Method for sizing natural- or synthetic-stone elements, particularly ceramic tiles, and machine for carrying out the method

Abstract: A method for sizing elements made of natural or synthetic stone, particularly ceramic tiles, and optionally also squaring such elements, by means of a dry abrasive tool (4) with wear compensation and single sizing of the element (5) being processed, avoiding the drawbacks of wet grinding and those linked to the complications due to size differences; the machine allows to rotate the abrasive tool (4) and to back it off axially, automatically and continuously in order to measure its wear and compensate for it at preset time intervals; application occurs particularly in ceramic factories for finishing the edges of tiles.

Description: The invention relates to a method and a machine for sizing elements made of natural or synthetic stone, particularly ceramic tiles, as defined in the precharacterising portions of claims 1 and 11 and as such disclosed in DE-A-4136252. The state of the art comprises a method which entails sorting the tiles or product elements in general, before grading, according to at least two sizes (with reference to the dimensions of the format). The sizing operation is performed on both pairs of sides of each element, in two successive steps, by intermediate rotation through 90°, by means of grinding machines with diamondized grinding wheels, installed bilaterally with respect to the transfer line of said elements and, only in the case of porcelainized gres tiles, generally by associating it with the requirement of said material to be mostly dressed on the exposed face, with the consequent compulsory need for water cooling. As regards the production of single-fired tiles, the adoption of sizing in the production would entail the installation of a new plant, owing to the need to use diamondized grinding wheels, since the possibility of dry sizing, using grinding wheels of the carborundum type, must be discarded at the outset due to the rate at which they wear, which entails frequent adjustments of the stroke of the corresponding spindle. The sizing method currently performed has first of all a drawback due to the use of diamondized grinding wheels, since it entails that the product absorbs water, or cooling liquid in general, and that therefore it is necessary to wait for the product to dry before packaging in order to prevent soaking of the container, which is generally made of paper-like material. Another severe drawback is the need to adopt a plurality of sizes of the product in order to contain the tolerance range within each size class, in order to comply with the planned tolerances in the use of the product. The adoption of a plurality of sizes is disadvantageous, mainly owing to the consequent multiplication of the number of articles caused by dividing each size into various color shades during the grading of the product. For example, for a square tile having a nominal side of 200 mm, two sizes become necessary: one for the range 199-200, the other for the range 200-201. In practice, this multiplication of the articles causes a disadvantageous increase in the number of operations, in the number of packages and in the spaces required: this occurs in the management of the plants, of the store and of sales, with a highly onerous increase in production and service costs. On the other hand, the adoption of sizing and of optional dry squaring is not substantially feasible, owing to the extent and rate of the wear of non-diamondized grinding wheels, for example of the carborundum type, which can be used in dry processes but indeed force frequent stops to adjust the axial position of the spindle. This prior state of the art is susceptible of considerable improvements as regards the possibility of eliminating the above mentioned drawbacks. The above leads to a need to solve the problem of finding a sizing method which allows first of all to avoid dividing the product that has the same nominal dimensions into various sizes and to obtain automatic and continuous sizing. Another aspect of the problem is to reduce the waiting time for packaging, which is necessary to avoid soaking of the containers. Another aspect of the problem is the overcoming of the severe consequences of actions required to cope with the rapid wear of dry grinding wheels, in order to make them conveniently usable in sizing. The invention solves the above problem by the method as claimed in claim 1 and the machine as claimed in claim 11. The dependent claims describe preferred embodiments of the invention. The method is for the dry sizing of natural- or synthetic-stone elements, optionally associated with squaring, eliminating the plurality of sizes; as regards the additional aspect of eliminating the drawbacks due to the rapid wear of the grinding wheels, the method provides for automatically compensating for said wear without any interruption of the process. In particular, the sizing method entails: centering the element which is moving for processing in a first plant section which advantageously has a plurality of consecutive stations, each of which is generally constituted by two mutually opposite motorized grinding wheels, each of which is assigned the task of removing thickness