3,866,667 [Application Number 05/423,448] was granted by the patent office on 1975-02-18 for lens blocking device.
This patent grant is currently assigned to Textron Inc.. Invention is credited to Lloyd F. Knight.
United States Patent |
3,866,667 |
Knight |
February 18, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
LENS BLOCKING DEVICE
Abstract
A lens blocking device is described in which a pair of
wedge-shaped prism rings may be adjusted relative to each other and
relative to the base of the device for incorporating into the lens
block a predetermined amount of prism at a predetermined meridianal
orientation. Graduations measured in diopters are marked on the
prism rings. Graduations measured in degrees are marked on the
base. The lens blocking device contains means for supporting a lens
blank to be blocked and for aligning the cylindrical and optical
axes of a premarked lens blank with the device. Means are described
for introducing a low melting point blocking metal into a mold
cavity defined by the prism rings, the base and the lens blank.
Means for cooling the blocking metal and for the easy removal of
the solidifed block with attached lens blank are also described.
Means are described for holding the prism rings in coaxial and
rotatable contact with each other and with the base.
Inventors: |
Knight; Lloyd F. (Belleair
Beach, FL) |
Assignee: |
Textron Inc. (Tampa,
FL)
|
Family
ID: |
23678942 |
Appl.
No.: |
05/423,448 |
Filed: |
December 10, 1973 |
Current U.S.
Class: |
164/334; 269/7;
269/58; 425/808; 451/390 |
Current CPC
Class: |
B24B
13/0052 (20130101); B23Q 1/54 (20130101); B23Q
3/086 (20130101); Y10S 425/808 (20130101) |
Current International
Class: |
B23Q
1/25 (20060101); B23Q 3/08 (20060101); B23Q
1/54 (20060101); B24B 13/005 (20060101); B22d
017/24 (); B22d 019/00 () |
Field of
Search: |
;164/332,334 ;249/88
;425/808 ;51/162,216LP ;269/7,58 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Husar; Francis S.
Assistant Examiner: Rising; V. K.
Attorney, Agent or Firm: Ball; Ronald F. Smith; Charles B.
Walpert; Gary A.
Claims
1. A lens blocking device comprising
a pair of wedge-shaped prism rings, one of which is rotatably
adjustable relative to the other about the axis of said one of said
rings,
said rings having contiguous plane faces which are inclined with
respect to the axis of said one of said rings,
means for holding said rings in axial alignment and in rotatable
contact with one another,
indicia on each of said rings for selecting a predetermined amount
of prism in diopters by rotating said rings relative to each
other,
a base,
means for holding said one of said rings in axial alignment and in
rotatable contact with said base,
indicia on said base for selecting a predetermined prism axis by
rotating said rings relative to said base,
means on the distal face of the other of said rings for engaging
the surface of a lens blank to be blocked,
said rings, base and lens blank surface defining a cavity shaped to
form a lens block,
and means for supplying molten metal to said cavity for molding
said lens block in said cavity and adhering it to said lens blank,
whereby a predetermined amount of prism in diopters and a
predetermined meridianal
2. A lens blocking device as defined in claim 1 wherein said lens
blank is provided with indicia designating the optical axis and
cylinder axis thereof and said base comprises means for aligning
said lens blank with
3. A lens blocking device as defined in claim 1 wherein said base
further comprises means for removing said block from said cavity
including a piston reciprocable in the direction of the axis of
said one of said
4. A lens blocking device as defined in claim 1 wherein said means
for holding said rings in axial alignment and in rotatable contact
with one another comprise a C-shaped retaining ring and means for
fixedly attaching said retaining ring to said one of said pair of
rings with the other of said pair of rings being rotatably disposed
axially between said retaining
5. A lens blocking device as defined in claim 1 wherein said means
for holding said rings in axial alignment and in rotatable contact
with one another comprise a plurality of permanent magnets disposed
in said one of said rings, said other of said rings being of
material attracted by said
6. A lens blocking device as defined in claim 1 wherein said means
for holding the distal face of said one of said pair of rings in
rotatable contact with said base comprise a J-shaped clamping
member in said base, the inner end of said member being adapted to
be received in an annular
7. A lens blocking device as defined in claim 1 further comprising
means for holding said lens blank in contact with said lens
engaging means.
