EP2205399B1

EP2205399B1 - Method of fabrication of grinding wheels and related plant

Info

Publication number: EP2205399B1
Application number: EP07849697A
Authority: EP
Inventors: Giuseppe Maternini
Original assignee: Davide Maternini SpA
Current assignee: Davide Maternini SpA
Priority date: 2007-10-18
Filing date: 2007-10-18
Publication date: 2011-09-07
Anticipated expiration: 2027-10-18
Also published as: ATE523292T1; EP2205399A1; WO2009050749A1

Abstract

The problems deriving from even slight dishomogenities of the density of filling of molds for forming abrasive wheels with a powdery abrasive.mixture induced by the levelling action by repeated passes of a scraper blade moving along the same line onto the surface of a mold table (2) are overcome by performing the. following succession of operations: - lifting a mobile bottom (4) of a mold table hole (3) at a first level equal to or lower than the level of the surface of the mold table; - depositing on the surface of the mold. table (2) a layer of a certain uniform of thickness of abrasive mixture while maintaining the mobile bottom (4) at said first lifted level; - lowering inside the hole (3) through the mold table (2) said mobile bottom (4) sustaining the mass of abrasive mixture bearing on its surface; - lifting said mobile bottom (4) to a second preestablished level equal to or lower than said first level; - removing the abrasive mixture of said deposited layer, from the surface of the mold table (2) levelling the abrasive mixture filling of said mold cavity; - lowering said mobile bottom (4) inside said hole (3). The method is effective even in case of forming by pressing at room.temperature ultrathin grinding wheels and/or incorporating one or more reinforcement nets, to be successively hardened in oven.

Description

TECHNICAL FIELD

This invention relates to the fabrication of grinding wheels and in particular of grinding wheels having a relatively small thickness (ultrathin).

BACKGROUND OF THE INVENTION

Fabrication processes of grinding wheels are well known as well as fabrication plants that tend to be ever more automated, with processing lines defined by a succession of workstations each dedicated to perform one or more operations.
A first fundamental step of fabrication of these consumable tools consists in weighting according to a certain dosage formulation the different solid and fluid components of the abrasive mixture and in mixing them together either simultaneously or preferably by adding them into the mix in more steps for obtaining a homogeneous powdery abrasive mixture of reproducible characteristics. This phase of preparation of the abrasive mixture is generally carried out in a batch-wise manner, producing a certain quantity of mixture.
A highly automated production line generally has a certain number of automated workstations for introducing in a mold cavity the various items that commonly compose the grinding wheel in an appropriate sequence, before reaching a press station.
Often, a first operation may consist in optionally introducing a first reinforcement metal grommet of the central hole of the grinding wheel on a central pin of the mold, and in laying on the bottom of the mold at least a first disk of reinforcement fabric, generally a net of resin impregnated fibers of a high tensile strength material preceded or not by the laying of a paper label printed with characteristic data of the grinding wheel.
Whether or not the construction contemplates the presence of reinforcement nets, in a successively reached workstation, the mold cavity or cavities of a planar mold table are filled with the premixed abrasive mixture released by a distributing hopper-drawer travelling forward and backward over the mold table and provided with front and rear containment and scraper blades.
In a successively reached automated workstation, at least a second disk of reinforcement net is laid onto the abrasive mixture and, in a successive workstation, a preprinted label with data and characteristics of the grinding wheel being fabricated is eventually laid on top of the reinforcement net, if so chosen in alternative to a prior placement of such a label onto the bottom of the mold.
In some cases, the process may contemplate placing two reinforcement metal grommets of the central hole of the grinding wheel on the same central pin of the mold: one before the filling with the abrasive mixture and a second placed after, before pressing.
Preferably, in fabricating particularly thin grinding wheels, a single reinforcement metal grommet of the central hole of the grinding wheel is placed on the central pin of the mold after filling it, in a workstation preceding the press workstation.
The press workstation include a press equipped with one or a plurality of plates for compacting the abrasive mixture inside respective mold cavities thus consolidating at room temperature the components introduced in the mold, shaping the grinding wheel in form of a "biscuit" consolidated only by pressure, but whose thermosetting binder has not yet been subjected to the hardening conditions (reticulation of the thermosetting resin binder, commonly of phenolic type).
The not yet hardened grinding wheels so fabricated, even if they are perfectly consolidated and may be handed without excessive care, are still flexible and plastically deformable. They will assume their final mechanical properties only after heat treatment in oven.
The high level of automation and high yield imposed by evident reasons of minimizing the fabrication costs of typically consumable tools such as grinding wheels are requirements that must in any case conjugate themselves with the need of ensuring the maximum homogeneity of the density of the finished wheel in consideration of the fact that they are destined to function at extremely high rotational speed.
Even more than the so-called de-burring grinding wheels, grinding wheels designed for cutting operations are tools having a relatively fast wear and the industry, whenever work conditions make it possible, prefers to use cutting wheels of reduced thickness for sensible savings in terms of reduced energy consumption and consumption of abrasive material.
Grinding wheels producers have on and on refined the fabrication technology toward the objective of producing cutting wheels of reduced thickness that could nevertheless ensure adequate mechanical characteristics. Present technology permits to fabricate grinding wheels of diameter that may reach up to about 230 mm and thickness from 1.0 mm to 3,5 mm.
In fabricating so-called "ultrathin" wheels of adequate mechanical sturdiness, the requisite of ensuring the maximum homogeneity of the density of the abrasive mixture filling the molds and a perfect planarity and parallelism of reinforcement nets that are generally incorporated in the consolidated mass of abrasive mixture on opposite faces of the wheel, become even more critical.
Moreover, in case the wheel incorporates one or more reinforcement nets, an accentuated thinness of the consolidated mass of abrasive mixture, deviations from perfect planarity and parallelism of the reinforcement net or nets may also cause failures.

