US3886695A

US3886695A - Method for grinding a gem stone

Info

Publication number: US3886695A
Application number: US361037A
Authority: US
Inventors: Maximo Elbe
Original assignee: COLORANT SCHMUCKSTEIN GmbH
Current assignee: HOGE RAAD VOOR DIAMANT
Priority date: 1973-05-17
Filing date: 1973-05-17
Publication date: 1975-06-03
Anticipated expiration: 1992-06-03

Abstract

A gem stone to be ground is placed against a rotating grinding wheel, and thereupon the gem stone is turned relative to the grinding wheel until an ideal orientation has been determined for the gem stone relative to the grinding wheel. The frictional heating of the gem stone is measured in the region of contact of the same with the grinding wheel, and the contact pressure at which the gem stone contacts the grinding wheel is controlled as a function of the measured temperature and in such a manner as to maintain the temperature constant. The respective facets of the gem stone are thereupon ground with the grinding wheel.

Description

United States Patent 11 1 Elbe June 3, 1975 [54] METHOD FOR GRINDING A GEM STONE 2,829,472 4/1958 Salzer 51/229 X 3,811,230 5/1974 Beck 51/229 X [75] Inventor: Maximo Elbe, Hamburg, Germany [73] Assignee: Colorant Schmuckstein Gmbl-l, Primary Examiner-Donald G. Kelly m g, Germany Attorney, Agent, or Firm-Michael S. Striker [22] Filed: May 17, 1973 [2]] Appl. No.: 361,037 [57] ABSTRACT A gem stone to be ground is placed against a rotating C 51 283. 51 216 LP. [322 grinding wheel, and thereupon the gem stone is turned Cl n B24b B24b 9/16 relative to the grinding wheel until an ideal orientation 0 I n u a e e l s e e u v I u l a n a e e q e u v h b d t I f a [58] Field of Search 51/283, 229, 216 LPzl27, as 6 ermmsd 6 gm 5l/l21, 122,123 R, 125, 125.5,322, 323,125/30 grinding wheel. The frictional heating of the gem stone is measured in the region of contact of the same with the grinding wheel, and the contact pressure at References Cited which the gem stone contacts the grinding wheel is UNITED STATES PATENTS controlled as a function of the measured temperature 1,173,174 2/1916 Coleman 51/229 X and in such a manner as to maintain the temperature 2,137,405 11/1938 Johns, 51/229 X constant. The respective facets of the gem stone are 2,427,064 MOUl X thereupon ground the grinding wheel 2,429,961 10/1947 Rakowitzky 51/283 0 2,542,698 2/1951 OBrien 51/229 4 Claims, 13 Drawing Figures l l l (/5 a //a /02 a, i .9 1 /5 l T 7 /2 loo 3 Pmmnm 1915 38861695 SHEET 3 PATENTEDJUH3 I975 SHEET FIG. 7b

' FIG. 70

FIG. 70'

FIG. 7a

METHOD FOR GRINDING A GEM STONE BACKGROUND OF'THE INVENTION The present invention relates to a method of grinding a gem stone, and to an apparatus for carrying out the method.

The term gem stone as employed in the present disclosure is intended to refer in general to all precious stones, and in particular to diamonds of all types, including diamonds which are used for industrial purposes. In other words, the invention is not intended to be limited to diamonds exclusively. nor is it intended to be limited to diamonds or other precious stones which are exclusively used for decorative purposes, but also those which are intended to be used for industrial and other applications.

In many respects, the prior art is not satisfactory insofar as the approach to the grinding of gem stones is concerned, as well as the equipment for effecting such grinding. It is therefore acknowledged in the industry that further improvements are highly desirable, but heretofore such futher improvements have not been forthcoming.

SUMMARY OF THE INVENTION It is, accordingly, a general object of the present invention to provide such further improvements and to overcome the disadvantages of the prior art.

More particularly, it is an object of the present invention to provide an improved method of grinding a gem stone which is not possessed of the prior-art disadvantages.

Another important object of the invention is to provide an improved apparatus or machine for grinding of gem stones.

The present invention is intended to make it possible to determine the ideal grinding direction for the facets of a gem stone, particularly a diamond, that is the ideal orientation which the diamond should have with respect to the grinding wheel as facets are being produced. Futhermore, the respective facet is to be ground to a specific predetermined depth (the term depth referring to the material removal beginning from the outer surface at which grinding originates) and to so polish it that at least one side of the gem stone can be completed by resort to the present invention without requiring any further additional processing steps. The angles of the facets should be reproducible, they should be capable of being produced in a short period of time and with maximum precision, and a minimum amount of the material of the gem stone should be lost during the production of the facets.

Of all gem stones, diamonds provide, of course, the most difficulties in attaining the objects of the present invention, because the diamond is the only gem stone in which it is necessary to determine an appropriate orientation of the stone relative to the grinding wheel for each facet, in order to produce a cut of high quality. This is not to say that in particular the apparatus and, of course, the method according to the present invention cannot be used with other gem-stones, as has indeed already been pointed out above. Such other gem stones may be all precious or even semi-precious stones, not only natural ones but alsotsynthetic ones, and in fact, 'evcn rglassQl-lowever, the invention can be best described by referringto' thediamond which is the most difficult workpiece to process.

