Machine and method for making an internal thread in a hollow workpiece

Abstract

A method for making an internal thread in a hollow workpiece, comprising the steps of making a first threaded section (T1) and making a second threaded section (T2) of the inner surface (102) of a through hole (101). Making the first threaded section (T1) involves inserting a first grinding tool (12a) into the first end portion (103a) of the through hole (101) and advancing the first grinding tool (12a), with respect to the hollow workpiece (100), until a first intermediate work-ending position is reached, located between the first (103a) and the second end portion (103b) of the hollow workpiece (100). Making the first threaded section (T1) involves inserting the second grinding tool (13a) into the second end portion (103b) of the through hole (101) and advancing the second grinding tool (13a), with respect to the hollow workpiece (100), until a second intermediate work- ending position is reached, located between the first (103a) and the second end portion (103b) of the hollow workpiece (100).

Description

[0001] MACHINE AND METHOD FOR MAKING AN INTERNAL THREAD IN A HOLLOW WORKPIECE

[0002] Field of application

[0003] The present invention relates to a machine and a method for making an internal thread in a hollow workpiece, in particular of a workpiece having a threaded and through internal cavity.

[0004] Prior art

[0005] In the prior art, internal threads have always been made by tapping or milling operations.

[0006] Tapping is a simple and well-known machining process that allows the internal threads to be made by using a tool, called tapping tool, which, being preformed with the negative of the thread to be made, works the internal cavity of the workpiece by rolling or material removal.

[0007] Thread milling is a machining technique used to produce threaded surfaces on metal workpieces using a tool called a thread milling cutter, which unlike the tapping tool has a multiplicity of cutting edges and is driven by a numerically controlled machine.

[0008] Both of these machinings, however, have limitations from the point of view of machining precision, making them unsuitable for making components for very demanding technological applications, such as robot actuators.

[0009] The latest generation robots require the use of actuators that at the same time must guarantee speed, lightness, rigidity and precision, which is why very small satellite roller screws (planetary roller screw) are being opted for among the various technologies available.

[0010] This leads to the need for very tight tolerances and low roughness, which is incompatible with the typical machinings carried out when making threads.

[0011] For this reason, in order to guarantee maximum precision at the surface level, in recent times it has been decided to replace the typical machinings by chip removal or rolling with a machining that is normally adopted in the finishing step, i.e. grinding.

[0012] In this regard, however, since it is not common to use grinding tools for internal thread machining, we soon had to face problems related to the slenderness ratio of components to be machined, which, being of greatly reduced diameter in relation to length, make it difficult to use commonly used grinding wheels.

[0013] Having a grinding wheel mounted on a wheel-holding shaft that has a length equivalent to that of the workpiece to be machined makes the grinding process complicated and uneconomical, as it requires the adoption of very conservative machining parameters (speed, feed, etc.) in order to avoid dynamic and vibration effects on the workpiece.

[0014] In addition, if the profile of the thread to be made requires an inclination between the axis of the grinding wheel and the axis of the workpiece, the problem of the slenderness of the grinding wheel holder is even greater, as it must be ensured that there is no collision with the internal diameter of the thread when the grinding wheel holding shaft is at maximum depth.

[0015] The object of the present invention is therefore to make available a machine and a method for making an internal thread in a hollow workpiece that are highly efficient.

[0016] Another object of the invention is to make available a machine and a method for making an internal thread in a hollow workpiece capable of working components with tolerances and roughnesses typical of the grinding process regardless of their slenderness ratio, in particular for components with a high slenderness ratio.

[0017] Summary of the invention

[0018] The above illustrated objects are achieved by a machine and a method for making an internal thread in a hollow workpiece having the characteristics of the following claims.

[0019] In particular, the machine comprises a workpiece-holding assembly and a grinding unit.

[0020] Preferably, the workpiece-holding assembly comprises a workpieceholding spindle having a through cavity extending along a central axis, in use corresponding to a central axis of a workpiece to be machined.

[0021] Preferably, the workpiece-holding assembly comprises a clamping member associated with the cavity and movable between a release configuration and a clamping configuration, in which it holds the workpiece within the cavity.

[0022] Preferably, the workpiece-holding spindle and said clamping member being integrally rotatable about said central axis of the through cavity.

[0023] Preferably, the grinding unit comprises a support provided with a first and a second arm extending transversely to said central axis of the through cavity, one facing the other.

[0024] Preferably, the first and second arms defining between them a working volume within which, in at least a first operating configuration of the machine, the workpiece-holding spindle is located.

[0025] Preferably, the first and second arms comprising respectively a first toolholding spindle and a second tool-holding spindle each coupled to a corresponding first and second grinding tool which, in said first operating configuration, are inserted in the working volume and oriented in the direction of the cavity of the workpiece-holding spindle.

[0026] Preferably, the first and second arms are rigidly connected to move integrally in translation and / or rotation with respect to said workpieceholding assembly.

[0027] Advantageously, preparing a grinding unit equipped with two grinding tools that are facing each other and rigidly connected allows the internal thread to be made by entering the cavity at different times and from two sides, allowing the use of tools with reduced slenderness and capable of working at greater speeds.

[0028] The adoption of a layout in which the two tools are anchored to the same grinding unit and cannot be moved with respect to each other, moreover, ensures that the mutual positioning between the workpiece-holding assembly and grinding tools remains unchanged in both machining steps, thereby obviating any problems related to a repositioning of the workpiece. According to a further aspect of the invention, the invention relates to a method for making an internal thread in a hollow workpiece.

[0029] Preferably, the method involves preparing a hollow workpiece provided with a through hole having an inner surface to be machined and extending along its own central axis between a first and a second end portion.

[0030] Preferably, said hollow workpiece has a slenderness ratio defined by a ratio between the length of the workpiece along its central axis and an inner diameter of the inner surface, greater than 3.

