GB2037196A - Gem stone polishing machines - Google Patents

Abstract

A gem stone polishing machine has a rotatable grain shaft 128 for mutally orientating a gem stone 18 and a rotating scaife 46. A signal dependent on drag between the stone and scaife is generated and a drive rotates the grain shaft to orientate the stone so as to give a high drag. Drag is generally related to polishing efficiency. The stone can thus be polished efficiently. In a preferred arrangement a signal dependent on the rate of stone feed is generated to confirm efficient polishing. <IMAGE>

Description

SPECIFICATION Gem stone polishing machines The invention relates to gem stone polishing machines for use especially, but not exclusively, in polishing diamonds. The invention is particularly applicable to gem stone polishing machines having a drive and control arrangement for polishing one facet after another automatically.

The British Patent Specification 1,421,331 describes a gem stone polishing machine in which a grain shaft is rotated in a predetermined manner to polish gem stones automatically. Because the grainshaft is rotated in a predetermined manner the gem stone, if a diamond, must have a predetermined structure to ensure proper polishing along the "grain" is used idiomatically and refers to the fact that diamonds may be anisotropic and show widely different rates of polishing depending on the direction of polishing. Machines of this type are sold under the Trade Mark Piermatic by Bonas Machine Company Limited.

The U.S. Patent Specification 3,886,695 describes a gem stone polishing machine capable of seeking the grain and "finding" the best polishing direction without turning the grain shaft in a predetermined manner. These machines do not appear to have become widely established. The stone temperature is measured or has to be controlled in orderto find the optical polishing direction. This is likely to be a prolonged process preventing repeated checking of the optimal polishing direction and leading to a considerable period of time during grain seeking in which the stone is not optimally aligned and leading to possible damage to the scaive. In addition it is believed that temperature is not unambiguously related to the diamond grain in polishing. Spurious events may lead to wrong stone-scaive orientation.

The mechanical arrangements adopted were not rigid and hindered the accurate control of stone/scaive movement.

The invention has as an object to provide a gem stone polishing machine capable of grain seeking, occasioning less scaive wear, reliable in operation and/or relatively convenient to adjust.

It is a further object of the invention to provide a polishing machine capable of controlling the various stages of facet polishing, reducing the requirements for skilled operatives and/or capable of polishing makeable stones i.e. having no predictable grain structure.

According to the invention there is firstly provided a gem stone polishing machine including a facet head for mounting a gem stone, a rotatable scaife for polishing a gem stone, a grain shaft for orientating the gem stone for polishing in a desired direction by the scaife, a grain shaft angle transducer arrangement for providing a signal dependent on different rotational positions of the grain shaft, a drive for rotating the grain shaft, a means for providing a signal dependent on drag between the gem stone and the scaive to thereby providing a signal dependent on polishing efficiency and enabling the grain shaft to be rotated so as to provide efficient polishing. It has been found that a drag signal provides a particularly convenient way for observing the efficiency of polishing. Variations in drag can be followed at very small delays.Thus an optimal drag direction can be found by a 360" turn of the stone which is quite brief yet generates an accurate series of signals. Generally speaking drag is reliably related to polishing efficiency.

Preferably the machine further includes a feed means for moving the facet head and scaive towards one another and a means for providing a signal dependent on the rate of feed to thereby enable the efficiency of polishing to be confirmed or another polishing direction to be adopted. In the event that the drag signal should not be properly related to polishing efficiency, the rate of feed signal can be used to enable remedial action to be taken, if necessary by arresting the polishing of the facet until other facets have been polished. The possibility of prolonged misalignment between stone and scaive can thus be significantly reduced enabling scaive-lives to be extended.Conveniently the machine further includes a means for urging the gem stone and scaive towards one another at a polishing force and a means for providing a signal dependent on the polishing force to thereby enable the polishing force to be set at a desired level or levels. The polishing force control provides the means for (a) increasing the reliability of the drag signal as the drag can vary with polishing force as well as the grain alignment.

Expensive devices for maintaining constant polishing forces need not be used as the polishing force control can be used, as will be explained, to keep the polishing force constant; (b) reducing scaive wear as the stone initially engages the scaive. At this time no grain-data are available so that there could be a chance of temporary misalignment of scaive and stone leading to undesirable wear or damage of the scaive. The polishing force control signal can be used to urge the stone gently onto the scaive whilst the stone is rotated continuously until drag-data are available; (c) enable the polishing force to be adjusted so as to obtain the best facet polish.

The aforesaid capabilities can be provided in stone-scaive mountings of different physical layouts including one in which the facet head is mounted on the grain shaft and a smoothing arm cantilevered from a pillar locating a grain housing mounting the grain shaft over a scaive, the grain shaft being rotatable in the grain housing to enable the grain shaft to be rotated. This resembles the tested prior art arrangements. Preferably the grain housing is slidable longitudinally with respect to the smoothing arm and the means for providing a drag signal includes a means for resiliently biasing the smoothing arm and grain housing towards one another and a transducer for providing a signal dependent on their relative displacement against the resilient means bias.Suitably the smoothing arm and grain housing are connected by a shaft mounted in rotary linear bearings to permit both relative displacement of the smoothing and grain housing longitudinally and angular displacement to obtain access to the facet head. The use of the rotary-linear motion about the same axis enables rigidity to be maintained whilst providing the extra freedom of motion for measuring drag.