Description
BACKGROUND OF THE INVENTION
This application relates to the blocking of lens blanks and more
particularly relates to an improved apparatus for the blocking of
lens blanks so that in subsequent generating operations a desired
prism can be ground into the lens.
In the manufacture of ophthalmic lenses, a lens blank is formed
into a finished lens by successively grinding and then polishing
first one surface and then the other. Usually one surface is
concave and the other convex, and the two surfaces have different
curvatures, so that the thickness of the lens varies at different
points. This variation in thickness gives the optical refraction
necessary to provide the desired correction in vision.
A lens blank having one of its two surfaces ground and polished is
termed a semi-finished lens. The subsequent generation of the
opposite surface is a more exacting operation because the second
surface must not only have the correct curvature, but must bear an
exact and precise relation to the previously finished surface, in
order for the lens to have the ophthalmic properties desired. This
precise location of the second surface with respect to the first
may require either or both of two adjustments or settings, one
called "prism" and the other called "axis." Setting for prism
involves a tilting of the second surface with respect to the first,
and setting for axis involves a rotation of the second surface with
respect to the first.
The problem of correctly relating the second surface to the first
has heretofore been met by mounting a lens blank on a lens block in
a conventional manner and then adjusting the block in the chuck of
a generating machine so that the desired amount of prism at the
correct meridian could be ground into the lens. One device for
incorporating prism into the lens in this manner is disclosed in
U.S. Pat. No. to Allen et al. 2,879,632.
Another technique that has been used is that in which the desired
amount of prism at the prescribed meridian is incorporated directly
into the lens block so that when the block is mounted in a
conventional manner in a grinding machine the desired prism is
ground into the lens. Prior art devices of this type are
exemplified by U.S. Pat. Nos. to Buckminster 3,049,766 and Prunier
3,195,197. These latter devices use a blocking system in which
molten metal of low melting point, while constrained in a mold, is
cast directly on the lens surface. The desired amount of prism is
incorporated into the block by either tilting the lens with pins as
the lens is presented to the mold, or by selecting and inserting
into the device one of several support rings which provide the
desired amount of tilt to the lens as it is presented to the mold.
These devices are complicated and expensive to construct and
maintain. In addition, in the tilting pin device molten metal may
infiltrate into the pin sleeve areas and interfere with the
blocking operation. In the selected support ring device, in order
to incorporate the desired amount of prism into the block, a large
inventory of support rings is required and the constant handling
thereof results in undesirable wear and replacement problems.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a mechanism for
incorporating prism directly into a lens block of the low melting
point metal type which is precisely accurate, simple to operate,
free of substantially all maintenance problems and requires no
substitution of parts.
The present invention is for a device for blocking lens blanks
which are ultimately to be provided with a prescriptive prism
correction, i.e., where one side of the lens blank is to be
finished thicker than its diametrically opposed side along a
predetermined axis through the optical center of the lens
blank.
When blocking a lens that is to be finished with a cylinder and
prismatic correction it is important that the amount of prism,
measured in diopters, be accurately introduced into the lens. It is
equally important that the prism axis be oriented in the prescribed
meridian and in proper relation to the cylinder axis of the
lens.
