GB2074480A - Working gemstones - Google Patents

Abstract

In order to provide a simple way of sensing the diameter or radius of a rotating gemstone while it is being worked, or the variation between the maximum and minimum diameter or radius, an image or shadow of at least one side of the rotating stone 1 is projected at right angles to the axis of rotation, onto a sensor 7 which senses any change in position of the edge of the image or shadow as the stone 1 rotates. <IMAGE>

Description

SPECIFICATION Working gemstones This invention relates to working a rotating gemstone, particularly to bruting, which is the grinding of the girdle of a rough gemstone priorto polishing the stone. Normally, the stone is rotated and abraded by a stationary tool (which in the case of many gemstones will normally be another gemstone) or by another rotating gemstone. It is important to cease bruting at the correct moment, to save consumption of valuable material. The standard way of knowing when bruting is completed is to examine the girdle by eye-when bruting is nearly finished, most of the girdle would be matt but small zones of the original, shiny surface remain, called nijven, and one normally continues until the nijven have nearly, though not completely, disappeared.With modern high speed machines, for instance as described in British Patent Specification No. 2 018 173, it is relatively time consuming to stop the machine and examine the stone, but it is nonetheless important to avoid over-bruting. Another similar operation is coning.

The invention provides methods as set forth in Claims 1 and 14, apparatus as set forth in Claims 7 and 15, and a machine as set forth in Claim 16. The remaining Claims set forth preferred features of the invention.

The invention therefore furnishes a simple method of determining when the working operation has been completed, and which can also be used for other purposes, and indeed the method can be used more generally in association with any workpiece which is being worked while being rotated at high speed.

The method can be used in various ways, among which can be mentioned: a) measuring the roundness of a gemstone by giving a signal representative of the difference between the maximum and minimum radii or diameters, for example either giving a reading of the absolute difference or giving a signal when the difference falls to a predetermined value; b) measuring the maximum radius or diameter, for example by giving a reading of the absolute value or by giving a signal when the maximum falls to a predetermined value; c) acting as a line monitor, to stop working if the stone falls off or chips or shifts on the dop or holder; d) acting as a quality control for the roundness of for example hand-bruted stones or even finished or polished stones.

The apparatus can give a reading of the absolute value of the parameter being measured, but preferably either stops working automatically or gives an audible or visible signal for an operator to stop the machine. In the latter case, accurate working can be achieved to predetermined parameters, e.g. to radius or diameter measurements which are accu rate to +3,u(, 3( = 10-' m). For instance, the apparatus can be programmed or set to stop bruting at any value between 1% and 41% non-roundness (difference between maximum and minimum radii or diameters as percentage of the minimum radius or diameter), say 1% or 5%. The apparatus can be incorporated in a complete system in which the bruted radius or diameter, or the difference, is predetermined by an iiiitial inspection of the stone and is input into the apparatus.

In general, it is preferred thatthe image or shadow of both sides of the stone be projected onto the sensor and that circuitry associated with the sensor cancels out equal movements of both sides of the image or shadow. In this way, machine vibrations are cancelled out.

The image or shadow can be of any suitable type, but is preferably a bright-field image. A bright-field image is produced by having a light source behind the stone and a focusing system, and a broad light source can be used. It is found that a broad light source is betterthan a point source as better edge values are achieved. Shadow projection can be achieved using a point source and no image-forming optics. The light source can be coherent or noncoherent, but for simplicity a non-coherent source, e.g. a tungsten lamp, can be used.

If the sensor is square, e.g. is a square array of photodiodes, the apparatus could be used to control the stroke in the bruting machine of the Specification referred to above; if the stones are not in contact, there is a gap through which light can pass, and this gap can be sensed, causing the stroke to be automatically controlled. For instance, a 400 x 300 array of photodiodes could be used (in this sense, "square" also means "rectangular").

Preferably, where bright-field illumination is used, and in general, it is preferred to project the image or shadow upwards. The presence of gemstone dust gives a problem, but it is easier to blow the dust (say continuously) off the projecting equipment than off the more delicate sensor and e.g. focusing optics.

The accompanying drawing is a schematic view of apparatus in accordance with the invention.

The drawing shows a diamond 1 which is rotating at high speed and having its girdle 2 bruted, the diamond being mounted on a dop 3. Below the diamond 1,there is a broad light source 4 and a condensor 5. Above the diamond 1 there is an imaging lens 6 and a linear photodiode array 7 which is connected to an electronics board 8 which is not rigidly connected to the array 7, in turn connected to a relay 9 for stopping the bruting machine (not shown).

The photodiode array 7 extends at right angles to the sheet of paper, and a suitable array may have 1728 individual photodiodes each 13,a x 13cm; there is a second series of dark photodiodes for compensation and serial output shift registers for both series.

The condensor 5 focuses an image of the source 4 at the centre of the imaging lens 6, giving the most efficient lighting with a small residual coherence.

The imaging lens 6 focuses a bright-field image of the diamond 1 on the array 7. It will be seen that the optical axis of the system is at right angles to the axis of rotation of the diamond 1.

In a preferred arrangement, the imaging lens 6 has a focal length of 75mm and the distances a, b andc are 300mm, 100mm and 100mm, respectively. This gives 3 x magnification of the diamond 1 atthe array 7, i.e. with 13,u spacing of the diodes, one can achieve 4.2cm resolution. Using this arrangement, stones of up to 5mm diameter (which are the most common diamonds) can be examined. If the stones are larger, more than one array can be spliced togetheron a beam splitter (forming an extended line detector), say to sense each edge of the stone.

