Abrasive device

Abstract: The abrasive polishing device has a carrier, typically in the form of a rotatable polishing head (10) and abrasive polishing pads (16) mounted on the carrier. Each polishing pad (16) includes an abrasive body (18) which is provided by a thermoplastic polymer impregnated with ultra-hard abrasive particles and which presents an abrasive polishing surface for performing an abrasive polishing action in use. The abrasive body (18) is formed with a regular array of recesses, typically narrow capillary-type passages therein which extend to the abrasive surface. The recesses result in improved cooling of the abrasive layer during a polishing operation.

Description: BACKGROUND TO THE INVENTION THIS invention relates to abrasive polishing devices. Conventionally, polishing of materials such as granite and marble is achieved using a polishing apparatus that has a rotating polishing head on which a number of polishing pads, typically with wear surfaces of silicon carbide, are mounted. The problem with the conventional polishing apparatuses of this kind is that the wear surfaces are rapidly worn down and require frequent replacement. US-A-3 517 466 discloses a rotatable abrasive wheel having means to rotate it, said wheel having a generally circular and substantially rigid base plate fixed directly to the rotating means, an elastomer pad conforming generally to the outlines of said base plate and having one face affixed thereto, said elastomer pad having affixed to the opposite face thereof a flexible disk member conforming generally to the outline of said base plate and said pad, said flexible disk member having dispersed over the working face thereof a series of cylindrical cavities of substantially equal depth, said cavities having disposed therein a generally cylindrical abrasive member having an abrasive dispersed over the operating face thereof, said cylinders protruding, respectively, below the working face of said flexible disk, substantially equal distances. WO-A-9 205 014 discloses an abrasive tool, such as a grinding wheel or a saw, comprising a support and a working portion secured to the support, the working portion comprising a mass of ultra-hard abrasive particles dispersed in a non-porous thermoplastic polymer matrix, the abrasive particle content of the working portion being at least 4 volume percent. SU-A-984852 discloses a diamond tool for cutting concrete comprising a carbide body with cavities for diamond abrasive grains held by a bond. The cavities consist of holes with square cross-section, parallel slots of rectangular cross-section, or intersecting slots. Patent Abstracts of Japan, Vol. 11, No. 188 (M-599)(2635) discloses a grinding tool comprising a metal base member carrying a ringlike abrasive body presenting an abrasive polishing surface for performing an abrasive polishing action in use. The abrasive body is formed with recesses therein which extend to the abrasive polishing surface. SUMMARY OF THE INVENTION The invention is set forth in the Claims. The ultra-hard material will typically comprise diamond or cubic boron nitride particles. The thermoplastic polymer is preferably selected from one or more of the following polymers: Polyetheretherketone (PEEK) such as that marketed by ICI under the trade name VICTREX®. Poly (amide-imide) such as that marketed by Amoco under the trade name TORLON®. Polyphenylene sulphide (PPS) such as that marketed by Phillips under the trade name RYTON®. Liquid crystal polymer (LCP) such as that marketed by Hoechst under the trade name VECTRA®. In a case where the ultra-hard particles are diamond particles, the particles will usually have a size in the range 2 µm (micron) to 300 µm (micron). Also, the particles will usually be present in the abrasive body in an amount of 3% to 30%, preferably 3% to 10%, by volume. The recesses can be in the form of narrow capillary passages extending perpendicularly to the polishing surface. The passages will typically be round in cross-section with a diameter of approximately 50 µm (micron). In the preferred application, the carrier is in the form of a rotatable polishing head and a plurality of abrasive polishing pads is mounted on the polishing head. The abrasive body is in the form of an abrasive layer mounted on a base, and the base is also made of a thermoplastic polymer. The abrasive layer and the base may have complemental, interengaged projections and recesses that secure the layer to the base. Alternatively, the abrasive layer may be attached to the base by an overmoulding process. Either or both of the abrasive body and the base can incorporate a colourant which identifies the abrasive capacity of the ultra-hard abrasive particles. Another aspect of the present invention provides a polishing pad in accordance with Claim 14. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows an axial view of an abrasive device; and Figure 2 shows an enlarged cross-section at the line 2-2 in Figure 1. DESCRIPTION OF EMBODIMENTS The illustrated abrasive device is a polishing apparatus which is used to polish a surface of a body of material such as granite or marble. The polishing apparatus includes a polishing head 10 in the form of a circular steel plate 12. The plate 12 is mounted on a central, rotatable shaft 14. A number of polishing pads 16 are secured to the surface of the plate 12. Each polishing pad 16 consists of an abrasive body in the form of an abrasive layer 18 mounted on a base 20. The abrasive layer 18 is provided by a suitable thermoplastic polymer, typically PEEK, impregnated with ultra-hard abrasive particles. The particles will usually be diamond or cubic boron nitride particles. The abrasive layer 18 is formed with a series of projections 22 extending from the surface remote from