In order to feed the stone in a desired manner, preferably the smoothing arm is movable on an upright axis with respect to the pillar and a member supports the smoothing arm from below, the member being movable to enable the facet head and scaive to be fed towards one another.

The supporting member may be a rotatable cam but most conveniently the member is a lever, a lead screw bears againstthe lever to locate the smoothing arm and a lead screw rotation transducer is provided to provide a signal dependent on the rate of feed. An accurate rate of feed signal can be obtained.

The polishing force can be detected by permitting an additional up-down motion. Preferably a means is provided for resiliently biasing the facet head towards the scaive and a transducer is provided for providing a signal dependent on vertical facet head displacement for providing a signal dependent on polishing force and enables the member to control the polishing force. Thus the polishing force can be measured by permitting up/down stone movement.

By maintaining a constant polishing force, the relative facet head-grain housing position will be the same so that the displacement of the cam or the lead screw can still serve as a reliable indication of the rate of feed.

The invention secondly provides a gem stone polishing machine including a facet head for mounting a gem stone, a rotatable scaife for polishing a gem stone, a grain shaft for orientating the gem stone for polishing in a desired direction by the scaife, a grain shaft angle transducer arrangement for providing a signal dependent on different rotational positions of the grain shaft, a drive for rotating the grain shaft, feed means for moving the facet head and the scaife towards one another, means for providing a signal dependent on the rate of feed thereby providing a signal dependent on polishing efficiency and enabling the grain shaft to be rotated so as to provide efficient polishing. Although the rate of feed data will not follow polishing efficiency variations as closely as the drag signal, this arrangement is capable of providing data sufficiently quickly for grain seeking.

To reduce scaive wear and make the rate of feed a reliable polishing efficiency indicator, preferably the machine includes a means for urging the gem stone and the scaife towards one another at a polishing force, and a means for providing a signal dependent on the polishing force to thereby enable the polishing force to be set at a desired level or levels.

The machine can be interfaced with sophisticated electronic control systems to provide automatic detailed control over most parts of the stone polishing process. The machine may have solenoid powered systems for quickly lifting the stone when electrical contact is made between the pot holding the stone and the scaive. Cantilever beam-strain gauge assemblies can be used to provide easily adjustable reliable polishing force and drag transducers. Various angular motions can be detected and controlled using suitable angle transducers such as gratings and light emitting diode/sensor systems.

Makeable stones can be polished automatically.

Faster polishing times for the individual facets can be obtained. Scaive wear can be reduced and polishing may be more easily controlled so providing greater repeatability.

Figure 1 shows a section seen in plan along line B-B in Figure 13 of a gem stone polishing machine of the invention; Figures 2A and B show adjacent parts of a partly sectioned side view of the machine of Figure 1. The section is along line K-K in Figure 3; Figure 3 shows a part section rear view of the machine of Figure 1.The section is along line L-L in Figure 2; Figure 4 shows an end view in the direction of arrow M in Figure3; Figure 5 shows a part section along line Q-Q in Figure 1; Figure 6 shows a part section along line P-P in Figures; Figure 7 shows a section along line N-N in Figure 3; Figure 8 shows a section along line H-H in Figure 2; Figure 9 shows a section along line G-G in Figure 2; Figure 10 shows a view in the direction of arrow J in Figure 2; Figure 11 shows a section along line A-A in Figure 13; Figure 12 shows a section along line C-C in Figure 13; Figure 13 shows a section along line F-F in Figure 2; Figure 14 shows a plan view of a grain head of the machine of Figure 1 partly assembled; Figure 15 shows the plan view of Figure 14 with the machine assembled;; Figure 16 shows an end view of a partly assembled machine in the direction of arrow E in Figure 12; Figure 17 shows position masks and PCB's for a cam drive motor of the machine of Figure 1; Figure 18 shows a section through the facet head of the machine of Figure 1; Figure 19 shows a bottom view of the facet head of Figure 18; Figures 20A and B show respectively a side and top elevation of a positioning mask and PCB for a grain shaft of the invention of Figure 1; Figures 21 and 22 show top elevations of beams for drag and polishing force transducers of the machine of Figure 1; Figure 23 shows a section through a modified facet angle adjustment mechanism forthe machine of Figure 1; Figure 24 shows a partial section through a mod~ ified smoothing arm height control mechanism for the machine of Figure 1; and Figure 25 shows a side view of the modified mechanism of Figure 24 and Figure 26 shows a graph of the drag variation with different rotational positions of the stone.

With reference to the Figures, a diamond polishing machine has a central rotating scaife at the periphery of which are mounted four diamond polishing heads which are operated and constructed in the same manner to overhang a scaife46.

Each diamond polishing head 2 (see Figure 1) incorporates a base plate 4 mounting a pillar shaft 6 and a pillar sleeve 8 surrounding the shaft 6 but spaced therefrom. A compression spring 10 is fitted into the lower part of the annular space 12 formed between the pillar shaft 6 and the sleeve 8 (see Figure2).