,It is known in the art that a diamond has hardness differentials of 124000, that is depending upon the orientation which the diamond is given with respect to the grinding wheel, the degree of hardness of the diamond (and therefore the degree of difficulty of grinding it) may vary within the above ratio. This means then that an economically and qualitatively acceptable grinding result on the diamond facets can be obtained only when the diamond is ground in a specific direction (for each facet), which direction is predetermined by the crystalline structure of the diamond. The specific most advantageous direction must be determined for each and every facet being produced.

The invention proposes three different approaches to this problem.

On the one hand, the specific most advantageous grinding direction can be determined by the amount of material which can be removed from the diamond per unit time. Evidently, in the soft grinding direction the thus-calculated grinding speed will be greater in a relationship of 400011 than in the hard grinding direction. Depending upon the orientation of the particular facet with respect to the crystal lattice the most advantageous ideal" grinding direction may occur between 1 and 4 times.

During grinding, even in the non-ideal grinding directions, a temperature will develop in the diamond and the holder in which the diamond is held for the grinding operation, which will result in an expansion of the holder which substantially equals and is opposite to the amount of material being removed. As long as the diamond is oriented in one of the non-ideal directions with reference to the rotating grinding disc, this heating causes the diamond to seemingly grow", whereas when the diamond is oriented in the ideal direction it will seemingly grow and also have material removed, so that the criterion for the amount of material removed is unambiguous. It is clear that the temperature changes resulting in a seeming growing of the diamond due to the expansion of the holder, necessitate a period during which the grinding apparatus must be adjusted to allow for this growth. Because this adjustment must be repeated as the ideal grinding direc tion for each and every facet is determined, it has been found that it is advantageous if the grinding temperature can be regulated. Once the diamond and the apparatus have reached a certain temperature during grinding of the first facet, a regulating arrangement can maintain this temperature constant during the grinding of all other facets, independently of the direction or orientation of the diamond relative to the rotating grinding wheel during the grinding of a particular one of the subsequent facets.

The manner in which the grinding temperature can be maintained constant, may be selected from various possibilities. The most advantageous one of these is to vary the pressure at which the gem stone contacts the rotating grinding wheel. Of course, electrical, infrared or other heating or cooling, for instance indirect heating or cooling of the holder which holds the gem stone, would have the same effect. The temperature sensing device is directly adjacent to the gem stone to be ground and may be a resistor having a negative or a positive temperature coefficient, or a thermal element whose change in resistance or voltagecaused by changes in the sensed temperature-is amplified and is transmitted to an instrumentality which changes the pressure at which the gem stone contacts the rotating grinding wheel.

Maintaining the grinding temperature constant has a significant advantage. namely in that the finished facet can be removed from further grinding at a substantial and well defined grinding depth, independently of the thermal expansion of the holder. Details of this will be discussed subsequently.

A second possibility is to determine the ideal grinding direction or orientation by determining the temperature which the gem stone assumes as it is being ground. It has been found that this grinding temperature will be greater if the diamond is ground in the ideal direction than in one of the non-ideal directions. To determine this, the pressure at which the stone contacts the rotating grinding wheel is maintained constant, and the temperature sensor influences the orientation of the gem stone with reference to the grinding wheel (i.e., changes the orientation) until the ideal grinding direction has been reached, which is determined by the fact that in this grinding direction the gem stone will become heated, during frictional contact with the rotating grinding wheel, to a higher temperature than in the other directions.

The third possibility envisions that the ideal grinding direction is pre-programmed for a particular cut of a gem stone, for instance to produce a brilliant cut, in that the optimum or ideal grinding direction for each facet is determined by appropriate orientation of a sample gem relative to the crystalline structure thereof, and the thus-obtained information is recorded in suitable manner, for instance on magnetic tape. Thereafter, all facets of other gem stones to be similarly cut are produced by grinding in the pre-programmed direction. It is, of course, to be assumed in this case that the gem stones to be ground have been previously oriented in the same manner as the sample stone, that is they must be inserted into the holder in the samemanner in which the sample stone was previously inserted.

All three possibilities mentioned above have the advantage that several arrangements of the same type can be used simultaneously and independently of one another in conjunction with a single rotary grinding wheel. Evidently, this is a highly economical proposition because the grinding apparatus itself can be utilized economically, the expensive grinding wheel can be very evenly and uniformly utilized, and the quality of each ground facet in terms of its planarity and its smoothness prior to polishing can be improved.

