«AXIAL MOVEMENT LINEAR GAUGE»

Technical Field

The present invention relates to an axial movement linear gauge, including a support and protection structure that defines a longitudinal axis, a spindle movable along the longitudinal axis with an external surface and at least one longitudinal groove achieved on such surface, a feeler for contacting a piece to be checked, a position transducer adapted to detect displacements of the feeler with respect to the support and protection structure and comprising at least one movable part connected to the spindle, guide devices for guiding movements of the spindle along the longitudinal axis with respect to the support and protection structure, and an antirotation system for preventing rotations of the spindle about the longitudinal axis .

The present invention further relates to an axial movement linear gauge including a support and protection structure that defines a longitudinal axis, a spindle movable along the longitudinal axis, a feeler for contacting a piece to be checked, a position transducer adapted to detect displacements of the feeler with respect to the support and protection structure and comprising at least one movable part connected to the spindle, guide devices for guiding movements of the spindle along the longitudinal axis with respect to the support and protection structure, and a longitudinal locking element that cooperates with the support and protection structure.

There are known in the art, for example from international patent application No. WO-A-97/46849, axial movement linear gauges, or heads, of the so-called "pencil" type, with a cylindrical spindle, which axially slides along a longitudinal direction within a casing, by means of a guide

device consisting of two axial bearings with recirculating balls, housed in the casing in longitudinally spaced out positions, each comprising through holes, within which the spindle is partially inserted. The spindle carries at one end a feeler for touching the piece to be checked, and at the other end a ferromagnetic core, that translates inside associated windings, as a consequence of axial movements of the spindle. Each axial bearing includes closed tracks in which rows of balls can roll. The tracks include longitudinal portions formed at positions corresponding to slits located on the internal surface of the bearing, along which the balls contact the spindle and enable it to axially slide with respect to the casing. In such pencil probe, in order to avoid axial rotations of the spindle with respect to the casing, that would cause problems of measurement repeatability, a pin is radially fixed to the spindle and carries at its free end an idle wheel, housed in an axial slit integral with the casing, for example achieved in a spacer element located between the two bearings. In such gauges the unavoidable clearance existing between the slit and the idle wheel may affect the accuracy and the repeatability of the gauge, above all in the cases in which it is required to utilize a feeler offset with respect to the longitudinal axis of the gauge. Furthermore, an antirotation device, as the one disclosed in WO-A- 97/46849 limits the stroke of the gauge and demands extra fine mechanical machining to be carried out on the spindle, to insert the pin with the idle wheel and on the spacer element for achieving the slit, with consequent additional expenses and non-negligible assembly time.

International Patent Application No. WO-A-01/38819, discloses a pencil probe, which offers improved performances in terms of measurement accuracy and repeatability with respect to the solution illustrated in WO-A-97/46849. In particular, WO-A-01/38819 illustrates a pencil probe, in which the same means (for example the balls of axial bearings with recirculating balls of the

type illustrated in WO-A-97/46849) that assure the guide of the spindle, carry out also antirotation functions of the spindle about the longitudinal axis. The spindle is machined in order to obtain thereon one or more grooves, for example V-shaped, on which the balls of at least one axial bearing with recirculating balls are coupled. The solution proposed in WO-A-01/38819 offers improved performances in terms of measurement accuracy and repeatability, thanks to the coupling between the balls of the bearing and V-shaped seat of the spindle, and offers a simpler structure, since it is not necessary to achieve the slit in suitable position and is not even necessary to fix any pins on the spindle. However, as the balls and the V-shaped seat achieve antirotation functions and guiding functions at the same time, their coupling must be carried out in a particularly precise way, that reflects to the necessity to carry out the axial bearings with recirculating balls and the V- shaped seat with very small tolerances, and therefore with high machining costs and high number of out-of-spec parts. Even the assembly needs time and particular care, because of small coupling tolerances indeed. On the other hand, in the gauge illustrated in WO-A-01/38819 it is not possible to increase the clearance between the spindle and the axial bearing with recirculating balls. In fact, as it can be noticed in the embodiments illustrated in figures 2-5 of the above cited International patent application, if the coupling between the balls and the V-shaped seat(s) were not particularly precise, for example if it were relatively backlash, the spindle, while functioning, could incline, because of external stresses and it could cause serious measurement errors .