The base plate 4 also mounts a cam housing 14 (see Figures 1,3 and 4) containing a rotatable cam 16 for lifting and lowering the diamond 18 driven by a bearing mounted layshaft 20 and a flexible coupling 22 by a cam motor 24 and reduction gear 26. The coupling 22 ensures conjoint rotation of an output shaft 28 and the layshaft 20 but accommodates minor misalignment between the shafts 20, 28. Secured to the top of the cam housing 14 is a housing 30 for a piston 32 having a roller 34 for engaging the cam 16 which roller is pivoted on a pin 36 fitting into a slot 38 in the housing 30 to permit up down movement of the piston 32 without permitting turning of the piston 32 around its longitudinal axis. The piston 32 is carried in a Glacier DU bush and the roller 34 is crowned to accommodate minor misalignments.

The base plate (See Figures 1,3, 5 and 6) finally mounts a housing 40 for a smoothing motor 42 and reduction gearing 44 for reciprocating the diamond 18 generally radially across the scaife 46 (See Figure 1) to encourage even wear. An output shaft 48 drives an eccentric roller 50 having an outer bearing mounted sleeve 52 through a flexible coupling 54.

The base plate assembly described can be adjusted at two corners 56 to pivot it about axes passing through the centre of a hardened crowned pin 57 adjacent the pillar shaft 6. The base plate assembly actuates a smoothing arm assembly to reciprocate it bodily with the diamond 18, enables the heightwise position of the smoothing arm assembly to be altered during polishing by the cam 16 whilst the spring 10 reduces the force with which the assembly would rest on the cam 16 by counterbalancing its weight.

The base plate 4 and the parts attached thereto support and move the smoothing arm assembly generally indicated at 58 (see Figure 2). The assembly 58 incorporates a smoothing arm housing 60 which is mounted around the pillar sleeve 8 and rotatable and axially movable with respect thereto by rotary-linear bearings 62 such as those made and sold by Rotolin Inc. under the name Rotolin. These bearings comprise a cage carrying cylindrically distributed balls. The housing 60 has an axially mounted hollow adjustment nut 64 which passes into the space 12 between the pillar and the sleeve to rest on the spring 10. The pillar shaft 6 extends to the top of the nut 64 and has an abutment 66 at the top to limit upward movement of the housing 60.

On one side of the housing 60 there is mounted (see Figure 2) an adjustment nut 68 bearing against one end of a spindle 70, the other end of which carries a roller 72 resting on the top of the aforementioned piston 32. The roller 72 pivots on a pin 74 guided in a slot 76 in the housing 60. Thus movement imparted by the rotatable cam 16 can be transmitted via the roller 34, the piston 32, the roller 72 and the spindle 70 to the smoothing arm assembly 58. The hollow nut 64 can be adjusted to vary the downward force exerted by the smoothing arm assembly 58 on the cam 16 and to relieve the weight on the cam 16. The motor 24 can thus operate under a light torque. The piston 32 has a large top area to permit the roller 72 to roll over that surface during the smoothing motion.

On another side of the housing 60 there are secured a pair of pins 78 each carrying through bearings 80, a pair of sleeves 82 which fit against opposite sides of the aforementioned eccentric sleeve 52.

Rotation of the eccentric sleeve 52 causes the smoothing arm assembly 58 to be reciprocated. The different bearing supported sleeves 52, 82 provide a low friction drive connection permitting also up and down movement of the smoothing arm assembly 58 with little drag. The pins 78 are mounted in a block 84 (see Figure 7) which can be shifted transversely to alter the position of the centre line of the arc of reciprocation of the smoothing arm assembly 58 (normally 50 approximately).

A smoothing arm shaft 86 (see Figures 1,8,9, 10) is rotatably mounted by bearings 88 in a generally horizontal attitude and held against axial movement by a ring 90 secured by means of a screw 92 bearing against one end of the housing 60 and an enlarged diameter portion 94 of the shaft 86 bearing against the other end of the housing 60. The shaft 86 is hollow for conveying electrical wiring to a grain head assembly. A latching block 96 is pressed on to the shaft 86 and secured by a screw 98. The block 96 carries a pivotable latch arm 100, whose pivot centre can be horizontally adjusted, and a thimble micrometer 102.The housing 60 carries a member 104 having a sprung ball 105 for engaging in a chamfered circular aperture 106 of the latch arm 100, for abutting the thimble micrometer 102 and mounting a printed circuit board (PCB) light emitting diodephototransistor arrangement 108 to detect the latching in position of the latch arm 100. The ball 104 and aperture 106 engage to hold the block 96 and the shaft 86 at a particular angle which can be set precisely by the thimble micrometer 102. During man ufacturethe latch arm 100 can be set approximately by adjusting its pivot position. The shaft 86 extends through the block 96 to a grain head assembly gen rally indicated at 110.To hold the block 96 and the assembly 110 at a predetermined angle whilst permitting sliding movement of the grain head assembly 110, a guide shaft 112 fixed to the assembly 110 is slidably received in a self-aligning bearing 114 of the super ball bushing type made and sold by Ransom, Hoffmann and Pollard Ltd. The bearing 114 has balls received in saddle cages which can pivot to a limited extent to accommodate imperfect parallelinity between the guide shaft 112 and the smoothing arm shaft 86. The bearing 114 can thus be set to take up play and permit accurate sliding movement of the grain head 110 without appreciably resisting the movement.