DISCUSSION OF THE PRIOR ART

The filling step of the mold cavity with the abrasive mixture has a great influence on the mechanical qualities of the finished grinding wheel. In particular, it has been observed that the way the filling of the cavity of the mold with abrasive mixture distributed by gravity fall into the cavity from the bottom of a distributor drawer moving over the plane of a mold table, with containment and scraper blades at the leading and at the trailing side of the travelling drawer that are elastically held in contact with the surface of the mold table, determines a significant dishomogenization of the density of free flowing powdery abrasive mixture fallen into the mold cavity.
The repeated scraping action along the same direction of translation of the distributor drawer tends to increase the density of the mixture that fills the cavity near opposite arcs of the circular rim of the mold, because of the dragging caused by the passage of the two scraper blades of the distributor drawer over the filled cavities. In addition, the speed at which the distributor drawer travels first in a direction and after in the opposite direction is relatively fast for comprehensible reasons of productivity. The document EP-A-1473 117 discloses a process according to the preamble of claim 1 and a machine according to the preamble of claim 12, respectively.

OBJECTIVE AND SUMMARY OF THE INVENTION

It has now been found a new and outstandingly effective process of fabricating grinding wheels, even ultrathin, in a completely automatic or semiautomatic manner in a high productivity fabrication line or automatic machine that ensures an enhanced homogeneity of the density of the abrasive mixture with which the mold cavity is filled, without penalizing productivity.
The new process overcomes the drawbacks and criticalnesses of known processes, sensibly improving reliability of the fabrication process that results in an almost total abutment of failure statistics of the fabricated grinding wheels.
According to the process of this invention, the filling of a preestablished quantity of abrasive mixture in a cavity of a mold table defined in a hole inside which a mobile bottom may be lifted at preestablished levels, comprises performing in succession the steps of:

lifting said mobile bottom at a first level equal to or lower than the level of the planar surface of the mold table;
depositing on the surface of the mold table a layer of a certain uniform thickness of abrasive mixture while maintaining the mobile bottom at said first lifted level;
lowering back inside the whole of the mold table the mobile bottom sustaining the mass of abrasive mixture bearing on its surface;
lifting said mobile bottom to a second preestablished level equal to or lower than said first lifted level;
removing the layer of abrasive mixture from the surface of the mold table levelling the abrasive mixture filling said mold cavity;
lowering back said mobile bottom inside said hole.