the polishing surface 24. Each base 20 is also made of a thermoplastic polymer, which will in most cases be different from that used in the layer 18. The base is formed with a series of recesses 26 complemental in shape and position to the projections 22 of the layer 18. In practice, the layer 18 is secured to the base 20 by an interference fit of the projections in the recesses, by thermal bonding of the projections into the recesses, or by ultrasonic welding of the projections in the recesses. In a typical case, the pads 16 have a thickness of between 5mm and 20mm. They may be fixed to the surface of the plate 12 in any conventional manner. As illustrated, the abrasive layer 18 is in each case formed with a regular array of recesses communicating with the polishing surface 24. In the illustrated embodiment, these recesses are in the form of narrow capillary passages 28 that extend for the full thickness of the layer 18 but which are nevertheless blind because of the presence of the base. The passages are generally circular in cross-section and it will be noted that they extend perpendicularly to the polishing surface 24. In a typical case, the passages have a diameter of around 50 µm (micron). In practice, the polishing head 10 is rotated and pressed against a surface which is to be polished by abrasive action. The polishing action is performed by the abrasive layers 18, which will of course wear down with use. However, given that the layers 18 have a fairly substantial thickness, it is not considered necessary to align the polishing surfaces 24 with one another very accurately at the outset. Should some of the polishing surfaces 24 initially protrude further from the polishing head than others, those surfaces will wear down preferentially, at a rapid rate, until all the surfaces are level, i.e. until the polishing head is properly "bedded in". The presence of the capillary passages 28 is considered to be advantageous for the reason that they can promote greater freedom in the abrasive cutting action performed by the abrasive particles. Furthermore the passages allow the coolant which is applied to the polishing zone during polishing to gain access to internal regions of the layer 18 and thereby provide an enhanced cooling function. According to a preferred feature of the invention, the polymer material of the layer 18, and possibly also that used in the base 20, can incorporate a visible colourant. The purpose of the colourant is to identify the abrasive capacity of the polishing pad 16, and thereby to enable consumers to select the appropriate pads for a particular job without difficulty. In a case in which the abrasive layer 18 incorporates diamond particles, the particles will typically have a size in the range 2 µm (micron) to 300 µm (micron) and will occupy 3% to 30% and preferably 3% to 10% by volume of the layer. The results of two series of tests which have been carried out with polishing pads according to the invention are set out below. Test 1 Polishing pads according to the invention where made up with the following specification for use in an automated, stagewise polishing apparatus employed to polish granite samples in Germany. PAD NO. ABRASIVE GRIT GRIT GRADE CONCENTRATION 1 De Beers Diagloss (Trade Mark) Medium 25 2 De Beers Diagloss (Trade Mark) Fine 20 3 De Beers Diagloss (Trade Mark) Ultra Fine 15 Medium grade diamond grit typically has a diamond particle size of about 90 µm (micron), fine grit a diamond particle size of about 60 µm (micron) and ultra fine grit a diamond particle size of about 5 µm (micron). The "concentration" values given in the above table are in accordance with normal usage of the term "concentration" as used in the abrasives industry. In practice, a concentration of 4,4 carats/cm 3 corresponds to a concentration value of 100. A concentration value of 25 corresponds to a value of 1,1 carats/cm 3 . Stated differently, the concentration values of 25, 20 and 15 seen in the above table correspond to values of 6,25%, 5% and 3,75% by volume. In polishing mixed types of granite, the polishing pads achieved lives in excess of 2000m 2 . Typical polishing times and resulting granite surface conditions are given in the following table. PAD NO. POLISHING TIME (Mins.) GLOSS VALUE 1 10 20 2 10 26 3 9 48 It was noted that these results are, in terms of tool life or polishing cost, far superior to those obtainable using conventional abrasives such as silicon carbide. It was also noted that higher gloss values were achievable when the polishing pads were used on black granite and fine grain granite than on coarser grades of granite. Test 2 A series of DIAGLOSS (trade mark) impregnated polymer polishing pads were made up for use in a manual, as opposed to automatic, granite polishing apparatus. The polishing pads that were made up included grit ranging from extra coarse (corresponding to a diamond particle size of about 190 µm (micron)) at a concentration value of 35 (corresponding to a value of 8,75% by volume), used for the roughing stage, to ultra fine (corresponding to a diamond particle size of 5 µm (micron)) at a concentration value of 12 (corresponding to a value of 3% by volume), used for final polishing. The pads were used to polish granite samples in India. Polishing rates up to 50% faster than the rates achieved for conventional abrasives were observed. Extended pad lives ranging from 450m 2 during the roughing stages to 600m 2 during the final polishing stages were achieved, accompanied by a more consistent polish. The pad life exceeded expectations and was far greater than experienced for conventional abrasive pads. It is believed that the reason why the results of Test 1 are superior to those of Test 2 lies in the difference between the polishing processes used.