The grain head assembly 110 (see Figures 1,2, 11, 12, 13) is free to slide on shaft 86 which is rigidly attached to the block 96. Assembly 110 is remote from the base plate assembly and carries a facet head assembley 116 attached to the lower end of an integral grain shaft 128 whose axis is fixed at 900 to the axis of shaft 86. The facet head assembly is normally positioned adjacent to the scaife surface 46. The grain head assembly 110 is prevented from rotating about the axis of shaft 86 by a parallel guide shaft 112 pressed into the grain head 118. This shaft runs in the bearing 114 rigidly attached to block 96.

The grain head 110 participates in all the motions of the smoothing arm and in addition is free to move axially with respect to the shaft 86.

A grain head housing 118 has a first horizontal bore 120 for securing the guide shaft 112 which is pressed into it and a second horizontal bore 122 for engaging the shaft 86 through rotary-linear bearings 124 (made by Rotolin Inc.). The housing 118 also has a vertical bore 126 in which a grain shaft 128 is mounted.

The smoothing arm shaft 86 extends right through the housing 118 and engages at the front extremity through a knife edged block, a drag transducer 130 retained in a cover 132 attached to the front of the housing 118.

The grain head shaft 128 has a slot 134 at the top engaging a transverse pin 136 on a gear box output shaft 138 (See Figure 14) to form a torsion-only coupling. A reversible grain motor 140 is fixed to the top of the housing 118 (See Figure 15). The pin-slot connection enables the shaft 128 to move axially without significant angular, relative displacement between the output shaft 138 and the grain head shaft 128.

A shaft angle transducer arrangement 142 is associated with the top of the shaft 128 to encode its angular position at different heightwise positions of the shaft 128. A slip ring and contact assembly 144 is arranged below the shaft angle transducer arrangement (see Figure 12). The contacts are mounted on an electrically insulated block 146 and slip rings 148 are wider than contacts 150 to permit their heightwise displacement. An arrangement for heightwise relative positioning of the grain head shaft 128 and housing 118 is arranged between the slip ring and contact assembly 144 and a pair of rotary linear bearings 152 (made by Rotolin Inc.) supporting the grain head shaft 128.

The shaft 128 has an annular ring 154 and the ring 154 (and thereby the shaft 128) can be biased downward by a polishing force transducer 156 extending between the shafts 86 and 112 from a cover 158 attached to the housing. The shaft 128 can be biased upward by a 8:1 reduction lever 160 pivotably con nected to a plunger 162 of a solenoid 164 housed in a cover 166 attached to the housing 118. Thrust bear ings 178 are used to ensure that upward and down ward bias do not appreciably hinder angular move ment of the shaft 128. A plate 180 limits movement of the long arm part of the lever 160 when the grain head is inverted for access. A weight 182 is slidably attached to the lever 160. The weight 182 can be slid to different positions to adjust the upward bias.A PCB light emitting diode-phototransistor arrangement 184 serves to detect operation of the lever 160.

The grain shaft 128 is thus arranged to participate in the smoothing arm reciprocation the heightwise movement thereof by the cam 16 and the pivotal movement about the shaft 86 of the latching block. In addition the different components on the grain head assembly 110 enable the shaft 128 to turn on its axis and the shaft 128 is movable up and down and movable sideways bodily with the grain head housing 118. The different bearings ensures that the turning movement and in particular the up and down movement and sliding movement along the smoothing arm shaft can take place at a low friction whilst permitting the other motions to be positively transmitted to the grain shaft 128.

A facet head assembly 116 designed along the lines of known facet heads for automatic diamond cutting machines is bolted to the lower end of the grain shaft 128 where it protrudes from the housing 118. (See Figure 18). The facet head 116 mounts a cylindrical diamond chuck 186 having a drawbolt collet which can be opened or closed by a nut 188 and rotated in accurately indexed steps by a facet motor 190 and reduction gear 192. The chuck 186 has a diamond holder or press pot which can be locked in position on a diamond shaft. A facet head housing 194 is pivoted for movement about a substantially horizontal axis at 196 on a mounting part 198 and the angle of the facet head can be set by a micrometer adjustment device 200 and locking toggle 202.

Ultra-flexible wiring 204 can pass from the mounting part 198 into a hollow interior of the grain shaft 128 to the slip rings 148. The ultraflexible wiring from the contacts forthe slip rings 148 passes out through an aperture (See Figure 16) in the rear of the grain head housing 118 to the passage 206 in the latching block 96 to provide an expansion joint arrangement which produces minimal resistance to grain head motion.

The facet motor 190 drives the diamond shaft through a worm and wheel reduction transmission.

