US2458384A - Grinding of lenses or of dies therefor - Google Patents

Description

Jan. 4, 1949. J. H. JEFFREE 2,458,384

GRINDING QF LENSES OR OF DIES THEREFOR Filed March 9, 1945 3 Sheets-Sheet 1 hwehfor I e vhn Jgffrec WAM, flewo x 91W Jan. 4, 1949. J. H. JEFFREE 2,458,384

GRINDING 0F LENSES OR 0F DIES THEREFOR Filed March 9, 1945 3 Sheets-Sheet 2 A Q Q Q VI/II/l/I/ N 3 RN N 5* g1 I 3 Joh H.Jeffrce a lnvenTor J. 4, 1949. J. H. JEFFREE 2,458,384

GRINDING OF LENSES OR OF DIES THEREFOR Filed March 9, 1945 3 Sheets-Sheet 3 D/PHT/ON OF T001 MOVfMf/VT FOCUS WOR/f SURF/7C5 TOOL [117m POI/170M) 2ND. FOCUS OPT/CAL AX/J P/VOT AXU TOOL Y Pas/770W 2. 7u r ocl/az r001 Pas no.

FOCUS. Z.

I WORKPIL'CF P/yar m/mvr- J emu 4* w? Patented Jan. 4, 1949 GRINDING or LENSES on or mas THEREFOR John Henry Jeifree, Cobham, England, assignor to Combined Optical Industries Limited. Slough, England, a British company Application March 9, 194 Serial No. 581,730 In Great Britain tober 26, 1943 Section 1, Public Law 90, August a, 1946 Patent expires October 26, 1963 2 Claims.

This invention relates to improved methods an apparatus for grinding and/or polishing curved surfaces of various kinds including aspheric surfaces, particularly lenses and the dies used for the moulding of lenses from transparent plastics, as described for example in Kingston United States Patent No, 2,314,838, dated March 23, 1943. The word lens in this specification is to be understood to include a mirror, where the context cal surfaces, and the grinding of lenses which give cylindrical and spherical corrections are well known. While, however, various kinds of optical systems are susceptible of improvement by the inclusion therein of aspheric lens or mirror surfaces, the production of such surfaces of suflicient precision by known methods is so difficult that little use of the same in optical systems has been made hitherto. The objects of this invention comprise the provision of methods and apparatus whereby aspheric as well as spherical surfaces can be ground and polished effectively and with considerable precision. The term aspheric" as used herein should be understood to include surfaces of revolutions such as paraboloids, ellipsoid-s and hyperboloids(generally though not exclusive 1y prolate in form) and also to include surfaces having curvatures of third order or of higher orders, which are symmetrically curved relative to an axis, including surfaces which are concave at the middle and convex near the periphery or vice versa. The present invention can beutilized in all such cases, its simplest application however being to the grinding and polishing of prolate surfaces of revolution which can be rotated on their axes of-symmetry during the process.

In accordance with the method herein described, the workpiece is first roughly formedto the desired surface (such for example as one of the surfaces of revolution), and this is rotated about its axis of symmetry. This surface is scraped (9r, when the polishing stage is reached. it may be polished), by the edge of a lapping tool similar to a profile cutter which is initially shaped roughly to fit the surface. This tool is held under sumcient pressure against the surface acted upon sufllcient movement in its own axial direction, 1. e.. in the direction of the axis of the curve imparted to the tool edge by grinding, to maintain contact between the edge and the surface. In the formation of surfaces other than prolate surfaces of revolution the axis about which thetool is caused to swing will not pass through a focus of the curved surface, but will be otherwise determined, as is later explained.

To render the grinding eflective, an abrasivelubricant composition, such as a suspension of finely divided mineral matter in a viscous liquid such as oil or glycerine is preferably-used on the workpiece, and both the surface of the workpiece and the edge of the tool are abraded thereby to attain their final shape appropriate to their initial shape and the relative movements of workpiece and tool. The result of the above is that when the desired shape of workpiece surface is 'attained the tool edge will continue to fit and make contact therewith, with considerable accu racy at the central portion of the tool edge; and

with an accuracy at other portions ofthe tool edge which, in some cases, is not quite so great.

