WO2011110947A1

WO2011110947A1 - Method of fabrication of grinding wheels and related automatic machine

Info

Publication number: WO2011110947A1
Application number: PCT/IB2011/000627
Authority: WO
Inventors: Giuseppe Maternini
Original assignee: Davide Maternini S.P.A.
Priority date: 2010-03-11
Filing date: 2011-03-08
Publication date: 2011-09-15
Also published as: IT1399287B1; ITVA20100023A1

Abstract

The abrasive mixture for filling a mold cavity of a mold table is let to free fall off a distributing drawer travelling at a certain height above the mold table, being expelled through a transversal slot, practically at a constant rate and homogenously for the whole width of the slot, by a drag-and-translate action of a belt conveyor constituting at least part of the bottom of the distributor drawer. The height from which the mixture is released is such to promote a vertical component of the speed of fall of the granular mixture and to make negligible or practically null any residual horizontal component of the speed of the granules. The facts that the granular mixture deposits as a layer of substantially uniform thickness, that the receiving mold table is substantially free of raised or depressed areas and that, by properly timing the motion of the belt conveyor during the flight of the distribution drawer in order to limit deposition of the blanket layer of abrasive mixture only over and in the closest neighborhood of the mold cavity footprint, prevent induction of significant "viscous" dragging phenomena of the filling of the mold cavity once the mobile bottom of the cavity is lowered carrying down with it mixture deposited thereon, and a scraper blade is moved over the flat mold table to sweep away the excess mixture deposited around the rim of the mold cavity. The method is effective also in case of forming by pressing at low temperature ultra thin grinding wheels incorporating one or more reinforcement fabrics, to be successively hardened in oven.

Description

"METHOD OF FABRICATION OF GRINDING WHEELS AND RELATED AUTOMATIC MACHINE"

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 highly automated production line generally has a certain number of automated workstations for introducing in a mold cavity the various items (reinforcement metal grommet of the central hole, a net of resin impregnated fibers of a high tensile strength material, a paper label printed), besides the premixed abrasive mixture before reaching a press station.

In a dedicated workstation, the mold cavity or cavities of a planar mold table are filled with the premixed abrasive mixture, for example released by a distributing hopper-drawer travelling forward and backward over the mold table and provided with front and rear containment and scraper blades of the mixture.

The press workstation includes 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 or in respect to a single reinforcement net, becomes even more critical.

The tiny thickness of the consolidated mass of abrasive mixture cannot tolerate even the smallest deviation from a perfect planarity and parallelism of the reinforcement net or nets.

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 molds, because of the dragging caused by the passage of the two containment and scraper blades on the front and on the rear side of the distributor drawer, the distributor drawer travelling at a relatively fast speed first in a direction and after in the opposite direction over the powder of abrasive mixture fallen into the mold.

A prior patent application in the name of the same Applicant, published with the number WO 2009/050749 Al, discloses an enhanced method that contemplates the filling of a preestablished quantity of abrasive mixture in a cavity of a mold table, through a hole of which a mobile bottom may be lifted at preestablished levels, 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; and

lowering back said mobile bottom inside said hole for continuing the fabrication and cooking the grinding wheel.

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.

By reducing even more the thickness of cutting wheels, also the method of the prior patent application, even if greatly enhanced, shows its limitations in ensuring an almost absolute filling homogeneity.

OBJECTIVE AND SUMMARY OF THE INVENTION

After deep studies it has been possible to establish that there is at least a tiny difference of the density of the filling granular abrasive mixture of the mold, probably due to a dynamical "bias" caused by the component of the horizontal translation speed (neither negligible or reducible, unless a reduced productivity of the forming machine is accepted) of the distributor drawer that is thus associated to the particles released over the table and inside the mold-cavities thereof.

To this problem and limitation of the above discussed prior art a novel and effective solution has been found that, besides overcoming the residual technical problem, simplifies the filling and levelling sequence that is completed in the same workstation, reducing the number of method steps, besides reducing significantly the excess of mixture to be released over the table-mold and thus the amount to be removed, recovered and recycled, improving the overall economy of the sequence.

Instead of releasing a levelled layer of mixture in contact with the surface of the table plane during the translation of a common distributor drawer with open bottom, the mixture is allowed to drop out from a certain height from a slit of the drawer, and is expelled at an almost constant rate, homogeneously along the whole extension of the output slit, because of a drag-and-translate t action of a conveyor belt, constituting at least in part the bottom of the distributor drawer. During its excursion, the distributor drawer equipped with such a mobile bottom, travels over the mold table at a certain height. The height of fall of the powder mixture is such to make the vertical component of the fall speed of the grains remarkably more significant than the residual horizontal component due to the horizontal translation speed of the distribution drawer when the mixture falls off freely from the drawer.