[0031] Preferably, it involves preparing a first grinding tool and a second grinding tool facing each other and placed at a distance that defines, between the grinding tools, a working volume within which the hollow workpiece can be placed.

[0032] Preferably, it involves making a first threaded section of the inner surface of the through hole by inserting the first grinding tool into the first end of the through hole and advancing the first grinding tool, with respect to the hollow workpiece, until a first intermediate work-ending position, located between the first and second end portions of the hollow workpiece, is reached.

[0033] Preferably, it involves making a second threaded section of the inner surface of the through hole by inserting the second grinding tool into the second end of the through hole and advancing the second grinding tool, with respect to the hollow workpiece, until a second intermediate workending position, located between the first and second end portions of the hollow workpiece, is reached.

[0034] According to each of the different aspects, the invention further comprises one or more of the following preferred features.

[0035] Preferably, making the first threaded section of the inner surface of the through hole involves positioning the first grinding tool in a first machining- start position facing the first end portion of the hollow workpiece.

[0036] Preferably, making the first threaded section of the inner surface of the through hole involves rotating the first grinding tool.

[0037] Preferably, making the first threaded section of the inner surface of the through hole involves imparting a first rotation to the hollow workpiece about its central axis.

[0038] Preferably, making the first threaded section of the inner surface of the through hole involves extracting the first grinding tool from the first end portion of the hollow workpiece.

[0039] Preferably, making the second threaded section of the inner surface of the through hole involves positioning the second grinding tool in a second machining-start position facing the second end portion of the hollow workpiece.

[0040] Preferably, making the second threaded section of the inner surface of the through hole involves rotating the second grinding tool.

[0041] Preferably, making the second threaded section of the inner surface of the through hole involves imparting a second rotation to the hollow workpiece about its central axis.

[0042] Preferably, making the second threaded section of the inner surface of the through hole involves extracting the second grinding tool from the second end portion of the through hole.

[0043] Preferably, the first and second arms are movable integrally with respect to the workpiece-holding assembly in translation along at least one direction transverse with respect to the central axis of the through cavity.

[0044] Preferably, the first and second arms are movable integrally with respect to the workpiece-holding assembly in rotation about at least one axis of rotation orthogonal with respect to the central axis of the through cavity. Preferably, the grinding unit comprises a frame.

[0045] Preferably, the support comprises a base for connection between the first and second arms and slidably associated with the frame to translate along at least a first operating direction, transverse with respect to the central axis of the through cavity.

[0046] Preferably, the grinding unit comprises a carousel rotatably associated with the frame for rotating about an axis of rotation orthogonal with respect to the central axis of the through cavity.

[0047] Preferably, the base of the support is slidably constrained to the carousel. Preferably, the support of the grinding unit is movable, with respect to the workpiece-holding assembly, along a second operating direction, orthogonal with respect to the first operating direction and the central axis of the through cavity.

[0048] Preferably, the frame is slidably associated with a reference plane or baseplate to move along said second operating direction towards and / or away from the grinding unit.

[0049] Preferably, the workpiece-holding assembly is movable with respect to the grinding unit along a third operating direction, parallel to the central axis of the through cavity.

[0050] Preferably, in any operating position of the machine, the first and second grinding tools are aligned along a common working axis.

[0051] Preferably, the clamping member of the workpiece-holding assembly comprises a vice provided with a plurality of jaws movable in synchrony towards and away from said central axis between an open position, corresponding to the release configuration, and a closed position, corresponding to the clamping configuration.

[0052] Preferably, the workpiece-holding spindle extends, along the central axis, between a first end portion and a second end portion, which in the first operating configuration of the machine respectively face the first and second arms to be accessible by the first grinding tool and the second grinding tool.

[0053] Preferably, the machine comprises a control unit associated with the grinding unit and the workpiece-holding assembly.

[0054] Preferably, the control unit is configured to drive a movement of the grinding unit with respect to the workpiece-holding assembly such that: - in a first working position, the first grinding tool is inserted into the cavity of the workpiece-holding spindle at the first end portion;

[0055] - in a second working position, the second grinding tool is inserted into the cavity of the workpiece-holding spindle at the second end portion. Preferably, the control unit is configured to impart a first rotation, in the first working position, and a second rotation, in the second working position, to the workpiece-holding spindle.

[0056] Preferably, the first rotation and the second rotation have opposite directions of rotation.

[0057] Preferably, the control unit is configured to drive the grinding unit along the first and / or second direction and in rotation about the axis of rotation to bring the first grinding tool into a first machining-start position facing the first end portion of the workpiece-holding spindle.

[0058] Preferably, the control unit is configured to rotate the first tool-holding spindle.

[0059] Preferably, the control unit is configured to impart the first rotation to the workpiece-holding spindle of the workpiece-holding assembly.

[0060] Preferably, the control unit is configured to drive the workpiece-holding assembly towards the first arm of the grinding unit, or vice versa, until the first working position is reached.

[0061] Preferably, the control unit is configured to advance the workpiece-holding assembly or the grinding unit along the central axis until a first work-ending position is reached by the first grinding tool.

[0062] Preferably, the control unit is configured to retract the workpiece-holding assembly or the grinding unit along the central axis until the first grinding tool is extracted from the first end portion of the workpiece-holding spindle. Preferably, the control unit is configured to drive the grinding unit along the first and / or second direction and in rotation about the axis of rotation to bring the second grinding tool into a second machining-start position facing the second end portion of the workpiece-holding spindle.

[0063] Preferably, the control unit is configured to rotate the second tool-holding spindle.

[0064] Preferably, the control unit is configured to impart the second rotation to the workpiece-holding spindle of the workpiece-holding assembly.

[0065] Preferably, the control unit is configured to drive the workpiece-holding assembly towards the second arm of the grinding unit, or vice versa, until the second working position is reached.

[0066] Preferably, the control unit is configured to advance the workpiece-holding assembly or the grinding unit along the central axis until a second workending position is reached by the second grinding tool.