The object of the present invention is to provide a linear gauge which enables to obtain high standard of performance in terms of measurement accuracy and/or repeatability and at the same time reduced production costs. This and other objects are achieved by a gauge, according

- A -

to claim 1 and claim 15.

A gauge according to the invention has reduced production costs thanks to the low cost of the single components and to the constructive and assembly simplicity. Other features of the invention will be more clear from the following detailed description, with reference to the enclosed sheet of drawings, given by way of non-limiting example only, wherein: figure 1 is a first longitudinal cross-sectional view of a gauge according to a first preferred embodiment of the invention, figure 2 is a second longitudinal cross-sectional view of the gauge of figure 1, figure 3 is an enlarged transversal cross-sectional view of the gauge of figure 1 taken along line III-III of figure 1, figure 4 is an enlarged transversal cross-sectional view of the gauge of figure 1 taken along line IV-IV of figure 2, and figure 5 is a longitudinal cross-sectional view of a gauge according to a second preferred embodiment of the invention, and figure 6 is a longitudinal cross-sectional view of a gauge according to a third preferred embodiment of the invention, figure 7 is an enlarged longitudinal cross-sectional view of the gauge of figure 6, figure 8 is a longitudinal cross-sectional view of a gauge according to a fourth preferred embodiment of the invention, figure 9 in an enlarged transversal cross-sectional view of the gauge of figure 8 taken along line IX-IX of figure 8, figure 10 is a longitudinal cross-sectional view of a gauge according to a fifth preferred embodiment of the invention, figure 11 is an enlarged transversal cross-sectional

view of the gauge of figure 10 taken along line XI-XI of figure 10, and figure 12 is an enlarged transversal cross-sectional view of the gauge of figure 10 taken along line XII-XII of figure 10.

A first preferred embodiment of a gauge according to the invention is illustrated in figures 1-4 and includes a support and protection structure 21 with a tubular casing 1, for example made of steel and with a substantially cylindrical shape, that defines a longitudinal geometric axis and an internal surface 2 substantially cylindrical and it includes, besides a main body, a portion with reduced diameter 6 and a threaded end portion 3. A spool 4 is housed within the casing 1 and defines an axial opening 7 and three external annular seats 8, 9, 10.

A spindle 11 with an external surface is inserted in and is movable with respect to the casing 1. A support element 13 for a feeler 14, adapted to contact a piece to be checked 47, is fixed to the spindle 11. A longitudinal groove 12 is achieved on the external surface of the spindle 11 and has a V-shaped transversal cross-section defined by two inclined and converging surfaces, and length substantially equal to the longitudinal extension of the spindle 11. An inductive differential position transducer 32 includes a primary winding 15 and two secondary windings 16 and 17, and a core 18 made of ferromagnetic material. According to a known technique, the windings 15, 16 and 17 are tightly wound about the spool 4, in correspondence of the annular seats 8, 9 and 10 respectively, whereas the core 18 is fixed, for example glued, on a stem 19 connected by means of a support 20 to the end of the spindle 11, opposite to end which carries the feeler 14. The connection among the stem 19, the support 20 and the spindle 11 can foresee for example the use of bonding agents. The spool 4 with associated windings 15, 16 and 17 is housed in and coupled to a liner 5, for example by glueing the windings 15, 16 and 17 to such liner. The external

surface of the liner 5 features a threaded portion screwed to the threaded end 3 of the casing 1.

