The present invention concerns smoothing mills for finish¬ ing the surfaces of manufactured articles or for preparing such articles for subsequent treatment.

Smoothing mills, particularly the disk type, are well-known tools and by means of an abrasive body, especially a rota¬ ting disk, are used for treating the surface of structures such as the body of a car, a piece of wooden furniture, floor- ing or thresholds of stone or wood, to obtain a good finish or to prepare the surface for further treatment such as painting .

Special devices enable the dust, produced by the abrasive body on the surface to which said body is applied, to be sucked up and carried into chambers placed for the purpose, in order to keep the environment cleaner and healthier. Suction and damping down the dust produced by such work is an important aspect of the process.

In one known model there is a fan fitted onto the motor shaft that rotates the plate-shaped part carrying the abra¬ sive disk.

The fan sucks up air and dust through a set of holes in the plate and in the abrasive disk and conveys it into a tube. Suction produced by the fan, moved by the same ro¬ tating shaft as that moving the abrasive disk, is in no way sufficient for the purpose. To overcome this insufficiency use is made of a separate self-propelled electric suction fan (of the bin-type vacuum cleaner) whose suction tube is connected to the smoothing mill .

A drawback to this system is the awkward presence of the electric cable feeding the fan as well as the high consump- tion of energy involved.

Other systeπjs comprise a fixed suction system having one or more articulated arms carrying suction tubes connected to one or more smoothing mills by couplings at the ends of said arms . These fixed installations require a considerable investment and maintenance costs are also high.

Further, coupling the smoothing mill to an articulated arm of the system limits movement, and consumption of energy is even higher than that for a mobile suction unit. Purpose of the present invention is to provide a smoothing mill comprising built-in means of dust suction which are more powerful, more effective and more efficient. A further purpose is to exploit the air discharged by the motor and the compressed air from the machine, which would otherwise be lost, and this saves a great deal of energy. Subject of the invention is a smoothing mill comprising a

suction duct for generated dust and a suction device made by fitting inside said suction duct one or more . ubular ejectors each comprising an initial truncated-cone shaped chamber and a final cylindrical chamber. Said ejectors are arranged coaxially in series, one par¬ tially inside another, with the truncated-cone shaped part preceding the cylindrical part, in the direction of suction. The compressed air enters the first of these ejectors through a coaxial nozzle. Internal diameter of the cylindrical chambers is equal to the lesser diameter of the truncated-cone shaped chambers preceding them.

Internal diameters of the second ejector are greater than the corresponding ones of the first ejector while internal diameters of the third ejector are greater than the cor¬ responding diameters of the second ejector, and so on. In this way a smooth passage of air in the cavities among the ejectors arranged in series is assured. The overall purpose is to obtain, for each ejector and by associated action of the truncated-cone and cylindrically shaped chambers, a high progressively multiplying capacity and suction power in the various stages of suction created by the ejectors arranged in series. The first ejector lies practically inside the truncated- -cone shaped chamber of the second ejector which in turn lies practically inside the truncated-cone shaped chamber of the third ejector.

In a preferred execution the smoothing mill is of the disk type . Said disk is mounted at the end of the shaft of a motor and comprises a suction chamber placed behind said disk

and communicating with the working area through a special¬ ly made aperture in the disk.

The dust suction duct communicates with said suction cham¬ ber through apertures made in the top of said chamber. The nozzle of the compressed air duct communicates with the first truncated cone-cylindrical ejector at a position practically on the front edge of said ejector. In a preferred type of execution the smoothing mill is worked by a compressed air motor, the compressed air dis- charged by said motor being used to feed the nozzle asso¬ ciated to the . runcated cone-cylindrical ejector. A valve for regulating opening and closing of the passage for fluid is fitted on the duct bringing in compressed air. Said valve comprises a shutter whose position in relation to the valve can be continuously adjusted by a ring nut.

By moving the lever that works the valve and adjusting ro¬ tation of the ring nut, the quantity of compressed air can be regulated as required. In another type of execution the smoothing mill is opera- ted by an electric motor, the nozzle associated to the truncated cone-cylindrical ejector being connected to a generator of compressed air by a flexible tube. The above-mentioned abrasive diak is supported by a plate, the surface that couples it to said disk being virtually annular and containing radial grooves which extend from a central lower zone to the outer circumference. Said abrasive disk is held in place by a retaining part with radial arms that compress said abrasive disk and, bending it, force it inside said grooves. The openings for dust suction through said plate and ab¬ rasive disk, and which are formed inside said lower central

zone of said plate, through the radial grooves referred to above ensure suction paths for said ejectors which will not become blocked up during operation even when the abra¬ sive disk is in contact with the work surface. The invention clearly offers many advantages.

By the use of a compressed air motor, the flow of com¬ pressed air discharged by the motor forms the primary fluid that actuating the dust suction system thus ensuring a ra¬ tional form of operation and and economy in consumption of energy by the machine.