Experiments with the first-mentioned proposal have shown that a distance sensor sensing the travel of the gem stone in the direction towards the plane of the grinding wheel, with subsequent differentiation, i.e., a grinding speed indicator can properly serve to adjust the drive which in turn moves the gem stone until the latter has reached its ideal orientation relative to the grinding wheel. The relatively low grinding speed in the ideal grinding direction or orientation, namely 3.6 mm. per hour (i.e., 1 micron per second) requires that the entire machine and all components thereof be very precisely made to exacting standards. It is, of course, well known that gem cutting, and particularly brilliant cuts, require that the surface of each facet be of high quality, that is that it be polished to a high degree of smoothness so that any tracks left by the grinding device disappear and cannot even be found under the jeweler's loupe. Moreover, the individual facets must extend at very precise angles relative to one another, because this relationship determines the brilliance of the finished gem stone. The same requirements essentially apply also to industrial diamonds, because the proper smooth polish of a facet determines a sharp angle relative to adjoining facets and thus provides for the desired sharp edge which is required of industrial diamonds, and. of course, the angles of the facets relative to one another are determinative of the manner in which the diamond can be used as a tool.

Economic grinding in terms of the time required can be carried out on a coarse diamond grinding wheel, but heretofore this was found to be unsatisfactory for subsequent polishing. This meant that the gem stone has always had to be polished by using a separate fine grinding wheel. in order to provide the same geometry for many facets, a particularly stringent degree of precision cooperation between the two grinding wheels and the grinding apparatus is necessary. If, however, the gem stone is supplied for contact with the grinding wheel via a spring parallelogram withoutpivots and without play in direction normal to the general plane of the grinding wheel, and if the gem stone is turned about its previously determined ideal orientation or grinding direction through small well-defined angles, while at the same time the contact pressure between gem stone and grinding wheel is reduced, then it has been found that a thoroughly acceptable polishing of the gem stone can be obtained even on a coarse grinding wheel without leaving behind any tracks on the stone. This eliminates the need for a subsequent operation and for the employment of a fine grinding wheel. Moreover, using a spring parallelogram arrangement the desired angles of the individual facets relative to one another can be obtained within narrow tolerance limits. A sliding arrangment of the apparatus according to the present invention assures that the gem stone is evenly moved over the entire grinding wheel, so that the latter is evenly used and worn away, particularly if several grinding arrangements are utilized with one and the same grinding wheel.

Since it is well known that diamonds are expensive, this also being true of industrial diamonds, it is hardly necessary to emphasize that any machine for grinding and polishing of a diamond must have as one of its important characteristics the requirement that it remove as little as possible of this expensive material while, however, properly grinding and polishing the diamond at all sides, that is on all regions where facets are to be provided. According to the present invention this requirement is met in that the precise center of the diamond is placed into the center of one axis ofa rotary component of the grinding machine, or at the point of intersection of two axes of respective turnable components. This will be discussed in more detail later, but it is pointed out that the necessary adjustment is advantageously carried out by means of a device, for instance a so-called profile projector, which permits a positioning of the center of the diamond with reference to the center of rotation of the one axis or the point of intersection of the two axes, with a deviation of less than 0.01 mm. One of the important determinatives for the proper adjustment is the holder which holds the uncut diamond or the hemispherically cut diamond in the grinding apparatus. lt is necessary that the gem stone be so adjusted that its precise center is located on the center of the axis of rotation of the holder. an adjustment which again is carried out by means of a profile projector. It is advantageous if the holder is itself of or is embedded in a material having a low coefficient of thermal conductivity, in order for the time constant of the grinding temperature to be small. so as to permit a more economical grinding operation.

A further consideration in determining the precision of the finish-ground facets is the possible inaccuracy which might be introduced by the journal for the grinding machine spindle. It has been found that a hydrostatically journalled grinding machine spindle having a deviation from true rotational movements of less than 0.2 microns will provide substantial improvements over a similar spindle which is mounted in a ball hearing. The latter permits movements of the spindle which act upon the gem stone as if the latter were being subjected to constant hammer blows.

In addition. the present invention provides for a possibility of maintaining the expensive rotating grinding wheel against deterioration, to the maximum possible extent, in that the grinding wheel is mounted on its flange not only by means of tension screws but also by means of additional pressure screws, which prevent it, at least largely, from becoming distorted as a result of the heating to which it is subjected during the grinding operation. Heretofore, such distortion was unavoidable, and it was then necessary to grind the grinding wheel in order to make its grinding surface planar again, resulting in a substantial loss of material.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description ofspecific embodiments, when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a somewhat diagrammatic side-elevational view of a machine according to the present invention, with certain portions omitted as not essential for an understanding;

FIG. 2A is a diagram illustrating the grinding operation when the gem stone is properly oriented with reference to the grinding wheel;

FIG. 2B is a diagram similar to FIG. 2A, but illustrating the grinding operation in a condition in which the gem stone is not properly oriented;

FIG. 3 is a diagram showing surface uneveness of a polished and a non-polished facet of a diamond which has been ground with a rotating grinding wheel;

FIG. 4 is a fragmentary partially sectioned enlarged detail view illustrating a temperature sensor;

FIG. 5 is a diagrammatic detail view partially in diagrammatic section of a temporary holder for a gem stone, together with a portion of a sighting screen of a profile projector;

FIG. 6 is a fragmentary partly sectioned detail view of a permanent holder for the gem stone;

FIG. 7 is a partly sectioned detail view of an enlarged scale of a portion of the machine shown in FIG. 1;