A guide device, for guiding axial movements of the spindle 11, with respect to the support and protection structure 21, includes for example an axial bearing with recirculating balls 22, housed into the casing 1 and including rolling elements, in particular balls 23, in contact with the external surface of the spindle 11. Such bearing 22 comprises a hollow support element 24 substantially cylindrically shaped, with a longitudinal through hole 25, in which the spindle 11 is inserted. The through hole 25 defines a cylindrical surface 26 of the support element 24, that has longitudinal slits 27. The hollow element 24 defines internal circulation tracks 30, where the balls 23 are seated. Each track 30 has a first longitudinal portion 31, a second longitudinal portion not illustrated in the drawings, substantially reciprocally parallel, and connecting portions 33 between these longitudinal portions. The slits 27 located on the cylindrical surface 26 are arranged on the first longitudinal portion 31 of each internal circulation track 30 and have specific dimensions so as to withhold the balls 23 within the associated rolling track 30, to enable such balls 23 to partially protrude with respect to the cylindrical surface 26 and contact the external surface of the spindle 11 when they are in the associated first longitudinal portion 31. The aforementioned international patent application No. WO-A-97/46849 provides a broader description of pencil probes including analogous axial bearings with recirculating balls.

As illustrated in the figures, the balls 23 that contact the external surface of the spindle 11 enable such spindle 11 to translate with respect to casing 1 along the longitudinal axis. An antirotation system comprises at least one antirotation element, in particular in the embodiment of the figures 1-4 the antirotation system comprises two antirotation balls 50

and 51, for example connected by means of calking or glueing, to suitable seats 52, 53, achieved on the support element 24 of the axial bearing with recirculating balls 22, in longitudinally opposite positions. The antirotation balls 50 and 51 are therefore in a fixed position with respect to the axial bearing with recirculating balls 22 and to the support and protection structure 21, and protrude towards the spindle 11 for engaging with the longitudinal V-shaped groove 12, preventing the spindle 11 to rotate about the longitudinal axis. When the spindle 11 translates with respect to the casing 1, the antirotation balls 50 and 51 cooperate with the surfaces of the longitudinal V-shaped groove 12, more specifically they slide. It should be noticed that the sliding friction between the antirotation balls 50, 51 and the V-shaped groove is extremely small, thanks to the reduced contact surface. However, the friction can be further reduced by foreseeing a minimum clearance between parts. Such clearance also facilitates the assembly operations without negatively affecting in any way the axial translation of the spindle 11, since the translation precision of the latter is defined only by the action of the balls 23 of the axial bearing with recirculating balls 22 on the external surface of the spindle 11. The antirotation balls 50 and 51 and the V-shaped groove 12 define the antirotation system of the gauge according to the invention. It should be noticed that the antirotation system does not limit the entity of the gauge stroke, contrary to what happens in the known gauges which include slit and pin, thus enabling to have greater application flexibility.

The gauge of figures 1-4 also comprises longitudinal positioning elements, more specifically a first and a second spacer element 34 and 35, housed in the casing 1 and arranged between the portion of reduced diameter 6 of casing 1 and the support element 24 of the bearing 22, and between the support element 24 and the spool 4,

respectively. In an alternative embodiment not illustrated, the antirotation elements, for example the antirotation balls 50 and 51, can be fixed to the spacer elements 34, 35. A thrust device including a mechanical thrust element, more specifically a compression spring 36, having ends respectively housed on a seat 28 of the support 20 for the stem 19 and on a seat 29 of the spool 4, thrusts the spindle 11 with respect to the support and protection structure 21 and enables to hold the spindle 11, when there is no contact between the feeler 14 and the piece to be checked 47, in a rest position defined by the abutment between surfaces of the support 20 and of the hollow support element 24. Abutment surfaces 37 of the support 13 for the feeler 14 and of the casing 1 are adapted to cooperate for defining the stroke limit of the spindle 11, further to contact between the feeler 14 and the piece 47 to be checked, in opposition to the thrust of the spring 36. A flexible, tubular-shaped sealing gasket 39 has one of its ends coupled to a seat 40 defined in the tubular casing 1, and the other end coupled to a seat 41 defined in the support 13 for the feeler 14. The electric connection between the windings 15, 16, 17 of the differential transducer 32 to external power supply, display and processing devices (schematically shown and indicated by reference number 43 in figure 1) occurs by means of wires of a cable 44. Longitudinal clamping elements comprise a threaded clamping ring nut 45 that, by coupling with the threaded portion of the housing 5, defines the mutual position of the casing 1 with respect to the housing 5 and thus defines the mutual position between the core 18 and the windings 15, 16, 17 of the transducer 32. When the gauge is in normal operating conditions, and there is no contact between the feeler 14 and the piece 47, the spring 36 urges the spindle 11 to a rest position. When