Together with the facts that during the fall of the mixture to form a blanket layer of substantially uniform width and thickness, the mold table is substantially flat, free of raised parts or depressions, and the blanket layer of mixture is deposited onto a restricted area containing the mold cavity, being possible to adapt the length of the relative slit and control the motion of the conveyor belt (timing it), is practically homogeneous and remains so even when the mobile bottom of the mold is successively lowered to a level corresponding to a pre- established fill volume of the mold cavity.

The limited amount of abrasive mixture that is deposited over a restricted neighbourhood of the mold cavity (hole through the table) may be sweeped off the surface of the mold table by a common scraper blade, moved from a disengagement position to a start position onto the surface of the mold table, pressing elastically on it, and advanced to an end-run position, to be thence lifted up and returned to the disengagement position. Thus the blade levels the abrasive mixture filler of the mold cavity when passing above it, in pushing the powdery mixture fallen around the rim of the cavity. In practice, the amount (weight) of mixture deposited on the mold table (only around the mold cavity) that is sweeped by the blade over the homogeneous filling of the cavity is reduced and does not induce significant phenomena of "viscous" dragging of the mixture filling the cavity above which the excess mixture is being swept away by the scraper blade.

As an option, similarly to what is illustrated in the prior patent application WO2009/050749A1, a pair of scraper blades may be used, both being lowered down to a median line of the mold cavity and carrying out a sweeping action along opposite directions of a half of excess mixture. When the respective end-run are reached, typically off an edge of the table, the blades are lifted up and returned to the stand-by position, coupled together again.

An example of automatic machine with these characteristics will be described hereinafter. The invention and preferred embodiment 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 biscuit form, simply consolidated by pressing in a mold similar to that disclosed and illustrated in Figure 2 of the above mentioned patent application in the name of the same applicant, modified in a way to implement the enhanced process of this disclosure.

Figures from 2 to 7 illustrate the actions performed in the work-station for loading into abrasive mixture the mold cavities, according to this disclosure.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The novel method of filling with a pre-established quantity of abrasive mixture 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 at command of an operator. In consideration of the common practice of introducing one or more reinforcement nets or fabrics in the mold of pre-formation 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 pre-established 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 bl, moving along a radial axis of the work-station, for picking up four circular reinforcement nets rl 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 when die stamping the reinforcement disks.

The radially shifting handling device bl carries the picked up quartet of disks exactly above the four 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 loading and levelling a certain amount of abrasive mixture inside each mobile bottom of the cavities of the mold table.

Commonly, this work-station C for loading a certain amount of abrasive mixture has, underneath the rotating table 5, lifting devices of the mobile bottoms 4 of the mold cavities at a desired level, such to determine with adequate precision the filling volume of abrasive mixture that upon pressing it at a preestablished load will produce a compact disk in form of a "biscuit" of a desired design thickness.

As already discussed above, the filling of the mold cavities with a certain quantity of abrasive mixture takes place by moving the distributor drawer cl over the surface of the mold table 2 that composes the rotating table 5 of the machine, radially toward the centre of rotation such that while travelling over the mold cavities, free flowing abrasive mixture contained in the distributor drawer cl is expelled through al least a transversal slit such to fall by gravity over the area or areas in which the mold cavities are realized and, after lowering the mobile bottoms of the cavities to a precisely designed retracted position and after the distributor drawer cl is returned o its stand-by position, the excess mixture is shaved off the plane of the table by the action of a scraper blade c2, elastically bearing on the flat surface of the table.

The abrasive mixture swept by the scraper blade falls in a hopper from where it is recycled back to a feed container c4, by means of the belt conveyor c5.

Supply of abrasive mixture into the drawer cl takes place by means of the feed belt conveyer c5, carrying the material out of the feed container c4.

The mold table with its four mold cavities in which the pre-established amount of abrasive mixture has thus 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, may deposit a printed paper label el onto the second reinforcement net.

In the successive work-station F, common automated handling devices, indicated with fl as a whole, place a metal grommet bml 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 components introduced in the mold cavities. Pressing is carried out at a pre-established load adapted to effectively consolidating the components of the grinding wheel in the form of a biscuit of compressed mixture that can be handled without excessive care. In the successive and last work-station H of the machine, the so consolidated grinding wheels are expelled by lifting the mobile bottoms of the mold cavities and the so consolidated grinding wheels are staked with gaps there between in suitable transfer racks to form packets hi that are then transferred by a conveyor h2 inside an oven for cooking at a temperature and for a time sufficient to ensure complete hardening of the thermosetting resin binder, generally a phenolic resin.

