GB2200582A - Working gemstones, and a device for examining gemstones for bruting - Google Patents

Abstract

A mechanism for reciprocation of a rotating gemstone in a bruting machine includes a driving shaft 24 which extends roughly parallel to a pivoted arm 39 and is connected to the arm 39 by way of an eccentric 31. The arm 39 is fixed to a 90 DEG arm 41 connected via a universal joint 42 and a post 44 to a reciprocable mounting plate 5 of one spindle 2. To increase the length of stroke of reciprocation, particularly at one end, the eccentric 31 and associated parts are moved along the driving shaft 24. In order to examine the stone as bruting proceeds, an image of the stone is projected onto a screen 54 using an optical system 51, 52, 53. The screen 54 has three lines including one which determines the ratio, normally 14%, between the table height and the girdle radius, and an associated slider is moved so that the join between two lines thereon is on the ratio determining line. If the outline of the image of the stone is on or just outside the two lines, one of the lines indicates the girdle radius. <IMAGE>

Description

WORKING GEMSTONES, AND A DEVICE FOR EXAMINING GEMSTONES FOR BRUTING A first aspect of this invention relates to a machine for working gemstones, comprising means for rotating two gemstones about respective axes such that the surface of the stones contact and grind each other, feed means for moving the stones relatively towards one another, to provide feed, and means for reciprocating the stones relative to one another.

The machine is for the preparation of rough gemstones, particularly diamonds, for facetting, particularly, but not exclusively, for bruting or coning the gemstones.

In each case, the stone will have been processed (normally by sawing in two) to provide a flat face called a table, which establishes a datum. Bruting is the formation of a girdle zone, normally of circular cylindrical shape, by grinding the periphery of the rough stone. Coning is the formation of a conical shape on the culet or lower side. The reciprocation of the stones will normally be along an axis at right angles to the table of at least one of the stones. Machines of this type are described in for instance GB-A-2 018 173, GB-A-2 082 100 and GB-A-1 046 743.

There are two problems in bruting, though they can also arise in other working operations. The girdle zone increases in depth, thus requiring a longer stroke for efficient operation. The stroke, if uncontrolled, can cause considerable chipping to the table. Since the two stones grind against each other, the two tables can pass beyond each other and cause considerable damage. GB-A-2 074 910 provides an electronic solution to the problem of controlling the reciprocation in order to increase the efficiency of the operation, but the arrangement is very expensive.

The first aspect of the invention provides a machine as set forth in Claims 1 or 5, and a method as set forth in Claim 6. Claims 2 to 4 set forth optional features of the first aspect of the invention.

In a general sense, the invention provides a simple mechanical arrangement for limiting the relative positions of the stones at the end of the stroke, to prevent one stone from overrunning the table position of the other stone and yet allowing the length of the stroke to be changed without varying this limiting position, i.e. varying the length of the stroke only by varying the position of one extremity (the second end of the stroke). However, in a variation, altering the stroke alters the relative positions of the gemstones at each end of the stroke, but alters the relative positions much more at the second end than at the first (table) end. This enables the tables to move much closer to the daylight position as bruting or working proceeds and as the stroke is lengthened.This avoids idle contact which can lead to polishing at the end of bruting - if the girdle is all the way round and the stones have been polished, the stones will not effectively grind each other again. In general. for a stone of 3 to 4 mm diameter, the stroke is such that the stones move from a position of 2 to 3 mm overlap at the table end of the stroke to a position in which there is no overlap (but no daylight).

The machine can be arranged so that a simple manual adjustment can be made by eye, which is within the skill of the operatives who normally carry out bruting.

If the contacting surfaces of the stones move in generally opposite directions relative to one another.

and particularly if the stones are rotated about parallel axes, the increase in stroke and the resulting increase in speed, reduces the danger of barrelling of the girdle at the culet end.

The second aspect of the invention relates to the control of bruting, and more particularly to a device for determining the girdle diameter of a gemstone which has a table and is to be bruted or is being bruted. One such device is shown in SA-A-76I7290, but it is not very practical in use. Another method of centering is shown in GB-A-2 080 712. but this can lead to imprecise results.

When bruting, the diameter and height of the girdle (which should be cylindrical) is of great importance for the correct facetting of the stone, and also for avoiding excessive removal of stone during working.

Specifications for a well made stone require a definite proportion of girdle height to stone diameter as well as a specific angle for the lower (culet) facets to the girdle plane, and for the culet to be located on the c-entral axis of the stone. At the end of bruting. the stones should be substantially circular with external imperfections removed around the girdle and just sufficient space provided for complete facetting.

