GB2074910A - Working gemstones - Google Patents

Abstract

In order to reduce the length of time taken to work a gemstone, e.g. to brute a gemstone, 1, in an operation in which the stone 1 is rotated and is abraded with a tool 2 which reciprocates relative to the stone 1, one automatically senses when the tool 2 is in or alternatively no longer in contact with the stone 1 and automatically changes the point at which the reciprocatory motion is reversed, thereby controlling and changing the length of stroke of the reciprocatory motion. <IMAGE>

Description

SPECIFICATION Working gemstones Background of the Invention The present invention relates generally to working gemstones, but more particularly, though not exclusively, to bruting or coning gemstones.

Bruting is performed by rotating the stone and abrading it with a tool in order to grind a girdle around the stone. Coning is a similar operation but is used to grind a conical surface on the stone, either on the table side of the girdle or on the other side. Depending on the actual gemstone, the tool can be another gemstone which can also be rotated so that it is worked at the same time. For instance, British Patent Specification No. 2 018 173 describes the mutual bruting of two stones which are rotated about parallel axes while one is reciprocated relative to the other.

Specifically referring to bruting, although the same comments apply more generally, it is normal to reciprocate the tool relative to the stone. The axial length of the part of the stone which contacts the tool increases during bruting, but it is very difficult to program a machine to take account of this increase in length by altering the stroke of reciprocation because the increase in length depends on the precise external shape of the stone. Thus, it is normal practice to have wasted motion at the end of a stroke during which the tool is not in contact with the stone, and this wastes time. In addition, the cross-feed is normally constant for each stroke, throughout the operation.Furthermore, the setting of the machine must be such that the longest possible (in the axial sense) girdle can be ground, for if the stroke is too short, an uneven girdle is machined which can in turn lead to chipping of the stone and failure of the cement holding the stone; so it is normal practice to have an excess stroke, even at the end of bruting. Particular difficulties are caused when the mutual bruting of two stones is being carried out because one stone may be nearly finished whilst the other is just starting.

The Invention The invention provides methods as set forth in Claims 1 and 1 1 and apparatus as set forth in Claims 6 and 12. The remaining Claims set forth preferred features of the invention.

Thus using the method of the invention, the working stroke is automatically reversed very shortly after the tool (e.g. another rotating stone) leaves contact with the stone, and the new working stroke begins, this saving waste motion and speeding up the whole operation (it is believed that working time can be reduced by up to two thirds). In operations such as coning and bruting, it is normally found that the stroke length will increase as the operation proceeds; the speed of cross-feed depends upon the amount of material that is removed in the respective stroke, and it has been realised that more material is removed during a long stroke than during a short stroke, so that in effect sensing the length of stroke enables the cross-feed rate to be dependent upon the stroke length at that time and to increase as the stroke length increases, thereby increasing the speed of operation.If the stroke length increases as the operation proceeds, to avoid too difficult setting at the beginning, the initial part of the operation can proceed with a fixed minimum-length stroke, the stroke length being increased automatically when the workingstroke length approaches to within a predetermined distance of the pre-set fixed length, a new stroke-length being set.

Often, the holder or dop on which the stone is set itself provides a reference face for one end of the stroke, and thus the automatic stroke reversal need only apply to the other end in such circumstances.

Using the invention, the working machine can be set up for a whole batch of graded diamonds, i.e. diamonds of roughly the same size, and in the case of bruting for instance, the speed of rotation and the cross-feed can be pre-set for the batch, leaving the automatic stroke control to take account of individual differences.

The sensing can be achieved by monitoring the rapid impacts caused by abrasion of the tool against the stone. This can provide a rapid response and background noise can be discriminated against.

Description of Preferred Embodiments The invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic view, showing two gemstones being bruted by mutual contact in a bruting machine generally as in the Specification referred to above; Figure 2 is a graph of distance (d) against time (t) for the reciprocatory motion, the curve only being drawn schematically; and Figure 3 is a schematic view of a second apparatus.

Figure 1 shows two diamonds 1, 2 which have been cemented onto dops 3, 4 in a conventional manner. The dops 3, 4 are held in holders (not shown) which are in turn held in chucks. As described in more detail in the Specification referred to above, both stones are mutually bruted, and a dop 4 is reciprocated parallel to its axis.

The rest of the apparatus of Figure 1 is only indicated very schematically. The chucks holding the dops 3, 4 are rotated by drives (e.g. electric motors) 5, 6 and for conveniency of representation, the drive 5 can be considered as being mounted on a cross slide for movement at right angles to the axis of rotation while the drive 6 can be considered as being mounted on a slide for movement parallel to the axis of rotation - in practice, both these movements can be effected by one of the drives. For cross-feed, the drive 5 has a nut 7 engaging a spindle 8 driven by a drive 9. For reciprocation, the drive 6 has a nut 10 engaging a spindle 11 driven by a fast-reversing drive 12.

For control of the apparatus, there is a thrust transducer in the form of a washer 13 contacting the nut 7 and there is a controller 14.

In general, the washer 13 and the controller 14 act as automatic sensing means for sensing when the diamonds 1, 2 are in contact; the washer 13 senses when the stones 1, 2 are no longer in contact in effect, by monitoring impacts due to the abrasion of the diamonds 1, 2 against each other.