contact occurs between the feeler 14 and a surface of the piece 47 (in any whatever manual or automatic known way, herein neither illustrated nor described) , the spindle 11 displaces with respect to the casing 1 in opposition to the action of the spring 36 along a rectilinear path, parallel to the longitudinal axis of the casing 1, guided by the bearing 22 and by the antirotation balls 50, 51. More specifically, the balls 23 contact - through the slits 27 - the external surface of the spindle 11 and roll at one side on them and at the other on internal surfaces of the spindle 11 due to the thrust that the spindle 11 undergoes. Possible rotation thrusts about the longitudinal axis to that the feeler 14 undergoes are contrasted by the coupling between the antirotation balls 50, 51 and the V-shaped groove 12.

The displacement of the spindle 11 causes the core 18 to displace within the windings 15, 16 and 17 and a corresponding output voltage variation to occur at terminals of the secondary windings 16 and 17, according to the known functioning principle of an inductive differential transducer. By means of the electric connection, comprising the wires of the cable 44, the voltage variation with respect to a zero condition (defined in a known way in a previous zero-setting phase of the gauge) is detected in the external devices 43 and the data relating to the amount of displacement from the rest position is displayed.

An axial gauge as the herein described one offers specific characteristics insofar as constructive and assembly simplicity, and reduced production costs are concerned.

Indeed, as the coupling between the balls 23 of axial bearing with recirculating balls 22 and the spindle 11 occurs along the external surface of the spindle 11 and it realizes guide functions only, such coupling can be embodied in a very simple way and with closer tolerances than, for example, the coupling illustrated in WO-A- 01/38819, in which at least some of the balls of the axial

bearing with recirculating balls engage with the V-shaped groove .

Even the V-shaped groove 12 and the antirotation balls 50, 51 can be realized with relatively wider tolerances, as previously illustrated, thanks to the clearance permissible in the coupling. Such clearance simplifies the assembly of the gauge, more specifically because the insertion of the spindle 11 inside the through hole 25 of the axial bearing with recirculating balls 22 is facilitated, so reducing the assembly time. Furthermore, the relatively wide tolerances enable to cut down the production costs of the single components, either because cheaper technologies and machining methods are sufficient and because the number of out-of-spec pieces is reduced. More specifically, the costs for realizing the V-shaped groove 12 of the spindle 11 and the axial bearing with recirculating balls 22 are considerably reduced with respect, for example, to the case illustrated in WO-A-01/38819. In figure 5 is shown in simplified way a longitudinal cross-section of an axial movement linear gauge, according to a second preferred embodiment of the invention, with a support and protection structure 70, comprising a tubular casing 55, inside which there is positioned an axial bearing with recirculating balls 56 completely similar to the axial bearing 22 of the figures 1-4. More specifically, the axial bearing with recirculating balls 56 is blocked between an abutment 59 achieved within the tubular casing 55 and a threaded ring nut 63 screwed on one end of the tubular casing 55. Two antirotation spheres 57, 58 are fixed to the axial bearing with recirculating balls 56 and engage with a V-shaped groove (not illustrated in figure 5) of a spindle 64, similarly to what has been illustrated with reference to the figures 1-4. Substantially, the gauge of the figure 5 includes guide devices and an antirotation system analogous to those illustrated in the figures 1-4.