Excessive displacement of the stone centre will result in an off-round stone, when an imperfection has just been removed, and insufficient displacement would produce too small a stone diameter. For holding the stone correctly during facetting, it is important that the girdle should be a reasonably parallel-sided straight cylinder.

With manual polishing, errors in the bruting operation can be compensated for during the facetting process by varying the angles from facet to facet to enable the facets to meet each other at specific points, and also to achieve an even girdle thickness. This compensation is not possible when automatic equipment is to be used, and uneven girdles are immediately apparent in stones produced on automatic equipment. In addition, excessive compensation requirements in manual processing require great skill and result in a substantial reduction in productivity.

It is normal practice to cement the table of the stone to a holder called a dop which is fixed onto a face plate which in turn is mounted on an end plate of a rotary spindle. The face plate forms what is known as a tapping chuck, and enables the stone to be adjustable off-centre by displacing the face plate in relation to the end plate. This adjustment is required to allow the circumference of the bruted stone to be formed in the best position relative to the rough stone, having regard to the quality and weight yield for the completed stone. The central axis of the bruted stone, i.e. the axis on which the culet will be formed, will coincide with the axis of the spindle.

The second aspect of the invention provides a device as set forth in Claims 7 or 8, and a method as set forth in Claims 9 or 12. Claims 10 and 11 set forth preferred features of the second aspect of the invention. The second aspect of the invention also extends to the bruting machine of Claim 13 and the method of bruting of Claim 14. The second aspect of the invention can be used in association with the first aspect of the invention.

The second aspect of the invention provides a suitably graduated system-to position and observe the progress of a gemstone during bruting. Bruting is terminated when the girdle has been ground down to the position of the girdle radius line.

The device, in association with the bruting machine, can give signifi#cant advantages in the initial centering of the stone, for re-centering during bruting, and for determining when to cease bruting. Constant visual examination and readjustment are required to ensure that the extent of lateral displacement is satisfactory, but the device enables proper adjustment to be made.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a bruting machine in accordance with the invention; Figures 2 and 3 each show two stones which are being bruted in the machine of Figure 1; Figure 4 shows a device for determining the girdle diameter before and during bruting; and Figures 5 and 6 show how the device of Figure 4 is used.

The bruting machine has two parallel axis spindles 1,2 for rotating two gemstones (not shown in Figure 1) about respective axes so that the surfaces of the stones contact and grind each other and mutually brute the stones. The spindle axes are in the same vertical plane, the spindle 1 being above the spindle 2.

The spindle 1 is carried in split-bearing housings 3, preferably with taper roller bearings. After accurate alignment, the bearing housings 3 are locked by means of locking screws 4. The bearing housings 3 are mounted upon a machined plate 5 which is fixed to the machine base. The spindle 2 is similarly mounted. but the second plate 5 is not fixed to the base. The plate 5 carries two further split bearing housings 6 with locking screws 7, carrying linear ball-bearings 8 travelling on a circular-section bar 9 which is fixed to the machine base by means of upright locking supports 10. This arrangement permits the plate 5 and the spindle 2 to reciprocate and also to move up and down due to movement about the axis 11 of the bar 9.

Up and down movement is effected by rotation of a feed screw 12 which is threaded through a projecting lug (not shown) on the second plate 5 and whose lower end engages a sliding surface on the machine base, allowing the feed screw 12 to reciprocate with the spindle 2. The feed screw 12 is shown fairly close to the axis 11. If desired, a second feed screw 13 can be provided, for fast feed, arranged similarly to the feed screw 12 but further away from the axis 11.

The arrangement for reciprocating the plate 5 is described below.

Each spindle 1,2 terminates in a precisely concentric cup fitting 14 having a threaded hole 15. An end plate 16 is precisely machined to fit the cup fitting 14 and to screw into, the hole 15 so that the end plate 16 engages perfectly against the face of the cup fitting 14 and is precisely centred on the respective spindle 1,2.

The end plate 16 carries bolts 17 and a face plate 18 is held against the end plate 16 by spring washers and nuts 19, the bolts 17 passing through oversized holes in the face plate 18. This provides a tapping chuck and enables the face plate 26 to be adjusted off-centre of the respective spindle 1,2 by tapping with a light hammer. A dop 20 is screwed into the face plate 18. In practice, the stones (not shown) will be cemented to the dops 20. Each end plate 16 can be removed from its cup fitting 14 and is interchangeable, as a result of having the end plates 16 identical and using the threaded holes 15.