In the present invention a low melting point blocking metal is
introduced into a water-cooled, generally cylindrically-shaped mold
cavity. The upper portion of the mold incorporates a lens support
ring, and a lens blank is positioned on the lens support ring and
serves as the upper closure of the mold. The blocking metal is such
that when the cavity is filled, and as the metal hardens, it
becomes attached to the lens blank. The hardened metal also forms
the block for the blank. The lens support ring may be adjusted
relative to the axis of the mold to position the lens blank in a
manner such that when the blocking metal is introduced into the
mold, the prescribed amount of prism may be incorporated into the
block at the desired meridianal orientation. The cast block is also
provided with aligned recesses which serve properly to position the
block, with the attached lens blank, in the chuck of the surfacing
machines subsequently used to grind and polish the lens to the
desired finish.
The semi-finished lens blank which serves as the upper closure for
the mold is first marked on the finished surface in a conventional
marking device to designate the optical center and cylindrical axis
of the lens. The support ring, which is integral with the upper one
of a pair of wedge-shaped prism rings mounted on the blocking
device, is then rotated with respect to its mating wedgeshaped
prism ring to align graduations on the two rings and provide the
required amount of diopter prism. The two aligned prism rings are
then rotated together with respect to the blocking device until the
prism axis is located in the required meridian. The premarked lens
blank is then placed on the lens block mold cavity with its
finished side against the support ring. The blank is adjusted
manually on the support ring until the marks on the blank are
aligned with reference indicia incorporated in the blocking device.
Metal blocking alloy is then introduced into the cavity beneath the
lens and solidified. The resulting lens block assembly with the
desired axis and degree of diopter prism accurately incorporated
therein is readily removed from the blocking device for subsequent
finishing of the lens.
There is a minimum number of moving parts to the device. Molten
blocking metal cannot penetrate into the prism adjusting mechanism.
The blocking device may be used to introduce prism of any amount
and at any axis into the block for any lens without requiring a
change or substitution of parts in the device. Adjustments for the
degree and axis of prism are made using direct readings from
graduations provided on the device. Either concave or convex lens
surfaces may be presented to the device for blocking.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a blocking device showing a lens blank in
dash line positioned on the device.
FIG. 2 is a vertical section, taken generally along the line 2--2
of FIG. 1, and showing a lens blank being blocked to incorporate a
predetermined amount of prism at a predetermined axis.
FIG. 3 is a view similar to FIG. 1, but showing a different setting
for prism and axis.
FIG. 4 is a front elevational view, partly in vertical section,
taken along the line 4--4 of FIG. 3.
FIG. 5 is an exploded perspective view of the adjustable
wedge-shaped prism rings of my invention together with a C-shaped
retaining ring.
FIG. 6 is a perspective view of a lens blocked in accordance with
my invention.
FIG. 7 is an exploded perspective view of the wedge-shaped prism
rings showing a modified means for maintaining the rings in contact
with one another.
FIG. 8 is a front elevation, partly in vertical section, of a
blocking device embodying the modification shown in FIG. 7.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, a base, generally designated 10,
having cooling passages 12 connected to cooling water inlet-outlet
14 has a central, cylindrically shaped bore 16. A piston 18 is
slidably mounted for axial reciprocation within bore 16. Coil
springs 20 are interposed between the head 22 of piston 18 and the
top of hollow mandrel 24. Springs 20 serve constantly to urge
piston 18 to its uppermost position within bore 16. Springs 20 are
held in place by bosses 26.
The head 22 of piston 18 has three diametrically aligned
cylindrical passages 28, 30, 32, in each of which is disposed a
light transmitting medium 34, such as a bundle of fiber optics.
Mounted on head 22 is a pair of generally conically-shaped
protrusions 36, 38. These protrusions 36, 38 are in diametral
alignment and are coaxial with passages 28 and 32, respectively.
Intermediate protrusions 36, 38 and diametrally aligned therewith
is centrally located hollow protrusion 40 having a lower
frusto-conical wall section 41 and an upper frusto-conical wall
section 42 connecting wall section 41 with cylindrical section 44.
Section 44 has a small bore (FIG. 1) axially aligned in the center
thereof. Bore 50 is axially aligned with passage 30. Within each of
the protrusions 36, 38 is a small bore 46, 48, respectively,
axially aligned with passages 28 and 32, respectively.
