AU639974B2 - Working a facet of a gemstone - Google Patents

Description

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l~i-yr~marar~i~~*r 63 974 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Form Short Title: Int Cl: Application Number: Lodged: rc~rr: r 62r tC

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f 6( Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: 4,$ic.

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TO BE COMPLETED BY APPLICANT Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: i *It THE DIAMOND TRADING COMPANY

LIMITED

7 Rolls Buildings, Fetter Lane, London EC4A 1NH, ENGLAND MARK CHRISTOPHER HANNA and ERIC

BLONDEEL

GRIFFITH HACK CO 71 YORK STREET SYDNEY NSW 2000 Complete Specification for the invention entitled: WORKING A FACET OF A GEMSTONE The following statement is a full description of this invention, including the best method of performing it known to me/us:- GH&CO REF: 17354-T/CLC:BC 4732A:BC

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M&C FOLIO: 230P55166 WANGDOC: 1201R 88.11.04 WORKING A FACET OF A GEMSTONE Background of the Invention The present invention relates to working a facet of a gem stone using a polishing member which engages the facet and effectively moves in one direction past the stone. Normally, the polishing member will be a rotating wheel, commonly called a scaife, and the scaife normally rotates about a vertical axis. Normally, the stone is held in a dop at the end of an arm of a hand e polishing machine, commonly called a tang, or an automatic machine can be used. The dop can be-turned about a vertical axis, relative to the arm, being held r The invention has particular applicability to polishing diamonds. Diamonds have a grain corresponding to the crystallographic planes; for each facet, there is a direction of polishing which is most effective. In other words, when polishing diamonds, the rate of polishing is strongly dependent on the orientation of the internal crystal structure of the stone to the polishing member and its direction of rotation. Grain is discussed for instance in GB-A-2 037 196.

1 i I I I laYLI-g~J~--L~ Determining the direction of polishing is part of the skill of a polisher. The polisher examines the stone and decides where the grain is. He turns the dop about the vertical axis until he believes the grain orientation is correct., and begins polishing. The polisher can sense if the stone is not being polished properly, and turns the dop round a little, about the vertical axis. When a facet has been finished, the polisher will know the setting of the dop for the other 'facets this is part of the polisher's skill. However, if a stone is right off-grain, it can dig in and spoil the scaife.

Particular difficulties have been encountered with stones with twisted grain structures, or dislocated n" grains or ring or ball naats in each case, there is ¥L t not a normal single crystal present. This is S particularly so with Australian diamonds, which appear to be polycrystalline, and grain identification is very difficvlt or impossible; in an extreme case, one facet could have three or more polishing directions.

Traditionally, working a facet of a gem stone comprised three steps, namely a first step in which the grain is sought by trying various grain orientations to see at which the stone works best, a second step in which substantially the whole of the facet is formed (blocked)

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69891/91 I 3 by grinding down the surface of the stone, and a third step in which the facet is smoothed so that all the scratch marks made when forming the facet are removed.

In order to smooth or form a high polish on the facet, material must be removed from the facet in such a way that the surface structure of the polishing member is no longer reproduced in the surface of the facet.

Traditionally, this can be done in a number of ways. The orientation of the stone could be changed to such a degree that the facet will still polish, but at a much smaller rate so that the depth of the scratches is less and the facet will be more highly polished. A higher rotational speed can be used. The specific loading can be reduced below about 3 M/mm 2 These ways can be combined.

SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a method of working a multi-grain facet of a gemstone using a polishing member which engages the facet and effectively moves in one direction past the stone as a whole, comprising rotating the stone continuously about an axis at right angles to the surface of the polishing member to work the facet.

SAccording to a further aspect of the present tfe 25 invention there is provided a method of working a facet of a gemstone of weight less than 10 pts using a ,0 polishing member which engages the facet and effectively i moves in one direction past the stone as a whole, comprising rotating the stone continuously about an axis at right angles to the surface of the polishing member to work the facet.

S:17354T/27.05.93 4 Preferably, the stone is rotated continuously until substantially the whole of the facet has been formed. In other words, the rotation of the stone is not merely to determine the grain, but is continued right through the polishing of the facet.

This provides the possibility of cheap and simple automation which does not require a skilled operator, Sbut can nonetheless provide good blocking or faceting.

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l This procedure is particularly useful for small or very small stones and for difficult stones. It can be used o on a hand-held tang. It is surprising that the polishing can be achieved at reasonable cutting rates, with good faceting and without ruining the polishing member ie the polishing'member life is good. The Sprocedure can be used to grind all facets, including tables.

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7 4 Vo$ I i Although no high skill is required, some precautions should preferably be observed. For difficult stones and stones having sharp edges, a diamond polishing member should be used. The specific pressure should be limited (but high enough) in order to prevent a sharp edge on the stone damaging the polishing member. For grinding normal quality stones, the specific pressure should be of the order of 3 N/mm 2 (at which pressure one can also observe the grain as discussed above), but for smooth faceting, one should use lower pressures. As the size of the facet increases as polishing progresses, it is useful to talk about the specific pressure on the finished facet initially, the pressure will be higher.