Other features of the improved method will, preferably, include the variation of the traverse of the tool edge across the workpiece, i. e. the continual displacement of the end positions of the 'tool edge, back and forth, during its reciprocawhile it is reciprocated back and forth across the causing thetool to swing about a focus (the more distant one) of the surface, while permitting it tion, to prevent the possibility of slight ridges being formed on the work, as might happen at the limits of movement of the edge if it always changed its direction from forward to back or vice versa at the same points; and also the carrying out of the desired movement ,of the tool, in such manner as to cause its edge continually to sweep the abrasive lubricant towards the centre of the workpiece. This can be done, in suitable cases, by causing the traverse of the tool to be slightly off centre. that is, by horizontal adjustment of the tool in its own plane.

The manner of practising the method will be more clearly understood by the description of apparatus, the operation and adjustment of which may be utilized inthe practice. One form of apparatus embodying the invention so far as it relates to apparatus, is illustrated in the accompanying drawings, in which Figure 1 is a plan view of such apparatus,

Figure 2 is a front elevation thereof, shown partly in section,

Figure 3 is a partial vertical section on line 3-3 of Figure 1, on an enlarged scale,

Figure 4 is a partial section along the plane of line IV-IV of Figure 2;

Figure 5 is a partial elevation looking from the left in Figure 2;

Figure '6 indicates diagrammatically the mounting of the workpiece for grinding a, hyperboloid (concave) surface;

Figure 7 indicates diagrammatically the mounting of the workpiece for grinding a concave ellipsoid (prolate) surface;

Figure 8 indicates diagrammatically the mounting of theworkpiece for grinding a concave ellipsoid (oblate) surface;

Figure 9 indicates diagrammatically the mounting of the workpiece for grinding a, prolate convex hyperbolic surface;

Figure 10 indicates diagrammatically the mounting of the workpiece for grinding a prolate convex ellipsoid surface; and

Figure 11 indicates diagrammatically the mounting of the workpiece for grinding an oblate convex ellipsoid surface.

Referring to the drawings, substantially upright frame members l3 are pivoted at their lower ends on rod I2 mounted on block II, which is secured on the fixed horizontal table I0. Members I3 carry a transverse pivot rod I4 at their upper ends, on which are pivoted two substantially horizontal frame members I5, these arms I5 being held against this pivot rod, in the formof construction illustrated, by leaf springs I5a. Members I5 carry at their opposite ends a transverse pivot rod I6 on which are pivoted arms H which have, integral therewith or rigidly secured thereto, a tool-supporting transverse member Ho.

The tool I8 is secured to member Ila, the tool being, in the form of construction illustrated, a flat metal sheet, the lower edge of which is shaped roughly to fit the curved surface which is to be ground, different tools being provided for the various surfaces such as hyperboloids, ellipsoids, etc., which are to be ground. As is indicated in Figure 3, the tool may be secured to member I la by means of a screw I8a. passing through a vertical slot I8b in the tool, and thence into member He the slot being wider than the shank of the screw, to permit some horizontal adjustment of the tool in its own plane (for a purpose hereafter explained), before tightening the screw, the head of which I8c is wide enough to have a firm bearing on the tool on both sides of the slot. The lower edge of the tool rests on the preformed surface of the workpiece I9, mounted on a workpiece holder which is rotated about its vertical axis by driving pulley 20L The workpiece may be, for example, a steel die which is to be ground and polished to a desired curved surface for use in the moulding of optical lenses from plastics, the curved surface shown in the drawings being ellipsoidal.