A source of illumination such as light bulb 52 is mounted atop
mandrel 24 and is connected to a conventional current source, not
shown. Light emitted from bulb 52 passes through light transmitting
medium 34 and is visible through bores 46, 48 and 50 when viewed
directly from above.
The head 22 of spring mounted piston 18 also contains protruding
segments 54 through one of which extends a cylindrical conduit 56
(FIG. 1) which serves as an inlet for molten metal.
Base 10 also contains horizontally disposed slots 58, 60
terminating in upwardly extending apertures 62, 64, respectively. A
J-shaped clamping member 66 is disposed within each of the slots
58, 60. A ball plunger 68 is disposed within base 10 beneath each
clamping member 66. The ball on the head of plunger 68 seats in an
aperture in clamping member 66.
A wedge-shaped lower prism ring, generally designated 70, having a
counterbored portion 72 seats on the upper annular surface 74 of
base 10. An annular groove 76 is cut into the wall 78 of
counterbored portion 72. Groove 76 is adapted to receive the inner
end of J-shaped clamping members 66. When the inner end of J-shaped
clamping member 66 is seated in groove 76, the lower prism ring 70
is held in axial alignment with and is in rotatable contact with
base 10.
Lower prism ring 70 is coaxial with the reciprocable piston 18. The
flat annular bottom surface 94 and the flat annular surface of
counterbored portion 72 of lower prism ring 70 are normal to the
axis of prism ring 70. Upper generally annular surface 80 of lower
prism ring 70 is inclined with respect to the bottom surface 94 of
the lower prism ring, thus giving the ring a wedge shape.
The upper surface 80 of lower prism ring 70 has an upwardly
extending, centrally located annular flange portion 82. The lower
prism ring has a generally cylindrical central bore 84, the wall of
which is inclined outwardly with respect to the axis of the lower
prism ring in the upward direction for purposes to be explained
more fully hereinafter. As shown in FIGS. 2 and 5, the wall of bore
84 is stepped to present a shoulder 86 within the bore.
The upper annular flange portion 82 of lower prism ring 70 has a
flat upper annular surface 90. Surface 90 is parallel to surface 80
and both surfaces are inclined with respect to the axis of lower
prism ring 70. Surface 90 serves as a seat for upper prism ring,
generally designated 102, as described hereinafter. For
approximately 270.degree. of its circumferential outer edge, upper
flat surface 80 connects with cylindrical wall 92. Wall 92 extends
downwardly and connects directly with the bottom annular flat
surface 94 of lower prism ring 70. For the remaining segment of
about 110.degree. of its circumference, flat surface 80 is
separated from wall 92 by sloping surface 96. A 90.degree. segment
of the surface 96 has prism scale graduations 124 marked thereon.
That portion of flat surface 80 that is angularly coextensive with
sloping surface 96 has corresponding prism scale graduations 126
marked thereon for purposes to be described more fully hereinafter.
Lower prism ring 70 has threaded apertures 98 adapted to receive
screws 100 (FIG. 1).
Axially aligned with lower prism ring 70 is wedge-shaped upper
prism ring, generally designated 102, having a flat annular bottom
surface 103 and a counterbored portion 104. Flat annular surface
106 of the counterbored portion of ring 102 is parallel to surface
103 and seats on the flat annular surface 90 of lower ring 70. The
wall of counterbored portion 104 of the upper ring is slightly
larger in diameter than the outer wall of annular flange 82 of the
lower ring and forms a sliding fit therewith. Upper ring 102 is
rotatable with respect to lower ring 70 about the axis of ring 70.
Contact between the wall of counterbored portion 104 and the outer
wall of flange 82 holds rings 70 and 102 in axial alignment with
one another. Flat annular surface 106 of ring 102 is contiguous
with flat annular surface 90 of ring 70. These faces are inclined
with respect to the axis of lower prism ring 70.