Claims: An abrasive polishing device comprising a carrier (10, 12) and a plurality of abrasive polishing pads (16) mounted on the carrier in spaced relationship, each abrasive polishing pad including an abrasive body (18) presenting an abrasive polishing surface (24) for performing an abrasive polishing action in use, characterised in that the abrasive body (18) of each polishing pad comprises a thermoplastic polymer impregnated therethrough with ultra-hard abrasive particles, and in that the abrasive body (18) of each polishing pad (16) is formed with a regular array of recesses (28) therein which extend to the abrasive polishing surface (24). An abrasive polishing apparatus according to claim 1 characterised in that the ultra-hard material comprises diamond or cubic boron nitride particles. An abrasive polishing apparatus according to claim 2 characterised in that the thermoplastic polymer is selected from PEEK, poly(amide-imide), polyphenylene sulphide and liquid crystal polymer. An abrasive polishing apparatus according to claim 3 characterised in that the ultra-hard particles are diamond particles with a size in the range 2 µm (micron)to 300 µm (micron). An abrasive polishing apparatus according to claim 4 characterised in that the diamond particles are present in the abrasive body (18) of each polishing pad (16) in an amount of 3% to 30% by volume. An abrasive polishing apparatus according to claim 5 characterised in that the diamond particles are present in the abrasive body (18) of each polishing pad (16) in an amount of 3% to 10% by volume. An abrasive polishing apparatus according to any one of the preceding claims characterised in that the recesses are in the form of narrow passages (28) extending perpendicularly to the polishing surfaces (24) of the abrasive body (18). An abrasive polishing apparatus according to claim 7 characterised in that the passages (28) are round cross-section capillary passages with a diameter of approximately 50 µm (micron). An abrasive polishing apparatus according to any one of the preceding claims characterised in that the carrier is a rotatable polishing head (10). An abrasive polishing apparatus according to any one of the preceding claims characterised in that the abrasive body of each polishing pad is in the form of an abrasive layer (18) mounted on a base (20). An abrasive polishing apparatus according to claim 10 characterised in that the base (20) is made of a thermoplastic polymer. An abrasive polishing apparatus according to claim 10 or claim 11 characterised in that the layer (18) and the base (20) have complemental, interengaged projections (22) and recesses (26) that secure the layer (18) to the base (20). An abrasive polishing apparatus according to any one of claims 10 to 12 characterised in that either or both of the abrasive layer (18) and the base (20) incorporate a colourant which identifies the abrasive capacity of the ultra-hard abrasive particles. A polishing pad (16) which is adapted to be mounted on a rotatable polishing head in spaced relationship to other polishing pads mounted on the same rotatable polishing head and which includes an abrasive body (18) presenting a polishing surface (24), characterised in that: the polishing pad has an elongate, symmetrical shape, the abrasive body is provided by a thermoplastic polymer impregnated therethrough with ultra-hard particles, the abrasive body presents a polishing surface (24) having a single, outer periphery, the polishing pad includes a base (20) on which the abrasive body is mounted, and the abrasive body includes a regular array of recesses (28) therein which extend to the polishing surface.

Inventors: SEXTON JOHN STIRLING, WRIGHT DEREK NORMAN, SEXTON, JOHN STIRLING, WRIGHT, DEREK NORMAN
Applicants: DE BEERS IND DIAMOND, DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1993
Main classification: B24D7
IPC codes: B24D3, B24D7, B24D11, B24D9