from each edge until finishing is achieved by reaching the preset size; checking said size, related to a first pair of sides, by means of a measurement and comparison apparatus; rotating through 90° the element at the exit of the first plant section, for the alternation of the second pair of sides; centering the element being processed in the second plant section; optionally positioning, for squaring, one of said elements or a plurality of elements, simultaneously with centering, in the second plant section; insertion between the successive pairs of grinding wheels, for sizing the second pair of sides, where necessary; geometric checking of the squaring by measuring the angle or by comparing the length of the diagonals; dimensional checking of the size of the second pair of sides. As to the compensating for the wear of the grinding wheels, which advantageously constitute a pair, the following is preferably provided: at time intervals advantageously preset according to the degree of precision of the size, to the workability characteristics of the element being processed and to the wear characteristics of the grinding wheel, each grinding wheel is backed off to measure the value of, and then compensate for, the wear that has occurred during the previous time interval; the backoff value of each grinding wheel must be enough to clear the control path of a control means bilaterally interposed between the working front and the front of said grinding wheel; the front of each grinding wheel returns to the working front by moving by an extent which is equal to the extent of the backoff, including the value of the grinding thickness of the edge assigned to said grinding wheel, plus the wear that has occurred in the previous time interval; the fraction of workload that has been neutralized in the backoff of one pair of grinding wheels is assigned to the next pair of grinding wheels; the fraction of workload that is neutralized in the backoff of the last pair of grinding wheels can be assigned to the preceding pair; according to a variation of the program, control of compensation of the wear of the last pair of grinding wheels of each plant section can be assigned to the size control section of the corresponding plant section; wear compensation is controlled by reading the size of the element being processed. The advantages achieved by the present invention are: the elimination of the need to resort to more than one size, with a very significant simplification in terms of processing, storage, administration and transport and a consequent considerable cost reduction; the possibility to use dry sizing and squaring, eliminating the waiting times and the associated costs for the drying of the items before placing them in the packages; the production -- by means of the squaring and of the subsequent checks -- of elements which are geometrically precise and particularly adapted for laying without gaps; the elimination of wet sizing and squaring, with the consequent elimination of the waiting times for the drying of the element being processed; the possibility of performing processing directly at the exit of the kiln, i.e., online, differently from what occurs in wet sizing and optional squaring, which must be performed in an adapted environment, separately from the production line: therefore, a further cost reduction; the possibility to work both with glazed products and with non-glazed products, consequently expanding the product range; higher productivity. Some embodiments of the invention are illustrated, merely by way of non-limitative example, in the accompanying five drawings, wherein: figure 1 is a partially sectional side view of the moving element of the machine according to the invention, installed so that it can slide axially in a direction which is normal to the direction of the line and guided on a fixed support, in the backed-off inactive position; figure 2 is a view, similar to figure 1, with the moving element in the forward active position; figures 3 and 4 are views of a similar machine, but on a support which oscillates longitudinally between the two end positions; figure 5 is a plan view of a generic sizing and squaring plant, with the individual sizing machines in the active position; figure 5a is a schematic plan view, illustrating the presetting of the device for compensating the wear of the element sizing grinding wheel; figure 6 is a plan view, similar to figure 5, which highlights the difference between the front working positions of each grinding wheel in order to illustrate them more clearly; figure 7 is an enlarged-scale perspective view of an element being sized in a plant section; figure 8 is a block diagram of a sizing and squaring plant according to the invention. The reference numeral 1 designates a motor, for example an electric one, from a spindle 2 of which a shaft 3 protrudes. The shaft 3 supports a grinding wheel 4 or abrasive tool for dry working, made for example of carborundum, which is adapted to grind the edge of an element 5 being processed and to perform any squaring. The reference numeral 6 designates a sensor, for example of the fiber-optic photocell type, which is arranged transversely and bilaterally in a position which is proximate to the edge of the element to be