The step of removing and the mixture in excess levelling the abrasive mixture filling the mold cavity is performed by a single pass of a scraper blade.
The sequencing in two distinct steps the filling of the mold cavity: the first step consisting in filling the mold cavity with a certain volume of abrasive mixture through the collapsing of the mass of free flowing abrasive mixture of the deposited layer over the retracting mobile bottom of the mold when lowering it inside the hole, and the second step consisting in removing the excess abrasive mixture of the deposited layer from the planar surface of the mold table and simultaneously levelling the mixture filling the mold cavity by a single pass of a scraper blade, proves to be extraordinarily effective in preventing dishomogenization of the density of the powdery filling of the mold cavity, because it significantly reduces the "drag" stresses induced by the passage of the scraper blade that sweeps off the table the mass of free flowing powdery mixture of the layer of uniform thickness initially deposited over the surface of the mold table.
Moreover, by performing the above two steps in distinct work-stations of a highly automated machine, productivity is fully preserved and in addition even the speed of translation of the scraper blade can be reduced for further reducing stresses on the mass of abrasive mixture filling the mold cavity.
An automatic machine made according to the present invention with these characteristics will be described later.
The invention and preferred embodiments thereof are defined in the annexed claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a simplified layout of an automatic machine for fabricating grinding wheels in green or biscuit form consolidated by pressing at room temperature of a known type as here specified by the applicant.
Figure 2 shows the layout of an automatic machine of the same type of the known machine of Figure 1, modified in a way to implement the novel process of this invention.