[0067] Preferably, the control unit is configured to retract the workpiece-holding assembly or the grinding unit along the central axis until the second grinding tool is extracted from the second end portion of the workpieceholding spindle.

[0068] Preferably, the machine comprises a profiling spindle configured to act, alternatively, on the first and second grinding tool.

[0069] Preferably, the profiling spindle is connected to the workpiece-holding assembly and is movable, with respect to the grinding unit, integrally therewith.

[0070] Preferably, the presence of an angular phasing assembly provided with a probe or sensor configured to measure the position of an existing thread within the hollow workpiece with respect to a position of the first or second grinding tool is envisaged.

[0071] Preferably, the angular phasing assembly is configured to drive the grinding unit and / or the workpiece-holding assembly (via the control unit) to place the respective first or second grinding tool in the correct finishing position.

[0072] Preferably, both the first threaded section and the second threaded section of the inner surface have an axial extension such as to exceed a centre line of the through hole.

[0073] Preferably, the step of creating the second threaded section provides for rejoining said second threaded section with the first threaded section to obtain a continuous thread between the first and second end portions.

[0074] Brief description of the drawings

[0075] The present invention will become clearer from the exemplary, therefore non-limiting, description of preferred, therefore non-exclusive, embodiments of a machine and a method for making an internal thread in a hollow workpiece, as illustrated in the attached drawing tables, in which:

[0076] • figure 1 shows a perspective view of a machine and a method for making an internal thread in a hollow workpiece according to the present invention;

[0077] • figure 2 is a front view of the machine of figure 1 ;

[0078] • figure 3 is a side view of the machine of figure 1 ;

[0079] • figure 3a shows a detail of figure 4;

[0080] • figures 4a-4i schematically show the steps of a method for making an internal thread in a hollow workpiece according to a present invention;

[0081] • figure 5 schematically shows a detail of the machine of figure 1 , in an alternative embodiment.

[0082] Detailed description

[0083] With reference to the attached figures, a machine for making an internal thread in a hollow workpiece according to the present invention has been indicated as a whole with 1 .

[0084] The term “hollow workpiece” 100 is preferably intended to define a substantially tubular element having a through hole 101 having an inner surface 102 to be machined and extending along its own central axis “Z” between a first 103a and a second end portion 103b.

[0085] Preferably, the hollow workpiece 100 is defined by a nut screw for a planetary roller screw, but it could be any tubular workpiece with internally threaded through cavity and, preferably, having a slenderness ratio greater than 3, more preferably greater than 5. In this text, “slenderness ratio” means the ratio between a length of the hollow workpiece 100 along its central axis “Z” and its internal diameter (i.e. diameter of the through hole 101 ).

[0086] Preferably, the machine 1 comprises a workpiece-holding assembly 2 and a grinding unit 3. The workpiece-holding assembly 2 and the grinding unit 3 are preferably associated with or constrained to a common baseplate 18. The workpiece-holding assembly 2 is configured to hold the hollow workpiece 100 and rotate it about its central axis “Z”.

[0087] More precisely, the workpiece-holding assembly 2 is configured to hold the hollow workpiece 100 from the outside and allow access to the through hole 101 from both end portions 103b.

[0088] Preferably, the workpiece-holding assembly 2 comprises a support structure 4, a workpiece-holding spindle 5 and a clamping member 6.

[0089] The workpiece-holding spindle 5 is integrally rotatable with the clamping member 6 with respect to the support structure 4 about a central axis “A” in use corresponding to the central axis “Z” of the hollow workpiece 100.

[0090] In this regard, the workpiece-holding spindle 5 has a through cavity 5a extending along the central axis “A”.

[0091] More precisely, the workpiece-holding spindle 5 (as well as the through cavity 5a) extends, along the central axis “A”, between a first end portion 7a and a second end portion 7b.

[0092] The through cavity 5a is therefore accessible both through the first 7a and through the second end portion 7b.

[0093] The support structure 4 is therefore provided with a through opening 4a in which the workpiece-holding spindle 5 is rotatably housed.

[0094] Special bearings are provided to hold the workpiece-holding spindle 5 in the through opening 4a with the option of (free) rotation.

[0095] A rotary actuator (not illustrated) is associated with the support structure 4 and operatively interposed between the support structure 4 itself and the workpiece-holding spindle 5 to rotate it about the central axis “A”.

[0096] In the preferred embodiment, said rotary actuator comprises a fixed body, a rotating shaft, a motor (preferably direct) and an angular measurement system to know the exact angular position of the workpiece and its speed during machining (characteristics not illustrated as they are known per se). The clamping member 6 is associated with the through cavity 5a and is movable between a release configuration and a clamping configuration.

[0097] In the clamping configuration, the clamping member 6 is configured to hold the hollow part 100 within the through cavity 5a, as exemplified in figure 3a.

[0098] In the release configuration, instead, the clamping member 6 is configured to allow the insertion and disengagement of the hollow workpiece 100 inside the through cavity, preferably through the first 7a or second end portion 7b.

[0099] It should be noted that, regardless of the configuration, the workpieceholding spindle 5 and the clamping member 6 are integrally rotatable about said central axis “A” of the through cavity 5a.

[0100] Preferably, the clamping member 6 of the piece-holding assembly 2 comprises a vice 8 provided with a plurality of jaws 8a movable in synchrony towards and away from said central axis “A” between an open position and a closed position.

[0101] The open (or mutually distanced) position corresponds to the release configuration.

[0102] The closed (or mutually close) position corresponds to the clamping configuration.

[0103] In the preferred embodiment, the vice 8 comprises three jaws 8a angularly equally-spaced about the central axis, with a relative offset of 120°.

[0104] Preferably, the jaws 8a each have a curved clamping end, or in any case shaped to maximize the contact surface with an extrados of the hollow workpiece 100.