The facet head includes a 24 preset position mechanical dead spot system for cutting 8 or 16 facets at desired angles. Power is supplied to the facet head through the contacts and slip rings on the grain shaft 128 of which two slip rings are connected to the facet motor, one to the eight dead spot facet head system, one to the 16 dead spot facet head system, two to the lift contacts for lifting the facet head when it contacts the scaife 46, and one to a common connection in the dead spot system. The number of slip rings can of course be varied in accordance with control requirements forthe facet head 116.

The facet head 116 thus provides a rotary motion about an axis inclined with respect to the scaife 46 in addition to the movements already imparted to the grain shaft 128. A metal contact clip 208 is mounted on the bottom of the facet head 116 (See Figure 19) for contacting the chuck 186 as will be described.

The overall arrangements thus provide a lightly movable floating smoothing arm/grain head/facet head assembly counterbalanced by the spring 10 and within this a lightly movable floating grain shaft/facet head assembly counterbalanced by the weight 182.

The various seals, bearings and fastening screws have been shown in the drawings but not discussed in detail. Sealing rings and labyrinth seal arrangements may be used wherever appropriate to reduce ingress of diamond dust. The bearings are all set to avoid play and yet permit light movement in the various senses described previously.

The different polishing heads each have their individual drive and control systems. A drive motor rotates the scaife 46 at a constant speed. The motor 42 and reduction gearing 44 reciprocate the smoothing arm assembly 58, grain head assembly 110 and the facet head 116, with the motor 42 rotating at constant speed.

The other motors i.e. the motor 24 for the cam 16, the motor 140 forthe grain shaft 128 and the motor 190 for the facet head 116 are driven only when required to rotate their respective shafts to the extent required. The operation of these motors is controlled by an electrically operated control unit, which receives information from the following sources.

The PCB arrangement 108 has a light emitting diode (LED)-phototransistor pair which cooperates to detect latching of the latch arm 100. This arrangement 108 provides an overall safety device and prevents automatic operation unless the latch arm 100 is in the proper position.

The PCB arrangement 184 is the same as arrangement 108 and optically detects a lowering of the lever 160. It provides a signal to indicate that lifting of the diamond is completed and that a fresh facet can be polished.

An arrangement (See Figure 17) employing two circular PCB's 210 cooperate with a circular mask 212 having orientation slot 214. The mask 212 rotates conjointly with the cam 16. The PCB's employing light emitting diode-phototransistor pairs are used to limit rotation of the cam 16 to the arc in which a snail cam shaped portion engages the roller 34 and imparts a constant upward motion thereto for a given arc of rotation of the cam 16. The maximum lift provided by the cam 16 is, say, 4 mm over a 3000 arc of rotation of the cam 16 in the direction shown by the arrow in Figure 4.

The arrangement 142 (See Figure 20), employing a PCB 216 with a pair of LED-phototransistor pairs, cooperates with a grain shaft positioning mask 218 to detect the grain shaft angle. The mask 218 has three concentric tracks with slots 220. The outer two tracks consist of say 32 equally spaced slots which are relatively displaced to provide an up and down counting ability. As the grain motor 140 is reversible it can cooperate with the PCB 216 so as to find a desired grain shaft angle even if at first the motor 140 causes the shaft 128 to overshoot. The innermost track has a single slot for indicating a zeroposition for the grain shaft 128 which is used only to position the diamond in the facet head for ease of inspection.The detailed operation of the arrangement 142 is described later on but the PCB-mask arrangement enables the grain shaft 128 to be positioned by controlled operation of the motor 140 in the required angular position.

The contact clip 208 can be made to rest on either a contact ring 222 or the presspot 224 of the chuck 186. Thus when the contact ring 222 or the presspot 224 contacts the scaife a circuit is completed to indicate the end of a facet polishing operation and set in motion the lift mechanism operated by the solenoid 164.

The foregoing positioning and detecting components do not physically influence the motions which they are intended to detect. The machine further includes however the drag and polishing force transducers 130 and 156 which influence the grain shaft position physically. Each transducer comprises a cantilevered beam 234 having a pair of strain gauges 236 on each side connected in a wheatstone bridge type ci rcuit to obtain a large output signal for a unit deflection and to make the beam deflection strain gauge output relationship largely independent of temperature variations. Each beam is mounted on a block at one end and is cantilevered out to engage the appropriate shaft at the other end. The block 226 of the polishing force transducer can be adjusted up or down by screws 228 in elongate slots 230 (See Figure 2).The shaft 86 has a screw and knife arrangement 232 to engage the free end of the beam of the drag transducer 130 which arrangement 232 can be adjusted axially on the shaft 86. The beams and gauges of the respective transducers have approximately the same full scale deflection.

The various adjustments fall in three categories: (a) manufacture adjustments; (b) initial adjustments necessary from time to time to maintain proper operation of the machine; and (c) regular adjustments necessary before an automatic polishing cycle can be initiated. Most of these adjustments are already employed for known polishing machines and the following description of the operation indicates adjustment operation only in general lines and details them only insofar as they relate to the invention.