At the rear ends, the spindles 1.2 carry toothed pulleys 21 driven by toothed timing belts 22 and in turn by toothed pulleys 23 carrying a driving shaft 24 driven by a main driving pulley 25. The shaft 24 is carried by upright supports 26 fixed to the machine base. Any suitable number of supports 26 can be incorporated.

The pulleys 21,23 and belts 22 rotate the spindles 1,2 at different speeds to avoid the rotation of the stones being synchronised. As long as there is no common factor, any suitable speed differential can be used, such as 258 or 50% or 70%. A large speed differential ensures that at the beginning of bruting, the high points of the stones engage frequently. In one example, the pulleys 21,22 on one side have respectively twenty-two and fifteen teeth and on the other side twenty-six and twenty-one teeth, the spindle speeds being for instance 137 and 170 r.p.m.

The reciprocating means comprises an eccentric 31 driven by the driving shaft 24. The eccentric 31 is movable manually in the axial direction of the driving shaft 24, being carried on a collar 32 and locked in position by a screw 33. In an alternative arrangement of the collar 32, it is carried on a keyway on the shaft 24 and makes a light frictional engagement with the shaft 24. This enables the collar 32 to be slid along the shaft 24 by hand while the machine is working. The eccentric 31 carries a press fitted bearing ring 34. and the degree of eccentricity provides for an amplitude of movement approximating to the average of the range which the reciprocating-means will be required to provide.The bearing ring 34 is surrounded by an annular follower 35 and is connected by means of a connecting member in the form of a bottle screw 36 to a housing 37 which carries a self-aligning transmission member or bearing 38. As an alternative to the bottle screw 36, the supports 25 can be adjustable in relation to the housing 37; if manufacturing tolerances are good, there is no need for any adjustment at all.

The bearing 38 can slide along a pivoted oscillatable member in the form of a rod 39 which is generally parallel to the driving shaft 24 and which is pivoted at 40; the rod 39 is connected to an arm 41 to form a bell-crank lever. The arm 41 is connected in turn to a universal joint 42 which is connected by way of a grub screw 43 to a post 44 fixed to the second plate 5. The grub screw 43 can be used to adjust the end positions of the stroke of the second plate 5. The grub screw 43 can be loosened and the entire assembly mounted on the second plate 5 moved along the bar 9 away from the grinding zone, to facilitate the interchange of the stone holding assemblies. This movement requires a loose driving belt 22, a condition which can be met using a toothed timing belt.

The bearing 38 and the housing 37 move along the rod 39 as the collar 32 moves. When this is done, the stroke of the reciprocation of the second plate 5 (and hence of the respective gemstone) is altered, increasing the angular oscillation of the rod 39 about the axis 40 without altering the eccentricity of the eccentric 31.

The closer the bearing 38 is to the pivot 40, the greater the oscillatory movement. When setting up the machine, the collar 32 is put in its position for the table end of the stroke, and the length of the bottle screw 35 is adjusted so that the arm 39 is precisely parallel to the driving shaft 24. The bottle screw 15 is then locked. If the collar 32 and associated members are moved along the driving shaft 24, there is no pivoting of the arm 39 about 40, and thus the second plate 5 does not change position.

If at a first end of the stroke of reciprocation, the arm or rod 39 is strictly parallel to the driving shaft 24, altering the position of the collar 32 and hence the length of the stroke does not alter the relative positions of the gemstones at the table end of the stroke as the arm 39 always returns to its parallel position at that end of the stroke. However, it is desirable to be able to alter the table end of the stroke slightly so that it increases in length during bruting, but much less than at the second end. In order to do this, the arm 39 is at an angle to the driving shaft 24 at each end of the stroke; at the table end of the stroke, the arm 39 will be inclined slightly away from the driving shaft 24. and that the other end of the stroke the arm 39 will be inclined more towards the driving shaft 24.

Figure 2 shows gemstones 45.46 relative to each other at the commencement of bruting, the gemstone 46 being the reciprocating stone carried on the spindle 2. Figure 3 shows the stones 45,46 after bruting has been partially completed. The dashed lines indicate the ends of the stroke. It can be seen in these Figures that the left-hand (table) end of the stroke remains in the same relative position whilst the right-hand end increases in length by the distance required to allow contact between the stones 45,46 over the entire bruted length.

There is an optical system whose axis is horizontal.

This hinders dust etc from settling on the lenses. The optical sysem has a high intensity lamp 51, a converging lens 52 and a magnifying lens 53, the magnifying lens 53 being focussed (directly or via mirrors) onto a special control device or screen 54 which is described below.

the bruting machine being arranged to permit the uninterrupted passage of the beam. As the screen 54 is beyond the driving shaft 24, the driving shaft 24 should be sufficiently below the level of the grinding zone to permit the passage of the light beam.