[0105] In the preferred embodiment, the jaws 8a are movable along respective substantially radial directions with respect to said central axis “A” between the closed position and the open position. The movement of the jaws 8a and the level of the clamping pressure are preferably managed by a hydraulic actuation system configured to move the jaws in a synchronized manner and not described in detail here as it is known per se.

[0106] Alternatively, in embodiments not illustrated, the clamping member could comprise a clamping gripper actuated by an axial movement of a conical reference surface which causes its uniform closure over the diameter.

[0107] In a further alternative, a hydraulic deformation blocking is provided, i.e. by increasing the pressure of a passage of oil that compresses a metal sheet which, thanks to its deformation, blocks the workpiece.

[0108] The grinding unit 3 instead comprises a pair of tool-holding spindles 12, 13 placed, in at least a first operating configuration of the machine, on opposite sides of the workpiece-holding spindle 5 with reference to the central axis “A”.

[0109] Advantageously, in this way it is possible to work, i.e. thread, the through hole 101 of the hollow workpiece 100 by acting separately from the two end portions 103a, 103b, without therefore requiring repositioning or rotation of the workpiece.

[0110] Preferably, the grinding unit 3 comprises a support 9 provided with a first arm 10 and a second arm 11 .

[0111] The first arm 10 and the second arm 11 comprise respectively a first toolholding spindle 12 and a second tool-holding spindle 13 each coupled to a corresponding first 12a and second grinding tool 13a.

[0112] The first 10 and the second arm 11 extend transversely to said central axis “A” of the through cavity 5a of the workpiece-holding spindle 5.

[0113] The first arm 10 faces the second arm 11 . The first 10 and the second arm 11 are spaced apart from each other along the central axis “A” and define a working volume “V” between them.

[0114] In the first operating configuration of the machine 1 , the workpiece-holding spindle 5 is located inside the working volume “V”.

[0115] In other words, in the first operating configuration of the machine 1 , the workpiece-holding spindle 5 is interposed, with reference to the central axis “A”, between the first 10 and the second arm 11 .

[0116] In this way, the first end portion 7a faces the first arm 10, while the second end portion 7b faces the second arm 11 .

[0117] Note that, in the first operating configuration, the first grinding tool 12a and the second grinding tool 13a are inserted in the working volume “V” and oriented in the direction of the cavity 5a of the workpiece-holding spindle 5.

[0118] Preferably, in any operating position of the machine, the first 12a and second grinding tool 13a are aligned along a common working axis “E”.

[0119] Note that, at least along the working axis “E”, the first 12a and the second grinding tool 13a are always kept at the same distance.

[0120] Preferably, in any case, the machine comprises a probe or sensor (not illustrated) for resetting the tool-holding shaft that allows to check exactly the position of a tool-holding spindle with respect to the one facing it.

[0121] The grinding tools 12a, 13a preferably comprise a shaft to the free end of which a grinding wheel is constrained.

[0122] In the preferred embodiment, each shaft extends along the working axis “E” between a constrained end, connected to the respective first 10 or second arm 11 , and a free end, away from the arm.

[0123] The first 12 and the second tool-holding spindle 13 are rotatable about their respective rotation axes, aligned with the working axis “E”.

[0124] The first 12 and the second tool-holding spindle 13 preferably comprise a fixed body, a rotation shaft placed on a system of bearings, a preferably direct motor and an angular measurement system, to know exactly the position and speed of the tool, characteristics not illustrated as they are known per se.

[0125] Note that the grinding tools 12a, 13a may be of a profilable or non- profilable type.

[0126] In this regard, in the preferred embodiment, the machine 1 comprises a profiling roller 20 configured to shape the grinding tools 12a, 13a, i.e. the abrasive grinding wheels, maintaining their shape and dimensional accuracy.

[0127] In particular, the machine 1 comprises a profiling spindle 21 configured to rotate the profiling roller 20.

[0128] Said profiling spindle 21 is preferably associated with the workpieceholding assembly 2.

[0129] In the preferred embodiment, in fact, the machine 1 comprises a single profiling spindle 21 configured to act, alternatively, on the first 12a and second grinding tool 13a.

[0130] Advantageously, this guarantees maximum uniformity of intervention on the two tools, which will be machined with an identical profile and by means of a common movement system.

[0131] In this regard, preferably the profiling spindle is connected to the workpiece-holding assembly 2 and is movable, with respect to the grinding unit 3, integrally therewith.

[0132] Advantageously, this makes it possible to facilitate control in profiling operations, exploiting the same reference system and the same machining axes.

[0133] Preferably, the profiling spindle 21 is located in a position radially external to the workpiece-holding spindle 5, with reference to the central axis “A”, but within an axial footprint thereof.

[0134] Advantageously, this reduces the travel / movement required to pass from the working to profiling positions, thereby reducing footprint and machine downtime.

[0135] In the preferred embodiment, the profiling spindle 21 is interposed, with reference to a second operating direction “C” orthogonal to the central axis “A”, between the support 9 of the grinding unit 3 and the workpieceholding spindle 5.

[0136] Preferably, both the first 10 and the second arm 11 comprise a respective motor (electric or hydraulic) connected to the respective tool-holding spindle for rotating it. In particular, preferably the first arm 10 comprises a first motor 12b (electric or hydraulic) connected to the respective first tool-holding spindle 12.

[0137] The second arm 11 comprises a second motor 13b (electric or hydraulic) connected to the respective second tool-holding spindle 13.

[0138] In the illustrated embodiment, each first 12b or second motor 13b overhangs from the respective first 10 or second arm 11 away from the other second 11 or first 10 arm, respectively.

[0139] In other words, preferably the first motor 12b is connected cantilevered to the first arm 10 away from the second arm 11 .

[0140] Similarly, the second motor 13b is connected cantilevered to the second arm 11 away from the first arm 10.

[0141] The first 10 and the second arm 11 are rigidly connected to move integrally in translation and / or rotation with respect to said workpieceholding assembly 2.

