CA1131955A - Multifaceted mirror assembly fixture - Google Patents

Abstract

Abstract:
The present invention relates to a fixture for assembling mulitfaceted mirrors having a plurality of faces disposed in respective planes that are angled relative to each other. The fixture is comprised of a base having a plurality of faces which correspond with the faces of the multifaceted mirror. Each face is accurately in the plane of a respective face of the multifaceted mirror. A plurality of locator blocks are provided, corresponding to the faces of the base. Each of the locator blocks have a mounting surface for removably engaging one of the faces of the base and a mirror support surface in the plane of the mounting surface and which extends beyond the base. A unit is provided for removably securing the locator blocks to respective faces of the base. A unit is provided for removably securing a plurality of individual mirrors respectively to the support surfaces of the locator blocks to position the mirrors at the faces of the multifaceted mirror. A unit is provided on the base for receiving and holding a support element positional adjacent the individual mirrors so the mirrors can be cemented to the support element while secured to the locator blocks to form the multifaceted mirror.

Description

Multifaceted Mirror Assembly Fixture This is a division of copending Canadian Pate~t Application serial No. 346,560 which was filed February 27, 1980.
Background of the Invention Rotating multifaceted mirrors produce a high speed scanning beam of light that can be modulated for many purposes in laser printers and electro-optical scanning systems. Such spinning mirrors rotate at high speeds and require optical accueacy to within seconds of arc to produce high quality results. This makes them very expensive to fabricate.
The invention involves a new construction for a multif~aceted mirror that is less costly to make without any sacrifice~in accuracy and reliability. It includes a ; method of making multifaceted mirror~, a fixture for assembling the mirrors, and the construction of the mir~r;ors~themselves~
Summ~arY of the Invention In aacordance with an aspect of the invention there is provided a fixture for assembling multifaceted mirrors having a plurality of`~faces disposed in respective planes that are angled relative to each other, said fixture comprising: (a) a base having a plurality of faces corresponding with the faces of said multifaaeted mirror, 'f , d~

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`` ~131955 each face of said base being accurately in the plane of a respective face of said multifaceted mirror; (b) a plurality of locator blocks corresponding to said faces of said base and said multifaceted mirror, each of said locator blocks having a mounting surface for removably engaging one of said faces of said base and a mirror support surface in the plane of said mounting surface and extending beyond said base; (c) means for removably securing said locator blocks to respective faces of said base; (d) means for removably securing a plurality of individual mirrors respectively to said support surfaces of said locator blocks to position said individual mirrors at the faces of said multifaceted mirror; and !e) means on said base for receiving and holding a support element positioned adjacent said individual mirrors so said individual mirrors can be cemented to said support element while secured to said locator blocks to form said multifaceted mirror.
A multifaceted mirror is made by cementing individual mirrors to a single-piece support element while the individual mirrors are held in accurate positions that are independent of the faces of the support. An asse~bly fixture has a base with faces that are accurately in the plane of the respective faces of the multifaceted mirror, and locator blocks with plane surfaces are attachable to ; each face of the base and provide mirror support surfaces extending beyond the base. The individual mirrors are secured to the mirror support surfaces of the locator blocks and held accurately in place while a support element is positioned adjacent the mirrors and the mirrors are cemented to the support element. A vacuum system is ~;~ preferred for securing the locator blocks to the base and for securing the individual mirrors to the locator blocks, and valves can be used to control the vacuum as the fixture and mirror components are assembled.

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113~55 Drawings ~
The present invention, taken in conjunction with the invention described in copending Canadian Patent Application Serial No. 346,560 which was filed on February 27, 1980, which will be described in detail hereinbelow with the aid of the accompanying drawings, in which:
Figure l is a plan view of a preferred embodiment of a multifaceted mirror made according to the invention;
Figure 2 is a side elevational view of the mirror of Figure l;
I0 Figure 3 is an enlarged fragmentary view of use of a spacer strip in making multifaceted mirrors according to the invention;
Figure 4 is an enlarged fragmentary view of use of a : trim strip in making multifaceted mirrors according to the invention;
Figure 5 is an enlarged fragmentary view of a trim strip cemented in place;
Figure 6 is a plan view of the base of an assembly fixture according to the invention;
Figure 7 is a cross-sectional view of the fixture base of Figure 6 taken along the line 7-7 thereof;
Figure 8 is a front elevational view of a locator block for use with the fixture base of Figures 6 and 7;
Figure 9 is a cross-sectional view of an assembled

