WO2018047103A1

WO2018047103A1 - Machining apparatus

Info

Publication number: WO2018047103A1
Application number: PCT/IB2017/055418
Authority: WO
Inventors: Enrico D'AMATO
Original assignee: BERSIGOTTI, Patrizia
Priority date: 2016-09-09
Filing date: 2017-09-08
Publication date: 2018-03-15
Also published as: EP3509789B1; IT201600091311A1; EP3509789A1; PL3509789T3; ES2924854T3

Abstract

There is described a machining apparatus (1, 100, 200), associable with a machine tool, to perform a surface finishing operation on a workpiece to be machined that comprises an abrasive belt (2) in contact with the workpiece to be machined so as to be driven in movement by the workpiece, and loop closed on a plurality of rollers (3, 4, 5), and a braking device (10) comprising a braking element (11, 111, 211) adapted to brake the roller (3) so as to generate a relative motion between the abrasive belt (2) and the workpiece to be machined.

Description

MACHINING APPARATUS

The invention relates to a machining apparatus for performing a surface finishing operation on a workpiece to be machined. In particular, the invention relates to an apparatus for performing smoothing (also called grinding) on a rotating workpiece that has been subjected to turning.

Surface finishing operations, such as smoothing or grinding, are machining operations required to give the desired surface finish to the surface of a workpiece that has been subjected to a stock removal operation (such as turning or milling).

These operations can be performed either manually or automatically.

In the case of manual machining, the operator carries out smoothing of the workpiece directly, for example using abrasive paper or with the aid of a portable tool in which a belt or disc made of abrasive paper run at a given rotation speed. Very often, when the workpiece is machined manually the process is carried out by the operator directly on the machine tool (such as a lathe) with which the mechanical machining processes were previously performed on the workpiece. During the machining operation, the workpiece to be machined rotates while the operator maintains the abrasive paper in contact with the rotating workpiece.

A drawback of this type of machining is that operations are long and laborious.

Another drawback is that the surface finish cannot be homogeneous and the quality of the worked surface depends greatly on the ability of the operator.

A further drawback of manual machining operations is that they are extremely dangerous for the operator, who, in order to machine the workpiece with the abrasive paper, must necessarily be located close to the rotating workpiece. As the rotating workpiece is typically made of metal material and can also rotate with significant rotation speeds, it is evident that the operator could be injured, even seriously, due to accidental contact with the rotating metal workpiece being machined.

In the case of automatic machining, it is necessary to use machine tools arranged to perform the aforesaid surface machining operations.

A drawback of this type of machining operation is that the processing cycle of the workpiece to be machined is more complex and longer, as the machined workpiece must be detached (for example from the lathe or cutter) and re-attached on the finishing machine tool (for example a grinder).

An object of the present invention is that of providing a machining apparatus to perform a surface finishing operation on a workpiece to be machined, in which the apparatus is improved with respect to prior art apparatus.

Another object is that of providing a machining apparatus with automatic operation that does not require the constant presence of an operator.

Yet another object is that of providing a machining apparatus that allows the operator to work in safety, eliminating the risk of injury during processing.

A further object is that of providing a machining apparatus capable of performing a surface finishing operation on the workpiece directly on the machine tool (for example, a lathe) with which the necessary stock removal operations were previously performed on the workpiece.

Yet a further object is that of providing a machining apparatus having an abrasive surface that, during operation, becomes worn to a lesser extent and in a more homogeneous manner with respect to the abrasive elements of prior art apparatus.

One more object is that of providing a machining apparatus that is inexpensive and easy to use.

According to the invention, there is provided a machining apparatus as defined in the appended claim 1 .

The invention may be better understood and implemented with reference to the accompanying drawings, which show an example of a non-limiting embodiment thereof, wherein:

Fig. 1 is an exploded perspective view of a first embodiment of a machining apparatus according to the present invention;

Fig. 2 is a plan view from above of the machining apparatus of Fig. 1 , in which the protection means have been removed;

Fig. 3 is a sectional view along the plane Ill-Ill visible in Fig. 2;

Fig. 4 is a plan view from above similar to Fig. 2, in which the apparatus is shown in three different operating positions;

Fig. 5 is a perspective view from below of the machining apparatus of Fig.