[0142] In this regard, the grinding unit 3 preferably comprises a frame 15 to which the support 9 is movably constrained.

[0143] Preferably, the support 9 comprises a base 14 for connection between the first 10 and the second arm 11 .

[0144] The support 9 therefore preferably has a substantially C-shaped conformation, defined by the two arms and the base.

[0145] Preferably, in this regard, the first 10 and the second arm 11 are movable integrally with respect to the workpiece-holding assembly 2 in translation along at least two directions transverse to the central axis “A” of the through cavity 5a.

[0146] More preferably, the first 10 and the second arm 11 are movable integrally with respect to the workpiece-holding assembly 2 in translation along at least a first operating direction “B” and a second operating direction “C”.

[0147] The first operating direction “B” is orthogonal to the working axis “E” and transverse with respect to the central axis “A”.

[0148] In a “zero” position, the first operating direction “B” is orthogonal to the central axis “A”.

[0149] The second operating direction “C” is orthogonal to the first operating direction “B” and the central axis “A” of the through cavity 5a.

[0150] Preferably, moreover, the first 10 and the second arm 11 are movable integrally with respect to the workpiece-holding assembly 2 in rotation about at least one axis of rotation “D” orthogonal to the central axis “A” of the through cavity 5a.

[0151] The axis of rotation “D” is preferably parallel to the second operating direction “C”.

[0152] Preferably, the base 14 of the support 9 is slidably associated with the frame 15 to translate along at least the first operating direction “B”.

[0153] In this regard, the base 14 preferably comprises a pair of runners or trolleys 16 that define a sliding connection (direct or indirect) with respect to the frame 15.

[0154] Said runners or trolleys 16 are movable along the first operating direction “B”.

[0155] It is also provided for the presence of a first linear actuator (not illustrated) configured to move the support 9 along the first operating direction “B”.

[0156] The first linear actuator is preferably a screw actuator, comprising a motor (electric or hydraulic) that rotates a recirculating ball or planetary roller screw that moves a carriage constrained to the support 9 (in particular to the base 14).

[0157] Alternatively, however, other known linear movement systems could be used, without deviating from the subject-matter of the invention.

[0158] Preferably, the grinding unit 3 comprises a carousel 17 rotatably associated with the frame 15 to rotate about the axis of rotation “D”.

[0159] The support 9 is constrained when rotating about the axis of rotation “D” to the carousel 17, thus allowing the inclination of the first 12a and the second grinding tool 13a, therefore of the working axis “E”, to be varied with respect to the central axis “A”.

[0160] The carousel 17 is associated with a rotary actuation system (not illustrated) configured to rotate it; preferably the rotary actuation system comprises a motor or gearmotor (hydraulic or electric) coupled to a toothed wheel integral with the carousel.

[0161] Note that, preferably, the base 14 of the support 9 is slidably constrained to the carousel 17.

[0162] In other words, the base 14 of the support 9 is configured to slide with respect to the carousel 17 along the first operating direction “B” and to rotate, with the carousel 17, about the axis of rotation “D”.

[0163] In this way, the orientation in space of the first operating direction “B” varies as the angular position of the carousel 17 varies.

[0164] Preferably, the support 9 of the grinding unit 9 is also movable, with respect to the workpiece-holding assembly 2, along the second operating direction “C”.

[0165] Preferably, in this regard, it is the frame 15 of the grinding unit 3 that moves, by means of a respective movement system, along the second operating direction “C” with respect to the baseplate 18.

[0166] The frame 15 of the grinding unit 2 is therefore movable along the second operating direction “C” towards and / or away from the workpiece-holding assembly 2.

[0167] In this regard, preferably, the workpiece-holding assembly 2 is also movable with respect to the grinding unit 3 along a third operating direction “F”, parallel to the central axis “A” of the through cavity 5a.

[0168] Note that it could be the workpiece-holding assembly 2 the one moving, as well as the grinding unit 3 (or part thereof).

[0169] In the preferred embodiment, the workpiece-holding assembly 2 moves along the third operating direction “F” with respect to the baseplate.

[0170] Preferably, the machine comprises a control unit 19 associated with the grinding unit 3 and the workpiece-holding assembly 2.

[0171] The control unit 19 is configured to drive a movement of the grinding unit 3 with respect to the workpiece-holding assembly 2.

[0172] Preferably, the control unit 19 is configured to drive the grinding unit 3 with respect to the workpiece-holding assembly 2 in at least a first working position and in at least a second working position.

[0173] In the first working position, the first grinding tool 12a is inserted into the cavity 5a of the workpiece-holding spindle 5 at the first end portion 7a.

[0174] In the second working position, the second grinding tool 13a is inserted into the cavity 5a of the workpiece-holding spindle 5 at the second end portion 7b.

[0175] Note that, in the first working position, the second grinding tool 13a is extracted from the cavity 5a of the workpiece-holding spindle 5.

[0176] In the second working position, the first grinding tool 12a is extracted from the cavity 5a of the workpiece-holding spindle 5.

[0177] The control unit 19 is configured to impart a first rotation, in the first working position, and a second rotation, in the second working position, to the workpiece-holding spindle 5.

[0178] Preferably, in order to allow making a continuous internal thread in the hollow workpiece 100, the first rotation and the second rotation have opposite rotation direction.

[0179] Preferably, the control unit 19 is configured to govern a machining procedure.

[0180] In particular, the control unit 19 is configured to drive the grinding unit 3 along the first “B” and / or the second direction “C” and / or in rotation about the axis of rotation “D” to bring the first grinding tool 12a into a first machining-start position facing the first end portion 7a of the workpieceholding spindle 5.

[0181] The control unit 19 is programmed to send to the movement systems of the grinding unit one or more signals representative of respective movements along the first “B”, second operating direction “C” or about the axis of rotation “D”.