2~5 fixture:holding individual mirrors and a support element for making a multifaceted mirror according to the invention Figure 10 is a cross-sectional view o~ an assembled : : mirror showing dimension's and angles held to high :~ 30 accuracies; and Figure ll is an enlarged plan view of a mirror fragment showing a prefered embodiment of a trim strip that diverts light from the mirror scanning path as the trim strip passes through a light beam.

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- , . -. ., : -DETAILED DESCRIPTION
The invention applies wherever the faces of a multifaceted mirror require substantial accuracy. Most multifaceted mirrors rotate on an axis and have faces that are angled relative to each other and spaced around the rotational axis. They can ha~e different numbers of faces at different angles and spacings relative to the rotational axis, and they can be used for many pur-poses. A typical use for such mirrors at present is in laser scanning devices where the mirrors rotate at high speed and have faces that ordinarily form a regular polygon in a plane perpendicular to the rotational axis.
Each reflective face of a polygonal mirror for laser scanning devices and other accurate optical uses must be optically flat, optically coated, and accurately located to within seconds of arc of a predetermined plane.
Forming reflective surfaces by grinding, polishing, and coating facets of a single piece of glass presently makes the completed mirrors very expensive. Also, making such 2Q mirrors with individual me~al faces that are independently ad~ustable has been attempted and is also expensive, troublesome, and inaccurate.
I fabricate and coat individual plane mirrors that are relatively inexpensive and then assemble these accurately to form a polygonal or multifaceted mirror, and I have found ways of doing this both accurately and ~ ~ economically to make better mirrors that cost less. By ;~ evaluating all the requirements carefully, I have been able to devise measures that insure accuracy o~ all : 3a relevant d~mensions and balance o~ the rotational mass so that the mirror rotates smoothly at high speeds that can e~ceed 25,000 rpm.
Individual plane mirrors can be made accurately ~lat br standard methods that are inexpensive. Many individual mirrors can also be coated simultaneously and accurately with whatever optical coatings are required, "..
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to keep coating costs to a minimum. Accurate dimensioning of individual mirrors in both size and mass is also relatively inexpensive.
I use a very accurate assembly fixture that 5 positions the individual mirrors precisely where desired and holds them accurately in place. The fixture itself is assembled of accurate components that hold the individual mirrors during fabrication and then disassemble to release the completed mirror. The entire fabrication can be accomplished on the fixture under clean conditions with a moderate level of skill and can achieve accuracies within a few seconds of arc.
A support element or core provides rotational or mounting support for the individual mirrors. The core is symmetrical and rotationally balanced and has an accur-ate rotational axis bore. It also has faces that lie parallel with and slightly spaced inward from the inner surfaces of the individual mirrors; and although the support element is mechanically accurate, its faces need not be optically accurate.
After positioning the individual mirrors in the assembled fixture, r place the support element radially inside the individual mirrors by fitting its axial bore over an axial locator pin on the fixture. This leaves small uniform gaps between the faces of the support element and the inner surfaces of the individual mirrors, and I in~ect epoxy or some other cement into these gaps to bond the individual mirrors securely to the support element.
The optical accuracy of the positions of the outer faces of tbe individual mirrors ls maintained by the assembly fixture and is independent of the faces of the support element. The -cement filling the gaps is uniformly dis-tributed around the optical axis to maintain rotational balance.
MIRROR
The resulting mirror 10 i`s sho~n in FIG. 1 as haying six re~lective faces; although 12 or more faces -. ~ .
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are common for polygonal mirrors, and any desired number of faces can be used. The faces of a multifaceted mirror made according to the invention need not form a polyhedron or a figure with a polygonal-shaped cross section, the mirror need not be rotatable, and the faces need not be in planes parallel to a rotational axis or any other line.
Epoxy 14 bonds individual mirrors 15 to support or core element 11 with sufficient adhesive strength to withstand the centrifugal force as mirror 10 rotates, and core element 11 provides a hub 12 and rotational support. Epoxy 14 is also preferred for bonding individual mirrors to a support element 1l, even if high speed rota-tion is not required of the resulting multifaceted mirror 10.
Support 11 can ~e formed of cast metal, ceramic, or plastic and need not 6e highly accurate, because the reflecting faces of mirrors 15 are positioned independently of the corresponding faces 16 of support or core 11. Faces 16 o~ support 11 are spaced radially inward from mirrors 15 to leave parallel and uniform gaps between the inner surfaces of mirrors 15 and core faces 16 so that epoxy 14 can be injected between faces 16 and mirrors 15 to form a secure bond.
The radii and angles to be accurately maintained for mirror 10 are best shown in the cross-sectional view of FIG. 10. Bore 13 of support element 11 is accurately concentric with and aligned with rotational axis 60 and has a radius rl as illustrated. The radius r2 of hub 12 is not a critical dimension, but the outer surface of hub 12 is also accurately concentric with axis 60 so that the hub provides a balanced rotational mass. Hub 12 can also be eliminated, and the disk~shaped body of support 11 can be fastened to a rotary element by screws parallel with the rotational axis.
The faces 16 of support element 11 are equally spaced from axis 60 by radii r3, which are accurate to ~, - . : ,. :
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precise mechanical tolerances. Faces 16 do not require optical polishing or exact optical location, however.
Inner surfaces 61 of mirrors 15 are equally spaced from axis 60 by radii r4 that are larger than radii r3 so that cement 14 filling the gaps between support element 11 and mirrors 15 is uniformly distributed and rotationally halanced around axis 60. Outer surfaces 62 of mirrors 15 are positioned with higb accuracy at radii r5 from axis 60 and held in place by the assembly fixture as explained below. Surfaces 62 are optically flat, uniformly coated, and positioned ~ithin a few seconds of arc to predetermined planes spaced around axis 60. Surfaces 62 are also accurately angled relative to each other.
As shown in ~rG. la, mirror surfaces 62 are parallel with rotat~onal axis 6~ and perpendicular to the plane 63 of the top surface of support element 11.
~urfaces 62 can also be o61ique to axis 60. Keeping surface 63 of support element 11 accurately perpendicular to axis 6~ and bore 13 c-oncentric ~ith axis 6~ insures 2a that the mirror rotates without ~Q~bling and provides an accurate linear scan path. rn actual practice, the plane of surface 63 and the opposite surface 73 of sup-port 11 is established only around the perimeter of support 11; and otherwise surfaces 63 and 73 are recessed sli9htly relative to the~r perimeters.
~; Balance of all the components of mirror 10 around ; the rotat~onal axis 60 is important for smooth running at high speeds that can exceed 25,~00 rpm and can cause vlbration regardless of mounting arrangements. Accuracy in component dimensions and mass distributlon insures a near per~ect rotational balance, and final balancing is achi~eved by removing tiny amounts of material from hub 12. Mirrors lacking a hub 12 can be balanced via slightly oversized holes for screws extending parallel - 35 with the rotational axis.
As best shown in FIGS . 3-5, each corner of core 11 between each of the faces 16 of core 11 is preferably - . ~. - - . .. .. .