The sequence of Figures from 3 to 6 illustrates in detail the actions performed in a first work-station for loading into abrasive mixture the mold cavities, according to the process of this invention.
The sequence of Figures from 7 to 10 illustrates in detail the actions performed in a successive work-station for removing the excess of abrasive mixture deposited on the mold table and for levelling the preestablished amount of abrasive mixture contained in the mold cavities.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The novel method of filling with a preestablished quantity of abrasive mixture of a mold cavity of the present invention will be illustrated in detail by making reference to an implementation of the filling operations using an automatic machine of known basic structure, being understood that the same operations may be performed even employing automatic machines or semiautomatic machines of different type from the one depicted in the drawings, for example using an automatic or semiautomatic machine having a rectilinear layout wherein mold tables are incrementally advanced through a succession of work-stations disposed in line, or by employing machines and/or apparatuses capable of carrying out the same sequence of operations of the method of the invention, whether in fully automatic fashion or under control of an operator.
In consideration of the preminent practice of introducing one or more reinforcement nets or fabrics in the mold of preformation of the grinding wheel, the machine shown in the drawings for purely illustrative purposes is designed for introducing two reinforcement nets. Nevertheless, the improved process of loading a preestablished quantity of abrasive mixture uniformly distributed in the mold is perfectly suitable also for fabricating grinding wheels without any reinforcement fabric embedded therein, even using the same automatic machine, by simply omitting the performance of the related operations of introduction of the reinforcement nets in the mold or molds.
Referring to Figure 1, the machine indicated with 1 as a whole, has a circular rotating table structure 5. In the example shown, a circular crown rotating table 5 comprises eight mold tables 2, each having four holes 3, occluded by respective mobile bottoms 4, for defining four mold cavities.
By angular increments, all the mold tables 2 reach in succession the eight work-stations A, B, C, D, E, F, G and H of the machine.
The station A is generally a "free" or service work-station that permits inspection of the mold cavities and removal of eventual residues, before the mold table advances to the work-station B, which is equipped with automatically controlled handling devices, indicated as a whole with b1, moving along a radial axis of the work-station, for picking up four circular reinforcement nets r1 from a belt conveyor b2 of a mechanism of controlled supply of the reinforcement nets. Optionally, the supply mechanism is covered by a hood b3 for realizing an at least partially confined region that may be heated in order to raise and maintain the temperature of the reinforcement nets several degrees above room temperature, if necessary, for favoring a complete relaxation in case undulations and/or warpings may have been induced by mechanical stresses during die stamping the disks.
The radially shifting handling device b1 carries the picked up quarted of disks exactly above four the cavities of the mold table 2 and deposit them onto the respective mobile bottoms 4 of the holes 3.
The same mold table 2 successively reaches the work-station C for levelling of a certain amount of abrasive mixture inside each cavity with mobile bottom of the mold table.
Commonly, this work-station C has, underneath the rotating table 5, lifting devices of the mobile bottoms 4 of the mold cavities at a certain level, such to determine with adequate precision the filling volume of abrasive mixture that upon pressing it at a preestablished load will produce the compacting of the material in form of a "biscuit" of a design thickness.
As already discussed in the introduction part of this description, the filling of the mold cavities with a certain quantity of abrasive mixture takes place by moving the distributor drawer c1 above the surface of the mold table 2 that composes the rotating table 5 of the machine, radially toward the center of rotation such that by passing over the mold cavities, the free flowing abrasive mixture contained in the distributor drawer c1 falls by gravity filling the mold cavities and is immediately shaved off and levelled therein by the action of two containment scraper blades c2 and c3, elastically bearing on the surface of the table, disposed at two opposite sides, respectively and alternately at the leading edge and at trailing edge, of the distributor drawer c1, during the repeated passage of the whole drawer c1 above the mold cavities, first advancing toward the center of rotation of the table and the second returning to its rest or stand-by position.
The abrasive mixture swept by the scraper blade falls in a hopper for where it is recycled back to a supply container c4 by means of the belt conveyor c5.
Feeding of abrasive mixture into the drawer c1 takes place by means of the feed belt conveyer c5, carrying the material out of the supply reservoir c4.
The mold table with its four mold cavities in which the preestablished amount of abrasive mixture has been introduced, reaches the work-station D equipped in a practically identical manner of the already described work-station B, in order to place over the levelled amount of abrasive mixture contained inside the mold cavities a second disk of reinforcement net r2, also in this case preferably preheated for relaxing eventual warps.
In the successive work-station E, common automated handling devices, not detailed in the drawings, deposit a printed paper label e1 onto the second reinforcement net.