The spindle 64 slides within the axial bearing with recirculating balls 56. A feeler 65 is coupled by means of

a support element 66 to a first end of the spindle 64. A ferromagnetic core 67, which slides inside windings 60 of a multi-seat and multi-winding inductive linear transducer of a known-type and thus not in detail illustrated, which is power supplied through electric wires 62, is coupled by means of a stem 68 to a second end of the spindle 64. The gauge of the figure 5 even includes a thrust device of a pneumatic known type, that is power supplied by a not illustrated compressed air source. The compressed air, through an input conduit 61, acts on a piston 69 connected to the spindle 64.

This second embodiment shows further advantages in terms of manufacturing and assembly simplicity, with respect to the gauge illustrated in the figures 1-4. In fact, when the gauge is assembled, the axial bearing with recirculating balls 56 is very easily inserted within the casing 55 through an aperture at an end of the tubular casing 55, and fixed in operative position only by means of the abutment 59 and the ring nut 63, the latter, once screwed on the tubular casing 55, blocks the axial bearing with recirculating balls 56 without the necessity of using other components, as spacer elements. Even the disassembly is extremely facilitated, because once the ring nut 63 is unscrewed, it is possible to extract the spindle 64, which pulls out, by means of the piston 69, the axial bearing with recirculating balls 56, that can thus be cleaned and/or replaced, as well as the other components of the gauge . In figures 6 and 7 there is illustrated an embodiment of the invention similar to that illustrated in figure 5, that is particularly advantageous, because it enables to reduce the quantity of treated air, more specifically compressed air. Many components of the gauge of figure 6 are similar or equal to correspondent components of the gauge of the figure 5, consequently they will be referenced to by the same numbers. The gauge illustrated in the figures 6 and 7 includes a support and protection structure 70 with a

tubular casing 55 that defines a longitudinal axis of the gauge and within which there is positioned a bushing 81 blocked in an analogous way to that described with reference to the axial bearing with recirculating balls 56 of the figure 5 between an abutment 59 and a ring nut 63. An annular sealing gasket 83 between the bushing 81 and the casing 55 ensures the seal. The bushing 81 is of the so- called "differential" type and shows characteristics similar to those illustrated in patent US-A-4616420, for example. In the specific case, as better visible in the figure 7, the bushing 81 includes two external annular projecting parts 84 and 85, longitudinally opposite to one another, in contact with the tubular casing 55 and two internal annular projecting parts 86 and 87, longitudinally arranged in different positions with respect to the external annular projecting parts 84 and 85, that act as guide for a spindle 64. The bushing 81 further comprises a transversal hole 88 in a substantially intermediate position with respect to the internal annular projecting parts 86, 87.

An antirotation element, more specifically a sphere 58 is fixed to the bushing 81, thus resulting stationary with respect to the bushing 81 and to the tubular casing 55, and engages with a V-shaped groove 82 of the spindle 64 which slides along the longitudinal axis within the tubular casing 55 and the bushing 81. A feeler 65 is connected to a first end 89 of the spindle 64, while to a second end 90, opposite to the first one, there is connected a stem 68 with smaller diameter than that of the spindle 64, that carries a ferromagnetic core 67 which slides within windings 60 of a linear inductive transducer, power supplied through electric wires 62. A seat for a mechanical sealing element, for example a metallic ring 91, is achieved in correspondence of the second end 90 of the spindle 64.

The V-shaped groove 82 has shorter length than the longitudinal dimension of the spindle 64 and more

specifically has a length of about a half of the latter, starting from the second end 90. Because of the limited length of the V-shaped groove 82, only the internal annular projecting part 87 projects onto a portion of the external surface of the spindle 64 interrupted by the V-shaped groove 82, while the internal annular projecting part 86 cooperates with a non-interrupted portion of the external surface of the spindle 64. The distance between the internal annular projecting parts 86, 87 and the length of the V-shaped groove 82 are such that the internal annular projecting part 86 never cooperates with the V-shaped groove 82 during the sliding of the spindle 64 between a rest position defined by the abutment between the feeler 65 and the ring nut 63, and an overstroke position defined by the abutment between the metallic ring 91 and an end surface of the bushing 81.