FIGS. 7a-7d are fragmentary detail views, showing various details of FIG. 7; and

FIG. 8 is a diagrammatictop-plan view illustrating the arrangement of tension and pressure screws used to counteract buckling or distortion of a diamond grinding wheel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Discussing firstly FIG. I, it will be seen that reference numeral I identifies an upright or column which is vertically adjustable so that it can be adjusted to the size of a diamond or gem stone II which is to be ground. The column I has an end portion that is received in a socket 2 wherein it is adjustably held by means of a screw 3. It will be evident that various ways well known to those skilled in the art can be utilized for permitting the column 1 to be raised and lowered in FIG. 1. The socket 2, in turn, is rigidly connected with a plate 5, for instance a brass plate the underside of which may be provided with two or more elongated grooves extending in the direction from the left towards the right in FIG. I, and in which grooves respective rails 6, for instance of steel, are received so that the plate 5 can be shifted radially towards and away from the rotatable grinding wheel 4. The movement of the plate 5 is effected by means of a motor 7 which is connected with the plate 5 in a manner well known to those skilled in the art. This arrangement provides for a particularly small degree of displacement of the column 1 in vertical direction (see FIG. 1), so that the stone 11 to be ground will slide radially over the surface of the grinding wheel 4 without undergoing any change in the contact pressure with which it engages the latter. Such displacement in vertical direction would be undesirable, contrary to intentional displacement to accommodate different size stones 11.

A further upright or column 8 extends in parallelism with the column 1, laterally spaced from the same as shown in FIG. 1. The columns 1 and 8 are connected with one another by means of two sets or

packages

9 and 10 of leaf springs whose opposite ends are connected to the respective columns as shown. Thus, the springs and the columns form a spring parallelogram. The lower end of the column 8 has a carrier 12 for the stone 11, which carrier is connected with a turnable shaft 8a extending through the column and rotating about the axis 13. The upper end of the column 8 carries a motor 14 for rotating the shaft 8a. The stone 11 is held in the carrier and with this arrangement the stone 1 l is supplied into contact 'with the upper surface of the grinding wheel 4 directly normal to this surface, without play and at a constant angle which is independent of the size of the stone. The motor 14 can turn the shaft 8a and therefore the stone 11 through 360. The purpose of this is to move the gem stone 11 to its ideal orientation relative to the grinding wheel 4, that is to turn it until it has assumed this ideal orientation in which it will be ground in the ideal direction which is dictated by the considerations outlined earlier.

Reference numeral 18 identifies a sensing device with amplifier and differentiator, which can sense the grinding speed, that is the speed at which material is being removed from the gem stone 11. Such devices are well known to those skilled in the art and require no detailed discussion. The device 18 controls the operation of the motor 14 so that the latter turns the shaft 8a and thereby the gem stone 11 until the latter has been moved to the ideal orientation relative to the rotating grinding wheel 4. In this position, the gem stone is retained until a facet has been ground on it. Immediately prior to completion of the grinding of the facet, the device 18 supplies a signal to the motor 14 via a flip-flop circuit, causing the motor 14 to turn the gem stone 11 in opposite directions from the previously selected ideal orientation, up to 10 angular degrees in each direction. The result of this movement is a polishing effect on the just-produced facet. and reference to FIG. 3 will show that within only a few seconds the relatively uneven surface produced during the grinding operation (see the upper line in FIG. 3) which has deviations from a planar condition of as much as 0.2 micron which could be determined by the naked eye, is polished to a smoothness (see the lower line in FIG. 3) where these deviations are below 0.01 micron. This is effected without having to change the coarse rotating grinding wheel 4, that is without having to have recourse to a fine grinding Wheel. Advantageously, the polishing operation is carried out while simultaneously the gem stone 11 is displaced by the motor 7 over the entire radial width of the grinding wheel 4, under constant reduction of the contact pressure between the gem stone 11 and the grinding wheel 4.

This reduction is effected in a manner which will now be described. It will be seen that a second regulating circuit is provided which utilizes a temperature sensor 25 shown in FIG. 4, a motor 15, a rigid connection 16 between the columns 1 and 8, and an eccentric 17. The temperature sensor 25 is connected with an amplifier (not shown, but conventional) which, in turn, supplies the signals received and amplified to the motor 15. The latter is mounted on the rigid support 16 and turns the eccentric 17 in such a manner that during small angular adjustments of the eccentric 17 the contact pressure with which the gem stone 11 engages the grinding wheel 4 is either reduced or increased without, however, permitting the gem stone 11 to move out of engagement with the grinding wheel. This makes it possible to maintain the grinding temperature to which gem stone 11 is heated by its frictional engagement with the grinding wheel 4, constant to within fractions of a degree centigrade with reference to a reference value that can be preset. This, in turn, means that the ideal direction or orientation can be determined independently of the temperature of the gem stone 11. In addition. the arrangement has the advantage that the grinding depth of each facet (the depth to which material is removed from the outermost surface of the gem stone until the grinding of the particular facet is completed) with an accuracy of 0.001 mm. Moreover, the device 18 whose upper end 19 cooperates with the spring package 10, will cause shortly before completion of the grinding operation of the respective facet a gradual reduction in the aforementioned contact pressure. This is the result of the signal supplied to the motor which causes the eccentric 17 to turn gradually until the motor 15 receives a further signal from the device 18 which causes it to turn the eccentric 17 through 180, whereby the gem stone 11 is raised out of contact with the grinding wheel 4 by a distance of up to approximately 1 mm. In this condition, the stone 1] is reoriented for grinding of the next facet, whereupon the motor 15 and the eccentric 17 are turned again until the temperature regulating arrangement described above is again operative. Initially, the new facet to be ground will offer a very small surface area to the grinding wheel 4 for contact with the same. This means that the initial amount of material removed is relatively high, so that the criterion for the high grinding speed is well defined and the new ideal orientation for the newly ground facet can be rapidly determined. The entire cycle will be completed the quicker, the less the temperature of the gem stone 11 is allowed to drop between the grinding of consecutive facets, and a comparison of FIGS. 2A and 28 will clearly indicate this, as well as the importance of the proper orientation of the gem with respect to the grinding wheel 4. It should be pointed out that this relatively high-speed operation is of considerable importance in terms of the economics ofgrinding gem stones, because it is now possible to machine grind gem stones relatively rapidly and to produce a quality of the surface finish which is at least equal to that obtained by hand grinding. Heretofore it was not possible to obtain such quality by resorting to'machine grinding.

The rapid operation is also in part dictated by the thermal time constant which determines the temperature rise as well as the constancy of the temperature. It has been found that it is advantageous if the holder for the gem 11, and the holder for the temperature sensor 25 (see FIG. 4) are of a material having a low coefficient of thermal conductivity, for instance chrome nickel steel, the type of steel designated in some parts of the industry as V2A steel, or ceramic material. FIGS. 2A and 2B diagrammatically and graphically show the conditions which develop at the gem stone when time and temperature are calculated at different contact pressures (between 0.7 and 1.7 kg.) and with the gem having different orientations relative to the grinding wheel. It will be seen in FIG. 2A that for instance at a contact pressure of 1.7 kg. the gem stone 11 will reach a grinding temperature of almost 150C. if it is oriented so as to have the ideal orientation relative to the grinding wheel, whereas FIG. 2B shows that if the orientation is not the ideal one, the grinding temperature will barely reach C. It might be assumed from this that it is most advantageous to select the ideal orientation in accordance with the highest grinding temperature, while maintaining the contact pressure constant. However, it has been found that even if very small thermal time constants are employed, this approach requires substantially more time for determining the ideal orientation than if the first of the three earlier mentioned approaches is used, namely to use the grinding speed as the determining factor. Utilizing the highest temperature as the determining factor has a particular disadvantage in that it is possible that the time required for determining the ideal orientation of the gem is longer than the time required for grinding a facet thereon. In this case, it would be possible that the facet has already been completely ground but would not be polished because insufficient time has elapsed for determining the ideal orientation of the gem relative to the grinding wheel.

It is pointed out that the signal which terminates the grinding and polishing operation for each facet need not necessarily be derived from a device such as the device 18 which was previously described. Other possibilities exist and it is merely necessary that whatever other arrangement is utilized be sufficiently sensitive and 0perate sufficiently accurately. For instance, optical arrangements, such as photo-siliconeelements, can be used, electrical arrangements which provide the signal as a result of changes in capacitance or inductivity, or even as a result of measuring the expansion or contraction of the gem, or else mechanical arrangements such as microcators.

It has already been pointed out earlier that the fact that thepolishing operation for each facet can be car ried out on the same coarse grinding wheel which is used to grind the'facet in the first place, without having to resort to a second operating step utilizing a fine grinding wheel, is of importance in terms of the economics of the automatic operation of the grinding prcedure. A more detailed description of the effect which is obtained with the present invention, as shown in FIG. 3, is therefore in order. The diagram in FIG. 3 directly compares the high and low points of the surface of the facet after grinding and after polishing. The high values in microns are relative. It is clear, however, that the gemmologist will immediately note the poor quality of the surface of a gem facet (even with the naked eye) if the height and depth variations amount to 0.2 microns. On the other hand, the lower line in FIG. 3, which represents the height and depth variations of the facet surface after only a few seconds of the polishing operation utilizing the same grinding wheel which has been used to produce the facet in the first place, is representative of a facet surface finish requiring the most sophisticated viewing and measuring devices before any traces of the grinding operation can be detected. This advantageous effect, utilizing the same grinding wheel which has been used to grind the facet in the first place, results from the fact that during small angular movement of the gem stone to and from its ideal orientation (i.e., during the polishing operation) particles of the grinding wheel will act upon the gem stone which are not the particles which have previously produced the low points or depressions in the facet surface. This means that during such movements the high points are ground off, before the previously produced low points come again in contact with those grinding wheel icles which produced them in the first place. This mean that the high points are removed 'as desired. If duri this operation the grinding pressure or contact pres is also reduced, then a surface is obtained Whose quaL ity will be thoroughly satisfactory not only for industrial diamonds but also for decorative gems without as pointed out before, having to use a fine grinding wh to obtain it.