Upper ring 102 has a centrally located bore 108. The wall of bore
108 is inclined outwardly with respect to the axis of the ring in
the upward direction. The upper annular surface 110 of ring 102 is
inclined with respect to the bottom surface 103. The inner edge of
upper surface 110 connects with upwardly extending annular flange
portion 112 having a thin annular upper edge 118 which serves as a
support ring for a lens blank 122. Prism scale graduations 128
extend through a segment of 90.degree. of the surface 110 of the
upper prism ring, as will be more fully described hereinafter.
Coaxial with the upper and lower prism rings is C-shaped retaining
ring 114. As shown in FIGS. 1 and 5, ring 114 extends in an arc of
about 270.degree., and is of the same diameter as lower prism ring
70. The inner diameter of ring 114 is less than the outer diameter
of upper prism ring 102. Holes 116, adapted to receive screws 100,
are drilled in ring 114.
The exploded view of FIG. 5 shows the relative positions of the
C-shaped retaining ring 114, upper prism ring 102 and lower prism
ring 70. As shown in FIG. 2, when assembled, screws 100 pass
through holes 116 and into threaded apertures 98 of the lower ring,
thus securing ring 114 to lower prism ring 70. Ring 102 is held
between rings 114 and 70 and is rotatable with respect to rings 114
and 70 about an axis common to all three rings.
As shown in FIG. 2, base 10 and piston 18 form the bottom wall of
mold cavity 120, and the upper and lower prism rings 102 and 70,
respectively, form the side wall of the cavity. Lens blank 122,
supported on the upper edge 118 of upper prism ring 102, forms the
top of the mold cavity.
Referring again to FIG. 1, base 10 has a flat upper surface 80.
Axis scale graduations 132 extend completely around base 10 on
circle having a diameter that is equal to the outer diameter of
lower prism 70. The axis scale graduations are in degrees and run
from 0.degree. through 180.degree. in the upper, clockwise
direction, as well as from 0.degree. through 180.degree. in the
lower, counterclockwise direction. The 0.degree. and 180.degree.
markings are on a diameter 130 that is coincident with the
diametrally aligned protrusions on piston 18. I refer to diameter
130 as the base apex line. The lower, counterclockwise axis scale
graduations 132 are used when a convex lens blank surface is
presented to the blocking device as shown in FIG. 2. The upper,
clockwise axis scale graduations are used when a concave lens blank
surface is presented to the blocking device.
The prism scales 128 on upper prism ring 102 are calibrated in
prism diopters. The calibrations run from 5 to 0. The 5 mark is
placed at the thinnest point on the ring. A plane in contact with
the upper edge 118 of ring 102 for the full circumference thereof
forms an angle of 2.degree. 44' with the plane of the bottom 103 of
the ring 102.
Prism scales 124 and 126 on lower prism ring 70 are also calibrated
in prism diopters, the calibrations running from 5 to 0. The 5 mark
on the lower prism ring is also at the thinnest point on the ring.
A plane in contact with the flat annular surface 90 of lower prism
ring 70 for the full circumference thereof forms an angle of
2.degree. 44' with the plane of bottom surface 94 of the ring
70.
The prism scale markings on both the upper and lower prism rings
are graduated from 5 to 0 with 5 at the thinnest point on each of
the rings, through an angle of 90.degree., as follows:
Corresponding Marking Angle ______________________________________
5.00 0.degree. 0' 4.75 18.degree. 12' 4.50 25.degree. 51' 4.25
31.degree. 47' 4.00 36.degree. 52' 3.75 41.degree. 25' 3.50
45.degree. 34' 3.25 49.degree. 28' 3.00 53.degree. 08' 2.75
56.degree. 38' 2.50 60.degree. 0' 2.25 63.degree. 15' 2.00
66.degree. 25' 1.75 69.degree. 30' 1.50 72.degree. 32' 1.25
75.degree. 31' 1.00 78.degree. 28' .75 81.degree. 23' .50
84.degree. 17' .25 87.degree. 9' 0.00 90.degree. 0'
______________________________________
It will be apparent from an examination of FIG. 1 that not all of
the markings of the prism scales 124, 126 and 128 are shown on the
drawing, the number of graduations shown being reduced for clarity.