The screen 54 is shown in Figure 4 and comprises two planar members, namely a rear, opaque, fixed member 55 and a front, transparent member 56. Each member 55,56 has straight lines marked on it, and these are shown more clearly in Figure 5 where the uninterrupted straight lines are those on the rear member 55 and the dashed straight lines are those on the front member 56.

The rear member 55 has two straight lines 57,58 intersecting at 900. namely a table line 57 which is to coincide with the table of the image of the stone 46 and an axis line 58 which is to indicate the axis of the stone 46. There is a third straight line 59, the girdle height line, which passes through the intersection of the table and axis lines 57,58 and which is inclined to the table line 57 by such an angle that at any point on the girdle height line 59 the ratio of the distance from the table line 57 to the distance from the axis line 58 is equal to the desired ratio of the girdle height to the girdle radius: normally, the desired girdle height is 14% of the girdle radius.The front member 56 has a girdle radius line 60, which is maintained parallel to the axis line 58 of the rear member 55. and a culet line 61, which joins the girdle radius line 60 and is inclined to the girdle radius line 60 at the desired culet angle; the desired culet angle is normally at 410 to the table. With respect to the axis line 58. the line 61 is inclined in the opposite sense to the line 59. Although it is stated that the lines 57,58 intersect and that the line 59 passes through the intersection, it is not necessary that the actual lines should be taken down as far as the intersection, it only being necessary that the imaginary extensions should intersect.

In use, the front member 56 can be held by hand on the rear member 55 and is moved with respect to the rear member 55 whilst keeping (i) the girdle radius line 60 parallel to the axis line 58 and (ii) the join between the culet line 61 and the girdle radius line 60 on the girdle height line 59 so that the girdle height line 59 acts as a locus. In order to enable the girdle radius line 60 to be kept parallel to the axis line 58, the rear member 55 can be provided with faint lines (not shown) parallel to the axis line 58. The front member 56 is moved as indicated in Figure 5 so that the outline of the image of the stone 46 is completely, but only just, outside the line 60,61. The spindle 2 is then turned through 1800 and the stone 46 is centered by tapping. The centering should be checked at the opposite pair of laterals, or on the flat if there is a flat.The stone 46 is then bruted until the girdle is on the girdle radius line 60. In practice, concavities or reentrants can be disclosed as bruting proceeds, and the screen 54 needs to be used a number of times during bruting. -Thus the machine operator can alter the setting in accordance with final requirements as bruting progresses.

In order to position the table of the image correctly on the table line 57, the screen 55 can have lateral adjus-tment, or the inclination of a mirror in the optical system can be altered. The axis line 58 is defined by the machine and should be accurately set up before beginning bruting.

Although it- would be possible to mount the screen 56 so that it can only track along the girdle height line 59, it is preferred not to do this as there is less flexibility in the arrangement. Figure 6 illustrates that if the front member 56 is used correctly as in Figure 5 and there is an opening at the culet, this opening can be closed by moving the front member 56 slightly to the right and thus somewhat lowering the girdle height. It is possible to take the girdle height down as far as 10%, though 12l/2% is a more suitable figure.

In theory at least, it would be possible to represent the lines 57-61 electronically in electronic imaging equipment.

Normally, the shaft 1, which merely rotates, will be fitted with a stone for finishing whilst the shaft 2 will carry a stone for initial processing. This avoids the necessity for examining the stone 45 on the spindle 1 using the screen 54. Furthermore, the stone 45 is above the stone 46 and is more clearly visible with a magnifying glass if further inspection is required.

When the stone 45 on the spindle 1 is completed, the end plate 16 and the whole assembly is removed and the other end plate 16 and assembly, carrying the now partly-processed stone, is fitted to the spindle 1.

The feed screw 12 is used during bruting, feeding normally being carried out in steps, say 5 microns every step, every four seconds, though at longer intervals, say every half minute, at the end of bruting. Even if there is no common factor in the speeds of rotation of the spindles 1,2, there is slight grooving at high feed rates due to synchronised tracking, and the feed rate should be cut at the end for a minute or two.

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

Claims (14)

Claims

1. A machine for working gemstones, comprising: means for rotating two gemstones about respective axes such that the surfaces of the stones contact and grind each other; feed means for moving the stones relatively towards one another, to provide feed; and means for reciprocating the stones relative to one another, the reciprocating means comprising: a driving shaft; an eccentric driven by the driving shaft and movable axially of the driving shaft; a pivoted oscillatable member connected to one of said rotating means for reciprocating the respective gemstone, and having an arm which is generally parallel to the driving shaft; a transmission member movable along the arm of the oscillatable member; and a connecting member for transmitting generally reciprocatary motion from the eccentric to the transmission member to thereby oscillate the oscillatable member; whereby the length of stroke of the reciprocation of the stone can be altered by moving the transmission member along the arm of the oscillatable member to increase the angular oscillation of the oscillatable member without altering the eccentricity of the eccentric.