The gauge illustrated in the figures 6 and 7 further comprises a thrust device of the pneumatic type, which is hereinafter described in detail. With respect to the gauge of the figure 5, such pneumatic thrust device does not include the piston 69, thus making the gauge simpler and more economic, without negatively affecting the reliability and/or the metrological performances of the gauge. The thrust device is operated by a service fluid, for example air, which can be input in pressure through a conduit 61, or sucked up through the same conduit 61, so modifying the pressure of a zone 92 between the spindle 64 and the windings 60 within the gauge. The pressure difference between the zone 92 and the external environment determines a displacement of the spindle 64 along the longitudinal axis. In substance, the pneumatic thrust device is adapted to provoke displacements of the spindle 64 towards the rest position and towards the overstroke position. More specifically, when the pressure within the gauge is higher than the external environment pressure, that is to say when compressed air is input in the gauge through the conduit 61, the spindle 64 and its associated

feeler 65 displace away from the windings 60 towards the overstroke position; on the contrary, when the pressure within the gauge is lower then the external environment pressure, that is to say, when air is sucked up in the gauge, the vacuum condition which tends to create within the gauge, enables the external environment pressure to thrust and maintain the spindle 64 and the feeler 65 in the rest position. The compressed air, input in pressure through the conduit 61, goes through the whole gauge thanks to the coupling with clearance between the windings 60 and the core 67 of the transducer, between the internal annular projecting part 87 and the interrupted portion of the external surface of the spindle 64, and finally between the internal annular projecting part 86 and the non-interrupted portion of the external surface of the spindle 64, and exits in correspondence to the ring nut 63. The air covers the inverse distance when it is sucked up in the gauge in order to bring it in a rest position. As previously mentioned, the gauge of the figures 6 and 7 enables to treat reduced air quantities (compressed or sucked) with respect to the gauge of the figure 5 without the necessity to use any piston, thanks to the coupling with reduced clearance between the internal annular projecting part 86 and the non-interrupted portion of the external surface of the spindle 64, which allows just a minimum amount of air to pass. It can be noticed that the spindle 64 embodies the piston function without any added elements . As the gauge of the figure 5, so the gauge of the figures 6 and 7 can be easily disassembled by unscrewing the ring nut 63 and extracting the spindle 64, which pulls even the bushing 81 out by means of the metallic ring 91. Then, through the hole 88, using for example compressed air, it is possible to clean the bushing 81 and the spindle 64, which tend to get dirty once they are in the rest position, that is to say, once the air is sucked up within the gauge.

A fourth alternative embodiment of a gauge according to the invention is illustrated in figures 8 and 9. In the following description, component parts of the gauge similar or equal to correspondent component parts of the gauges of the figures 1-7 will be referenced to by the same numbers. The gauge of the figures 8 and 9 includes a support and protection structure 70 with a tubular casing 55 that defines a longitudinal axis of the gauge and has an end portion 93, internally threaded and with an external transversal abutment surface 98 for an integral bushing 100. The integral bushing 100 is longitudinally hollow for a spindle 94 to pass and includes a longitudinal clamping element, for example a clamping ring nut 95, adapted to be screwed to the threaded end portion 93 of the tubular casing 55, and a bushing 96 integrally embodied with the ring nut 95. The bushing 96 is for example of the differential type illustrated in patent US-A-4613420 and has features similar to those of the bushing 81 of the figures 6 and 7, with two external annular projecting parts 84 and 85 and two internal annular projecting parts 86 and 87. The latters act as guide for the spindle 94 and define with it guide zones 105. It should be noticed that, for the sake of simplicity, the external and internal annular projecting parts are not illustrated in detail in the figure 8, and numbers 84, 85 and 86, 87 refer substantially to zones where such projecting parts are embodied. Advantageously, a non-illustrated annular sealing gasket can be placed between opposite cylindrical surfaces of the integral bushing 100 and the tubular casing 55 to ensure the seal. The integral bushing 100 further includes an abutment surface 99 that cooperates with the abutment surface 98 achieved on the tubular casing 55 and defines an operative position of the bushing 96. As the abutment surfaces 98 and 99, that are external surfaces, can be easily machined and therefore can be obtained with very reduced machining tolerances, the operative position is defined in a very precise way. It should be noticed that,

