Machine tool for making holes in elements intended for fibreglass extrusion

Abstract: It is provided a machine tool for making holes in elements intended for fibreglass extrusion, comprising a control unit (11) for the translatory advance motion of a spindle (5) equipped with a tool-holding head (6) and adapted to enable stopping of the spindle itself in the presence of axial resisting forces acting on the tool higher than a predetermined value.

Description: The present invention relates to a machine tool for making holes in elements intended for fibreglass extrusion. It is known that for manufacturing glass fibres or filaments or other similar materials, appropriate centrifugal extruders or rotors are used that consist of bodies having a substantially toroidal shape peripherally provided with a very high number of radial through holes, twenty/twenty-five thousand holes for example, of a very small diameter, generally in the order of 0.6:1.2 mm. The raw material in a melted state intended to form the glass fibre is supplied to these extruders that are driven in rotation so as to eject, by centrifugation, the melted glass contained therein through the radial holes, in order to form thread-like elements that are further stretched and made thin by vertical jets of hot gases before they are laid on the underlying conveyor belts. Some known techniques for executing the above mentioned radial holes in centrifugal extruders avoid using drilling tools and replace them with laser beams or EDM (electron discharge machining) methods. These techniques however have a drawback, in that holes are not made so perfectly cylindrical as they actually should be for enabling an optimal extrusion condition and, in addition, use of a laser beam in particular may give rise to work hardened areas in the steel material of which the extruder is made. The extruder structure therefore appears to be weaker and the extruder duration of live may become greatly reduced. Another more traditional known technique is based on the use of machine tools provided with points each of which is engaged in a tool-holding head coupled to a spindle. Appropriate drive means makes the spindle perform a rotary work motion and other actuating means move the spindle with a translatory advance movement. This known technique however, while enabling holes of perfectly cylindrical shape to be obtained by drilling of the mechanical type and avoiding work hardening in the material, has a serious drawback in that it is subjected to frequent interruptions in working because the material of the extruder to be drilled, generally high-carbon chrome-nickel steel, may sometimes have some lack of homogeneity in terms of hardness, which causes tool breakages at the areas of greater resistance to tool penetration. Practically, since, as already pointed out, a very great number of holes are to be made, taking into account the fact that point tools are to be often replaced, the production rates are very reduced and, as a result, the overall working costs of the elements intended for extrusion of glass fibres increase. It is also known from document DE 4000948 a machine tool presenting a spindle equipped with a tool holding head supporting a point tool. Actuating means makes the spindle perform a translatory advanced motion, while a control unit allows stopping of the translatoy motion of the spindle in case of predetermined axial resisting forces acting on the tool. Document CH 523116 discloses a machine-tool drive in which an engine provokes simultaneously a rotation and a descent of the tool and a rise of a net screw coaxial and engaged to the tool shaft. During the rise, the net screw load a spring increasing tool pressure against the piece to be worked. The spring can be also pre-loaded by means of a control screw so as to increase or decrease the maximum axial forces of the tool. Under this situation, the technical task underlying the present invention is to provide a machine tool, as defined in claim 1, for making holes capable of substantially obviating the above mentioned drawbacks. Within the scope of this technical task, it is an important aim of the invention to provide a machine tool for making holes capable of substantially eliminating the possibility of breaking the point tools even when workings are carried out on materials having confined areas which in an unforeseeable manner are of higher hardness than all the other areas having the expected hardness and which therefore cannot be penetrated by tools provided for normal drilling. The technical task mentioned and the aim specified are substantially achieved by a machine tool for making holes as described in the appended claims. Description of a preferred but non-exclusive embodiment of a machine tool for making holes in accordance with the invention is now given hereinafter, by way of non-limiting example, with reference to the accompanying drawing in which the only figure is a longitudinal sectional view of the machine close to an extruder element to be submitted to drilling operations. With reference to the drawing, the machine tool for making holes in accordance with the invention is generally identified by reference numeral 1. It comprises a base 2 with which actuating means 3 is integral, said means being adapted to carry out the translatory advance movement along a horizontal axial direction 4 of a spindle 5 provided with a first tool-holding head 6 to be coupled thereto by fitting along the same axial direction 4. A point tool 7 can be fixed to the tool-holding head 6, said tool 7 being capable of radially drilling an extruder element 8 so as to form radial holes 9 therein which are intended for extruding glass fibres. The rotary work motion of the spindle is performed by drive means 10 of a type known per se. In an original manner, the actuating means 3 comprises a control unit 11 for the translatory advance movement of spindle 5 adapted to enable the spindle to stop in the axial direction 4 in the presence of resisting axial forces exceeding a predetermined value and acting on the tool 7. More particularly, actuating means 3 comprises a support body 12 integral with the base 2 and defined by a hollow cylinder having a front opening 12a and a head 12b disposed at the back. Slidably coupled within the hollow cylinder 12 is a piston element 13 defining a slide adapted to house the spindle 5 and