2. The machine of Claim 1, and arranged so that at a first end of the stroke of reciprocation of the stone, the arm of the oscillatable member is parallel to the driving shaft, whereby altering the stroke does not alter the relative positions of the stones at the first end of the stroke.

3. The machine of Claim 1, and arranged such that at each end of the stroke of reciprocation of the stone, the arm of the oscillatable member is at an angle to the driving shaft, the angle at a first end of the stroke being opposite to the angle at the second end of the stroke, and the angle at said second end being substantially greater than the angle at said first end, whereby altering the length of stroke alters the relative positions of the stones at each end of the stroke, but alters the relative positions much more at said second end than at said first end.

4. The machine of Claim 2 or 3, and arranged for use with gemstones which have flat faces which are mounted on the respective rotating means, with the flat faces of the stones generally facing each other and the stones being reciprocated so that at said first end of the stroke the flat faces nearly coincide and at said second end of the -stroke the stones overlap to a substantial extent.

5. A machine for working gemstones, substantially as herein described with reference to, and as shown in, Figures 1 to 3 of the accompanying drawings.

6. A method of working gemstones, comprising rotating two gemstones about respective axes such that the surfaces of the stones contact and grind each other, moving the stones relative towards one another to provide feed, and reciprocating the stones relative to one another, the method being carried out on the machine of any of the preceding Claims.

7. A device for determining the girdle diameter of a gemstone which has a table and is to be bruted, the device comprising: a member having marked thereon two straight lines intersecting-at 900 namely a table line which is to coincide with the table of the image of the stone and an axis line which is to indicate the axis of the stone, and a third straight line, the girdle height line. which passes through the intersection of the table and axis lines and which is inclined to the table line by such an angle that at any point on the girdle height line the ratio of the distance from the table line to the distance from the axis line is equal to the desired ratio of the girdle height to the girdle radius; and a second member which coacts with the first member and has marked thereon a first straight line, the girdle radius line, which will be maintained parallel to the axis line of the first member, and a second straight line, the culet line, which joins the girdle radius line and is inclined to the girdle radius line at the desired culet angle; in use the first and second members being superimposed and movable one with respect to the other whilst maintaining the girdle radius line parallel to the axis line and the join between the culet line and the girdle radius line coincident with the girdle height line so that the girdle radius line represents the girdle position and the distance of the girdle radius line from the axis line represents the radius of the girdle.

8. A device for determining the girdle diameter of a gemstone which has a table and is to be bruted, the device being substantially as herein described with reference to, and as shown in, Figures 4 and 5 of the accompanying drawings.

9. A method of centering a gemstone which has a table and is to be bruted, and/or determining the girdle diameter, comprising using the device of Claim 8 or 9.

10. The method of Claim 9, wherein the gemstone is centered and the girdle diameter is determined by maintaining the girdle radius line of the second member parallel to the axis line of the first member, and moving the first and second members with respect to each other so that the join between the culet line and the girdle radius line of the second member is coincident with the girdle height line of the first member, the girdle radius line representing the girdle position and the distance of the girdle radius line from the axis line representing the radius of the girdle.

11. The method of Claim 10 varied in that the join between the culet line and the girdle radius line of the second member is positioned between the girdle height line and the table line, thereby reducing the girdle height.

12. The method of centering a gemstone which has a table and is to be bruted, and/or determining the diameter of the girdle, substantially as herein described with reference to, and as shown in, Figure 5 or Figure 6 of the accompanying drawings.

13. A bruting machine arranged to carry out the method of Claim 9, and comprising two parallel axis rotatable spindles each fitted with means for holding a gemstone, the holding means permitting off-centre adjustment of the gemstones relative to the axes of the respective spindles, the spindles being offset but facing one another and capable of reciprocation relative to each other in the direction of the axes, in order to brute one gemstone against the other, the bruting machine being arranged to permit the uninterrupted passage of a beam of light across the bruting area to enable a magnified image of one of the stones to be formed on said device.

14. A method of bruting using the bruting machine of Claim 13, comprising forming the magnified image of one of the stones on said device, adjusting the axis of the stone relative to the axis of the respective spindle so that the stone is centred. and bruting the stone down to a girdle diameter indicated by said girdle radius line.