contrarily to the embodiment of the figures 5 and 6 wherein the position of the bushing 81 is substantially defined by a part which is difficult to machine (the abutment 59, within the tubular casing 55) , the operative position of the bushing 96 is more precise and it is possible to prevent any longitudinal alignment errors of the bushing 96 and therefore sliding perpendicularity errors of the spindle 94. The spindle 94 has a plurality of longitudinal grooves 97, for example three or more, substantially similar and parallel to each other, one of which cooperates with an antirotation element, in particular a sphere 58, fixed to the bushing 96 and therefore stationary with respect to it and to the tubular casing 55. In a non-illustrated alternative embodiment, a pin connected to the spindle can engage with a slit achieved on the bushing. The longitudinal grooves 97 have length such that they are opposite to both of the internal annular projecting parts 86 and 87 during the entire stroke of the spindle 94. By virtue of the longitudinal groves 97, the guide zones 105 are reduced with respect to the embodiment of the figures 1-7. Therefore, possible problems on sliding caused by dust or other foreign material present in the environment and accumulated between the guide zones, are extremely limited. A feeler 65 is connected to a first end 102 of the spindle 64, while to a second end 103, opposite to the first one, there is coupled a stem 68, with a smaller diameter than the diameter of the spindle 64, that carries a ferromagnetic core 67 that slides within windings 60 of a linear inductive transducer, power supplied through electric wires 62. At the second end 103 of the spindle 64 there is achieved a seat for a mechanical sealing element, for example a metallic ring 104. The gauge shown in the figures 8 and 9 is of the pneumatic type and includes a thrust device completely similar to that illustrated with reference to the figures 6 and 7. More specifically, a service fluid, for example air, is

input, or sucked up through a conduit 61. Such air goes through the whole gauge thanks to the coupling with clearance between the windings 60 and the core 67 of the transducer, the grooves 97 of the spindle 94, the guide zones 105 between the spindle 94 and the internal annular projecting parts 86 and 87, and the coupling with clearance between the ring nut 95 and the spindle 94. It should be noticed that the very coupling between the ring nut 95 and the spindle 94 allows to limit the amount of treated air, notwithstanding the presence of the longitudinal grooves 97.

Such gauge is particularly advantageous because it can be operated by very low measuring forces, or even just by the force of gravity, without using any spring. In fact, as mentioned before, the longitudinal grooves 97 enable to restrict the guide zones 105 between the spindle 94 and the internal annular projecting parts 86 and 87, and thus to reduce the friction. Moreover, by virtue of the fact that the integral bushing 100 guarantees with high accuracy the sliding of the spindle 94 and therefore of the stem 68, it is possible to embody the inductive position transducer with a relatively wide clearance between the ferromagnetic core 67 and the windings 60, therefore further reducing the possibility to have undesired friction within the gauge and reducing the zones of deposits accumulation, without negatively affecting the gauge performances in terms of electrical repeatability. In substance, the feeler 65 can be led into contact with the piece to be checked (or towards the rest position) just by means of the force of gravity, and then it can be advantageously led back towards the rest position (or into contact with the piece) by means of the pneumatic thrust device.

A fifth alternative embodiment of a gauge according to the invention is shown in figures 10, 11 and 12 and presents further advantages in terms of cost and realization time saving. As many component parts of the gauge are equal or similar to correspondent component parts of the gauges of

the figures 1-9, the same reference numbers will be utilized.