ground and at a distance from said ground edge. The reference signs S1, S2, S3, ... SN designate the thicknesses of the edge of an element being processed that each grinding wheel must remove for sizing, where S1 is equal to zero or greater than zero (figure 5a). The reference numeral 7 designates at least one sleeve which supports, so that it can slide in a longitudinally guided manner, the motor 1 on at least one guide 8. The reference numeral 9 designates a linear actuator, for example of the step type, which actuates the movement of the at least one sleeve 7. The reference numeral 10 designates the supporting frame. The reference numeral 11 (figure 3) designates at least one pair of oscillating elements, for example leaf springs, which support the motor 1, protrude from a base 12 and are actuated by a linear actuator 13. The reference numeral 14 designates conveyor belts or elements for conveying the elements being processed. The reference numerals 15, 16 and 17 designate the frames of the machine sections that compose the plant. The reference numerals 18 and 19 designate the devices for measuring and checking, respectively, the size of a pair of edges and for measuring and checking the size of the other pair and the squaring. The reference numeral 20 designates the device for centering the elements being machined. The reference numeral 21 designates the device for presetting for squaring, which in each instance can act on the rear edge or on the front edge of said elements. The reference numeral 22 (figure 8) designates the first section of the sizing plant. The reference numeral 23 designates the second section, which optionally also comprises squaring; the reference numerals 24, 25 designate the two measurement and control sections with optional automatic feedback to perform the necessary corrections of the active position of the grinding wheel or wheels, advantageously on the last pair thereof which is part of the machines 1. The reference numerals 26 and 27 designate the device for optional correction after comparison between the read measurement and the reference measurement. The sizing method is carried out as follows. Sizing of the first pair of sides of the element 5 being processed occurs in the first plant section 15, where the element 5, conveyed by the line 14, is centered in the device 20 for centering with respect to the longitudinal axis of said first section, after positioning the pairs of grinding wheels 4, which work dry and are rotationally actuated by the motors 1 and moved into position by the respective linear actuators 9 or 13, so that each one reaches the working front according to a preset program, determining the distance d from the front of the corresponding plant section. The various and subsequent pairs of grinding wheels 4 gradually reduce the width of the element 5 until one reaches the dimension that corresponds to the preset size, which is checked by the device 18, while said element 5 continues through the rotation station (the so-called carousel) and then enters the second plant section 17. Here the element 5 undergoes an additional centering operation which is simultaneous with, or subsequent to, a positioning step for squaring, which is performed one element at a time by sets of elements 5, with subsequent insertion between the pairs of grinding wheels 4 which are adapted to perform, where necessary, simultaneously with squaring, also the sizing of the other dimension of the second pair of sides of the element being processed. This is followed by dimensional checking of the second size and by geometric checking of squareness, by measuring the angle or by comparing the lengths of the diagonals. As regards the machine, in execution of the method related to the compensation of the wear of the grinding wheels 4, compensation occurs mainly as follows. At preset time intervals, depending on the degree of precision of the size, on the workability characteristics of the element 5 and on the wear characteristics of the grinding wheel, the linear actuator backs off each grinding wheel 4 by an extent which is sufficient to clear the path related to the sensor 6 and then returns the grinding wheel to the working front, and so forth. During the cycle for compensating the wear of one pair of mutually opposite grinding wheels 4, or even of a single grinding wheel, the fraction of workload that is neutralized during backoff is assigned to the next pair of grinding wheels 4 or to the next grinding wheel, except for the last fraction, which can be assigned to the preceding grinding wheel or pair of grinding wheels. According to a variation of the program, it is possible to assign control of the wear of the last grinding wheel or pair of grinding wheels to the section 18 for size control of the corresponding plant section. In this case, compensation of the wear of the grinding wheel or pair of grinding wheels 4 is actuated by reading the size of the element 5 being processed. When each grinding wheel 4 reaches its wear limit, an adapted sensor, not shown, produces the backoff of said grinding wheel, the issuing of an adapted alarm and operational presetting for tool replacement. During replacement, the workload related to the grinding wheel is assigned to other grinding wheels, as in the case of wear compensation. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A method for sizing elements made of natural or synthetic stone, particularly ceramic tiles, which comprises the sizing and optional squaring, on both pairs of sides, of each natural- or synthetic-stone element (5) in two successive steps by means of an intermediate rotation through 90°, characterized in that sizing and optional squaring are achieved by means of a dry grinding process with continuous compensation of the wear of at least one grinding wheel (4). The method according to claim 1, characterized in that a single size is used for the dimensions of each element (5). The method according to claim 1, characterized in that it comprises the steps of: sizing the element (5) in a first plant section, with subsequent removal of thickness of each edge of a pair of sides until finishing occurs by reaching the size; checking the dimensions of the size by measurement and comparison; rotating through 90°; centering the element (5) in a second plant section and performing optional squaring: sizing the second pair of sides, where necessary; geometrically checking the squaring, if performed; checking the dimensions of the size of the second pair of sides; compensation of the wear of the at least one grinding wheel (4) being provided. The method according to claim 1, characterized in that the wear of the at least one dry grinding wheel (4) is compensated according to time intervals which are preset according to the degree of precision of the element (5) being processed and to the wear characteristics of the grinding wheel. The method according to claim 4, characterized in that wear compensation on the at least one grinding wheel (4) is performed after backing off from the working front in order to measure the value of the wear. The method according to claim 5, characterized in that the grinding wheel backoff value between the working front and the front of said grinding wheel (4) is sufficient to clear the control path of an interposed control means. The method according to claim 6, characterized in that the front of each grinding wheel (4) returns to the working front by moving by an extent which is equal to the extent of the backoff, including the value of the grinding thickness of the edge of the element (5), plus the wear that has occurred in the previous time interval. The method according to claim 7, characterized in that the work load fraction of one pair of grinding wheels (4) is assigned to the next pair of grinding wheels (4). The method according to claim 7, characterized in that the workload fraction that is neutralized in the backoff of one pair of grinding wheels (4) is assigned to the preceding pair of grinding wheels (4). The method according to claim 3, characterized in that control of the compensation of the wear of the last pair of grinding wheels (4) of each one of said first and second plant sections is assigned to the size control section of the corresponding plant section: wear compensation being actuated by reading the size of the element being processed. A machine for sizing elements made of natural or synthetic stone, particularly ceramic tiles, characterised in that it comprises at least one dry abrasive tool (4) which is made to rotate about its own axis and moves axially with respect to an element (5) being processed, first dry grinding means for first pair of sides of said element (5) being processed, second dry grinding means for second pair of sides of said element (5) being processed and continuous compensation means of the wear of at least one dry abrasive tool (4). The machine according to claim 11, characterized in that at least one sensor (6) is interposed - with its own path - between said element (5) being processed and each one of said abrasive tools (4). The machine according to claim 12, characterized in that said abrasive tool (4), rotationally actuated by a motor (1), is allowed to slide axially on at least one guide (8) by means of a linear actuator (9) which is supported on a frame (10). The machine according to claim 13, characterized in that said abrasive tool (4) rotationally actuated by the motor (1) is allowed to slide axially by means of a linear actuator (13) which acts on a pair of oscillating elements (11). A plant for treating natural or synthetic stone elements, particularly ceramic tiles, characterized in that it comprises a machine as defined in claim 11. The plant according to claim 15, characterized in that it comprises elements (14) for conveying the natural- or synthetic-stone elements (5) which are sized and/or squared by dry abrasive tools (4), after passing through a centering device (20) and optionally a squaring device (21), with subsequent measurement and control sections with optional automatic feedback. The plant according to claim 15, characterized in that it further comprises a device (26, 27) for size correction after comparison between the read measurement and the reference measurement, said device being present in each one of plant sections (15, 17) of said plant.

Inventors: SANTI GABRIELE, SANTI, GABRIELE
Applicants: PRAGMA S R L, PRAGMA S.R.L.
Application year: 1998
Main classification: B24B7
IPC codes: B24B47, B24B9, B24B7, B24B49