The grain shaft 128 is arranged exactly perpendicularto the scaife 46 by adjusting the pillar shaft 6 at the corners 56 of the base plate 4 and then finely adjusting the thimble micrometer 102 on the latch block 96. The necessary adjustment can be determined by using first a levelling cone and then an actual stone with the ultimate purpose of obtaining a proper culet (bottom tip) on the stone. If necessary the latch arm 100 is adjusted to ensure proper latching.

A stone is then placed in the press pot 224 after its proper polishing angle has been determined. The grain head assembly 110 and facet head 116 are turned upward until a latch mounting 234 rest against a pin 235 on the housing 60 just past the over-centre position. The diamond holder 222 is then inserted in to the collet chuck 186 and locked in place in the facethead 116 by use of the nut 188. The grain head assembly 110 and the facet head 116 can then be turned back so that the stone is above the sacife 46. The micrometer nut 200 is then operated to give the desired facet angle and clamped in position using the toggle device 202. The contact clip 208 is placed in contact with the ring 222. The stone, which has been shaped roughly, is then brought to within 1 mm of the scaife 46 by operating the nut 68.The latch arm 100 can then be engaged and automatic polishing can commence to polish a predetermined 16 or 8 facets on the stone.

At the start the cam 16 is stationary to hold the smoothing arm assembly 58 in the maximum height position. The smoothing motor 42 starts up; the grain motor 140 rotates the facet head continuously.

The roller 34 rests lightly on the cam 16; the lever 160 by means of the weight 182 urges the cantilever beam of the polishing force transducer 156 slightly upward to give a zero-polishing force signal; and the screw and knife arrangement 232 is set so as to cause the cantilever beam of the drag transducer 130 to bias the grain head assembly 110 against the latch block 96 and give a zero-drag signal. Where necessary the machine is adjusted so that the above conditions apply.

The cam motor 24 rotates to lower the diamond at a high (say 20 r.p.m.) speed and, as soon as the transducer 156 indicates a change in the beam position and hence contact between the diamond and the scaife 46, the motor 24 slows down (to say 2 r.p.m.). The polishing force can then be increased in steps by slow turning of the motor 24. The resultant lowering of the smoothing arm assembly 58 and the grain head assembly 110 does not appreciably shift the facet head 116 but deflects the beam of the transducer 156 increasingly and hence increases the polishing force.

Thus sudden contact between the diamond and scaife can be avoided, reducing wear of the scaife and accidental damage which can ruin a scaife.

Whilst the polishing force is being stepped up, the grain motor 140 continues to rotate at high speed. In this way, the chance that the initial alignment betwees the diamond and the scaife is the least favourable from a wear point of view and is maintained sufficiently long for the scaife to be damaged is avoided. It should be kept in mind that diamonds are polished using diamond powder and a "hard" edge on the rough diamond could well damage unfavourably aligned diamond particles.

As the polishing force increases, drag increases.

At a predetermined force level, the drag transducer 130 becomes operative and generates drag data relating to the different grain shaft angles as encoded by the shaft angle transducer 142. A graphical representation of the drag data is shown in Figure 26. The transducer 142 and the drag transducer 130 may be connected so that a drag datum is stored for analysis in relation to each recorded grain shaft angle. The start position forthe grain shaft angle encoding is not critical as tne counter can be made to re-set to zero after the full number of counts for a 360 shaft rotation.

After a sweep of 360% a processor of the control system can select the grain shaft angle (as at A in Figure 26) at which drag is greatest and rotate the grain motor subsequently to locate the shaft 128 at that angle. The angle can be maintained and altered if necessary after subsequent sweeps some time later. The effect is that the facet is polished at a most favourable grain shaft angle.

This grain seeking facility speeds polishing and enables stones to be polished having no predeter mined grain. The operative can set a diamond without reterence to the grain. T he processor tor select ing the grain shaft angle may be designed to operate with various degrees of sophistication. A shaft 128 may oscillate about the previously determined optimum drag angle to check that the angles on either side of the selected angle remain less favourable and if not initiate a repeated sweep. The processor may select one of a number of equivalent maximum drag angles and cut at the appropriate angles in succession. The processor may prematurely terminate the polishing of a facet if no favourable angle presents itself; commence another facet; and return to the difficult facet later in the overall polishing operation.

As the diamond facet is polished the area in contact with the scaife 46 increases, increasing drag but the cam motor 24 can rotate the cam 16 at slow speed so as to keep the polishing force constant by lowering the grain head assembly 110 in step with the lowering of the facet head 116 so as to keep the beam deflection of the polishing force transducer 156 constant. The rate of rotation of the motor 24 can then be utilised by the processor to act as a confirmation of the drag data and to interrupt the polishing of a facet where progress of polishing is slow despite the setting of the shaft 128 at the angle giving the highest drag.If say polishing at 0 as shown in Figure 26 were not to result in efficient polishing, the orientation could not be changed to polish at 180to polish atthesecond highest drag peak B.

The polishing force transducer can also be set to vary the polishing force in accordance with a predetermined scheme so as to speed up the overall polishing operation and decrease if desired the polishing force in the final stages.