In the successive work-station F, common automated handling devices, indicated with f1 as a whole, place a metal grommet bm1 for the reinforcement of the spindle hole of the wheel, on a central pin of the mold (not traced in the drawing), as last filling operation of the mold cavities.
In the successive work-station G takes place the pressing at room temperature of the elements introduced in the mold cavities, carried out at a preestablished load sufficient to determine an effective consolidation of all the components of the grinding wheel in the form of a biscuit that can be handled without excessive care and that in the successive and last work-station H of the machine is expelled by lifting the mobile bottoms of the mold cavities. Commonly, the so consolidated "green" grinding wheels are staked on suitable basements to form packs h1 that are then transferred by a conveyor h2 inside an oven for cooking at a temperature and for a time sufficient to ensure reticulation of the thermosetting resin binder, generally a phenolic resin.
The criticalnesses and drawbacks primarily attributable to a dishomogenity of the density of filling with abrasive mixture mold cavity, already discussed in the introduction part of this description, are practically eliminated by the novel method of filling of the present invention, which as in the sample embodiment of Figure 2, may be implemented even using a preexistent automatic machine, by introducing modifications that are not excessively relevant in the organization and in the functional devices of few of the work-stations of a rotating table machine of the type described above in relation to Figure 1.
Figure 2 represents a simplified layout of an automatic machine made or obtained by modifying an existing machine according to the present invention for implementing the improved process of loading of the mold cavities.
In order to implement the process of the invention in a rotating table machine having a basic architecture as the one described above, the loading of a predetermined quantity of abrasive mixture in mold cavities in two distinct steps performed in two successive work-stations of the machine for retaining unchanged the productivity (i.e. without prolonging the cycle of automatic operation), the organization of the work-stations incrementally reached by the mold tables 2, may be modified as illustrated.
The staging over two successive work-stations: C1 and C2, of the steps of loading the mold cavities with the preestablished amount of abrasive mixture, is enabled by combining in a single work-station, re-nominated E&F in Figure 2, the operations of loading the metal grommet of reinforcement of the central hole of the grinding wheel and of the printed paper label, thus shifting of one position the station D at which the second reinforcement net is loaded.
The automatic handling devices f1 of introduction of the quartet of reinforcement metal grommets f2 over the central pins (observable in successive drawings) of the four molds of each mold table 2 and the automated handling devices for picking up and placing the quarted of paper labels, are commonly of different form and quite compatible to be installed in a same work-station without particular problems. Moreover, both operations, beside not being particularly critical, may be quickened for remaining within the residence time of the mold tables 2 in the work-stations of the machine. Therefore, this simple reorganization will permit to use an existing basic structure of automatic machine for implementing the process of this invention.
The series of Figures from 3 to 6, illustrates the operations that are performed in the first loading work-station C1.
The partial elevation views, sectioned in part, permit to observe a common architecture of the structure of a mold table 2, part of the rotating table 5 of a machine and the support and lifting structures of the mobile bottoms 4 of the mold cavities, the lifting structures being present in all or only in few of the work-stations, according to requirements.
Each mold table 2 has holes the inner diameter of which is defined by cylindrical inserts 3, commonly of a hard metal alloy, resistant to abrasion, defining the mold cavities.
The mobile bottoms 4 of the mold cavities are commonly sustained by a single mobile base plate 8, sliding along upright guides and along the four pins 9 that define the central hole of the grinding wheels being fabricated. Three screws 7 allow adjustment of parallelism of the mobile bottoms 4 in respect to the plane of the table. This trimming device is useful for prearranging a certain bias inclination of few degrees or of a fraction of a degree of the mold bottoms, for volumetrically compensating for a residually small increment of density of the abrasive mixture filling the cavity near the part of the circular rim of the cavity that is last passed over by the scraper blade moving over the surface of the table.
The upright guides (not detailed in the drawing) connect the mold plate to a fixed rest plate that together with the upright guide rods, constitutes a robust suspension structure, indicated as a whole with 10 in the drawings, solidly connected to the rotating table 5.
Commonly the mobile bottoms 4 have a superficial wear disk (not detailed in the figures) of a hard metal alloy that is held fast on a underlying disk by a plurality of permanent magnets. By removing the wear disk, it is possible to access the head of the screws 7 for trimming the spatial inclination of the mold bottoms in respect to the horizontal plane of the rotating mold tables of the rotating table 5.
The mobile base plate 8 bears at its lower end stop onto the fixed rest plate of the suspension structure indicated as a whole with 10 from which extend upright the central pin 9, when the lifting stems 11 of the mobile bottoms 4 of the molds are retracted and thus disengaged from the suspension structure 10 by central actuator devices of the work-station.