In general terms the specific pressure will preferably be less than about 6 N/mm 2 and, dependent on the final surface quality required, the pressure is preferably between about 1 and about 5 N/mm 2 "The speed of rotation should preferably be within a 4r44 t certain range. If it is too low, the polishing member may be ruined when starting up, and efficiency drops (and polishing member temperature increases) if the speed is too high. For a 10 pt stone, on a rotating scaife, a 44 t n€Ireasonable speed is 1 to 4 rpm.

It$ As in normal practice, it is good practice to choose a good polishing facet as the first facet.

S:17354T/2705.93 S:1 7354T/27,05.93 I L l-.l~-asmcr~-xl- c w; 6 In at least preferred embodiments the procedure is particularly economical for small stones. The overall polishing time for stones up to 10 pt is shorter than using hand grain seeking in the latter case, the stone has to be inspected several times to find the grain, which leads to a significant time loss. Furthermore, as non-skilled operators are used and as automation is easily incorporated, several tangs can be employed on the I same polishing member, making the procedure economical for small and medium-sized stones.

iStones with a single grain orientation (non-naated stones) can be worked on traditional scaifes. However, stones with non-uniform crystallographic structures (naated stones) can be worked on coated scaifes; the heat generation is larger but the scaife life is not significantly decreased if the scaife is kept in condition in the normal way.

In at least preferred embodiments, a good surface finish can be achieved. For instance, if a constant load is used, a good surface can be achieved by arranging for the specific pressure on the finished facet to be lower than about 3 N/mm 2 If the grain is twisted, can still be achieved.

S:1 7354T/27.05.93 x~ i i I~EX~-WYI i -I*urau~ 7 Normal procedures to fix the stone in a dop which has a contact ring; the facet has been worked to the correct depth when the contact ring touches the polishing member. Using this procedure, there is improved accuracy of detecting when this occurs as the contact ring can be observed from all angles as it rotates over the surface of the polishing member. This provides accurate working, and a simple circuit can be arranged to provide automatic lift-off of the dop when the facet has been fully worked. Thus semi-automatic or automatic equipment can be provided from traditional tools.

For at least preferred embodiments of the present invention, the machinery required is extremely simple.

For at least preferred embodiments of the present invention, the rotation of the stone greatly reduces the incidence of broken culets (the pointed bottom part of the stone).

Preferably in accordance with a third procedure of the invention, a facet which has already been formed is 20 smoothed by rotating the stone continuously about said 4 axis.

"r In general, the third procedure removes the t requirement for a zoeting motion of the tang or polishing machine, i.e. the reciprocation of the polishing machine transversely to the motion of the polishing member.

However, if such a zoeting motion is used, the polishing member can still be considered to effectively move in one direction past the stone.

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8 In all thre-procedures, the facet being worked can be a substantial distance from said axis. This minimises digging in because the stone is effectively transversing the polishing member by an amount equivalent to twice said distance (the radius of rotation of the stone).

Preferred Embodiment The invention will be further described, by way of example, with reference to the accompanying drawing, in which; Figure 1 is a schematic elevation of the end of a hand-held tang; and Figure 2 is a sketch of the polished surface of a facet of a diamond.

The polishing member is a rotating scaife 1 which rotates about a vertical axis to the left of the drawing. The polishing machine is hand-held, and comprises a tang 2 which carries a traditional polishing head 3. The polishing head 3 includes a blocking holder 4 which carries a dop 5 holding a diamond 6, the dop being fitted to the holder 4 to provide an 'eight position' click dop. There is an angle Betting knob 7.

In a known way, the axis 8 of the diamond is at a preset, but adjustable, angle a to the surface of the scai.e s here, the angle a is shown as 471/20.

44 4 *4 iL1 j- In a known way, the dop 5 can be turned or indexed about the axis 8 to change the facet being polished, through steps of 450.

The polishing head 3 is mounted on the tang 2 for free rotation about a vertical axis 9, ie at right angles to the surface of the scaife 1. The polishing head 3 is connected by a shaft 10 to a 480:1 worm gearbox 11 which in turn is driven by a small 24 v Portescap dc motor S12. The gearbox 11 is a standard component and is not described in detail, but it includes a flanged collar 13 which is splined or keyed onto the shaft 8, and a worm gear 14 rotatably mounted on the collar 13 and biassed against the flange by a friction regulating spring forming a friction clutch (for operator convenience and motor protection). The motor 12 can be powered by a 9 v dry cell located under the heel of the tang (not shown), such that the polishing head 3 rotates at a slow speed, eg 1 to 5 rpm, preferably 3 rpm. In one arrangement, the radius of the offset, ie the distance d between the intersections of the axes 8, 9 with the surface of the scaife 1, is about 15 or 16 mm, but this can be changed as appropriate for instance an 8 mm or 6 mm distance may be preferred. To give a specific example which however is generally applicable, the scaife diameter is 330 mm and it rotates at 3000 rpm. The diamond 6 is rotated across a 40 mm wide diamond-impregnated track 1, on the outer part of the scaife 1. These relative speeds are preferred and can be calculated for tracks or scaifes of different diameters.