Any residual difference of the abrasive mixture filler density in the mold cavity, is almost nullified with the novel method of filling according to this disclosure that may even be retrofitted in an existent automatic machine, through not excessively relevant modifications of the overall organization and of the functional equipments of the workstation C for introducing the abrasive mixture in a mold cavity.

The series of Figures from 2 to 7, illustrates the operations that are performed in the abrasive mixture filling work-station C. For simplicity's sake, the drawings show a mold table 2 with a single mold cavity, though the mold table 2 may have a plurality of mold cavities, each replicating the structure of the single illustrated cavity, for example four cavities as in the layout example of an automatic machine of Figure 1.

The detail elevation views, partly sectioned, permit to observe the architecture of the mold table 2, forming, together with other juxtaposed identical mold tables the rotating table 5 of a machine (Figure 1), and the support and lifting structures of the mobile bottom(s) 4 of the mold cavities, these lifting structures being present in all or only in few of the work-stations of the machine or even only in the loading workstation C.

In the shown example, the mold table 2 has a hole, the inner diameter of which is defined by a cylindrical insert 3, commonly of a hard metal alloy resistant to abrasion.

The mobile bottom 4 is commonly sustained on a single mobile base plate 8, sliding along the respective upright guide 9 and along the pin 6 that defines the central hole of the grinding wheels being fabricated. The fixed bottom plate 8 and the upright guide 9 are part of a robust suspension structure, indicated as a whole with 10 in the drawings, that is solidly connected to the rotating table.

The mobile bottom 4 may 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 screws that may optionally be present for trimming the spatial inclination of the mold bottoms in respect to the horizontal plane of the mold tables of the rotating table, according to an arrangement disclosed and illustrated in the above mentioned prior patent application of the applicant.

The mobile base plate 8 bears at its lower end-run onto the fixed rest plate of the suspension structure indicated as a whole with 10, from which extend upright the central pins 9. This occurs when the lifting stems 11 of the mobile bottoms 4 of the molds are retracted and thus disengaged from the suspension structure 10 by actuator devices of the work-station.

The metal grommet of reinforcement of the central hole of the grinding wheel is slipped over the central pin 6 before pressing the grinding wheel.

As already said,, the rotating table rotates by increments advancing the mold tables 2 that constitute it, from a workstation of the machine to the successive.

In Figure 2, when a mold table 2 arrives at the filling work-station C, the lifting stems 11 are still in their rest position, disengaged from the overhanging rotating structures for allowing the rotating table of the machine to turn, bringing the mold table 2 to the station C. In these conditions of disengagement of the lifting stems 11, the bottom 4 of the mold cavity is at its lower end-run (fully retracted) position and over the surface of the mobile bottom 4 may be present a first reinforcement net rl , having been placed there 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 rl resting on them, to the same level or slightly smaller level (of few tens of a millimetre, corresponding to the thickness of the eventual reinforcement net laying onto the mobile bottom).

As shown in Figure 3, the distributor drawer cl, containing a certain volume of abrasive mixture M, is suspended at a certain height over the mold table 2, and at least part of its bottom is closed by a belt conveyor T circulating on pulleys, one of which is moved by an electrical motor, controlled for timing the activation of the belt conveyor. The distributor drawer cl, is first advanced along a radial axis of the work-station C as far as "flying" over the whole area of the mold table 2. While flying over the area of the mold cavity, circulation of the conveyor belt T that constitutes the bottom of the distributor drawer Cl is activated, such to expel by drag-and- translate mechanism a time constant quantity of abrasive mixture M through the slit F, causing it to drop from the end portion of the profile of the conveyor belt over the area that comprises the mold cavity. An appropriate timing of the motion of the belt T limits the deposition of a layer Mt of a pre-established desired uniform thickness (equal to or slightly exceeding the height of the filler to be introduced in the mold cavity) of abrading mixture, over the area of the mold cavity and in the closest neighbourhood of it onto the substantially planar surface of the table.

Once the distributor drawer cl has returned to its rest or stand-by position (generally above a fixed part of the table) as shown in Figure 4, the lifting stems 11 are retracted such to put the mobile bottom 4, that carries the mass Mc of abrasive mixture of the uniformly deposited layer bearing on the mobile bottom 4, at a height corresponding to a pre-established fill volume of the mold cavity that normally corresponds roughly to the thickness of the deposited layer.

When the bottom of the mold is lowered to this level, a scraping blade c2 is lowered to press elastically on the planar surface of the mold table 2, as shown in Figure 5, and thence it is moved along a radial direction, sweeping the relatively small excess amount of abrasive mixture M fallen on the surface of the mold table, around the rim of the mold cavity, over the filling Mc of the cavity, levelling its content, and pushing the excess mixture M as far as making it fall off the table into a collecting hopper c3, placed at the inner edge (in a radial direction) of the mold table 2, as shown in Figure 6.