When the polishing of the facet is about completed the ring 222 contacts the scaife, the presspot 224 having been ground away to provide a flush diamond-presspot surface. The contact clip 208 then initiates a lift routine. The lift routine may also be initiated if no favourable and effective polishing angle can be found.

At the start of the lift routine, the solenoid 164 is activated and pivots the lever 160 to raise the grain shaft 128 thus lifting the stone clearofscaife and deflecting the polishing force transducer 156. The cam motor 24 then is rotated at high speed back to its maximum lift position whilst the facet motor 190 rotates to bring the diamond to the proper angle for the next facet. The lift solenoid 164 and then de energised and the cam 16 once more rotated to lower the diamond in the manner described previously. The PCB-phototransistor arrangement 184 provides a signal to indicate completion of lift and the later operation involving turning of the cam 16 to lower the grain head cannot commence unless that signal is received.

The operation is repeated until all facets at the proper angle have been polished after which auto matic polishing is ended. Other facets can then polished automatically at a different angle with the clip 208 in contact with the presspot 224, after the ring 222 has been pushed back by hand. The diamond can then be taken out of the chuck 186 and re-set for polishing the facets on the opposite side in an analogous manner.

The machine can be utilised to facilitate the sensitive setting up of the axis of the grain shaft 128 at 900 to the scaife face 46.

An arm having a scaife contacting roller at one end and a diameter suitable for clamping in the facet head collet chuck 186 (see Figure 18) at the other end may be inserted in the aforementioned chuck and locked in place. By adjustment of nut 68 (Figure 2), the grain head 110 and hence the facet head 116 may be made to approach the face of a stationary scaife 46. When the roller on the arm contacts the scaife, an output from the polishing force transducer 156 will be obtained which is directly proportional to the vertical position of the grain shaft 128. The grain shaft may then be set to rotate continuously and the output from the polishing force transducer displayed in analogue form on a suitable moving coil meter on the control panel.If the grain shaft is not at 900 to the scaife face, the reading on the meter will change over a 3600 rotation.

By adjustment of both the base plate levelling means 56 and the thimble micrometer 102 the angle of the grain shaft can be altered until a null reading is obtained from the panel meter indicating that the grain shaft is accurately at 900 to the scaife face. Thus "trial and error" levelling and use of the levelling cone are obviated and the whole setting up procedure is simplified. This piece of equipment may also usefully be used to assess the flatness and run-out of the scaife itself at any time.

The motors may be easily controlled by the flexible system to incorporate programmed speed control and braking arrangements to reduce overshoot and system cycle time at the same time as maintaining accuracy of control.

With reference to Figure 23, instead of the micrometer adjustment device 200 there is provided a digital angle transducer 300 having a mask 302 on an exteriorthimble part 304 and a light emitting diode 306 and sensor 307 on a sleeve fixed to mounting part 198. The thimble 304 is connected by means of a lead screw to a pin 308. The facet angle is thus adjustable manually as before but a digital read out of facet angle can be obtained to facilitate adjustment. It also becomes possible to motorisethe thimble 304 so as to permit the facet angle to be adjusted automatically.

With reference to Figures 24 and 25, instead of the rotatable cam 16, a lever-lead screw arrangement may be used. The roller 72 rests, in operation, on a non-rotatable liftable saddle 400 which in turn bears on a short arm of a lever402 pivoted on shaft 404.

The long arm ofthe lever402 is urged upwards by the residual downward force exerted by the smoothing arm assembly and is located by the end of a lead screw 406 controlled by a rotatable member 408 by means of a worm gear 410 driven by an electric motor411.

A mask 412 is rotatable conjointly with the member 408 and provides a signal of the lead screw axial position through a light emitting diode-sensor arrangement 414 on a fixed part of the assembly.

The leadscrew 406 is constrained from rotation by a crosspin 416 which is free to slide in a slot 418 in the leadscrew housing 420. The lever402 provides a 5:1 reduction in movement so that the mask 412 and arrangement 414 enable the smoothing arm assem blyto be lowered in very small accurately determined increments. The motor410 may be returned rapidly, especially if a low final polishing force level is used, so making a separate solenoid driven lift mechanism unnecessary.

The smoothing arm shaft may be arranged radially with respect to the scaife and perform a smoothing action by sliding to and fro in a radial direction. Con venientlythen the drag force transducer is arranged to measure the force transversely of the smoothing arm shaft exerted by the scaife on the grain head.

The transducers may be of any suitable type and include transducers using pneumatic circuits to exert a desired polishing force without appreciably altering the grain head/grain shaft position.

The position masks may be carbon potentiometers or Moire-fringe type transducers.

The range of stones which can be polished is widened. Stones having no predictable crystalline structure (including so-called makeables) can be polished. The operator can use the machine without much knowledge of the crystalline structure of stones.

The speed of polishing can be increased as favourable grain shaft angles and cutting forces can be selected. The repeatability of facets can be improved as the lift mechanism acts promptly without appreciable inertia.

Scaife life can be increased as the diamond approaches the scaife slowly and the polishing force is only applied gradually.

Using suitable aids levelling and setting up can be performed more easily.