At the top 6 of the fixed central pins 9 of the molds will be placed the metal grommet of reinforcement of the central hole of the grinding wheels before pressing them.
The rotating table assembly rotates during phases of advancement of the mold tables 2 from a station to the next.
Lifting stems 11 for positioning the mobile bottoms 4 of the molds at preestablished levels must be present at least below the work-stations C1 and C2 dedicated to the loading of the abrasive mixture.
As depicted in Figure 3 when a mold table 2 arrives at the work-station C1, the lifting stems 11 are still in their rest position disengaged from the overhanging rotating structures for allowing the angular increment movement of the rotating table of the machine that brings a new mold table 2 to the station C1. In these conditions of angular advancement and necessity of disengagement of the lifting stems 11, the bottoms 4 of the mold cavities are at their lower end-run or fully retracted position and over the surface of the mobile bottoms 4 is present a first reinforcement net r1, having been deposed therein in the preceding work-station B.
The first action is performed by the stems 11 that lift the base plate 8 to a level such to bring the mobile bottom of the mold cavities with the first reinforcement net r1 resting on them, to a level few tens of a millimeter, in case of ultrathin wheels, and up to a maximum of about 3 mm, in case of wheels particularly thick, lower than the level of the surface of the mold table.
Successively, as shown in Figure 4, the distributor drawer c1, that according to an essential aspect of the method of this invention does not carry any scraper blade held in contact with the surface of the mold table, is first advanced along a radial axis of the work-station C1 as far as flying over the whole area of the mold table 2 at a height from the surface of the mold table appropriate for depositing a layer of abrasive mixture M of a certain and substantially uniform thickness over the mold table, along the track of radial movement, forward and backward of the distribution drawer filling any depression in correspondence of the mold cavities as determined by said first level of lifting of the mobile bottoms 4, as depicted in Figures 4 and 5.
Once the distributor drawer c1 has returned to its rest or stand-by position above a fixed portion 5' of the machine table, the lifting stems 11 are retracted and disengaged from the overhanging structure of the rotating table, thus lowering the mobile bottoms 4 of the mold cavities to their end run rest position, carrying thereon the abrasive mixture of the deposited layer bearing on their surface, as depicted in Figure 6.
With the lifting stems 11 disengaged, the rotating table is advanced until the same mold table reaches the successive work-station C2.
As schematically illustrated in Figure 7, in the work-station C2, common automated devices (not shown in the drawing) for example in the form of support arms or slidable bridge supports moved automatically by means of common pneumatical or electromechanical actuators, lower along a median line of the mold table 2, a pair of scraper blades c11 and c12 to elastically bear onto the surface of the mold table 2, sinking through the thickness of the layer M of abrasive mixture deposited in the preceding work-station C1.
In practice, the two scraper blades typically thin, penetrate through layer M abrasive mixture one close to the other without any significant separation distance between them.
At this point, the stems 11 of the underlying lifting structure of the work-station lift the mobile bottoms 4 carrying the first reinforcement net and the layer of uniform thickness of abrasive mixture up to a second level which is precisely established for volumetrically determining the amount of powdery abrasive mixture to be compacted. This second level is generally lower or in the limit equal to the first level of lifting of the same mobile bottoms during the deposition over the surface of the mold table of the layer of uniform thickness of abrasive mixture (Figures 4 and 5).
Thereafter, the two scraper blades c11 and c12 move in opposite radial directions, sweeping the abrasive mixture of the deposited layer M, passing over the two pairs of mold cavities, respectively, levelling the powdery content filling them, as far as dropping the excess abrasive mixture that had been deposited with uniform thickness over the mold table, into the collecting hoppers c3 and c4, respectively disposed one at the outer side and one at the inner side of the rotating circular crown table, along the radial axis of the work-station C2, as depicted in Figure 9, where are also observable the respective belt conveyors c5 and c5' that lift the recovered excess mixture into the supply reservoir c4 (Figure 2).
Figure 10 illustrates the retraction of the lifting stems 11 out of engagement with the overhanging structure of the rotating table and the movements of lifting off the surface of the table and of returning to a median stand-by position of the two scraper blades c11 and c12, ready to lower themselves again onto the surface of a new mold table 2 that will reach the station (thus replicating the cyclic situation represented in Figure 7).
With the mobile bottoms 4 of the molds returned again in their fully retracted (lowered) position, the mold table 2 will advance as far as reaching the work-station D for receiving the second reinforcement net (also in this case preferably preheated as done for the first reinforcement net) onto the levelled volume of abrasive mixture, before progressing through the terminal phases of loading of the paper label e1 and of the metal grommet bm1 of reinforcement of the central hole of the grinding wheel onto the central pin 6 of the molds, as already described with reference to the machine layout of Figure 2.