Similarly, not all of the degree markings of the axis scale 132 are
shown; again, the number illustrated being reduced for clarity.
In order to introduce the proper amount of prism at the correct
prism orientation into a lens to be blocked on the lens blocking
device of the invention, the upper prism ring 102 is first rotated
with respect to the lower prism ring 70 to register the desired
prism scale graduation 128 on the upper ring 102 with the
appropriate prism scale graduation 126 on the lower ring 70. The
upper and lower rings are then rotated as a unit to the appropriate
axis scale graduation 132 to orient the prism angle in the proper
meridian.
As stated above, prism scales 124 and 126 are marked on lower prism
ring 70. The angular placement of the graduations is the same for
each scale 124 and 126. Scale 126 is used to align the graduations
on the upper prism ring 102 with the graduations on the lower prism
ring 70. Scale 124 is then used to align the amount of prism with
the graduations on the axis scale 132.
As a specific example, and as shown in FIGS. 1 and 2, if a lens is
to be blocked to generate a minus curve of 3.5 diopters of prism at
an angle of 43.degree., the amount of prism is obtained by aligning
the 3.5 marking on scale 128 with the 3.5 marking on scale 126. The
aligned prism rings are then rotated as a unit relative to the base
to a point where the 3.5 marking on scale 124 is aligned with the
43.degree. graduation on the axis scale 132. This 43.degree. point
will be the apex of the prism that will be incorporated into the
lens in subsequent blocking and generating operations.
Another example is illustrated in FIGS. 3 and 4. There the lens is
to be blocked to generate a minus curve of 5 diopters of prism at
an angle of 10.degree.. The marking 5 on scale 128 of upper prism
ring 102 is aligned with the marking 5 on scale 126 of lower prism
ring 70. The upper and lower rings 102 and 70, respectively, are
then rotated as a unit relative to base 10 until the 5 marking on
scale 124 is aligned with the 10.degree. graduation on the axis
scale 132. This 10.degree. point will be the apex of the prism that
will be incorporated into the lens.
Once the proper amount and orientation of the prism has been
established by rotating the prism rings as described, a lens blank
122 previously given a thin coating of material that will assure
good adhesion thereto of the metal blocking alloy and marked in a
manner well known to those skilled in the art to designate the
cylinder axis (indicated by the dash line 133 in FIG. 1), as well
as the optical axis of the lens (indicated by the intersection of
short vertical line 134 and line 133), is brought into contact with
protrusion 40 extending upwardly from the head 22 of spring mounted
reciprocatable piston 18. The lens blank is then urged downwardly
against the resistance of coil springs 20, until the lens blank
seats on the upper edge 118 (lens support ring) of upper prism ring
102. The previously marked cylinder axis of the lens is then
aligned with the diametrically aligned bores 46, 48 of the
protrusions 36, 38, respectively, and the optical axis of the lens
is aligned with bore 50. This alignment is facilitated by observing
light emitted by light bulb 52 and transmitted upwardly through
passages 28, 30 and 32 of the head 22 by the bundles of light
transmitting fiber optics 34 disposed therein. This light will, of
course, pass through the lens 122 and is visible when viewed from
above the lens. It will be noted that when aligned, the cylindrical
axis 133 of the lens is also aligned with the base axis line
130.
After the axis and optical center of the lens are properly
positioned, a holding member 136 having a pressure pad 138 of
resilient material at the lower end thereof secures the lens blank
to the blocking device. The finished surface of the lens blank now
forms the upper wall of mold cavity 120. Molten metal blocking
alloy is then injected through conduit 56 into the mold cavity 120
beneath the lens. Cooling liquid 140 circulating through base 10
causes the blocking metal to solidify, resulting in a lens-block
assembly with the desired amount of prism and prism axis
incorporated in the assembly.