The regulation of the temperature of the gem Stone that is the maintenance of the temperature at constahi level, has a further advantage in that it prevents loosen ing of the gem in the holder due to the significant dif.

ferences in the coefficient of expansion of diamonds C) and the holder material (greater than ]O C The desired maintenance of the temperature at a constant level can, of course, be obtained in other ways than by changing the contact pressure, for instance by means of infrared radiators or electrical heaters, to name just two possibilities.

Various different types of holders for holding a gem to be ground are known in the art, and they can be used in the machine according to the present invention. However, it has been found that it is particularly advantageous if the gem stone 11 to be ground is first co nected to a temporary holder 30 (see FIG. 5) and afte an adjustment, which will subsequently be described, is transferred to and held in a permanent holder 31. Known holders are inform of clamps which require that the diamond be rondiert, i.e., that it be rubbed against another diamond. This involves an additional operating step which can be carried out only by a highly trained specialist and which leaves behind in the thus-treated diamond hairline fissures which remain visible under magnification even after the gem has been ground. The problem is avoided by connecting the diamond in its raw form to a pin 32, for instance by means of an adhesive, whereupon it is turned manually relative to a profile projector 34 (a device known in the gemological art) in contact with an abutment 33. A viewing screen of the projector 34 is provided with circles of different sizes which are concentric, as shown, and a shadow profile of the stone 11 is produced on this viewing screen. It is then merely necessary to so turn the stone 11 until by reference to these various circles the largest shadow profile is obtained. Thereupon, the holder 30, into the recess of which a suitable adhesive such as cement or the like has been previously introduced, is pushed against the diamond 11 (the pin 32 is slidable in the passage of the holder 30, as shown) so that the diamond is now arrested in the optimum orientation which has been determined with the aid of the profile projector 34. The distance identified by the double-headed arrow 36, which can be measured on the screen of the projector, represents a direct measure of the physically inaccessible exact center of the diamond. The radius of the largest circle covered by the shadow profile of the diamond can be controlled by a micrometer measurement. The pin 32 is now removed from the holder 30 and after the temperatureresistant cement has been hardened, the diamond 11 can be ground in the holder 30 in which it is centered in such a manner that a minimum loss of material will occur during grinding. This is sufficient for gems having a grinding angle of up to 45, for instance diamonds on which a brilliant cut is to be produced. However, the diamond has been given a bed in this manner.

In many instances, however, such a mounting in the holder is not sufficient, because there are not really any satisfactory adhesives for diamonds, especially for diamonds already having ground facets. Also, there a circumstances when it is desired that a diamond is to be ground at an angle of up to i.e., with its facets 1nclined to the girdle plane at such a degree, n Yvhlch case the diamond must either be further secured in the holder 32 by having the upper facets or the tableengaged by a holding device, or it must be secured in a holder which is of metal, for instance copper, aluminum or steel.

FIG. 6 shows this second possibility of a permanent holder 31 into which the diamond can be tra after it has been first centered in the temporary holder 0 of FIG. 5. It will be seen how the diamond secured 1 the holder 30 can be transferred into the holder 31, 1' which purpose the metal which is to ho d the mond is advantageously heated to a t s p between 350 and 600C. before the diamond 11 with the holder 30 and the cement 35 is pushed into the pernt metallic holder 31. The insertion can be guided y means of a guide tube 37. It is advanta h shell 38 of the h ld can b opened in order to permit the removal f h h ld 31 with the diamond II which might otherwise be difficult in the event clampmg due to thermal expansion should take place.

It has been d clear that the present invention Seeks to provide a method and apparatus which permits the automatic grinding of the various facets of a gem stone, such as a diamond. This requires that the angle between the individual facets (which has been previously calculated) must actually be obtained during grinding, or that deviations in excess of only a few minutes must not be permitted. If the grinding angle is maintained exactly within a tolerance of a few minutes, then it is possible to produce a gem stone, for instance a brilliant, whose facets are ground with such precision that it will have the desired brilliance. To obtain the requisite positioning of the facets during grinding, the apparatus according to the present invention uses an arrangement which is shown in detail in FIG. 7. Generally speaking, the arrangement in FIG. 7 makes it possible to control all movements electrically and to fix the gem stone in all positions necessary for grinding facets having certain predetermined facets relative to one another. It is important in this connection to differentiate between two types of angles, namely an angle included by one facet and the adjacent facet of a ring of such facets, and an angle included between two rings of facets. The arrangement in FIG. 1 can be set for both possibilities and positively arrested in either setting.

The diamond or other gem stone 11 will be seen in FIG. 1 to have its exact center 40 located on the point at which the vertical axis 13 (see also FIG. 1) intersects a horizontal axis about which a carrier 41 can pivot. The carrier 41 can turn about the axis 13 which extends normal to the plane of the upper surface of the grinding wheel 4 (see FIG. 1). The carrier 41 should be understood to correspond to the carrier 12 shown in FIG. 1 and is configured with an approximately cylindrical cross section in the illustrated embodiment. It surrounds the holder 31 for the gem stone 11, which holder is mounted turnable about its own longitudinal axis. The upper end of the carrier 41 is provided with two

ratchet wheels

42, 42. The shaft 8a rotatable about the axis 13 carries above the ratchet wheels 42, 42 a pulley 43 from which a taut rope or the like extends via

guide rollers

44 and 45 to a spring 46 which can be tensioned, for instance a watch spring or the like. This spring is shown only diagrammatically but it will be understood that it serves to maintain the rope 47 in taut condition.