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113~55 formed with a retainer slot 17 that includes a bore or enlargement 18. A trim strip 20 can then be inserted in retainer slot 17 and anchored in bore 18 to extend between and overlap the edges 22 of individual mirrors 15 as illustrated. The head 21 of retainer strip 20 pref-erably fits just over the edges 22 of mirrors 15 both for improved appearance and for security against any mirror 15 breaking loose and flying outward during rotation.
Trim strip 20 is preferably an extrusion of metal or other 1~ high tensile strength material that is cut to length.
It can have many different shapes for head 21 and anchorage end 23 fitting enlargement 18.
Retainer slot 17 can also be used to hold a spacer strip 25 as shown in FIG. 3 to insure proper spacing of individual mirrors l5 during assem~ly. Spacer strip 25 is preferably ~ormed of a resinous material that does not bond to epoxy or whatever cement is used to bond mirrors 15 to core 11, and spacer strip 25 is removed after the cement 14 has set. The space left by strip 25 after its removal then leaves room for insertion of trim strip 20 as shown in FIG. 4. Another cement 14a is then used to fill spaces around trim strip 20 and bond trim strip 20 securely in place. Gement 14a preferably has approximately the same densi~y as cement 14 so that any irregularities in the proportions of the two cements does not upset rotational balance.
A preferred shape for head 21 of trim strip 20 is best shown in FIG. 11. As a space or junction between mirror faces 62 rotates through a scanning beam 70, it can reflect lighk along a scan path 71 extending through an angle 72 to cause noise in the signal. When reflective surfaces are ground on a single glass element, a little rounding between them is unavoidable; and a brief light reflection from this causes a noise signal as scan-nfng beam 70 passes from one face to the next. Head 21of trim strip 20 can be shaped as shown in FIG. 11 to reduce this noise to a minimum.