related drive means 10 for governing the work movement. Practically, the piston element 13 has a length slightly lower than that of the inner cavity of cylinder 12 and during its translatory movement it can emerge by one end 13a thereof turned towards the tool-holding head 6 from the front opening 12a of cylinder 12. In addition, the actuating means 3 comprises a power unit 14 having a movable drive member 15 defined by a cylindrical rod adapted to carry out said translatory advance movement having strokes of predetermined length, upon interposition of a kinematic mechanism 16 converting the rotary motion of an electric motor 17 into a rectilinear motion. The control unit 11 is operatively interposed between the drive member 15 and the piston element 13 and is comprised of a chamber 18 defined between the head 12b and the second end 13b or back face of the piston element 13. Chamber 18 is filled with a predetermined amount of oil and therefore takes the volume of said oil. The cylindrical rod 15 is slidably engaged in a through hole 12c of head 12b and has an end portion 15a fitted in chamber 18. Also provided is fluid-operated means 19 communicating with chamber 18 through a small channel 20, formed in the side wall of cylinder 12 close to head 12b and adapted to discharge oil from said chamber upon overcoming of a calibration pressure corresponding to the presence of said axial resisting forces on the tool higher than said predetermined value and to admit the discharged oil again into chamber 18 in case of pressures lower than said calibration pressure. The fluid-operated means 19 consists of a fluid-operated circuit comprising a pressure relief valve 21 regulated on said calibration pressure, a one-way valve 22 disposed in parallel with valve 21 and a hydraulic accumulator 23 in communication with both said valves. Also provided to advantage is a tool-changing unit 24 integral with the cylinder 12 and adapted to pick up a tool to be replaced due to wear or breakage, from spindle 5 and to fit a new tool on the end of the spindle itself. The tool-changing unit 24 comprises a revolving turret 25 provided with a symmetric arm 26 adapted to support at least two tool-holding heads at its ends, i.e. the first tool-holding head 6 and a second tool-holding head 27. Obviously, by means of appropriate modifications, tool-changing units having three or more tool-holding heads could be provided. In addition, the tool-changing unit 24 comprises drive means 28 both for the translatory motion of turret 25 in the direction of said advance motion, that is along an oscillation axis 29 parallel to the axial direction 4, and for the rotary motion of said turret about the oscillation axis 29. Since in its operating position tool 7 is close to the extruder element 8 surface, in order to enable displacement of turret 25 so that the tool-holding head 26 or tool-holding head 27 may be removed from or fitted on the spindle, base 2 is movable on slide guides 30 by displacement means 31 consisting for example of a hydraulic actuator, operatively coordinated with the drive means 28 so as to move the cylinder 12 close to or away from the extruder element submitted to working. For a complete operating automation of the tool-changing unit 24, a sensor 32 detecting tool breakage may be provided to which the drive means 28 can be interlocked. Finally and advantageously, the control unit 11 may comprise a counter 33, connected to the hydraulic accumulator 23 and adapted to signal the number of oil discharges carried out by means 19 so as to identify the corresponding number of interruptions in the translatory motion of the spindle, that is the number of unexecuted through holes 9. Operation of a machine tool for making holes described above mainly as regards structure, is as follows. The translatory advance motion of spindle 5 and consequently of point tool 7 is carried out by penetration of the end portion 15a of the drive member 15 into chamber 18. This penetration causes a displacement of the piston element 13 corresponding to maintenance of a constant volume in the oil amount present therein, being oil incompressible. If the advance motion of point tool 7 into the extruder element 8 is counteracted by anomalous axial forces due to the presence of particularly hard areas in the material to be drilled, the oil pressure in chamber 18 increases and, upon reaching the calibration pressure of the pressure relief valve 21, causes discharge of part of the oil into the accumulator 23. In this way, while the drive member 15 is completing its intended advance stroke, the piston element 13 instantaneously stops and the point tool 7 is not submitted to dangerous stresses that could cause breaking of same. During the backward stroke of the drive member 15, since a lower pressure is restored in chamber 18, the oil contained in accumulator 23 is admitted again to the chamber itself that, as a result, will house the same oil amount as at the beginning. If replacement of the point tool 7 is required, due to wear or because it is broken, although the last-mentioned occurrence is practically possible substantially only due to periodic wear, the tool-changing unit 24 carries out removal of spindle 5 of the first tool-holding head 6 and fitting of the second head 27 thereon with the related point tool. For this operation, the displacement means 31 operates backward movement of base 1 relative to the extruder element 8 so as to create an appropriate room to enable a translatory motion of turret 25 sufficient for disengagement of the first tool-holding head 6 from the spindle. After carrying out rotation of the symmetric arm 26 and a new backward translatory motion of turret 25 for fitting the second tool-holding head 27 on the spindle, base 2 is brought back to the extruder element 8 at the intended operating distance. The invention achieves important advantages. Actually, since the point tools are practically never submitted to too many stresses, they have a greatly increased duration of life and therefore replacement of them has a much more reduced frequency. It is to point out that, above all in case of execution of a very high number of holes, as it is necessary for making extruder elements designed to form glass fibres, the production rates undergo almost inexistent interruptions and the production costs of these elements are greatly reduced.

Claims: A machine tool for making holes in elements intended for fibreglass extrusion, comprising: at least one point tool (7); at least one spindle (5) equipped with a tool-holding head (6); drive means (10) for carrying out the rotary work motion of the spindle (5); and actuating means (3), adapted to make the spindle (5) perform a translatory advance motion, comprising: a support body (12); a slide (13) slidably coupled to said support body (12) and adapted to house said spindle (5); a power unit (14) integral with said support body (12) and having a movable drive member (15) adapted to carry out said translatory motion with strokes of predetermined length, a control unit (11), operatively interposed between said drive member (15) and slide (13), for said translatory advance motion adapted to enable stopping of the translatory motion of said spindle (5) in the presence of axial resisting forces acting on the tool (7) that are higher than a predetermined value; characterized in that said support body (12) is hollow and said slide (13) is slidable within said hollow support body and has one end (13a) turned towards said tool-holding head (6), in that said control unit (11) comprises a chamber (18) filled with a predetermined amount of oil and confined between one head (12b) of said hollow support body and a second end (13b) of said slide, and fluid-operated means (19) in communication with said chamber (18) and adapted to discharge oil from the chamber itself upon overcoming of a calibration pressure corresponding to the presence of said axial resisting forces higher than said predetermined value and to admit the discharged oil again into said chamber in case of pressures lower than said calibration pressure, and in that said drive member (15) is slidably engaged in a through hole (12c) of said support body head (12b) and has an end portion (15a) fitted in said chamber. A machine as claimed in claim 1, characterized in that said support body (12) is defined by a hollow cylinder and said slide (13) is defined by a piston element slidable within said hollow cylinder, and in that said drive member (15) is defined by a cylindrical rod slidably engaged in a through hole (12c) of said cylinder head (12b) and having an end portion (15a) fitted in said chamber. A machine as claimed in claim 1, characterized in that said fluid-operated means (19) comprises: a pressure relief valve (21); a one-way valve (22) disposed in parallel with said pressure relief valve (21); and a hydraulic accumulator (23) in communication with said valves (21, 22). A machine as claimed in claim 1, characterized in that it comprises a tool-changing unit (24) integral with said support body (12) and adapted to pick up a tool to be replaced from said spindle (5) and fit a new tool onto the spindle itself. A machine as claimed in claim 4, characterized in that said tool-changing unit (24) comprises: a revolving turret (25) for supporting at least two tool-holding heads (6, 27); and drive means (28) for the translatory motion of said turret in the direction of said advance motion and rotation of same about an oscillation axis (29) parallel to said advance direction. A machine as claimed in claim 5, characterized in that said tool-changing unit (24) comprises a sensor detecting tool breakage (32), and in that said drive means is interlocked with said tool-breakage detecting sensor. A machine as claimed in claim 5, characterized in that it comprises: a base (2) integral with said support body (12); slide guides (30) for said base in the direction of said advance motion; and displacement means (31) for said base, operatively coordinated with said drive means controlling the translatory motion of the revolving turret. A machine as claimed in claim 1, characterized in that said control unit (11) comprises at least one counter (33) adapted to signal the number of oil discharges carried out by said fluid-operated means (19).

Inventors: MONTANI LUIGI, MONTANI, LUIGI
Applicants: TECHINT SPA, TECHINT COMPAGNIA TECNICA INTERNAZIONALE S.P.A.
Application year: 1997
Main classification: B23Q11
IPC codes: B23B47, B23Q11, B23Q17