The tool is supported for the desired movements as follows. An upright standard 2I is secured to the fixed table Ill at one side thereof. This is shown broken away, but may extend down below the level ofthe table for a required distance of several feet. Bracket 2| I is secured to this standard at a desired level above (or below) table II! by means indicated as the screw 2I Ia, the transverse flange of the standard being, conveniently, provided with a vertical slot, or suitably spaced openings, for this purpose. This bracket carries a bolt 2 I2 extending transversely beyond its inner surface, and may be slidably engaged, on one of its lateral surfaces, by a vertical bar 22 having perforations 22a extending therethrough at various points. The edge of this bar is insllding contact with bolt 2I2, fitting in a slotted guide 4 2I2a. thereon, with which it is held resiliently in contact, as by means of spring 222, which may be removably extended from bar 22 to a pin 2IIb on bracket 2. The pivot rod I6 is extended into one of the perforations 22a of bar 22, so that the arms I5 are approximately horizontal when the lower edge of the tool is resting on the workpiece I9. The tool I8 is rigidly secured to bar 22 to move therewith, as by securing one of the arms I! to the bar 22 by a pin extended into another one of the perforations 22a of the bar.

An arrangement is therefore provided whereby the tool rests by gravity on the workpiece I9 and is free to move upwards and downwards in accordance with the curvature of the workpiece, as the bar 22 slides to a sufllcient extent over the surface of the pivot bolt M2. The tool will also swing backwards and forwards about bolt 2I2 as a pivot, as its lower end is swung backwards and forwards across the workpiece by the means presently to be described. The pivoted frame I3, I5 serves to restrain movements of the template, and to prevent endwise movements, as will be discussed hereafter. If desired, the tool edge may be held down against the surface of the workpiece by spring pressure in addition to gravity, as by means of the spring I'Ib indicated.

The tool and the work are mutuallyabraded until, from an original rough form, they reach forms that fit each other at all positions of the working stroke. Since their angular relation to each other varies over the stroke, according to a predetermined law, the curve they acquire is determined by the law of this variation, as well as by the rough curve from which they started. The case is analogous to that of ordinary grinding of spheres together, where the curvature reached depends partly on the initial rough forms of tool and work, but the kind of curve (sphere) is that whereby the tool and work can fit each other in any position. In my machine, however, it is a surface and an edge that must fit, instead of two surfaces, and the conditions under which they fit are restricted by the position of the pivot controlling the angular position of the tool.

The apparatus as shown is arranged for the grinding of a prolate surface of revolution, specifically an ellipsoidal surface. Such a prolate surface, approximating in section to a conic section, concave or convex, may be worked with the edge of a tool so moved that its plane always passes through a point on the axis of the surface at or near a focus (the more distant focus) of the said conic section. The pivot 2I2 must therefore be positioned approximately at such point. The inclination of the tool to the surface then varies with its traverse, from perpendicular at the central position to oblique at others.- Clearly, the only type of curved surface capable of continually fitting an edge of a sheet so moved is a prolate one, that is, one with curvature decreasing away from the centre.

- For various types of prolate surfaces, the pivot 2I2 must be positioned at other points than that shown, to lie approximately at a focus of the desired curvature, and for other types of curvature other adjustments must be made, as will be discussed hereafter.

The driving means will now be described, which provide for traversing the template edge across the workpiece, with the variation of stroke which has been referred to above as. desirable. A vertical spindle IIlI at the right hand end of table I0 is rotated from motor 35 by worm and wormwheel 34, 31 and belt 38, which passes over the encased.

pulley 33 on the spindle oi the wormwheel and the driven pulley 25 on spindle ll.- The workholder 20 is rotated by another belt which passes around vertical axis extending upwardly from carrier 23,

, pulley 26 on spindle IOI and pulley 21" on the and this disc is rotated by the engagement of a small pinion 39 on crankpin 2 with the large pinion 40 secured to disc 21. Disc 21 is provided with a diametrically-extending slot 42 in which is mounted a screw 44 on which a slider 43 is adjustably mounted. This slider carries a driving pin 45 on which slotted bar 28 is mounted, this bar being secured in adjusted position by two locking washers 46 on pin 45. The traverse of the tool 18 is effected through stirrup 30 secured to template carrier Ila, universal joint 3|, and bar 28. A vertical pin 29, projecting upwardly from carrier 23 through a slot of bar 28, maintains ghe correct relationship between these two memers.