Suction of dust by a number of ejectors, and therefore in several stages, gives rise to increased effects appreciab¬ ly raising the power of suction, capacity and efficiency. The presence of radial grooves on the working surface of the abrasive disk improves suction of dust produced while the smoothing mill is in operation.

Said radial grooves collect within them a large proportion of the dust produced during rotation, interception and suction being more immediate. Under all conditions , there- fore, these radial grooves ensure suction o-f air from out¬ side close to the external circumference of the rotating abrasive disk, preventing interruption in the suction flow during work. Characteristics and purposes of the invention will be made still clearer by the following examples of its execution illustrated by diagrammatically drawn figures. Fig. 1 The invented smoothing mill with compressed air motor and two-stage suction, longitudinal section. Fig. 2 Plan view of the mill partially cut. away. Fig. 3 Detail of the two-stage suction device.

Fig. 4 Detail of the three-stage suction device.

Fig. 5 Smoothing mill with electric motor, longitudinal section. The smoothing mill comprises a body 1 of die-cast plastic material, provided with a handle 2. A compressed air motor 40 is installed inside said body 1. On the end of the shaft 6 of said motor, a plate 7 has been mounted to which plate is fitted an abrasive disk 9, with a pad 8 of elastic material interposed between them. In said pad 8 there is a central hole 10 and two or more radial grooves 8s, angularly spaced one from another and extending from said central hole 10 to the outer edge of said abrasive disk 9.

The abrasive disk 9 is locked in place by a retaining part 11, this in turn being fixed to the end of the shaft 6 by a screw 12.

In the plate 7 there is an opening 7a whose position cor¬ responds to that of the central hole 10 in the pad, and similarly there is a central hole 9a in the disk 9. On the back of the plate 7 there is a suction chamber 13 with cylindrical walls 14 of a flexible material.

Said chamber 13 communicates through a hole 15 at its top with a longitudinal duct 16 made in the handle 2. Inside said handle 2 are two more ducts, practically pa¬ rallel to the first, respectively a duct 17 carrying com- pressed air and a duct 18 for discharging it.

Compressed air is received from a flexible tube 19. On the compressed air entry duct 17, upstream of the motor 30, is mounted a control valve 20 that can be worked by the operator by an external lever 21 acting on a shutter 20 with a pin and return spring 23.

The end position of the shutter 22 when the valve 20 is

- 1 - is opened, obtained by moving the lever 21, can be adjusted by means of a knurled ring nut 24 supported by th-e lever 21, so that micrometric regulation of the rotating speed of the abrasive disk can easily be done by increasing or reducing the passage opening of the valve 20.

Having started up the motor 40, the compressed air returns through the duct 18 from where it flows into the nozzle 26 in line with a tubular ejector 3 which, in the .direction of forward movement of the compressed air, comprises an initial truncated-cone shaped part 27 and a second cylin¬ drical part 28 whose internal diameter is equal to the les¬ ser diameter of the truncated cone.

The ejector 3 sucks up air and dust from the working area through the duct 16, the hole 15 in the chamber 13 on the plate 7, the holes 7a in said plate, those 9a in the abra¬ sive disk 9, and hole 10 in the rubber pad 8. The ejector 3 is surrounded by a second ejector 4 formed of a tubular body comprising an initial truncated-cone shaped part 29 and a second cylindrical part 30. The jet flowing from the first ejector 3 therefore pene¬ trates into the second ejector 4 at the position of the second cylindrical part 30.

A further flow of air is created in the cavity between the first ejector 3 and the second ejector 4, making the flow passing from the first ejector more powerful.

A flexible discharge tube 31 connected to the end of the body of ejector 4, carries discharged compressed air from the motor, the dust produced by the process and outside ^■ air sucked in from the working area towards a collecting chamber .

In the figures the path followed by the compressed air is

is indicated by arrows and continuous lines while those taken by the mixture received from the ejectors and car¬ ried to the collecting chamber are indicated by arrows and dotted lines. The best suction is obtained when the end of the nozzle 26 is placed practically flush with the front edge of the first ejector 3. Fig. 4 shows a variation. The second ejector 4 is in turn lodged inside a tubular body constituting a third ejector 5 comprising an initial truncated-cone shaped part 41 and a second cylindrical part 42.

The end of said tubular body is connected to the discharge tube for dust, like the tube 30 in Fig. 1. Fig. 5 illustrates a further version.

Rotation of the abrasive disk 9 is obtained by an electric motor 50 fed through an electric cable 52.

A flexible tube 53, for feeding in compressed air as the primary fluid for the dust suction system, is directly con- nected to the nozzle 26 coaxial with the first ejector 3. In locking the abrasive disk 9 the retaining part 11 -is aided by the radial arms which bend the abrasive sheet in¬ side the radial grooves 8s in the rubber pad. The coordinated openings in the disk 9 - plate 7 and pad 8, corresponding to the central lower area of the working surface of the plate 7, keep said central lower area in communication with the outer air even when the abrasive disk is pressed against a flat surface. This makes suction of dust from the process more efficient still, as has been already explained.