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1~31~5 g This is accomplished by making head 21 nearly pointed with nearly flat sides 65 extending outward from mirror surfaces 62 to converge along a line 66. This orients faces 65 to divert light from beam 70 outside the scanning path 71. Flat side faces 65 also accommodate hooks 64 that overlap the edges 22 of mirrors 15, both for appearance and mechanical security. Reducing noise as beam 70 passes from one surface 62 to the next cooperates with other features of my mirror to increase accuracy.
A~SEMBLY FIXTURE
A multifaceted base 30, su~h as shown in FIGS.
6 and 7, is the main element af an assembly fixture for ~aking mult~faceted mirrors according to my inventiorl.
Base 30 is prefera61~ formed of hardened steel and has ~aces 31 that accurately conform to the planes of the faces 62 of mirrar 10 to witbin seconds of arc. Base ~aces 31 then form reference surfaces for accurately ~.
positioning mirrors. 15, and t~is i.s preferably done with vacuum.
A vacuum system connects to a socket 32 at the bottom of base 30, and vacuum passageways 33 lead from socket 32 to each face 31 of base 30. A screw 34 inter-sects each passageway 33 to form a manually operable valve for opening and closing each passageway 33. The pressure oP the vacuum system is also preferably adjustable in a generally known way.
: Locator blocks 40 as best shown in FIGS. 8 and 9 are formed to fit each face 31 of base 30 with a refer-ence surface 41 that is accurately plane. The lower portion 39 of reference:. surface 41 provides a mounting : surface engaging a face 31 of base 30, and an upper portion 49 of reference surface 41 provides a support surface fQr an individual mirror 15. Each block 40 has a vertical bore 42 plugged at the bottom by a pin 43, and holes 44-46 intersect bore 42 and open at the reference face of block 40. A screw 47 intersects bore 42 between holes 44 and - . - . ~ . .. -, .; , ~ . . : . .. .