It will be seen that the throw of the eccentric or crankpin '2 determines the length of stroke. of the working edge of the template back and forth over the workpiece, this length varying as crank-pin 24| is adjusted nearer to, orfurther away from, the centre of disc 24. Also, the adjusted position ofpin 45 lengthwise of the slot in link 28, through which it extends. determines what may be called the mean mid-position of the tool strokes in relation to the axis of the workpiece, which may, or may not, coincide with that axis in different cases. And the throw of the eccentric driving pin 45 determines the varia tion of dispacementof the tool stroke. That is .to say, the rotation of pin 45 about its centre causes the whole stroke to be displaced in the lengthwise direction of the stroke, so that its extreme positions at both ends will continuously vary, a series of such variations being completed each revolution of the disc 21. Also, the adjust-- frame l3, i5, i1, connected to the tool, is to pre--.

vent or restrain undesired movements thereof during operation. Thus, unless permitted to a small extent by looseness of bearings, rotation of the tool about the axis of. the workpiece, or about any vertical axis, is prevented, as well as its rotation in its own plane. The one rotation which is permitted, during operation, is that about the axis of pin 2|2, this being an axis normal to the axis of the workpiece, by which the correct inclination of the tool edge is determined at all times. As to moVementsLof translationof the tool edge, the hinged frame permits vertical movement thereof, in accordance with the curved contour of the surface tr aversed by'the edge. It also, of course, permits movement of the tool normal ornearly normal to its own plane, which constitutes its working stroke. A third possible movement of translation, a horizontal movement of the tool in its own plane, is prevented by the construction as shown, during operation.

As to initial adjustments, however, or adjustments when the apparatus is stopped, the tool may be shifted horizontally in its own plane, by suitably adjusting the position of screw 18a in slot i8b of the tool, as described above in connection with Figure 3, so that the tool may be maintained somewhat off centre in this plane, to enable it to sweep the abrasive-lubricant continuously towards the centre of the workpiece.

I A slight rotation of the tool in its own plane, for

adjustment, is also permitted by the screw and slot construction described, and such adjustment by tilting in this plane may be required in connection with the lineal adjustment referred to, or because of faulty alignment of other parts. A further possibility of adjusting the tool position by tilting it in its own plane may, if desired, be given by pivotally connecting the member H to the base In, as indicated at Ila, along a horizontal axis normal to the pivot 12, suitable pro-.

edge portions of the curved work surface, in cases in which such edge portions differ markedly from the curvature of the remainder of the surface. The utility of this expedient in particular cases can be determined readily by test.

As has been stated above, correct inclinations of the tool edge for the grinding of, and maintaining contact with, various prolate surfaces of revolution are obtainable by locating the pivot H2 at the distances from the surface of the workpiece, along the optical (i. e., the symmetrical) axis of the surface acted upon (or closely adjacent thereto), which represent in each case the'position of a focus of the curvature in question. In the case illustrated an ellipsoid is ground, with the Pivot H2 in about the relative position shown, the tool tilting symmetrically with respect to the axis of the workpiece. If a hyperboloidal surface is to be ground, the pivot 2i2 must be positioned some distance below the surface of the table I0, bracket 2! I being secured to the lower part of the upright frame member 2i, and the arms I5 and I! then being connected to vertical bar 22 at perforations 22a, which are shown in the drawing as being at the upper end of the bar. If, on the other hand, a paraboloidal surface is to be ground, the focus at which pivot 2l2 should be located is, theoretically, at an infinite distance above the workpiece, and the tool, the edge of which is roughly formed to a corresponding parabola, should be substantially upright, i. e. parallel to the axis of the workpiece, at all times. In practice, sufilciently accurate results can be obtained in grinding small dies or lenses by locating the pivot 2l2 at a height of about 2 feet above the surface of the work, or,

say, about four times the relative height shown in the drawings. If the device be used for grinding or polishing a spherical surface the pivot 212 normal to the surface, at all times.