In use, a diamond is sawn, or if 'it is a makeable a table is polished. A circular girdle is bruted (ground) adjacent the sawn face or table. The sawn face or table is inserted into the dop 5 so that the girdle is held by the dop 5. The motor 12 is energised and thIe tang 2 is brought down so that the diamond 6 rests on the scaife 1, and is rotated about the axis 9.

In a first procedure not in accordance with the present invention, after the dop 5 has rotated several times, say ten times, about the axis 9, the tang 2 is raised and the motor 12 de-energised so that the facet can be examined for scratch marks. The scratch marks indicate the direction in which the scaife 1 should move past the diamond 6. By using the stop-start switch, the dop 5 is turned to the correct orientation. When the motor 12 is de-energised, the gearbox 11 locks the polishing head 3 in position. The. tang 2 ,s then lowered and the facet is polished. Once the grain orientation has been determined for one facet, the correct grain orientation is known for the other facets (assuming that the diamond 6 is a single crystal this is not so with twisted or naated stones), and the blocking process is completed by polishing all eight facets.

S:17354T/27.05.93 [Ni o*) r sO 19 Lodged..... 11 Figure 2 illustrates the surface of a naated stone which has been polished with a specific pressure above about 3 N/mm 2 The facet has two areas 21, 22 in which the crystallographic orientation is different. As the stone 6 rotates on the scaife 1, there are moments at the 11 or 5 o'clock positions (indicated by the double arrow) where the area 21 polishes, and moments at the 9 or 3 o'clock positions when the area 22 polishes. As shown, scratches will appear, showing the various polishing directions. This is also true if there are multiple naats, each area of different crystallographic orientation having scratches indicating the polishing direction.

If multiple polishing directions are required, the procedure described below can be adopted. However, the procedure described below can also be used without inspecting the grain at .ll, particularly for small and medium-sized stones. In the second procedure, in accordance with an embodiment of the present invention, the motor 12 is energised continuously for the entire blocking process, i.e. until the facet has been fully formed, though not necessarily smoothed the speed of rotation of the polishing head 3 can remain the same.

Smoothing can be performed on the same scaife 1, or can be performed on a different scaife. In order to produce a high polish, the material must be removed in such a S:17354T/27.05.93

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way that the surface structure of the polishing member is not reproduced in the surface of the facet. In accordance with the third procedure of the invention, this is done with continuous rotation of the head 3 whilst maintaining a constant polishing load the speed of rotation of the polishing head 3 can remain the same. As noted above, the constant polishing load will cause a decreasing specific loading as the facet size increases.

The third procedure can also be carried out on stones which have been blocked in the normal manner, particularly larger stones (eg having a size greater than 10 pts).

Throughout this specification, weights are given in points (pts), as is customary in the art. 1 point is 0.002 g.

The present invention has been described above purely by way of example, and modifications can be made within the spirit of the invention. t.1Pi j,

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Claims (9)

1. A method of working a multi-grain facet of a gemstone using a polishing member which engages the facet and effectively moves in one direction past the stone as a whole, comprising rotating the stone continuously about an axis at right angles to the surface of the polishing member to work the facet.

2. The method of claim 1, wherein the stone is a small stone of weight less than 10 pts.

3. A method of working a facet of a gemstone of weight less than 10 pts using a polishing member which engages the facet and effectively moves in one direction past the stone as a whole, comprising rotating the stone continuously about an axis at right angles to the surface of the polishing member to work the facet.

4. The method of claim 1, 2 or 3, wherein the specific pressure on the facet is less than a pressure substantially equal to 6 N/mm 2 S. 17354T/27.05.93 j Ut-lLU Ll: I/J4-I/CLC:BC i. 'yi 4732A:BC *444q0e 0P 0 4404 *a a r* @0 4.rr *48444 09 14 The method of any of Claims 1 to 4, wherein the stone is rotated about said axis at a speed substantially in the range from 1 to 5 rpm.

6. The method of any of Claims 1 to 5, wherein the stone is rotated about the axis until substantially the whole of the facet has been formed.

7. The method of any of Claims 1 to 5, when used to smooth the facet.

8. The method of Claim 7, wherein the specific pressure on the facet is less than a pressure substantially equal to 3 N/mm 2

9. The method of any of the preceding Claims, wherein the axis is a substantial distance from said facet, whereby the facet rotates about a substantial radius. A method of working a multi-grain facet of a gemstone substantially as herein described with reference to the accompanying drawings. I i li i~ i: i 15

11. A method of working a facet of a gemstone of weight less than 10pts, substantially as herein described with reference to the accompanying drawings. Dated this 16th day of January 1992 THE DIAMOND TRADING COMPANY LIMITED By their Patent Attorneys GRIFFITH HACK CO V44* 9 9 *I t o Sf3ll 94 a S *e