1 ;

Fig. 6 is a side view of a braking device comprised in the machining apparatus of Fig. 1 ;

Fig. 7 is a perspective view of the braking device of Fig. 6;

Fig. 8 is a perspective view from above of a second embodiment of a machining apparatus according to the present invention, in which the protection means have been removed;

Fig. 9 is a plan view from above of the machining apparatus of Fig. 8;

Fig. 10 is an exploded perspective view of the machining apparatus of Fig.

8;

Fig. 1 1 is a perspective view from above of the machining apparatus of Fig. 8 with the protection means present;

Fig. 12 is a perspective view from above of a third embodiment of a machining apparatus according to the present invention, in which the protection means have been removed;

Fig. 13 is a plan view from above of the machining apparatus of Fig. 12; Fig. 14 is a perspective view of a braking device included in the machining apparatus of Fig. 12;

Fig. 15 is a sectional view of the braking device of Fig. 14 along the plane XIV-XIV.

With reference to Figs. 1 -15, three different embodiments of a machining apparatus are visible, each identified respectively as a whole by the reference numeral 1 (Figs. 1 -5), 100 (Figs. 8-1 1 ) and 200 (Figs. 12-15).

The machining apparatus 1 , 100, 200 according to the present invention is arranged to perform a surface finishing operation on a workpiece to be machined (not depicted), in particular smoothing or grinding.

The machining apparatus 1 , 100, 200 comprises an abrasive belt 2 having an active surface 2a arranged to contact the workpiece to be machined so that the abrasive belt 2 is driven in movement by the motion (e.g. rotational) of the workpiece to be machined.

The machining apparatus 1 , 100, 200 further comprises a plurality of rollers 3, 4 and 5 intended to support the abrasive belt 2. The abrasive belt 2 is loop closed on the plurality of rollers 3, 4 and 5 and these latter are positioned spaced apart from one another to tension the abrasive belt 2.

In use, the workpiece to be machined rotates with a given rotation speed, which can even be very high, and moves the abrasive belt 2 that is mounted on rollers 3, 4, 5.

The machining apparatus 1 , 100, 200 comprises a braking device 10 (Fig. 2; Fig. 8; Fig. 12). The braking device 10, in turn, comprises a braking element 1 1 , 1 1 1 , 21 1 of the roller 3 configured to brake this roller 3, so as to generate a relative motion between the abrasive belt 2 and the workpiece to be machined.

It is understood that in other versions, not depicted, the braking device 10 can also be mounted in a different position to the position illustrated in the accompanying figures, so that the braking element 1 1 , 1 1 1 , 21 1 acts on another roller 4, 5 of the plurality of rollers.

The machining apparatus 1 , 100, 200 comprises a pair of arms 26, 27, both hinged to a single supporting element 28 on opposite sides with respect to one another.

The pair of arms 26, 27 is arranged according to a symmetrical configuration with respect to a longitudinal axis X of the machining apparatus 1 , 100, 200.

Each arm 26, 27, at an end thereof opposite the end hinged to the support element 28, rotatingly supports a respective roller 4, 5 in an idle manner. A torsion spring 29 is associated with the first arm 26 and tends to maintain it in the position in which the respective roller 4 is at the maximum distance possible from the longitudinal axis X.

An adjustment plate 30 is instead associated with the second arm 27 (Figs. 4 and 5; Figs. 8 and 9; Fig. 12), and is provided with three adjustment holes 31 to which a respective position of the second arm 27 corresponds. In a tooling phase of the machining apparatus 1 , 100, 200, the operator sets the position of the second arm 27 fixing it to one of the three holes 31 , as a function of the size and of the shape of the workpieces to be machined. It should be noted that the first arm 26 takes a position that is a function of the position of the second arm 27 so as always to maintain the symmetrical configuration of the pair of arms 26, 27 with respect to the axis X.

Consequently, the pair of arms 26, 27 can take three different positions, indicated respectively with P1 , P2 and P3 in Fig. 4 for the first embodiment of the machining apparatus 1 by way of example; the further two embodiments 100 and 200 also make it possible for the pair of arms 26, 27 to take said positions P1 , P2 and P3.

With regard to the position of the rollers 4 and 5, as the pair of arms 26, 27 passes from the position P1 to the position P2 and finally to the position P3, the distance of the rollers 4 and 5 from the longitudinal axis X increases. In other words, the possibility of adjusting the position of the arms 26, 27, allows them to be taken to the most suitable positions as a function of the workpiece to be machined. For example, the pair of arms 26, 27 will be in position P1 to machine a workpiece with a small diameter, and in position P3 to machine a workpiece with a large diameter.