[0182] In this first machining-start position, the first grinding tool 12a is located near or in contact with the first end portion 103a of the hollow workpiece 100, while the second grinding tool 13a is arranged outside the cavity 5a of the workpiece-holding spindle 5.

[0183] The control unit 19 is thus configured to rotate the first tool-holding spindle 12 about the respective axis of rotation.

[0184] The control unit 19 is further configured to impart the first rotation to the workpiece-holding spindle 5 of the workpiece-holding assembly 2.

[0185] Note that rotating the first tool-holding spindle 12 and imparting the first rotation to the workpiece-holding spindle 5 can be steps carried out simultaneously or in sequence.

[0186] The control unit 19 sends to the workpiece-holding spindle 5 and to the tool-holding spindle 12 respective signals representative of the machining specifications, i.e. of speed, torque and other relevant parameters.

[0187] The control unit 19 is therefore programmed to drive the workpiece-holding assembly 2 towards the first arm 10 of the grinding unit 3, or vice versa, until the first working position is reached.

[0188] In the preferred embodiment, the control unit 19 sends to the workpieceholding assembly 2 a signal representative of a movement (direction, advancement, etc.) of the workpiece-holding assembly 2 along the third operating direction “F”.

[0189] In any case, the opposite would also be possible, with the advancement of the grinding unit 3 with respect to the workpiece-holding assembly 2, depending on the structure of the machine.

[0190] The workpiece-holding assembly 2 is therefore advanced along the central axis “A” until a first work-ending position is reached by the first grinding tool 12a.

[0191] Preferably, in the first work-ending position, the first grinding tool 12a has travelled, with respect to the workpiece-holding spindle 5, a stroke greater than or equal to half a length of the thread to be made.

[0192] The workpiece-holding assembly 2 (or the grinding unit 3) is then retracted along the central axis “A” until the first grinding tool 12a is extracted from the first end portion 7a of the workpiece-holding spindle 5.

[0193] The control unit 19 is therefore configured to drive the grinding unit 2 along the first “B” and / or second direction “C” and / or in rotation about the axis of rotation “D” to bring the second grinding tool 13a into a second machiningstart position facing the second end portion 7b of the workpiece-holding spindle 5.

[0194] Preferably, the extraction step of the first grinding tool 12a and the positioning of the second grinding tool 13a in the second machining-start position are contextual, the two tool-holding spindles being rigidly connected.

[0195] In this second machining-start position, the second grinding tool 13a is located near or in contact with the second end portion 103b of the hollow workpiece 100, while the first grinding tool 12a is arranged outside the cavity 5a of the workpiece-holding spindle 5.

[0196] The second tool-holding spindle 13 is rotated about the respective axis of rotation.

[0197] The control unit 19 is further configured to impart the second rotation to the workpiece-holding spindle 5 of the workpiece-holding assembly 2.

[0198] Also in this case, rotating the second tool-holding spindle 13 and imparting the first rotation to the workpiece-holding spindle 5 can be steps carried out simultaneously or in sequence.

[0199] The control unit 19 is then configured to drive the workpiece-holding assembly 2 towards the second arm 11 of the grinding unit 3, or vice versa, until the second working position is reached.

[0200] In the preferred embodiment, the control unit 19 sends to the workpieceholding assembly 2 a signal representative of a movement (direction, advancement, etc.) of the workpiece-holding assembly 2 along the third operating direction “F”.

[0201] In any case, the opposite would also be possible, with the advancement of the grinding unit 3 with respect to the workpiece-holding assembly 2, depending on the structure of the machine.

[0202] The workpiece-holding assembly 2 (or the grinding unit 3) along the central axis “A” until a second work-ending position is reached by the second grinding tool 13a.

[0203] Preferably, in the second work-ending position, the second grinding tool 13a has travelled, with respect to the workpiece-holding spindle 5, a stroke greater than or equal to half a length of the thread to be made.

[0204] More preferably, the machining performed by the second grinding tool 13a runs again at least in part an end section of the machining performed by the first grinding tool.

[0205] The control unit is therefore configured to retract the workpiece-holding assembly 2 (or the grinding unit 3) along the central axis “A” until the second grinding tool 13a is extracted from the second end portion 7b of the workpiece-holding spindle 5.

[0206] It should be noted that, in the preferred embodiment, the machine 1 comprises an angular phasing assembly 22, configured to detect the specifications of a pre-existing thread inside the hollow workpiece 100 and provide the control unit with a signal having an informative content representative of such specifications.

[0207] The phasing assembly 22 preferably comprises a probe or sensor 23 configured to measure the position of the existing thread with respect to the position of the grinding tool 12a, 13a.

[0208] Preferably, the phasing assembly 22 comprises a movable arm 24 with respect to each grinding tool 12a, 13a to the free end 24a of which the probe or sensor 23, preferably a feeler, is mounted.

[0209] The control unit 19 is configured to:

[0210] - receive from the phasing assembly a signal representative of the distance between the existing thread and the position of the grinding tool 12a, 13,

[0211] - drive the grinding unit 3 and / or the workpiece-holding assembly 2 to place the respective grinding tool 12a, 13a in the correct finishing position.

[0212] In some embodiments, not illustrated, the machine 1 comprises two workpiece-holding assemblies 2 movable independently between a working position, in which the respective workpiece-holding spindle 5 is placed in the working volume “V”, and a loading / unloading position, in which the respective workpiece-holding spindle 5 is positioned outside the working volume “V” in order to allow positioning a workpiece to be machined and / or picking up the worked workpiece.

[0213] This advantageously significantly increases the productivity of the machine.

[0214] In use, the machine 1 as described above allows to implement a method for making an internal thread in a hollow workpiece according to the present invention.

[0215] It should be noted that in the description of the method, the same reference numbers used so far in the description of the machine 1 will be used for the same technical characteristics.

