IL99993A - Working a facet of a small gemstone - Google Patents

Description

WORKING A FACET OF A SMALL GEMSTONE m op i n v nN3 vnut The Applicants ;= GERSAN ESTABLISHMENT, a Liechtenstein Company, of Staedtle 36, 9490 Vaduz, Liechtenstein.

The In lentors 1. Hanna, Mark Christopher, of Watermolendreef 58 , St.Miklass 2700, Belgium. 2. Blondeel, £ric, Edgard Tinelstraat 13, Bruges 8200, Belgium.

BACKGROUND„OF. THE _INVENTION The present invention is a second division of Patent Application No. 088454 and relates to working a facet of a small 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 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 in a friction bearing.

The invention has particular applicability to polishing diamonds of a weight less than 10 pts.

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) 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.

Trarditionally, 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 2 reduced below about 3 N/mm . These ways can be combined.

In the case of small stones of a weight of less than 10 pts, this procedure would be too costly, especially since it requires a skilled worker to polish the stone. For this reason the present procedure omits the step of determinating the various grain orientations, permitting the work to be carried out by semi-skilled or even unskilled personnel.

THE INVENTION In accordance with the present invention, the stone is rotaed about an axis at right angles to the surface of the polishing member. Thus the stone will be secured with its axis at a preset angle to the polishing member, corresponding to the facet angle, but is put to the polishing member in no fixed grain orientation.

Normally, the rotation of the stone will be effected using a motor, but all procedures of the invention can be carried out on a hand-held tang.

In accordance with a first procedure, the stone rotated continuously until substantially the whole of the facet has been formed. In other words, the rotation of the stone 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, but can nonetheless provide good blocking or faceting. This procedure is particularly useful for small or very small stones and for difficult stones. It can be used 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 - i.e. the polishing member life is good. The procedure can be used to grind all facets, including tables.

Although no high skill is required, some precautions must 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 2 " order of 3 N/rara . at which preeeure one ran -also- "~ "\ obcorvo the grain ac dioQUSoed 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 be 2 less than about 6 N/mm , and, dependent on the final surface quality required, the pressure is preferably 2 between about 1 and about 5 N/mm .

The speed of rotation should be within a 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 reasonable speed is 1 to 4 rpra.

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

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-ekilled operators are used and as automation is easily incorporated, several tangs can be employed on the same polishing member, making the procedure economical for small and medium-sized stones.

Stones 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 signi icantly decreased if the scaife is kept in condition in the normal way. Ά 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 2 facet to be lower than about 3 N/mm . If the grain is twisted, higher specific loads should be used, but a good surface can still be achieved.

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 an 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.

The machinery required is extremely simple.

The rotation of the stone greatly reduces the incidence of broken culets (the pointed bottom part of the stone).

In accordance with a gecorrd.. procedure of the invention, a facet which has already been formed is smoothed by rotating the stone continuously about said axis. la general, the second rocedure removes the requirement for a zoeting motion of the tang or polishing machine, ie 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.

Injx&h these... 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.

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 5 being fitted to the holder 4 to provide an 'eight position* click dop. There ie an angle setting 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 scaife 1 - here, the angle a is shown as 47i/a°.

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 45°.

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 12. 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 15, forming a friction clutch (foe 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 6peed, 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 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 the tang 2 is brought down so that the diamond 6 rests on the scaife 1, and is rotated about the axis 9.

In the present procedure, 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 materail must be removed in such a way that the surface structure of the polishing member is not reproduced in the surface of the facet. In accordance with the second procedure of the invention, this is done with continuous rotation of the head 3 whilst mainaining 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.

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.

Claims (8)

Claims

1. 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.

2. The method of Claim 1, wherein the specific pressure on the facet is less a pressure substantially equal to 6 N/mm .

3. The method of Claim 1 or 2, wherein the stone is rotated about said axis at a speed substantially in the range from 1 to 5 rpm.

4. The method of Claim 1, 2 or 3, wherein the stone is rotated continuously about the axis until substantially the whole of the facet has been formed.

5. The method of any of Claims 1 to 3 when used to smooth a facet of a gemstone.

6. The method of Claim 5, wherein the specific pressure on the facet is less than a pressure substantially equal 2 to 3 N/mm .

7. The method of any of the preceeding Claims, wherein the axis is a substantial distance from said facet, whereby the facet rotates about a substantial radius.

8. A method of working a facet of a gemstone of weight less than 10 pts, substantially as herein described with reference to the accompanying drawings. FOR THE APPLICANTS, EidE.GLUC SMAN, PATENT ATTORNEY.