The support element 28 is fixed through screws to a supporting base 32 through which the machining apparatus 1 , 100, 200 is associated with a machine tool, such as a lathe or a cutter. The supporting base 32 can be mounted on guides of known type (not depicted) to allow the machining apparatus 1 , 100, 200 to be moved on the machine tool so as to position it in the required machining position.

The machining apparatus 1 , 100, 200 further comprises protection means 33, 133 arranged to isolate the portion of apparatus arranged close to the operator (in particular the roller 3, the braking device 10 and the abrasive belt 2), from the operator.

Figs. 1 -5 show a first embodiment of the machining apparatus identified as a whole by the number 1 .

In the machining apparatus 1 , the rollers 3, 4 and 5 intended to support the abrasive belt 2 are all idle rollers. The braking device 10 (Fig. 6 and 7) comprises a braking element 1 1 that has a contact surface 1 1 a intended to contact a roller, which in the version illustrated is the roller 3. The contact surface 1 1 a evidently has a shape complementary to the shape the roller 3, i.e., a concave shape having a radius of curvature equivalent to the radius of the roller.

The braking element 1 1 can be made of a metal alloy, such as bronze.

In other versions, the braking element 1 1 can also be made of other materials suitable to develop a friction force (between braking element 1 1 and roller 3) sufficient to brake the roller also in the presence of a limited thrust force of the braking element 1 1 . However, it should be noted that the material with which the braking element 1 1 is made must have a greater hardness with respect to the material with which the roller 3 is made to avoid damaging this latter.

The braking element 1 1 is fixed to a supporting base 12. This latter is in turn mounted on a moving slide 13.

The moving slide 13 rests on a support plate 14 and can slide with respect to this latter.

Therefore, the braking element 1 1 is mounted on the moving slide 13, which consequently allows the braking element 1 1 to be moved toward or away from the roller 3.

For this purpose, the braking device 10 comprises adjustment means 15 arranged to move the moving slide 13. The adjustment means 15 comprise a knob 16 and a pin 17 fixed thereto, wherein the knob 16 is arranged to rotate the pin 17.

The pin 17 comprises a threaded portion 18 arranged to engage, through a threaded connection, with a projecting portion 19 of the moving slide 13 so that the slide 13 can move together with (i.e., in an integral manner) the pin 17.

The adjustment means 15 further comprise a safety element 20, for example configured as a lever. It must be understood that in other versions, not depicted, the safety element 20 can be configured as any device suitable to block accidental rotation of the pin 17, such as a nut-lock nut. The safety element 20 is arranged to prevent accidental rotation of the pin 17 that could, for example, be triggered by the vibrations generated by the machining apparatus 1 during operation.

The safety element 20 is connected to the pin 17 at the threaded portion 18, in particular in an area adjacent to the projecting portion 19.

In this way, an area 21 is identified on the pin 17 having a longitudinal extension that defines the extent of the maximum possible movement of the pin 17 following rotation of the knob 16, indicated by the letter C in Fig. 6.

The pin 17 further comprises a thickening 22 that delimits an end portion 23 thereof. The end portion 23 is arranged inside a specific seat 24 (Fig. 3) made in the support plate 14 in a position such as to be aligned and concentric with respect to the pin 17.

The braking device 10 comprises an elastic element 25 arranged to exert an elastic force F (Figs. 2-4) on the moving slide 13; the force being oriented so as to maintain the braking element 1 1 pressed against the roller 3. The elastic element 25, which can for example be configured as a helical spring, is housed in the seat 24 and is positioned so as to surround the end portion 23. In use, the compressed elastic element 25 exerts a thrust caused by the elastic force F on the thickening 22 so as to thrust the pin 17, and consequently the moving slide 13, in the same direction as the force F.

Acting on the adjustment means 15 it is possible to adjust the intensity of the force F, and therefore the force with which the braking element 1 1 presses against the roller 3. In fact, a rotation of the knob 16 corresponds to a linear movement (translation) of the pin 17. If the pin 17 moves toward the elastic element 25, this latter is further compressed and therefore the elastic reaction force F that the elastic element 25 exerts on the pin 17 increases. Vice versa, if the pin 17 moves away from the elastic element 25, this latter is less compressed and the elastic force F exerted on the pin 17 decreases.

In use, the operator adjusts the position of the braking element 1 1 acting on the adjustment means 15. Adjustment is of empirical nature and is performed by the operator while the workpiece to be machined is rotating. Adjustment of the position of the braking device 10 ends when the operator, based on experience, obtains a running speed of the abrasive belt 2 suitable to carry out the surface finishing operations required.

Said protection means 33 comprise a lower element 34 and protective casing. The lower element 34 can for example be fixed to the support plate 14. The protective casing 35 is removably fixed to the lower element 34, for example through threaded screws.

It should be noted that in a version, not depicted, the protective casing 35 comprises a movable wall 35a, which is hinged to one of the walls adjacent thereto to rotate with respect to these latter. In this way, the movable wall 35a allows an operator to access the inside of the machining apparatus 1 to perform the necessary maintenance or set up operations of the apparatus (such as replacing the worn abrasive belt 2, or changing the position of the pair of arms 26, 27) without removing the protective casing 35 from the lower element 34.

Figs. 8-1 1 show a second embodiment of the machining apparatus identified as a whole with the number 100. In the machining apparatus 100, the rollers 4 and 5, intended to support the abrasive belt 2, are idle rollers.

In said second embodiment the braking element 1 1 1 of the braking device 10 is represented by a rotary damper on the output shaft 1 16 of which a rotation axle 1 12 of the roller 3 is mounted.

The rotary damper 1 1 1 comprises a body with an orifice filled with a special high viscosity fluid (of silicone type) and permanently sealed. This fluid is made to seep through a hole or a groove in a rotating blade integral with the output shaft 1 16 obtaining the damping effect. The damping torque that is obtained is determined by the viscosity of the fluid and by the shape of the orifice.

The machining apparatus 100 further comprises a support plate 1 13 fixed by means of screws to said supporting base 32 (Fig. 10). The rotary damper 1 1 1 is fixed by means of screws to said support plate 1 13; a spacer ring nut 1 14, through which the output shaft 1 16 is inserted, is interposed between the support plate 1 13 and the rotary damper 1 1 1 . The rotation axle 1 12 of the roller 3 is housed inside a bearing 1 15 supported by said support plate 1 13.

The rotary damper 1 1 1 is adapted to exert on the roller 3 a damping torque adapted to damp the rotation of the roller 3, slowing the running speed of the abrasive belt 2; this damping torque being a function of the viscosity of the fluid and of the shape of the orifice. By choosing the type of rotary damper 1 1 1 appropriately, a running speed of the abrasive belt suitable to obtain the desired surface finishing operations is achieved.

As shown in Fig. 1 1 , said protection means 133 are represented by a casing with a first element 138 that surrounds and encloses said roller 3 within it; a first and a second plate 134 and 135, of which one (134) has larger dimensions with respect to the other (135), extend from said first element 138. The plate 134 comprises a slit 136 provided at its ends with two projecting elements 137, said slit 136 being adapted to allow the operator to observe the abrasive belt 2 during use of the apparatus 100 and said projecting elements 137 acting as references for approximately detecting the running speed of the abrasive belt 2. In particular, the running speed of the belt 2 is detected based on the time in which a given point identified on the belt 2 travels along the slit 136, starting from one projecting element and reaching the other 137; a table (not shown in the figures), will indicate the corresponding running speed, as a function of the time the belt 2 takes to travel along the slit 136 from said point identified thereon.

Finally, a third embodiment of the machining apparatus 200 according to the present invention is shown in Figs. 12-14.

In the third embodiment the braking element 21 1 of the braking device 10 is represented by a magnetic clutch, on the shaft 212 of which the roller 3 is integrally mounted. The magnetic clutch is adapted to exert on the roller 3 a given braking torque that is a function of the magnetic force generated by the presence of a magnetic field created by a permanent magnet or by a direct current coil.

In the embodiment illustrated in Figs. 14 and 15, the magnetic clutch 21 1 comprises a magnetic disc 213, which interacts with a friction disc with magnets 214 integral with the shaft 212 of the roller 3. The magnetic coupling between the magnetic disc 213 and the friction disc 214 generates a braking torque on the shaft 212 of the roller 3, slowing the running speed of the abrasive belt 2.

In the alternative case of the magnetic clutch 21 1 with a coil supplied with electric current, the rotation speed of the roller 3 is also adjustable proportionally to the electric current supplying the coil.

The magnetic clutch 21 1 is fixed by means of screws to a support 216. The shaft 212 is inserted, and is free to rotate, inside a ball bearing 215 housed inside the support 216. Advantageously, a protective casing 217 is, in turn, fixed to the support 216.

Also in this case, due to the magnetic clutch 21 1 a running speed of the abrasive belt 2 suitable to perform the desired surface finishing operations is obtained.

The third embodiment of the machining apparatus 200 is provided with protection means identical to the protective means (133) present in the second embodiment 100.

As the abrasive belt 2 runs at a constant speed, it is evident that the active surface 2a of the belt becomes worn in a homogeneous manner. Once the abrasive belt 2 has been completely worn, the operator replaces the worn belt with a new abrasive belt.

Thanks to the invention, a machining apparatus 1 , 100, 200 that has the advantage of operating automatically is provided. In fact, once the machining apparatus 1 has been set up by the operator in a tooling phase prior to machining, the apparatus can operate autonomously, without requiring the constant presence or action of the operator.

An advantage of the apparatus according to the invention is that it has no "active" driving means arranged to move the abrasive belt 2, which instead moves thanks to the movement (typically, a rotation movement) of the workpiece to be machined. In fact, unlike prior art solutions provided with driving means (for example an electric motor that allows the apparatus to operate automatically), the absence of these driving means has the advantage of decreasing the footprint, weight and cost of the machining apparatus 1 , 100, 200. The machining apparatus 1 is also advantageous in the case in which the driving means are of the type operated manually by an operator, for example by operating a handwheel, as in this case operation is not automatic and requires the action and constant presence of the operator during machining.

In this way, once the operator has preliminarily adjusted the position of the braking element 1 1 with respect to the roller 3 through the adjustment means 15, a constant running speed of the abrasive belt 2 is set. This allows another advantage of the machining apparatus 1 to be obtained, given by the fact that wear of the abrasive material of the abrasive belt 2 is substantially homogeneous along the whole of its extension, without localized wear typical of prior art solutions occurring. This increases in the useful life of the abrasive belt 2, which consequently requires to be replaced less frequently. In fact, localised wear of the abrasive belt 2 typical of prior art solutions can frequently lead to premature breakage of the abrasive belt 2, which must be replaced often with a consequent increase in costs.

Another advantage of the machining apparatus 1 , 100, 200 is that it provides an apparatus that is economical to produce that also allows operating costs to be decreased.

A further advantage of the machining apparatus 1 is that it is easy to use and to assemble on a pre-existing machine tool, such as a lathe.

Yet another advantage of the machining apparatus 1 is that it is very flexible, as it is capable of operating in three different positions P1 , P2, P3 as a function of the overall dimensions (in particular, of the maximum diameter) of the workpieces to be machined.

A further advantage of the machining apparatus 1 is that it can operate guaranteeing the safety of the operators and of the surrounding environment. In fact, thanks to the fact that operation of the apparatus 1 is completely automatic, during machining the operator is not required to take action on the workpiece to be machined, or on the abrasive belt 2 as this latter is driven in movement by the rotation of the workpiece, and runs with the desired speed due to the presence of the braking device 10 that was previously adjusted by the operator. Moreover, the protection means 33, 133 allow the apparatus 1 to be effectively isolated from the surrounding environment in order to protect people and objects in close proximity thereto.

Claims

1 . A machining apparatus (1 , 100, 200), associable with a machine tool, to perform a surface finishing operation on a workpiece to be machined, comprising:

an abrasive belt (2) having an active surface (2a) arranged to contact a workpiece to be machined, said abrasive belt (2) being driven in movement by the motion of the workpiece to be machined,

a plurality of rollers (3, 4, 5) intended to support said abrasive belt (2), wherein said abrasive belt (2) is loop closed on said plurality of rollers (3, 4, 5) and said rollers (3, 4, 5) are positioned spaced apart from one another to tension said abrasive belt (2),

said apparatus (1 , 100, 200) being characterized by further comprising a braking device (10) associated with a roller (3) of said plurality of rollers (3, 4, 5) and comprising a braking element (1 1 , 1 1 1 , 21 1 ) adapted to brake said roller (3) so as to generate a relative motion between said abrasive belt (2) and said workpiece to be machined.