[0216] In any case, it should be noted that all the characteristics of machine 1 , even if not explicitly identified or used in the following description of the process, are to be considered applicable to it.

[0217] This does not mean that the method is not implementable even with machines potentially different from the one described so far.

[0218] The method involves preparing the hollow workpiece 100 and preparing a first grinding tool 12a and a second grinding tool 13a facing each other and placed at a distance that defines, between the grinding tools 12a, 13a, a working volume “V” within which the hollow workpiece 100 can be placed.

[0219] Preferably, said hollow workpiece 100 has a slenderness ratio defined by a ratio between the length of the workpiece along its central axis “Z” and an inner diameter of the inner surface, greater than 3.

[0220] The grinding tools 12a, 13a preferably have the characteristics described above in relation to the machine 1 .

[0221] In particular, preferably the first 12a and the second grinding tool 13a are aligned along a common working axis “E” at any step of the method.

[0222] According to the invention, a first threaded section “T1” and a second threaded section “T2” of the inner surface 102 of the through hole 101 are made with two machinings which are carried out, respectively, starting from the first end portion 103a and the second end portion 103b of the through hole 101 .

[0223] Preferably, therefore, the creation of the first threaded section “T1” is performed by inserting the first grinding tool 12a into the first end portion 103a of the through hole 101 and advancing the first grinding tool 12a, with respect to the hollow workpiece 100, until a first intermediate workending position, located between the first 103a and the second end portion 103b of the hollow workpiece 100, is reached.

[0224] The creation of the second threaded section “T2” is similarly followed by inserting the second grinding tool 13a into the second end portion 103b of the through hole 101 and advancing the second grinding tool 13a, with respect to the hollow workpiece 100, until a second intermediate workending position, located between the first 103a and the second end portion 103b of the hollow workpiece 100, is reached.

[0225] In greater detail, the first grinding tool 12a is positioned in a first machining-start position facing the first end portion 103a of the hollow workpiece 100.

[0226] The first grinding tool 12a is rotated.

[0227] A first rotation is imparted to the hollow workpiece 100 about its central axis “Z”.

[0228] The first threaded section “T1” of the inner surface 102 of the through hole 101 is then made by advancing the first grinding tool 12a, with respect to the hollow workpiece 100, until the first work-ending position is reached.

[0229] The first grinding tool 12a is then extracted from the first end portion 103a of the hollow workpiece 100.

[0230] The second grinding tool 13a is then positioned in a second machiningstart position facing the second end portion 103b of the hollow workpiece 100.

[0231] It should be noted that, preferably, the extraction of the first grinding tool 12a from the first end portion 103a of the hollow workpiece 100 is contextual to the positioning of the second grinding tool 13a in a second machining-start position.

[0232] At this point (or before), the second grinding tool 13a is rotated and a second rotation is imparted to the hollow workpiece 100 about its central axis “Z”.

[0233] Preferably, the first rotation and the second rotation are opposite to each other (i.e. counter-rotating).

[0234] The second threaded section “T2” of the inner surface 102 of the through hole 101 is then made by advancing the second grinding tool 13a, with respect to the hollow workpiece 100, until the second work-ending position is reached.

[0235] Preferably, said step of making the second threaded section “T2” provides for rejoining said second threaded section “T2” with the first threaded section “T1 ” to obtain a continuous thread between the first 103a and the second end portion 103b.

[0236] More preferably, both the first threaded section “T1 ” and the second threaded section “T2” of the inner surface 102 have an axial extension such as to exceed a centre line “M” of the through hole 101 .

[0237] In other words, the first “T 1 ” and the second threaded section “T2” have an overlapping portion, where the two machinings intersect to give continuity to the thread made.

[0238] Preferably, therefore, the sum of the stroke travelled by the two grinding tools 12a, 13a between the machining-start position and the work-ending position is greater than an axial length, evaluated along the central axis “Z”, of the through hole 101 .

[0239] It should be noted that the method according to the present invention can be implemented both to make the entire internal thread of the hollow workpiece, starting from a tubular blank with a smooth inner surface, and starting from a semi-finished product having an internal thread already made by other machinings (turning, tapping, etc...) that needs to be rerun to achieve a higher level of precision.

[0240] The invention, therefore, achieves the intended purposes and achieves important advantages.

[0241] In fact, the adoption of a machine and a method that does not require any repositioning of the hollow workpiece and guarantees the mutual position between the two grinding tools makes it possible to perform fast and precise machining, overcoming the drawbacks of the prior art.

[0242] In addition, the use of two grinding tools acting from opposite sides of the hollow body makes it possible to use components with a high slenderness ratio (i.e. “stubby”), reducing the risk of vibratory phenomena during machining and allowing the speeds involved to be increased.

[0243] The above, together with the use of a workpiece-holding spindle movable along the working axis, further reduces misalignments and the error of intersection between the two threaded sectors and greatly increases the precision and reliability of the machining.

[0244] This is further improved by the adoption of a single profiling roller, the position of which with respect to both grinding tools is defined. In this way, both possible alignment errors and non-conformities while making the thread are minimised, since both tools are profiled by the same profiler.

Claims

CLAIMS1 . Method for making an internal thread in a hollow workpiece, comprising the steps of:- preparing a hollow workpiece (100) provided with a through hole (101 ) having an inner surface (102) to be machined and extending along its own central axis (Z) between a first (103a) and a second end portion (103b);- preparing a first grinding tool (12a) and a second grinding tool (13a) facing each other and placed at a distance that defines, between the grinding tools (12a, 13a), a working volume (V) within which the hollow workpiece (100) can be placed;- making a first threaded section (T1 ) of the inner surface (102) of the through hole (101 ) by inserting the first grinding tool (12a) into the first end portion (103a) of the through hole (101 ) and advancing the first grinding tool (12a), with respect to the hollow workpiece (100), until a first intermediate work-ending position, located between the first (103a) and second end portion (103b) of the hollow workpiece (100), is reached;- making a second threaded section (T2) of the inner surface (102) of the through hole (101 ) by inserting the second grinding tool (13a) into the second end portion (103b) of the through hole (101 ) and advancing the second grinding tool (13a), with respect to the hollow workpiece (100), until a second intermediate work-ending position, located between the first (103a) and second end portion (103b) of the hollow workpiece (100), is reached.