Overall the machine enables the polishing operation to be more accurately supervised using electric signal outputs which can be interfaced conveniently with a processor performing the required functions so as to provide a flexible control system requiring little attention from an operator.

Claims (25)

1. A gem stone polishing machine including a facet head for mounting a gem stone, a rotatable scaife for polishing a gem stone, a grain shaft for orientating the gem stone for polishing in a desired direction by the scaife, a grain shaft angle transducer arrangement for providing a signal dependent on different rotational positions of the grain shaft, a drive for rotating the grain shaft, a means for providing a signal dependent on drag between the gem stone and the scaive thereby providing a signal dependent on polishing efficiency and enabling the grain shaft to be rotated so as to provide efficient polishing.

2. A machine according to claim 1 further including a feed means for moving the facet head and scaive towards one another.

3. A machine according to claim 2 further including a means for providing a signal dependent on the rate of feed to thereby enable the efficiency of polishing to be confirmed or another polishing direction to be adopted.

4. A machine according to claim 2 or claim 3 further including a means for urging the gem stone and scaive towards one another at a polishing force and a means for providing a signal dependent on the polishing force to thereby enable the polishing force to be set at a desired level or levels.

5. A machine according to any of the preceding claims in which the facet head is mounted on the grain shaft and a smoothing arm cantilevered from a pillar locates a grain housing mounting the grain shaft over a scaive, the grain shaft being rotatable in the grain housing to enable the grain shaft to be rotated.

6. A machine according to claim 5 in which the grain housing is slidable longitudinally with respect to the smoothing arm and the means for providing a drag signal includes a means for resiliently biasing the smoothing arm and grain housing towards one another and a transducer for providing a signal dependent on their relative displacement against the resilient means bias.

7. A machine according to claim 6 in which the resilient means and drag transducer are in the form of a cantilever beam carrying a strain gauge.

8. A machine according to claim 7 in which four strain gauges are mounted two on each side of the beam to measure simultaneously in extension and compression and are connected in a Wheatstonebridge circuit.

9. A machine according to any of the preceding claims 5 to 8 in which the smoothing arm and grain housing are connected by a shaft mounted in rotary linear bearings to permit both relative displacement of the smoothing and grain housing longitudinally and angular displacement to obtain access to the facet head.

10. A machine according to claim 7 and claim 9 in which the shaft extends through the grain housing to one side thereof, the cantilever beam is mounted on said one side and means are provided for adjusting the position of one end of the cantilever beam in an axial direction of the shaft.

11. A machine according to any of claims 5 to 10 in which the smoothing arm is movable on an upright axis with respect to the pillar and a member supports the smoothing arm from below, the member being movable to enable the facet head and scaive to be fed towards one another.

12. A machine according to claim 11 in which the smoothing arm and pillar are mounted with respect to one another by a pillar shaft and rotary-linear bearings.

13. A machine according to claim 11 or claim 12 in which the member is a rotatable cam and a cam angle transducer is provided to provide a signal dependent on the rate of feed.

14. A machine according to claim 11 in which the member is a lever, a lead screw bears against the lever to locate the smoothing arm and a lead screw rotation transducer is provided to provide a signal dependent on the rate of feed.

15. A machine according to claim 13 or claim 14 further including a lift mechanism for raising the facet head quickly on completion of the polishing of a facet.

16. A machine according to claim 15 in which the lift mechanism includes a solenoid arranged to lift the facet head with respect to the smoothing arm.

17. A machine according to any of the preceding claims 11 to 16 in which a means is provided for resiliently biasing the facet head towards the scaive and a transducer is provided for providing a signal dependent on vertical facet head displacement for providing a signal dependent on polishing force and enables the member to control the polishing force.

18. A machine according to claim 17 in which the grain shaft is mounted in rotary linear bearings to permit axial and angular movement, and the biassing means is a cantilever beam acting in the grain shaft to urge the facet head towards the scaife and carrying a strain gauge to provide the polishing force signal.

19. A machine according to claim 18 in which the cantilever beam is adjustable to adjust beam deflection.

20. A machine according to any of claims 11 to 19 in which a spring means is arranged to partly counterbalance the weight of the smoothing arm which is slidable and angularly movable by rotary linear bearings with respect to the pillar.

21. A machine according to claim 1 in which the spring means is adjustable.

22. A machine substantially as shown and described with reference to Figures 1 to 22.

23. A machine substantially as shown and described with reference to Figures 1 to 22 and modified by Figure 23.

24. Agem stone polishing machine including a facet head for mounting a gem stone, a rotatable scaife for polishing a gem stone, a grain shaft for orientating the gem stone for polishing in a desired direction by the scaife, a grain shaft angle transducer arrangement for providing a signal dependent on different rotational positions of the grain shaft, a drive for rotating the grain shaft, feed means for moving the facet head and the scaife towards one another, means for providing a signal dependent on the rate of feed to thereby provide a signal dependent on polishing efficiency and enable the grain shaft to be rotated so as to provide efficient polishing.

25. A machine according to claim 24 further including a means for urging the gem stone and the scaife towards one another at a polishing force, and a means for providing a signal dependent on the polishing force to thereby enable the polishing force to be set at a desired level or levels.