The optical system can be varied as appropriate, according to the distance available, and for instance a compound lens system can be used with a further lens between the imaging lens 6 and the array 7 to give a magnification of say 10 x to 20 x.

The information presented by each diode is a pulse of current proportional to the amount of light incident on the diode. This pulse of current is processed by a design development board included on the electronics board 8, to produce a voltage output, proportional to light. The design development board produces the necessary clock, scan and power wave forms that are required by the array 7. A video processor element is included in order to convert the current output pulses to voltage and also to remove clock feed-through signals.

Also included on the electronics board 8 is a high speed comparator with hysteresis to effect data reduction and digitisation by converting the video wave form to a TTL compatible signal. This and other clock and synchronization signals are presented to a pair of counter and latch networks that enable width and relative position information regarding the image of the diamond 1 to be extracted. The extraction information forms a 12 bit data highway and via handshake controls are fed to a Motorola microprocessor system. The processor is used to make as many measurements of diameter that are possible in one synchronized revolution of the bruting machine spindle. For instance, with a speed of 3000 rpm, and a 7-10 megacycle clock rate, it is possible to programme to take 36 measurements in each revolution, i.e. at every 10 degrees, and this is satisfactory for bruting control.When all measurements are made, the processor makes a statistical analysis of width, i.e. mean diameter, standard deviation, maximum and minimum and total number of measurements.

In general terms, the output can be an indicated absolute value, though the preferred arrangement is to connect the output to a relay 9 which automati callystopsthe bruting machine when the maximum and minimum diameter difference falls to a predetermined value. The bruting machine itself can be arranged to stop the reciprocation when the diamonds are not in contact and to feed them apart.

If the bruting machine is that described in the Specification referred to above, there will be another diamond behind the diamond shown in the drawing, the other diamond reciprocating as it rotates. There will thus be some interference with the projection of the image, and this can be dealt with in various ways. For instance, the image can be sensed only when the other diamond has reciprocated to the end of its stroke; alternatively, the diameter of both stones can be sensed when they are together using the extended line detector referred to above and then the diameter of the single stone 1 when the other one has reciprocated away; or again, the diameter of both stones can be measured continuously and the difference apart of the axes be measured using a linear transducer, for instance a simple capacitance transducerwhich has sufficient accuracy, and the measurement fed into the electronics board 8.

In a second embodiment, the array7 is a square array, analogous optics and electronics being used.

Claims (4)

1. A method ofworking a gemstone using a machine which rotates the stone, comprising projecting an image or shadow of at least one side of the rotating stone in a direction generally at right angles to the axis of rotation, onto a sensor which senses any change in position of the edge of the image or shadow as the stone rotates, the sensor giving a signal which is representative of the radius or diame terofthe stone.

2. The method of Claim 1, wherein the sensor gives a signal which is representative of the difference between the maximum radius and the minimum radius, or maximum diameter and minimum diameter, ofthe stone.

3. The method of Claim 1 or 2, wherein the sensor gives a signal when a predetermined parameter is reached.

4. The method of claim 3 wherein the signal automatically causes the machine to cease working the stone.

Claims4to 16 renumbered 5to 17 respectively.

4. The method of Claim 1 or 2, wherein the sensor gives a signal which is a measurement of an absolute dimension.

5. The method of any one of the preceding Claims, wherein an image or shadow of both sides of the stone are projected onto the sensor and equal motions of both sides of the image or shadow in the same direction are cancelled out

6. The method of any one of the preceding Claims, wherein a bright-field image is projected.

7. Apparatus for use in association with a machine for working a gemstone and which rotates the stone, comprising means for projecting an image or shadow of at least one side of a gemstone rotating in the machine in a direction generally at right angles to the axis of rotation of the gemstone, a sensor upon which the image or shadow will be projected, for sensing any change in position of the edge of the image or shadow as the stone rotates, and means foe giving a signal representative of the radius or diameterofthe gemstone.

8. The apparatus of Claim 7, wherein the project ing means is for projecting a bright-field image.

9. The apparatus of Claim 7 or 8, wherein the sensor includes a photodiode array and the projecting means is for projecting an image or shadow of the stone onto the array.

10. The apparatus of Claim 7, 8 or 9, wherein the image or shadow is projected upwards onto the sensor.

11. The apparatus of Claim 7, 8 or 9, wherein the signal giving means includes means for indicating an absolute value of the radius or diameter of the gemstone.

12. The apparatus of Claim 7,8 or 9, wherein the signal giving means includes means for indicating when the maximum and minimum radius or diameter difference of the gemstone falls to a predetermined value.

13. The apparatus of Claim 12,furthercomprising means responsive to the indicating means for stopping the machine when the maximum and minimum radius or diameter difference falls to the predetermined value.

14. A method of working a gemstone using a machine which rotates the stone, substantially as herein described with reference to, and as shown in, the accompanying drawing.

15. Apparatus for use in association with a machine for working a gemstone and which rotates the stone, substantially as herein described with reference to, and as shown in, the accompanying drawing.

16. A gemstone working machine including means for rotating the stone and apparatus as claimed in any one of Claims 7 to 13 and 15.

New claims or amendments to claims filed on 6-5-81 Superseded claims New or amended claims: