EP2988889B1

EP2988889B1 - Apparatus for continuous casting and method to assemble said apparatus for continuous casting

Info

Publication number: EP2988889B1
Application number: EP14728320.4A
Authority: EP
Inventors: Daniele COMAND; Enzo FRANCESCHINIS
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 2013-04-23
Filing date: 2014-04-23
Publication date: 2017-06-07
Anticipated expiration: 2034-04-23
Also published as: ITUD20130054A1; WO2014174446A1; US20160067770A1; EP2988889A1; CN105555437A

Description

FIELD OF THE INVENTION

The present invention concerns an apparatus for continuous casting that allows to cast metal products such as blooms, slabs with a rectangular section, billets and bars with a double "T" section, also known as "beam blanks".
In particular, the apparatus for continuous casting according to the present invention is configured to allow to install a crystallizer of the plate type.
The present invention also concerns a method to assemble the apparatus for continuous casting.

BACKGROUND OF THE INVENTION

Apparatuses are known for continuous casting comprising a support structure, hereafter indicated as mold body, provided to support a crystallizer.
The mold body, depending on its constructional configuration, can allow to install crystallizers of the tubular type or plate type.
Crystallizers of the tubular type allow to obtain metal products with very simple and even cross section, for example blooms or billets with a round or square section, or with a more complex cross section but of limited size.
Known crystallizers of the plate type are generally used for casting billets, blooms, slabs or Beam Blanks.
Said crystallizers of the plate type comprise four plates connected to each other to define, with their surface which during use faces toward the inside, a casting cavity for the passage of the liquid metal.
The plates of the crystallizer are reciprocally connected by means of a frame which is mounted in the mold body. The frame is provided with four frame elements, and one of the plates is associated to each of them.
The four frame elements are reciprocally connected in correspondence to their lateral edges, enclosing the plates inside them. The plates are coupled with each other and are kept compact with each other in correspondence to respective connection edges by the action of reciprocal connection between the frame elements.
In particular, it is provided that the frame elements are connected to each other by connection means which can comprise holes, threaded or not, into which screws, stud bolts are screwed, centering or clamping pins are inserted and/or elastic elements are associated.
The plates have lateral edges shaped to define together respective same-shape couplings.
Direct connection means are not provided between the plates, since they could require an increase in thickness of the plates or could stiffen the structure too much, causing the onset of internal tensions because of thermal stresses. On the other hand the connection between the plates and the frame is particularly complex and uneconomic in terms of assembly time and the number of components used. Merely by way of example, some examples of connection between plates and support frame are shown in documents JP-A-S61.137651 , US-A-4.002.317 and US-A-6.041.848 . These known solutions are not only particularly complex and uneconomic in terms of time with regard to assembly, but also they do not guarantee a correct and precise reciprocal positioning of the various plates, to the detriment of the final quality of the cast product.
Ports to introduce and discharge a cooling liquid, usually water, may also be associated to the frame. The cooling liquid provides to keep the plates of the crystallizer constantly cooled.
One purpose of the present invention is to make an apparatus for continuous casting that allows to assemble the crystallizers simply and quickly in the mold body.
Another purpose of the present invention is to reduce the complexity of connection between the plates of a crystallizer of the plate type.
It is also a purpose of the present invention to make an apparatus for continuous casting that provides to reduce the number of components necessary for the reciprocal connection between the plates of a plate-type crystallizer.
It is also a purpose of the present invention to perfect a method to assemble an apparatus for continuous casting that is simple and fast.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claims, while the dependent claims describe other characteristics of the invention or variants to the main inventive idea.
In accordance with the above purposes, an apparatus for continuous casting comprises a support frame and a plurality of plates coupled to each other to define a tubular element, or crystallizer, provided with a casting cavity through which, during use, the molten metal is cast. Positioning members are provided to position the plates with respect to the support frame. The positioning members comprise on each occasion attachment devices or thrust devices.
According to one feature of the present invention, only a first plate of the plates is attached to a wall of the support frame, by means of corresponding attachment devices.
According to another feature of the present invention, thrust devices are associated to the other plates and are configured to thrust them toward the inside of the support frame and to compress them with respect to each other, in correspondence with respective connection edges and against the at least one first plate.
In this way it is no longer necessary to provide positioning members between parts of the support frame as was provided in the state of the art: it is enough to attach the first plate to the frame using the corresponding attachment devices and subsequently to compress the other plates with respect to each other and against the first plate with the thrust devices.
The number of attachment components is greatly reduced with respect to the state of the art, giving advantages both in terms of production costs and also in terms of assembly times.
The first plate in practice constitutes a reference system for positioning the other plates. The action of compression exerted by the thrust devices compacts the plates with respect to each other, defining the casting crystallizer. The action of indirect connection of the thrust devices allows the other plates, unlike the first one, to adapt to possible deformations/dilations that can arise due to heat stresses. In this way the onset of internal tensions can be prevented, which could deform the crystallizer as a whole and compromise its working life.
According to one form of embodiment of the present invention, the thrust devices comprise thruster elements mounted on the support frame and configured to thrust the other plates, unlike the first one, toward the inside of the frame. In this way the thruster elements allow to regulate the reciprocal pressure between the plates.
According to another form of embodiment, the thrust devices comprise elastic blocks attached to the external surface of the plates not constrained to the wall of the support frame and the thruster elements, mounted on the latter, and configured to compress the elastic blocks.
By regulating the action of the thruster elements on the elastic blocks it is possible to regulate the action of compression that the plates exert between them and, at the same time, guarantee possible settling of the plates due to vibrations, deformations or induced heat dilations.
The present invention also concerns a method to assemble a casting apparatus as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the present invention will become apparent from the following description of some forms of embodiment, given as a non-restrictive example with reference to the attached drawings wherein:

fig. 1 is a schematic representation in section of an apparatus for continuous casting according to the present invention;
fig. 2 is a section view from II to II in fig. 1;
fig. 3 is a section view from III to III in fig. 1;
fig. 4 shows an enlarged detail of fig. 3;
figs. 5 and 6 show an enlarged detail of fig. 3.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one form of embodiment can conveniently be incorporated into other forms of embodiment without further clarifications.

DETAILED DESCRIPTION OF SOME FORMS OF EMBODIMENT

With reference to fig. 1 an apparatus for continuous casting is indicated in its entirety by the reference number 10 and comprises a support structure or mold body 11, to which a plate-type mold 12 is associated.
The mold 12 develops longitudinally along a longitudinal axis Z which can have a rectilinear development, if the apparatus 10 is installed in a continuous casting machine of the vertical type, or it can have a slightly curved development if the apparatus 10 is installed in a curved continuous casting machine.
The mold 12 comprises a support frame 26 and a tubular element or crystallizer 27, disposed in the support frame 26.
The support frame 26 can also have a tubular configuration as shown in figs. 1-3 or, alternatively, it may be provided with a plurality of frame elements suitably connected with each other to allow to insert the crystallizer 27 inside it. The frame elements can comprise, for example, a plurality of plates, blades or metal sheets solidly coupled with each other, for example by welding.
Between the support frame 26 and the crystallizer 27 an interspace 51 is defined, which takes into account possible heat dilations to which the crystallizer 27 is subjected during use.
The crystallizer 27 comprises a plurality of plates 28a, 28b, 29a and 29b connected to each other, in ways which will be described hereafter, to define a through casting cavity 30 for the passage of the melted metal.
The through cavity 30 has substantially the same section shape as that of the product that is cast, in this case a double "T" shape to obtain products such as beam blanks.
Each plate 28a, 28b, 29a and 29b is provided with a plurality of passage channels 31 for the cooling water that extend through for the entire length of the plate 28a, 28b, 29a, 29b.
The passage channels 31 can be made completely in the thickness of each plate 28a, 28b, 29a, 29b, using holing operations for example, as shown in figs. 1-3. Other forms of embodiment, not shown in the drawings, possibly combinable with forms of embodiment described here, provide that the passage channels 31 are defined by longitudinal grooves open toward the outside and subsequently closed by closing elements.
Each passage channel 31 has an entrance end 32a and an exit end 32b located in fluidic communication by means of connection channels 33a and 33b made transversely in the thickness of each plate 28a, 28b, 29a and 29b.
The entrance ends 32a are closed in their end part by a closing flange 34, attached in the mold body 11.
The exit ends 32b, on the other hand, are closed by an upper flange 24, also attached to the mold body 11.
Each plate 28a, 28b, 29a and 29b is provided in its turn with respective connection edges 35 (fig. 3) suitably shaped to define reciprocal same-shape couplings between the plates 28a, 28b, 29a and 29b.
One or more connection members are attached, in a known manner, to the support frame 26, and provide to feed and discharge the cooling liquid to/from the passage channels 31.
In the form of embodiment shown in figs. 1 and 2, the connection members comprise entrance sleeves 37a and exit sleeves 37b, fluidically connected respectively to the entrance ends 32a and the exit ends 32b of the passage channels 31.
Some forms of embodiment can provide that the entrance sleeves 37a and/or the exit sleeves 37b serve one or more passage channels 31.
Positioning members 50 are provided to position the crystallizer 27 with respect to the support frame 26.
In particular, the positioning members 50 are suitable to determine the reciprocal positioning of the plates 28a, 28b, 29a, 29b and the external walls of the support frame 26.
In the forms of embodiment shown in figs. 1-6, the positioning members 50 comprise attachment devices 52 provided to attach a first plate 28a of the crystallizer 27 to a wall of the support frame 26.
According to one form of embodiment, the attachment devices 52 are configured to maintain, during use, the first plate 28a in a fixed and reciprocally predetermined position with respect to the support frame 26. In this way the first plate 28a defines a reference and precise positioning system for all the other plates 28b, 29a, 29b.
Some forms of embodiment of the present invention provide that the first plate 28a is the one disposed toward the extrados of the curve of the mold 12.
With reference to fig. 3, it is provided that the attachment devices 52 comprise a plurality of screws 53 that can be associated with the first plate 28a. In particular, the screws 53 can be inserted in through holes 54 made in the thickness of the support frame 26.
The screws 53 screw into threaded holes 55 made on the external surface of the first plate 28a and transverse to the longitudinal axis Z.
In the form of embodiment in fig. 3, between the first plate 28a and the wall of the support frame 26 there is a spacer 56 with a distancing function.
Some forms of embodiment of the present invention provide that the wall of the support frame 26 to which the first plate 28a is attached is provided with reference elements, such as protruding abutment portions, which define references for the correct positioning of the first plate 28a with respect to the support frame 26, and consequently also for the other plates 28b, 29a and 29b.
Thrust devices 57 (fig. 3) are provided to determine the reciprocal positioning of a second plate 28b, opposite the first plate 28a, of a third plate 29a that is positioned in two first connection edges 35 of the first 28a and the second 28b plate, and of a fourth plate 29b, opposite the third plate 29a, which is positioned in two second connection edges 35 of the third plate 29a and the fourth plate 29b.
In particular, the thrust devices 57 are configured to compress, in correspondence with their respective connection edges, the second plate 28b, the third plate 29a and the fourth plate 29b against the first plate 28a. The action of compression is exerted in combination with a thrusting action of the plates 28b, 29a, 29b toward the inside of the support frame 26.
According to possible forms of embodiment of the present invention, the thrust devices 57 can be chosen from a group comprising threaded elements, screws, nuts, pins, actuators, worm screw mechanisms, rack mechanisms, motors or a possible combination thereof.
One possible form of embodiment may for example provide that the thrust devices 57 are mounted on the support frame 26 and have a terminal contact end that is selectively movable toward the inside of the support frame 26 and that contacts, during use, the plates 28b, 29a, 29b. One possible form of embodiment may for example provide that the thrust devices 57 comprise one or more screws mounted in through holes made in the thickness of the walls of the support frame 26.
The screws are made to protrude toward the inside of the support frame 26 and, by adjusting the entity of screwing, it is possible to regulate the thrust action exerted on the plates 28b, 29a, 29b.
According to one possible form of embodiment, the thrust devices 57 are mounted on the support frame 26 and are configured to thrust the plates 28b, 29a, 29b toward the inside of the support frame 26, in order to compress them with respect to each other and against the first plate 28a.
Some forms of embodiment, one of which is shown in fig. 4, provide that the thrust devices 57 comprise a plurality of elastic blocks 58 attached to the external surface of the second plate 28b, of the third plate 29a and of the fourth plate 29b, in this case to the external surface of the fourth plate 29b, and on which thruster elements 64 act during use. The thruster elements 64 are mounted on the external surface of the support frame 26 and are configured to compress the elastic blocks 58.
The elastic blocks 58 comprise a containing body 59 with a substantially cylindrical shape, provided to contain a plurality of elastic elements 60 inside it.
In the form of embodiment shown in fig. 4, the elastic elements 60 comprise cup type springs, although in other forms of embodiment the elastic elements 60 can comprise compression springs of the helical type, conical disc springs or leaf springs or suchlike.
The containing body 59 comprises a container 61 provided with an aperture 65 for the introduction of the elastic elements 60.
The aperture 65 of the container 61 is partly closed by a lid 62 that provides to maintain the elastic elements 60 compressed inside the container 61, and generates a first preloading thereof.
The container 61 is housed, by mechanical interference, in a respective blind hole 63 made in the thickness of the second plate 28b, the third plate 29a and the fourth plate 29b.
The reciprocal connection between the container 61 and the lid 62 can be the threaded type or, in other forms of embodiment, by same-shape coupling or interference, for example providing snap-in attachment teeth of the non-releasable type.
The lid 62 is provided with a hole 67 which allows the thruster elements 64 to cooperate with the elastic elements 60. Inside the container 61, and between the elastic elements 60 and the lid 62, a small plate 66 is interposed that protrudes toward the outside through the hole 67.
Suitable abutments 68 are provided in the small plate 66 and in the lid 62, to prevent the small plate 66 from exiting from the container 61.
The thruster elements 64 comprise a thrust screw 69 which is screwed into a threaded hole 70 made through through the thickness of the support frame 26.
In particular, the threaded hole 70 is made in a position coordinated to the one in which the blind hole 63 is provided for housing the elastic block 58.
The thrust screw 69 of each second attachment device 57 presses against the small plate 66, compressing the elastic elements 60 inside the containing body 59.
The action of compression of the elastic elements 60 associated respectively to the second plate 28b, the third plate 29a and the fourth plate 29b is transmitted onto these.
The overall effect of compression of the elastic elements 60 therefore translates into an effect of compression of the second plate 28b, the third plate 29a and the fourth plate 29b against the first plate 28a, which is the only one directly attached to the support frame 26.
Assembling the crystallizer 12 with the support frame 26 comprises a first operation of coupling only the first plate 28a inside the support frame 26, disposing it in contact against a wall of the latter. If necessary, it may be provided to interpose the spacer 56 between the first plate 28a and the support frame 26 as described above.
The first plate 28a is attached in the support frame 26 by the attachment devices 52 which exert a holding action of the first plate 28a against the wall of the support frame 26.
A subsequent operation is provided to introduce the second plate 28b, the third plate 29a and the fourth plate 29b inside the support frame 26, disposing them in reciprocal contact with their connection edges 35 and with the connection edges 35 of the first plate 28a.
Some forms of embodiment provide that the operation of introducing the second plate 28b, the third plate 29a, and the fourth plate 29b into the support frame 26 occurs simultaneously.
In this case dedicated equipment can be provided that, before the insertion, reciprocally connects the second plate 28b, the third plate 29a and the fourth plate 29b, disposing them in the position they will assume during use.
The equipment, together with the second plate 28b, the third plate 29a and the fourth plate 29b is used to allow the simultaneous insertion of the latter inside the support frame 26.
Other forms of embodiment can provide that the insertion of the second plate 28b, the third plate 29a and the fourth plate 29b occurs in sequence.
Once the second plate 28b, the third plate 29a and the fourth plate 29b are inserted in the support frame 26, the thrust devices 57 are activated. The activation of the thrust devices 57 allows to compress the second plate 28b, the third plate 29a and the fourth plate 29b against the first plate 28a.
In fact, while the first plate 28a is held by the attachment devices 52 against the support frame 26, the second plate 28b, the third plate 29a and the fourth plate 29b are thrust toward the inside of the support frame 26 and the reciprocal cooperation between the connection edges 35 determines an assembled condition.
The fact that no direct connection is provided between the plates 28b, 29a, 29b and the support frame 26 but only a reciprocal compaction thereof, allows to confer on the crystallizer 27 a greater adaptability to the stresses to which it is subjected during use, and also prevents the onset of tensions inside all the plates 28a, 28b, 29a, 29b.
Other forms of embodiment of the present invention, shown for example in figs. 1-3 and 5 and 6, provide that the positioning members 50 comprise first positioning devices 71a and 71b (figs. 1 - 6) provided to determine a precise positioning of the first plate 28a and the second plate 28b, and second positioning devices 72 provided to determine a precise positioning of the third plate 29a and the fourth plate 29b with respect to the support frame 26 in an axial direction, that is, along the longitudinal axis Z, and a transverse direction.
The first positioning devices 71a, 71b and the second positioning devices 72 comprise a plurality of pins 73 (figs. 5 and 6) suitable to be inserted in respective through holes 74 made in the thickness of the support frame 26 and according to an axis that is located transverse to the longitudinal axis Z.
The position of the pins 73 in the through holes 74 is maintained by connection members 75 which, in this case, comprise screws 76 inserted into through holes 77 made in a head 78 of the pin 73.
Each first positioning device 71a, 71b (figs. 5 and 6) comprises a first housing seating 79, respectively 80, made blind in the thickness of the first 28a and the second 28b plate.
The first housing seatings 79 of the first positioning devices 71 a (fig. 5) are configured to constrain the position of the first 28a and the second 28b plate with respect to the support frame 26 both in a direction parallel to the longitudinal axis Z and also in a transverse direction, for example orthogonal, to the longitudinal axis Z.
The first housing seatings 80 of the first positioning devices 71b (fig. 6) on the other hand are configured to constrain the position of the plates 28a, 28b in a transverse direction, in this specific case, orthogonal, to the longitudinal axis Z and to leave the movement free in a direction parallel to the longitudinal axis Z.
Each second positioning device 72 comprises a second housing seating 81 made blind in the thickness of the third 29a and the fourth 29b plate (figs. 2 and 3).
Finally, the second housing seatings 81 of the second positioning devices 72 (figs. 2 and 3) are configured to constrain the position of the plates 29a, 29b in a direction parallel to the longitudinal axis Z and to leave the movement free in a transverse direction, in this specific case, orthogonal, to the longitudinal axis Z, to allow any necessary adaption if the plates 28a, 28b, 29a, 29b are regenerated and if the thickness of the spacer 56 is modified.
In fact, because of wear on the plates 28a, 28b, 29a, 29b, the surface of the latter that is internal during use is subjected to a mechanical working causing removal of material, so that the plates 28a, 28b, 29a, 29b change their total thickness.
The first plate 28a changes its total thickness and, in order to maintain the positioning of the internal profile unchanged with respect to the machine, a spacer 56 is inserted, with a calibrated thickness with respect to the support frame 26. These operations produce very slight movements of the theoretical center of the plates 28b, 29a, 29b.
In general, the positioning of the crystallizer 27 in the support frame 26 is given by the abutment to which the first plate 28a is attached. A rigid positioning constraint makes it impossible to assemble the crystallizer 27 in the support frame 26.
In this respect, both the first housing seatings 80 and the second housing seatings 81 have a greater size compared to the size of the diameter of the pin 73 in the direction in which a movement of the plates 28a, 28b, 29a, 29b is allowed, while they have a tolerance coupling, play coupling or interference coupling in the direction in which movement is prevented.
The freedom of movement of the plates 28a, 28b, 29a, 29b with respect to the pins 73 allows to take into account possible thermal dilations to which the latter are subjected during use, and thus prevent the onset of internal stresses that could deform the crystallizer 27, generating cast products with unwanted geometric and microstructural characteristics, as well as reducing the useful life of the crystallizer 27 itself.
According to possible forms of embodiment, not shown in the drawings, in the first housing seatings 79, 80 and/or in the second housing seatings 81, lining bushings can be inserted, made with anti-wear material, that is, a material harder than the material that the plates 28a, 28b, 29a, 29b are made of.
The bushings can be inserted by interference into the first housing seatings 79, 80 and/or the second housing seatings 81, and can in turn have a seating, circular or oblong in shape, in which the end parts of the pins 73 are disposed, as described above. The seatings of the bushings can be configured to allow a movement of the plates 28a, 28b, 29a, 29b in a vertical, horizontal or transverse direction with respect to the longitudinal axis Z.
In this way, the stresses due to possible deformations or heat dilations to which the plates 28a, 28b, 29a, 29b are subjected to act directly on the bushings which preserve the housing seatings 79, 80 and 81 from wear and from possible plastic deformations.
This solution guarantees a correct positioning over time of the crystallizer 27 with respect to the support frame 26.
It is clear that modifications and/or additions of parts may be made to the apparatus for continuous casting as described heretofore, without departing from the field and scope of the present invention. It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of apparatus for continuous casting, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