2. Method according to claim 1 , wherein the first (12a) and second grinding tools (13a) are aligned along a common working axis (E) at any machining stage.

3. Method according to claim 1 or 2, wherein both the first threaded section (T1 ) and the second threaded section (T2) of the inner surface(102) have an axial extension such as to exceed a centre line (M) of the through hole (101 ).

4. Method according to any one of claims 1 to 3, wherein said step of creating the second threaded section (T2) provides for rejoining said second threaded section (T2) with the first threaded section (T1 ) to obtain a continuous thread between the first (103a) and second end portions (103b).

5. Method according to any one of claims 1 to 4, wherein making the first threaded section (T1 ) of the inner surface of the through hole (101 ) involves imparting a first rotation to the hollow workpiece (100) about its central axis (Z) and making the second threaded section (T2) of the inner surface of the through hole (101 ) involves imparting a second rotation to the hollow workpiece (100) about its central axis (Z); said first rotation and said second rotation being opposite to each other.

6. Method according to any one of the preceding claims, wherein said hollow workpiece (100) has a slenderness ratio defined by the following relation:L / D > 3 wherein:- L corresponds to a length of the hollow workpiece (100) evaluated along the central axis (Z);- D corresponds to an inner diameter of the through hole (101 ).

7. Machine for making an internal thread in a hollow workpiece (100), comprising:- a workpiece holding assembly (2) provided with:• a workpiece-holding spindle (5) having a through cavity (5a) extending along a central axis (A), in use corresponding to acentral axis (Z) of a workpiece (100) to be machined;• a clamping member (6) associated with the through cavity (5a) and movable between a release configuration and a clamping configuration, in which it holds the workpiece (100) within the through cavity (5a); said workpiece-holding spindle (5) and said clamping member (6) being integrally rotatable about said central axis (A) of the through cavity (5a);- a grinding unit (3) comprising a support (9) provided with a first (10) and a second arm (11 ) extending transversely to said central axis (A) of the through cavity (5a), one facing the other, with said first (10) and second arms (11 ) defining between them a working volume (V) within which, in at least a first operating configuration of the machine, the workpiece-holding spindle (5) is located; said first (10) and second arms (11 ) comprising, respectively, a first tool-holding spindle (12) and a second tool-holding spindle (13) each coupled to a corresponding first (12a) and second grinding tool (13a) which, in said first operating configuration, are inserted in the working volume (V) and oriented in the direction of the through cavity (5a) of the workpiece-holding spindle (5); wherein said first (10) and second arm (11 ) are rigidly connected to move integrally in translation and / or rotation with respect to said workpieceholding assembly (2).

8. Machine according to claim 7, wherein said first (10) and second arms (11 ) are integrally movable with respect to the workpiece-holding assembly (2) in translation along at least one direction transverse to the central axis (A) of the through cavity (5a).

9. Machine according to claim 7 or claim 8, wherein said first (10) and second arms (11 ) are integrally movable with respect to the workpieceholding assembly (2) in rotation about at least one axis of rotation orthogonal to the central axis (A) of the through cavity (5a).

10. Machine according to claim 8 or claim 9, wherein the grinding unit (3) is movable, with respect to the workpiece-holding assembly (2), along a second operating direction (C), orthogonal with respect to the first operating direction (B) and the central axis (A) of the through cavity (5a).11 . Machine according to any one of the preceding claims, wherein the workpiece-holding assembly (2) is movable with respect to the grinding unit (3) along a third operating direction (F), parallel to the central axis (A) of the through cavity (5a).

12. Machine according to any one of the preceding claims, wherein, in any operating position of the machine, the first (12a) and the second grinding tool (13a) are aligned along a common working axis (E).

13. Machine according to any one of the preceding claims, wherein the workpiece-holding spindle (5) extends, along the central axis (A), between a first end portion (7a) and a second end portion (7b), which in the first operating configuration of the machine (1 ) respectively face the first (10) and second arms (11 ) to be accessible by the first grinding tool (12a) and the second grinding tool (13a).

14. Machine according to claim 13, comprising a control unit (19) associated with the grinding unit (3) and the workpiece-holding assembly (2) and configured to drive a movement of the grinding unit (3) with respect to the workpiece-holding assembly (2) such that:- in a first working position, the first grinding tool (12a) is inserted into the through hole (5a) of the workpiece-holding spindle (5) at the first end portion (7a);- in a second working position, the second grinding tool (13a) is inserted into the through hole (5a) of the workpiece-holding spindle (5) at thesecond end portion (7b).

15. Machine according to claim 14, wherein the control unit (19) is configured to impart a first rotation, in the first working position, and a second rotation, in the second working position, to the workpiece-holding spindle (5).

16. Machine according to claim 15, wherein the first rotation and the second rotation have opposite directions of rotation.

17. Machine according to any one of the preceding claims, comprising a profiling spindle (21 ) configured to act, alternatively, on the first (12a) and second grinding tool (13a).

18. Machine according to claim 17, wherein the profiling spindle (21 ) is connected to the workpiece-holding assembly (2) and is movable, with respect to the grinding unit (3), integrally therewith.

19. Machine according to any one of the preceding claims, comprising an angular phasing assembly (22):- provided with a probe or sensor (23) configured to measure the position of an existing thread within the hollow workpiece (100) with respect to a position of the first (12a) or second grinding tool (13a) and- configured to drive the grinding unit (3) and / or the workpieceholding assembly (2) to place the respective first or second grinding tool (12a, 13a) in the correct relative position for finishing the existing thread.

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