In some cases other than the prolate surfaces of revolution, the inclination of the tool will not vary symmetrically with respect to the axis of the workpiece as described above. When acting upon oblate surfaces of revolution the template in its mid position will be inclined, to the vertical, and may remain so throughout its traverse. In such cases the position of the pivot will be found to lie on the periphery of a circle tangent to the central portion of the curvature to be traced, which circle has a radius equal to one half the radius of the central portion of the oblate surface. Considerations indicating the position of the pivotal point upon such periphery need not be iven here. The correct position will be found to be the one which, when used as a pivot as described, will cause the tool edge always to maintain contact (at least as to its central portion), with the surface to be'ground, and this point can always be found, either by theoretical considerations checked up by trial, or by trial and error, or empirical, methods, until the correct position is ascertained. In all such cases, brackets similar to the bracket 2| I, shown in the drawings, but carrying a pivot pin similar to the pin 212 at the required distance from the axis of the workpiece, will be provided.

Among other modifications of the apparatus to fit different cases, it may be said that astigmatic surfaces having different curvatures in different directions may, if symmetrically curved relative to an axis, be worked while rotating the workpiece on this axis, by making the inclination of the tool depend on the angle of rotation of the workpiece as well as on the extent of traverse of the tool. In all such cases the principle holds good that a point must be found through which the pivot 2 I2 is extended, about which the bar 22 will swing, this being so located that the tool edge will always incline at such angles to the surface of the workpiece as to continue to fit the same when the desired final shape of the surface is attained.

Reference has been made to the abrasivelubricant composition. In the case where an aspheric surface of great precision is imparted to a steel die the abrasive-lubricant composition may be changed during the grinding operation. A suspension of carborundum in oil may first be used, then a suspension of finely divided alumina in oil or glycerine and finally a suspension of r diamantine in oil or glycerine. Polishing may be effected in the same machine, the tool in that case having a flexible edge or an edge of leather or soft fabric.

I claim:

1. In apparatus for grinding or polishing optical surfaces, including aspheric surfaces, the combination of a workpiece holder, means for rotating it about an axis of symmetry of a workpiece thereon, a pivot member, a tool carrier, means for supporting said carrier in sliding engagement with said pivot member, to swing thereabout, with the edge of a tool carried thereby pressed against the surface of a workpiece on said workholder, and means for swinging said edge across said surface with a varied stroke, comprising, a rotatable eccentric pin, a carrier pivoted thereto, a disc journalled on said carrier and rotated by driving connection from said pin, a pin eccentrically mounted on said disc, and driving connections between said last named pin and said tool carrier.

2. In a method of forming oblate conicoidal surfaces of revolution on workpieces, the steps which consist in forming a workpiece surface roughly to the required curvature, shaping a tool edge roughly to fit the same, rotating the workpiece about the optical axis of the said surface.

swinging the tool back and forth over said surface about an axis passing through a point on one side of said optical axis and on the periphery of a circle tangent to the centre portion of said surface and having a radius of curvature half that of the surface to be formed on the said central portion of the workpiece, while maintaining the shaped edge of the tool pressed into contact with the surface, and permitting the tool to move in its axial direction in conformity with the form of the curved surface engaged by its edge, the said second-mentioned axis being at such a distance along said periphery from said optical axis that the tool in swinging will tilt at various angles of inclination to the surface of the workpiece during its travel across the same, such that the tool edge will tend to fit the surface of the workpiece when the inaccuracies of the latter have been removed.

JOHN HENRY JEFFREE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 761,207 Clark May 31, 1904 857,791 Clark June 25, 1907 947,774 Germain Jan. 25, 1910 995,393 Witmer June 13, 1911 1,221,280 Brockbank Apr. 3, 1917 1,343,522 Robertson et a1. June 15, 1920 1,383,863 Scheuerle July 5, 1921 1,520,703 Ericsson Dec. 30, 1924 1,639,012 Tillyer et a] Aug. 16, 1927 1,827,748 Holman Oct. 20, 1931 2,176,154 Shannon Oct. 17, 1939 2,352,386 Holman June 27, 1944 FOREIGN PATENTS Number Country Date 755,354 France Sept. 4, 1933

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