At the end of the radial sweeping, the scraping blade c2 lifts up and returns in its stand-by position, as shown in Figure 7.

In this series of figures it is also possible to observe the retraction of the lifting stems 11 out of engagement with the rotating table structure, such that with the mobile bottom 4 of the mold returned again to its fully lowered position, the mold table 2 is free to advance as far as reaching the work-station D, for receiving eventually a second reinforcement net (also in this case preferably preheated as for the first reinforcement net) onto the levelled layer of abrasive mixture of pre- established thickness and substantially homogeneous density, and thence progressing through the terminal phases of loading of a paper label el and of a metal grommet bml of reinforcement of the central hole of the grinding wheel slipped over the central pin 6 of the mold, as already described with reference to the machine layout of Figure 1.

Claims

C L A I M S

1. 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 thermosetting binder, at least a metal grommet of reinforcement of a central hole of the wheel and optionally one or more reinforcement fabric of resin impregnated fibers of a high tensile strength material, and hardening in oven the fabricated biscuit, characterized in that the loading of a predetermined amount of abrasive mixture into at least a mold cavity hole of a planar mold table, closed by a mobile bottom liftable at different levels, comprises performing in succession the following steps:

lifting said mobile bottom at a first level equal to or negligibly lower than the surface plane of the mold table;

depositing on a restricted area containing said cavity of the planar surface of the mold table a layer of uniform thickness of abrasive mixture allowed to free fall by gravity from a height while maintaining the mobile bottom at said first lifted level;

lowering said mobile bottom sustaining the mass of abrasive mixture of said layer of uniform thickness deposited on its surface to a second lifted level corresponding to an established fill volume of the mold cavity;

removing the excess abrasive mixture of said layer from the planar surface of the mold table deposited in said restricted area around the rim of the cavity footprint and levelling the abrasive mixture filling the mold cavity by a single stroke of a scraper blade moving from a stand-by position off the cavity area, lowering itself onto the planar mold table, sliding thereon over the filled mold cavity as far as sweeping the excess abrasive mixture off the mold table, lifting itself and returning to said stand-by position;

further lowering said mobile bottom inside said hole to a pressing position.

2. The process of claim 1, wherein said free fall from a height deposition of a layer of uniform thickness of abrasive mixture, takes place by expelling said mixture through a slot at either the leading or trailing edge of a distribution drawer travelling at a height above the mold table, at least part of the bottom of the drawer being a belt conveyor activated to expel said mixture while flying over said restricted area.

3. The process according to claim 1, characterized in that it is performed by advancing an assembly of said mold table and relative mobile bottoms of mold cavities in succession through a plurality of work-stations, at least the workstations of deposition on the surface of said mold table of said layer of abrasive mixture of uniform thickness and the work-station of removal of excess mixture and levelling of mold cavity fillings being equipped with said lifting mechanisms of said mobile bottom.

4. 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.

5. 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 blade moves in a radial direction.

6. 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, optionally together with at least a paper label, one or more 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 established levels referred to the level of the surface of the mold table for modifying the depth of the mold cavity, the loading of a certain amount of abrasive mixture inside each mold cavity with mobile bottom of a mold table being performed in one of said work-stations having a distributing drawer containing a volume of abrasive mixture, flying at a certain height above said mold table, at least part of the bottom of the distributor drawer being constituted by a conveyor belt circulating on free-wheeling and motor driven control pulleys;

said conveyor belt defining an outlet slot of a layer of abrasive mixture of uniform width and thickness in cooperation with an upper lip of an aperture in the leading or trailing wall of said flying distributor drawer for releasing off the conveyor belt circulating over a pulley the mixture of said outlet layer in a uniform stream falling over a restricted area of said mold table containing one or several mold cavities by timing the drive motor of at a least a control pulley of said conveyor belt.

7. The machine according to claim 6, having positioning means of the mobile bottom of said or said mold cavities of a mold table adapted to

lift and sustain said mobile bottom at a first level coinciding with or lower than the surface plane of the mold table;

retract inside the hole of the mold table the mobile bottom, sustaining the mass of abrasive mixture received thereon upon depositing a homogenous layer of abrasive mixture over the restricted area of the mold table containing said cavity, to a lower level corresponding to an established fill volume of the mold cavity with abrasive mixture;

lower the mobile bottom inside the hole of the mold table to the lowest pressing position level, disengaging said positioning means from the mold table for advancing toward a successive work station.