2. The apparatus according to claim 1 , wherein said rollers (3, 4, 5) are all idle rollers and wherein said braking element (1 1 ) is mounted on a moving slide (13) that allows said braking element (1 1 ) to be moved toward or away from said roller (3).

3. The apparatus according to claim 2, wherein said braking device (10) comprises adjustment means (15) arranged to move said moving slide (13).

4. The apparatus according to claim 3, wherein said adjustment means (15) comprise a knob (16) and a pin (17) fixed thereto, wherein said knob (16) is arranged to rotate said pin (17); wherein said pin (17) comprises a threaded portion (18) arranged to engage, through a threaded connection, with a projecting portion (19) of said moving slide (13) so that said slide can move together with said pin (17).

5. The apparatus according to claim 4, wherein said adjustment means (15) comprise a safety element (20) arranged to prevent the accidental rotation of said pin (17) and is connected to this latter at said threaded portion (18).

6. The apparatus according to one of claims 2 to 5, wherein said braking device (1 1 ) comprises an elastic element (25) arranged to exert an elastic force (F) on said moving slide (13), said force (F) being oriented so as to maintain said braking element (1 1 ) pressed against said roller (3).

7. The apparatus according to claim 1 , wherein said braking element (1 1 1 ) of the braking device (10) is represented by a rotary damper on the output shaft (1 16) of which a rotation axle (1 12) of the roller (3) is mounted, said rotary damper (1 1 1 ) being adapted to exert on the roller (3) a damping torque adapted to dampen rotation of the roller (3), slowing the running speed of the abrasive belt (2).

8. The apparatus according to claim 7, wherein said rotary damper (1 1 1 ) comprises a body with an orifice filled with a high viscosity fluid and permanently sealed, said fluid being made to seep through a hole or a groove in a rotating blade integral with the output shaft (1 16), obtaining the braking effect, said damping torque being determined by the viscosity of the fluid and by the shape of the orifice.

9. The apparatus according to claim 8, wherein said rotary damper (1 1 1 ) is fixed by means of screws to a support plate (1 13) in turn fixed to a supporting base (32), a spacer ring nut (1 14), through which the output shaft (1 16) is inserted, being interposed between the support plate (1 13) and the rotary damper (1 1 1 ), said rotation axle (1 12) of the roller (3) being housed inside a bearing (1 15) supported by said support plate (1 13).

10. The apparatus according to claim 1 , wherein said braking element (21 1 ) of the braking device (10) is represented by a magnetic clutch, on the shaft (212) of which the roller (3) is mounted integral, the magnetic clutch being adapted to exert on the roller (3) a given braking torque that is a function of the magnetic force generated by the presence of a magnetic field created by a permanent magnet or a direct current coil.

1 1 . The apparatus according to claim 10, wherein said magnetic clutch (21 1 ) comprises a magnetic disc (213) that interacts with a friction disc with magnets (214) integral with the shaft (212) of the roller (3), the magnetic coupling between the magnetic disc (213) and the friction disc (214), generating said braking torque on the shaft (212) of the roller (3), slowing the running speed of the abrasive belt (2).

12. The apparatus according to claim 1 1 , wherein said magnetic clutch (21 1 ) is fixed by means of screws to a support (216), the shaft (212) is inserted, and is free to rotate, inside a ball bearing (215) housed inside the support (216).

13. The apparatus according to one of the preceding claims, comprising a pair of arms (26, 27) both hinged to a single supporting element (28) on opposite sides with respect to one another so as to be arranged according to a symmetrical configuration with respect to a longitudinal axis (X) of said machining apparatus (1 ), wherein each arm (26, 27) at an end thereof, opposite the end hinged to said supporting element (28), rotatingly supports a respective roller (4, 5) in an idle manner.

14. The apparatus according to claim 8, wherein a torsion spring (29) is associated with a first arm (26) and tends to maintain said first arm (26) in the position in which the respective roller (4) is at the maximum possible distance from said longitudinal axis (X); and wherein an adjustment plate (30) is associated with a second arm (27) and is provided with three adjustment holes (31 ) to which a respective position of said second arm (27) corresponds.

15. The apparatus according to one of the preceding claims, comprising protection means (33) arranged to isolate from the operator the portion of apparatus arranged in proximity of the operator; wherein said protection means (33) comprise a lower element (34) and a protective casing (35) removably fixed to said lower element (34).