However, in a general sense, a transducer can be used to measure force, impact or acceleration and the transducer can be in any suitable position. The controller 14 is arranged such that it responds to sudden, abrupt changes in signal from the transducer and discriminates against background noise. There is also mechanical stroke limitation, though the length of stroke will be arranged to be greater than that expected to be required. The height of the stone girdle at any stage of bruting is not uniform and, except near the completion of bruting, the cross-section of the stone is not circular; furthermore, material is removed essentially by chipping, though on a fine scale.

This means that disappearance of a contact signal in any one stroke does not necessarily indicate the point at which the stroke direction should be reversed. In practice, the disappearance of the signal is monitored over a number of stroke cycles and the controller 14 then resets the stroke length to the largest value indicated. The controller 14 is also arranged so that if working does not recommence, reciprocation continues for say 50 cycles and then the bruting machine is stopped, in case one of the diamonds 1, 2 has fallen off or has been badly chipped. The controller 14 has the further function of controlling the drive 9 to increase the rate of cross-feed as the stroke length increases. The controller 14 can be a standard micro-processor.

The drive 12 can be an electric stepping motor, say stepping at 2,000 steps per second, and instead of the lead screw arrangement shown, a cam arrangement, for instance an equiangular cam, can be used.

In general, the end faces of the dops 3, 4 can be used as reference surfaces to that the controller 14 can be arranged to automatically reverse the stroke when the end surface of the dop 4 has passed slightly to the right of the end surface of the dop 3, when the diamonds 1, 2 will no longer be in contact. However, this is not essential, and this reversal can also be carried out automatically using the transducer.

The controller 14 can be arranged so that the automatic stroke control does not begin immediately. It is found to be an advantage to set an initial stroke length, which will be long enough for all the diamonds in that batch.

An alternative would be to set an initial stroke length which is somewhat greater than that expected, letting the controller 14 begin automatic stroke limitation as soon as the first cycle is completed.

Bruting proceeds as follows: A new dop 3 or 4 is placed in position (there will normally be one part-bruted diamond in the machine).

The drive 9 executes fast feed and the drive 12 begins reciprocation.

Immediately prior to the beginning of working, the drive 9 is switched over to normal feed.

The drive 12 executes a number of fixed length strokes until time t1 - this is schematically indicated in Figure 2 as being after only three strokes, but it will be after many more.

At t1, the stroke length detected is a predetermined distance short of the fixed length stroke, and automatic stroke limitation will take over. As the girdle is bruted, its axial length will increase and the stroke length will increase, as indicated schematically in Figure 2, and the crossfeed rate will also increase.

When bruting has been completed, which can be sensed as described in a Patent Application No. 8013407 filed the same day as the present Application under the Marks Er Clerk Folio 39145 (a copy of which is filed herewith), the drive 12 is stopped with the stone 2 in its extreme right-hand position and the drive 9 executes fast reverse speed.

Figure 3 shows a different way of sensing when the stones 1, 2 are in or not in contact with one another, and is similar to the arrangement described in the Patent Application referred to above. For convenience, the second stone 2 and stop 4 are not shown, but they may rotate about an axis behind the axis of the dop 3. A broad light source 21 and a condenser 22, together with an imaging lens 23, project a bright-light image of the zone of contact between the stones 1, 2 onto a linear photodiode array 24 which is shown as connected through an electronics board 25 to a controller 14. The photodiode array 24 may be a simple linear array extending parallel to the axes of the dops 3, 4 though the preferred arrangement is to have a square (i.e. geometrically square or rectangular) array of photodiodes for also sensing the outside edges of the stones 1,2 and thus controlling both stroke and diameter. More specific details of the arrangement are given in the Patent Application referred to above.

Claims (12)

1. A method of working a gemstone, comprising rotating a stone and abrading it with a tool which reciprocates relative to the stone; and automatically sensing when the tool is in or alternatively is not in contact with the stone and dependent on such sensing automatically changing the pont at which the reciprocatory motion is reversed, thereby automatically changing the length of stroke of the reciprocatory motion.

2. The method of Claim 1, wherein the automatic sensing is effected by detecting when the tool is in contact with the stone.

3. The method of Claim 1 or 2, wherein the sensing is effected by monitoring impacts due to abrasion of the tool against the stone.

4. The method of Claim 1 or 2, wherein the sensing is effected by projecting an image or shadow of the rotating stone at right angles to a plane containing the axis of the stone and the zone of contact between the stone and the tool, onto a sensor.

5. The method of any one of the preceding Claims, wherein the gemstone is being bruted or coned.

6. Apparatus for working a gemstone, comprising means for holding and rotating a gemstone; means for holding a tool for abrading the stone; means for reciprocating the tool relative to the stone; means for automatically sensing when the tool is in or alternatively is not in contact with the stone; and means, dependant on the sensing means, for automatically changing the point at which the reciprocatory motion is reversed, thereby changing the length of stroke of the reciprocatory motion.

7. Apparatus according to Claim 6, wherein the automatic sensing means detects when the tool is in contact with the stone.

8. The apparatus of Claim 6 or 7, wherein the automatic sensing means includes a transducer for monitoring impacts due to abrasion of the tool against the stone.

9. The apparatus of Claim 6, wherein the sensing means includes a sensor and means for projecting an image or shadow of the rotating stone onto the sensor.

10. The apparatus of any one of Claims 6 to 9, and being a bruting or coning machine.

11. A method of working a gemstone, substantially as herein described with reference to the accompanying drawings.

12. Apparatus for working a gemstone, substantially as herein described with reference to, and as shown in, Figures 1 or Figure 3 of the accompanying drawings.