A coil 48 of an electromagnet acts upon a pin 49 which can extend between the projections of the

ratchet wheels

42 and 42 in order to provide for a switch-over of the arrangement. The coil 48 and the pin 49 are operated by an electrical signal which originates when a facet is completely ground and polished. In this condition, the carrier 41 is slightly lifted so that the gem stone 11 is out of engagement with the grinding wheel by the aforementioned approximately 1 mm. distance. When one of the

ratchet wheels

42, 42 is disengaged by operation of the pin 49, the holder 31 turns about the axis 50, until the next projection of one of the ratchet wheels becomes engaged. The distance in circumferential direction of this rotary movement corresponds to the amount by which the stone 11 must be turned, in order to be able to grind the next adjacent facet of the same ring of facets of which one facet has just been completed. By selecting

ratchet wheels

42, 42 having a requisite number of projections which are equally distributed over their circumference, it is possible to pre-program the grinding of the facet rings having a desired predetermined number'of facets;

When a facet ring has been completed, that is when all of its facets have been ground, the device is to be ca pable to switch-over so that the next facet ring can be automatically ground. For this purpose, the carrier 41 with the holder 31, the

wheel

42, 42, the coil 48 and the pin 49 is pivotable in the direction of the doubleheaded arrow 51 approximately about the aforementioned horizontal axis which passes through the center 40 of the gem 11. The position illustrated in FIG. 8 shows the arrangement in the leftmost possible position, that is at a 0 pivotal deflection. It will be seen that moving towards the'right from this position there are provided six arresting portions 52, for instance holes, so that the arrangement can be arrested in six additional positions by engagement with respective ones of the arresting portions 52. The spacing of these positions in the direction of the double-headed arrow 51 corresponds to the size of the angle included between the facets of individual facet rings relative to one another. An electromagnet 53 is provided, having a pin 54 which can enter into the respective holes 52. A movement of the arrangement in the direction towards the next hole 52 can be initiated by the tension of the rope 47 when the

ratchet wheels

42, 42 are stationary, and this pivoting from one position to another takes place so rapidly (both when the arrangement is adjusted to provide for grinding of the successive facets of a single ring, as well as when it is adjusted to provide for grinding of succesive rings) that a noticeable cooling of the stone need not be feared).

In some instances, for instance where industrial or tool diamonds are to be ground, it is necessary to form certain ground radii on the diamond. For this reason, the carrier 41 must be capable of being continuously pivoted about the horizontal axis passing through the center 40, as well as about the vertical axis 50. In this case, the spring 47 must be provided with a drive which effects the desired pivoting.

Coarse sintered grinding wheels which are used in accordance with the present invention, are not planar because of the sintering operation. Of course, they are processed so as to become planar, but they tend to warp nevertheless, especially when they are mounted on a flange by means of tension screws 21. The present invention provides a way of overcoming this problem by locating intermediate circumferentially successive ones of the tension screws 21 respective pressure screws 22 (compare FIGS. 8 and 1). This means that even in the installed operating condition the grinding wheel can be rapidly readjusted without having to remove the many precisely adjusted devices cooperating with it, simply by operating one or more of the pressure screws 22 in such a manner as to reestablish the planarity of the grinding surface of the wheel. The necessary measurements can be carried out by appropriate measuring devices.

It is advantageous that more than one stone canbe ground simultaneously on one and the same grind n wheel, using the apparatus according to the present invention. In particular, if more than two stones are ground simultaneously, then the uniformity and plan ity of the surface of the grinding wheel will, in fact, improve during use, so that the surface quality of the facets of the ground stones will similarly improve. Also, y grinding two or more stones simultaneously, the grinding wheel will be worn uniformly which is impor from an economic point of view because gr g wheels are expensive.

It has already beenpointed out earlier how it 15 P ble to provide a pre-orientation of a sample S on and to subsequently use the information thereby obtained for programming the ideal orientation of later stones to be ground, with the information for the sample stone having been obtained for each facet tobe ground. It should be mentioned in this connection that the ideal orientation or direction of grinding in this case can. be experimentally determined with respect to the sample stone, without resorting to the other possibilities disclosed in the introduction to the present specification. Of course, if such a pre-programming operation is desired, that is if the subsequent stones are to be cut in accordance with the information obtained by grinding the sample stone, then it is clear that the subsequent stones must be inserted into the grinding apparatus in the same orientation in which the sample diamond was previously inserted, or at most within a tolerance of a few degrees, because otherwise there will be no proper correspondence of the ideal grinding direction or orientation of the gems to be ground and of the sample stone or gem.