Upon solidification of the blocking metal, reciprocatable head 18
is withdrawn downwardly into the base a short distance by means,
not shown, and then returned under the force of slightly compressed
coil springs 20 against the solidified block. This causes the
solidified block and attached lens blank to be jarred loose from
the blocking device. The removal of the block and attached lens
blank is facilitated by the outward inclination of bore 84 of the
lower prism ring 70 and the outward inclination of bore 108 of
upper prism ring 102, as described above. Shoulder 86 within bore
84 of the lower prism ring prevents jamming of the solidified block
against the outer wall of bore 84 on withdrawal of the head 18 as
described.
The completed lens-block assembly when removed from the blocking
device is shown in FIG. 6 with the lens blank 122 attached to the
block, generally designated 142. The shank 144 of block 142 is of
uniform dimension in the axial direction. The head 146 of the block
is thicker, i.e., has a greater dimension in the axial direction,
on one side thereof than on its diametrally opposed side. This
difference in thickness represents the amount of prism that is
incorporated in the lens-block assembly. It will be understood, of
course, that the upper and lower prism rings may be adjusted so
that no prism is introduced into the block. This, for example, will
be the case where the 0 graduation on the prism scale 128 is
aligned with the 0 graduation on the prism scale 126.
The tang portion 148 of block 142 has diametrally aligned
conically-shaped recesses 150 and 152 corresponding to protrusions
36 and 38 on piston 18. Center recess 154 having center opening 156
that extends completely through the block, and corresponding to
protrusion 40 on piston 18, is diametrally aligned with recesses
150 and 152. The three recesses function as axis aligning means
when the block is presented to the chuck of a lens grinding machine
for subsequent generation of the desired surface on the unfinished
surface of the lens blank. Sprue 158, through which the molten
blocking metal flowed into mold cavity 120 through conduit 56, is
shown adjacent the tang of the block.
A modification of the means for maintaining the upper and lower
prism rings in axial alignment, and without the necessity of
retaining ring 114, is shown in FIGS. 7 and 8. A plurality of
cylindrically shaped permanent magnets 160 are shrunk fit into
correspondingly shaped openings 162 drilled into the slightly
raised, upper flat annular surface 164 of wedge-shaped lower prism
ring 166. The bottom surface 168 of wedge-shaped upper prism ring
170 is countersunk to form an annular recess having a corresponding
flat annular surface 172 that seats against surface 164 and is
rotatably held in place thereon by magnets 160. Shoulder 174 which
has a sliding fit in the annular recess of bottom surface 168
maintains the two rings in axial alignment. The two contiguous flat
surfaces 164 and 172 are inclined with respect to the axis of lower
prism ring 166. Lower ring 166 is held in axial alignment and in
rotatable contact with base 10 by J-shaped clamping members 66 in
the same manner as described in connection with the embodiments
shown in FIGS. 1 through 5.
Operation of the modified embodiment illustrated in FIGS. 7 and 8
is identical with that described in connection with the embodiment
shown in FIGS. 1 through 6. It will be understood that while the
strength of magnets 160 is great enough to maintain the upper and
lower prism rings 170 and 166, respectively, in rotatable
engagement with one another, the magnets are not so strong as to
interfere with the rotation of the two rings with respect to each
other to align the graduations on the prism scales as desired, nor
do the magnets interfere with the rotation of the aligned prism
rings with respect to base 10 in order to orient the prism to the
desired meridian.
The terms and expressions which have been employed here are used as
terms of description and not of limitation, and there is no
intention, in the use of such terms and expressions, of excluding
equivalents of the features shown and described, or portions
thereof, it being recognized that various modifications are
possible within the scope of the invention claimed.
* * * * *