Claims

Apparatus for continuous casting comprising a support frame (26) and a plurality of plates (28a, 28b, 29a, 29b) coupled to each other to define a tubular element (27) provided with a casting cavity (30), positioning members (50) being provided to position said plates (28a, 28b, 29a, 29b) with respect to said support frame (26), said positioning members (50) comprising each time attachment devices or thrust devices, characterized in that only one first plate (28a) of said plates is attached to a wall of said support frame (26), by means of corresponding attachment devices (52), and in that thrust devices (57) are associated to the other plates (28b, 29a, 29b) to thrust them toward the inside of said support frame (26) and to compress them with respect to each other, in correspondence with respective connection edges (35) and against said first plate (28a).
Apparatus as in claim 1, characterized in that said thrust devices (57) are mounted on said support frame (26).
Apparatus as in claim 2, characterized in that said thrust devices (57) comprise elastic blocks (58) attached to the external surface of said plates (28b, 29a, 29b) and thruster elements (64) attached to said support frame (26) and configured to compress said elastic blocks (58).
Apparatus as in claim 3, characterized in that each of said elastic blocks (58) comprises a containing body (59) attached to said plates (28b, 29a, 29b) and configured to contain a plurality of elastic elements (60).
Apparatus as in claim 4, characterized in that said containing body (59) is housed in a hole (63) made in the thickness of said plates (28b, 29a, 29b).
Apparatus as in claim 4 or 5, characterized in that said containing body (59) comprises a container (61) provided with an aperture (65) for the introduction of said elastic elements (60), said aperture (65) being closed by a lid (62) to contain said elastic elements (60).
Apparatus as in any claim from 2 to 6, characterized in that each of said thruster elements (64) comprises a thruster screw (69) and a threaded hole (70) made in said support frame (26) and into which said thruster screw (69) is screwed.
Apparatus as in any claim hereinbefore, characterized in that said attachment devices (52) comprise a plurality of screws (53) able to be associated with said support frame (26) and with one first plate (28a).
Apparatus as in claim 8, characterized in that said support frame (26) is provided with through holes (54) into which said screws (53) are inserted, and in that said at least one first plate (28a) is provided with threaded holes (55) into which said screws (53) are screwed.
Apparatus as in any claim hereinbefore, characterized in that said positioning members (50) comprise positioning devices (71, 72) provided to position said plates (28a, 28b, 29a, 29b) with respect to said support frame (26) in an axial and transverse direction.
Method to assemble an apparatus for continuous casting comprising the positioning by means of positioning members (50) of a plurality of plates (28a, 28b, 29a, 29b) to a support frame (26) to define a tubular element (27) provided with a casting cavity (30), characterized in that it comprises the attachment of only a first plate (28a) of said plates to a wall of said support frame (26) by means of attachment devices (52), and a second operation of coupling the other plates (28b, 29a, 29b) with respect to each other and to said first plate (28a), thrusting them toward the inside of said support frame (26) and compressing them in correspondence with respective connection edges (35) by the action of thrust devices (57).
Method as in claim 11, characterized in that the attaching of said first plate (28a) provides an action of holding said at least one first plate (28a) to said support frame (26).
Method as in claim 11 or 12, characterized in that said second operation provides to regulate the pressure action exerted by said thrust devices (57), associated with said support frame (26), against said other plates (28b, 29a, 29b).
Method as in claim 13, characterized in that during said second operation, thruster elements (64) of said thrust devices (57) act against respective elastic blocks (58) attached to the external surface of said plates (28b, 29a, 29b).
Method as in any claim from 11 to 14, characterized in that said second operation provides to insert said plates (28b, 29a, 29b) into said support frame (26), disposing them in reciprocal contact with their connection edges (35).