~131~55 ~ 10-45 for manually controlling a vacuum as explained more fully below.
An abutment plate 50 secured to the underside of base 30 by screws 51 provides a reference surface per-pendicular to face surfaces 31 for locating blocks 40.Notches 52 ~n abutment plate 50 are centered under each face 31 of base 30. One of the locator blocks 40 is positioned against each face 31 of base 30 to rest on abutment plate 50 ~ith pin 43 extending into notch 52 and plane reference surface 41 conforming securely and accurately to the plane of surface 31.
A shallow recess 48 in reference surface 41 of blocks 40 communicates with hole 44 so that vacuum applied at surface 3I via passage 33 and screw valve 34 i.n base 30 evacuates recess 48 and holds locator block 40 accurately against face 31. A vacuum system for holding locator bllcks 40 on base 30 has the advantages of easy assembly and disassembly and also makes presence of any dirt or inaccuracies in assembly more readily detectable.
2Q After mounting surfaces 39 of locator blocks 4~ are positioned accurately in contact with faces 31 of base 30 and held in place by respective vacuums~ in-dividual mirrors 15 are secured to the support surfaces 49 of locator blocks 40. This is done by placing each individual mirror 15 accurately against a support surface 49 and opening screw ~alve 47 to bring the vacuum applied to mounting surface 39 into communication with support surface 49 via passageway 42 and holes 45 and 46 that open into shallowly recessed slots 38, This e~acuates ~lots 38 and applies a vacuum to mir~ror 15 for holding the mirror securely in place and also indicating its accurate engagement with support surface 49.
Then core 11 is inserted into the assembly of locator blocks and mirrors to be positioned radially inside mirrors 15 and accuratelr centered by bore 13 in hub 12 being fitted over a locator pin 53 accurately , .. . ..
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centered in base 30. Core 11 and mirrors 15 are supported in the axial direction by an accurate reference surface 54 formed around the upper perimeter of base 30. If core 11 has retainer slots 17 and if spacer strips 25 are desired, they are inserted between mirrors 15 to extend outward between locator ~locks 40.
When everrthing is accurately assembled and securely held in place, epoxy or other suitable cement is in~ected into the open gaps between core 11 and mirrors 15 as schematically shown in FIG. 9 for cementing the mirrors to the core. After the cement has set, spacer strips 25 are removed; and trim strips 20 are inserted in retainer slots 17 to protect the edges of mirrors 15 and present a clean and neat outward appearance. The vacuum is shut off and locator blocks 40 are removed to release the assembled mirror from the fixture. The assem-bled mirror is then balanced and tested and is ready for use.
Many variations in the illustrated details are 2Q possible. A vacuum system and valving can be arranged in other ways, and locator blocks 40 can have many dif-ferent configurations. The optimum arrangement preserves the h~gh accuracy available with minimal expense in ma-chining and shaping base 30 and locator blocks 40. Making the individual mirrors 15 is well understood and relatively inexpensive; and core ll can be a simple casting, since its accuracy is not critically involved in the result, By keeping the assembly highly clean and holding all the components to high accuracies~ end results within a few seconds of arc can be obtained without prohlbitive expense.
Multifaceted mirrors having shapes other than a regular polygon as illustrated can also be made according to the invention by using different shaped bases and locator blocks of an assembly fixture and different shaped support elements so that each individual mirror is held accurately in a predetermined plane and bonded securely , . ,, . :; ' : - ~
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to an ad~acent support. Workers skilled in the art of optical assemblies will understand the available alter-natives and their relative merits once the basic concepts - are understood.

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Claims (7)

Claims:

1. A fixture for assembling multifaceted mirrors having a plurality of faces disposed in respective planes that are angled relative to each other, said fixture comprising:
(a) a base having a plurality of faces corresponding with the faces of said multifaceted mirror, each face of said base being accurately in the plane of a respective face of said multifaceted mirror;
(b) a plurality of locator blocks coresponding to said faces of said base and said multifaceted mirror, each of said locator blocks having a mounting surface for removably engaging one of said faces of said base and a mirror support surface in the plane of said mounting surface and extending beyond said base;
(c) means for removably securing said locator blocks to respective faces of said base;
(d) means for removably securing a plurality of individual mirrors respectively to said support surfaces of said locator blocks to position said individual mirrors at the faces of said multi-faceted mirror; and (e) means on said base for receiving and holding a support element positioned adjacent said individual mirrors so said individual mirrors can be cemented to said support element while secured to said locator blocks to form said multifaceted mirror.

2. The fixture of claim 1 wherein said means for securing said locator blocks to said base includes a vacuum system with vacuum passageways opening at each of said faces of said base.

3. The fixture of claim 2 including valves in said base for opening and closing each of said vacuum passageways.

4. The fixture of claim 2 wherein said means for securing said individual mirrors to said locator blocks include vacuum passageways in said locator blocks communicating with said vacuum passageways in said base and opening at said mirror support surfaces.

5. The fixture of claim 4 including valves in said locator blocks for opening and closing said vacuum passageways to said mirror support surfaces.

6. The fixture of claim 1 including an abutment plate secured to said base to position said locator blocks in a direction perpendicular to the planes of said faces of said base.

7. The fixture of claim 1 including a centering pin accurately located on the axis of said mirror for receiving and holding a central axial hole of said support element.