Claims

A fabrication process of a grinding wheel by forming a biscuit consolidated by pressing in a mold a certain amount of powdery abrasive mixture containing a thermal setting binder, at least a metal grommet of reinforcement of a central hole of the wheel and optionally one or more reinforcement nets of resin impregnated fibers of a high ensile strength material and hardening in oven the fabricated biscuit, whereby the loading of a predetermined amount of abrasive mixture into a mold cavity hole of a mold table, closed by a mobile bottom that is lifted at a preestablished level, comprises performing in succession the following operations:
- lifting said mobile bottom at first level equal or lower than the level of the surface of the mold table;

- depositing on the surface of the mold table a layer of a certain uniform of thickness of abrasive mixture while maintaining the mobile bottom at said first lifted level; characterized by thereafter

- lowering inside the hole of the mold table said mobile bottom sustaining the mass of abrasive mixture deposited on its surface;

- lifting said mobile bottom to a second preestablished level equal to or lower than said first level;

- removing the abrasive mixture of said layer from the surface of the mold table levelling the abrasive mixture filling said mold cavity;

- lowering said mobile bottom inside said hole.
The process according to claim 1, wherein the removal and levelling of the abrasive mixture is performed by a scraper blade in a single pass.
The process according to claim 1, characterized in that it is performed by advancing an assembly of said mold table and mobile bottom in succession through a plurality of work-stations, at least the work-stations of deposition on the surface of said mold table of said layer of abrasive mixture of uniform thickness and the work-station of removal and levelling being equipped with lifting mechanisms of said mobile bottom.
The process according to claim 3, wherein said mold table has a plurality of holes with respective mobile bottoms defining as many mold cavities, moved by unified lifting means.
The process according to claim 4, wherein said mold cavities are disposed on one side and on the other side of a median line of said mold table, the operation of removal and levelling being performed by a pair of parallel scraper belts that are lowered onto the surface of the table along said median line and moved in opposite directions for clearing the mold table from the deposited mixture at a reduced speed, in the same time interval taken by a single pass performed with a single scraper blade.
The process according to claim 3, wherein several co-planar mold tables form a circular crown rotated by angular increments for bringing each mold table to reach in succession said work-stations, said distributor drawer and said or said means for removing and levelling moving along a radial direction.
The process according to claim 1, further comprising the operations of laying a first reinforcement net onto the surface of the mobile bottom and a second reinforcement net onto the levelled abrasive mixture respectively before and after execution of said sequence of operations.
The process according to claim 7, wherein said reinforcement nets are preheated before introducing them into the mold.
The process according to claim 1, wherein said metal grommet of reinforcement of the central hole of the grinding wheel is placed on a central pin extending through a hole of said mobile bottom of the mold as last operation before pressing.
The process according to claim 7, wherein a paper label is placed over said reinforcement net.
The process according to claim 7, characterized by the fact that on the mobile bottom of the mold cavity is placed a paper label electrostatically precharged for distending eventual warps before laying thereon said first reinforcement net.
Automatic or semiautomatic machine for fabricating a biscuit consolidated by pressing in a mold of a certain amount of powdery abrasive mixture containing a thermosetting binder, together with at least a paper label, a first and a second reinforcement nets of fibers of a high tensile strength material impregnated with a resin, and at least a metal grommet of reinforcement of a central hole of the wheel, to be successively hardened in oven, having an assembly of coplanar mold tables forming a circular crown that is rotated by angular increments for bringing each mold table in succession through a plurality of work-stations of the machine, each mold table having one or a plurality of holes, inside which a respective mobile bottom is liftable in the hole at preestablished levels from the level of the surface of the mold table for modifying the depth of the mold cavity, characterized in that the loading of a certain amount of abrasive mixture inside each mold cavity with mobile bottom of a mold table is performed in two distinct and successively reached work-stations;
- the first of said two successive work-stations has lifting means of the mobile bottoms of the cavities of the mold table and a mixture distributing drawer moving from a stand-by position to an extended position flying above the surface of the mold table for depositing thereon a layer of a certain thickness of abrasive mixture while maintaining the mobile bottom of the mold cavities on which at least a first reinforcement net has previously been placed, at a lifted level equal to or lower than the level of the surface of the mold table, lowering the mobile bottom of the mold cavity after completing the deposition of said layer of abrasive mixture;

- the second of said two successive stations has lifting means of the mobile bottom of the mold cavity of the table to a second preestablished level equal to or lower than said first level, at least a scraper blade and means for lowering it onto the surface of the table and for sliding it over the surface of the table for removing the abrasive mixture of said layer deposited on the table, levelling the content of abrasive mixture filling said mold cavities in a single pass, hoppers for recovering the abrasive mixture removed from the surface of the table and means for recycling the recovered abrasive mixture to a supply reservoir of the preceding work-station of deposition of the layer of abrasive mixture.
The automatic or semiautomatic machine of claim 12, wherein each mold table has a plurality of said mold cavities distributed on one side and on the other side of a median line of said table tangent to a circumference of said circular crown, a pair of scraper blades and respective means for lowering them together along said median line and for moving the two blades in opposite radial directions, recovering the removed abrasive mixture into two hoppers for recycling the mixture, respectively placed at the inner side and at the outer side of said mold table.