Finally, it is also pointed out that an emergency switch may be provided which may act upon the motor and rapidly lift the gem out of contact with the grinding wheel (either triggered manually or automatically) if this should become necessary for any reason. Relative to FIG. 7 the following should be noted:

At the center 40 there is a horizontal axis 103. This axis or shaft 103 (See FIG. 7a) consists of at least one section in the FIG. 1 behind the gem stone l1 resp. the holder 30. The carrier 41 is stepped up at the side in the direction of the axis. On this axis 103 the carrier 41 is pivotable supported by a mechanical sleeve I04 surrounding the horizontal axis 103 (see FIG. 7b). In case of two mounting plates, one of which is to be seen in FIG. 7, two sections of the axis 103 can be provided, one behind and another before the holder 30 in FIG. 1. In this case the lower end of the carrier will be bifurcated.

The respective position of the carrier 41 about the horizontal axis 103 will be settled by the pin 54 ope ated by the electromagnet 53. The pin enters in the holes forming the arresting portions 52.

The ratchet wheels 42, 42' have radial extending teeth or projections. The

ratchet wheels

42, 42 are connected to another in such a way that the teeth of the one wheel are staggered against the teeth of the other wheel. This means the teeth of the one wheel are arranged in the projection of the gap between the teeth of the other wheel. In FIG. 70 tooth 104 of the wheel 42 is arranged in the projection of the gap 105 b tween the

teeth

106, 107 of the wheel 42. The gaps b tween the teeth are larger than a tooth.

The pin 49 movable by the electromagnetic actuator 48 has on its end a nose 108 (see FIG. 70) extending in a gap between two teeth of one of the wheels 42 or 42' (see FIG. 7d). The actuator 48 moves the pin 49 between two positions. In the higher position the nose 108 extends in a gap between two teeth of the wheel 42, in the lower position the nose extends in a gap b tween two teeth of the wheel 42. In both positions the rope 47, which is biased by the spring 46, arranged i a winding roller at 46, draws the unit about the axis 50 that one side of a tooth engages the nose 108. If the pin will be moved in the other position the rope 47 rotates the carrier until a tooth of the other wheel contacts the nose.

The carrier 41 is provided with a groove 109 as mounting device of a counter-holder 110 for support the diamond in case of position of the carrier in an angle that means during the grinding of facets. During the grinding operation with vertical directed carrier 41 the counter-holder 110 will be removed.

FIG. I shows the grinding wheel 4. This grinding wheel is mounted on a spindle 20. This spindle is mounted in a bearing 23 with very small tolerances. It is preferred to use a hydrostatic bearing of the Kugellagerfabrik Fischer, Germany. The spindle is driven by a unit 100, according to the arrow 101.

As it can be seen in FIG. 1 the rigid support 16 extending from the column 1 is mounted in a level lower than the link or leaf 10. The motor 15 which may be a geared engine, has a shaft 102 on which is mounted the eccentric 17 so that the eccentric contacts the link or leaf 10. If the eccentric rotates the link or leaf 10 and also the column 8 with a shaft 8a will be lowered or raised.

The sensing device 18 with an upper end or sensor 19 is a device of the firm Novotechnik Stuttgart, Germany, with the name Linostat type F 205. This device 18 is a measury instrument for a distance.

In the example the temperature sensor 25 is a device of the firm Philipps, Germany or Netherlands, type NTC.

The profile projector 34 is a device of the firm Ernst Zeiss, Wetzlar. It has an enlargement factor of 1:100 at least.

As another possibility in respect to the device l8 microcators are named. These are known instruments of the firm Johannson, Sweden, type Microcator 500 A/4. This dial gauge functions without gears, therefore with high exactness.

The word bed, previously mentioned, means that with the described means the diamond has been embedded in a holder-material adapting itself to the shape of the embedded part of the diamond.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of constructions differing from the type described above.

While the invention has been illustrated and described as embodied in a machine for the grinding and polishing of gem stones, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

1. In a method of grinding a gem stone, particularly a diamond, the steps of placing a gem stone to be ground against a rotating grinding wheel; turning said gem stone relative to said grinding wheel for determination of an ideal orientation of said gem stone relative to said grinding wheel; measuring the frictional heating of said gem stone; controlling the contact pressure at ground-through up to 10 from said ideal orientation while reducing said contact pressure.

3. A method as defined in claim 1, further comprising the step of polishing said gem stone; and wherein all of said steps are carried out on the same grinding wheel and without re-positioning said gem stone.

4. A method as defined in claim 1, wherein the step of measuring comprises sensing the seeming growing of said gem stone under the influence of said frictional heating.

Claims

1. In a method of grinding a gem stone, particularly a diamond, the steps of placing a gem stone to be ground against a rotating grinding wheel; turning said gem stone relative to said grinding wheel for determination of an ideal orientation of said gem stone relative to said grinding wheel; measuring the frictional heating of said gem stone; controlling the contact pressure at which said gem stone contacts said grinding wheel, as a function of the temperature of said gem stone and in such a manner as to maintain said temperature constant; determining the amount of material removal which takes place during the preceding steps; terminating the turning of said gem stone when said ideal orientation is reached; and grinding respective facets of said gem stone with said grinding wheel.

2. A method as defined in claim 1, and further comprising the steps of turning the gem stones-immediately prior to completion of a respective facet being ground-through up to 10* from said ideal orientation while reducing said contact pressure.