The gauge of the figures 10-12 includes a support and protection structure 70 with a tubular casing 55 that defines a longitudinal axis of the gauge and has an end portion 93, internally threaded and with an external transversal abutment surface 98 for an integral bushing

106. The integral bushing 106 is longitudinally hollow for a spindle 107 to pass and includes a longitudinal clamping element, for example a clamping ring nut 108, adapted to be screwed to the threaded end portion 93 of the tubular casing 55, and a bushing 109 integrally embodied with the ring nut 108, for example of the differential type disclosed in patent US-A-4616420 with two internal annular projecting parts in correspondence of zones 110 and 111 which act as guide for the spindle 107 and define with it guide zones 112, visible in the figures 11 and 12. Advantageously, a non-illustrated annular sealing gasket can be placed between opposite cylindrical surfaces of the integral bushing 106 and the tubular casing 55 to ensure the seal. The integral bushing 106 is referred and blocked with respect to the tubular casing 55 in a way analogous to that illustrated with reference to the figures 8 and 9, by means of an abutment surface 99 that cooperates with the abutment surface 98 achieved on the tubular casing 55. As in the previous embodiment, such operative position is defined in a particularly precise way, by virtue of the easiness of mechanical machining of the abutment surfaces 98, 99. A feeler 65 is connected to a first end 116 of the spindle

107, while to a second end 117, opposite to the first one, there is coupled a stem 68, with a smaller diameter than the diameter of the spindle 107, that carries a ferromagnetic core 67 that slides within windings 60 of a linear inductive transducer, power supplied through electric wires 62. The internal annular projecting parts 110 and 111 have a

plurality of longitudinal grooves 113, for example three or more, which enable to reduce the overall surface of the guide zones 112, thus obtaining a surface reduction effect of the zones subjected to deposits accumulation, analogously to the gauge of the figures 8 and 9.

The gauge includes an antirotation system, better visible in figure 11, with an antirotation element, for example a pin 114 stuck in a transversal hole of the spindle 107. The pin 114 is coupled with a suitable longitudinal slit 115 achieved in the bushing 109 and prevents rotations of the spindle 107 with respect to the tubular casing 55. Alternatively, the gauge can include an antirotation system of the type illustrated with reference to the previous figures, with an antirotation element stationary with respect to the support and protection structure that engages with a suitable longitudinal slit of the spindle. The gauge of the figures 10-12 is of the pneumatic type, with a thrust device completely similar to that illustrated with reference to the figures 6-9, with a conduit 61 for inputting or sucking up a service fluid, for example air. As in the embodiments of the figures 8 and 9, the amount of treated air is limited by the coupling between the ring nut 108 and the spindle 107, notwithstanding the presence of the longitudinal grooves 113. The hereinbefore described gauges can be modified, without departing from the scope of protection of the invention. For example, the axial bearing with recirculating balls can be different from that illustrated and can contain rolling bodies different from balls, as for example cylindrical or other known-shaped bodies, and a number of tracks different from four, for example three, arranged at 120° one from the other .

A gauge according to the invention can include more than two antirotation elements (50, 51; 57, 58) that can be arranged along a same direction, or along different directions. Consequently, the spindle will have more than one groove, each at directions along which the antirotation

elements are arranged.

The antirotation elements (50, 51; 57, 58) can be spherical- shaped, as previously described, or can have a different shape, for example an elongated shape and/or with a rotation surface for cooperating with the longitudinal groove. Even the longitudinal groove can have a profile different from the V-shaped illustrated one, for example U- shaped or right angle shaped, and can have length shorter than the longitudinal extension of the spindle. In another alternative embodiment, the axial bearing with recirculating balls (22;56) or the bushing (81; 96; 109) are replaced by two or more known-type bushings to which antirotation elements can be connected for engaging with the longitudinal groove of the spindle. In a further alternative embodiment, the axial bearing with recirculating balls (22;56) or the bushing (81) are replaced by a linear bearing with balls retainer, for example of the type described in patent US-A-5236264. In such embodiment, the antirotation elements can be connected to the support and protection structure, for example to the external casing, or to other elements stationary with respect to the external casing (as sleeves which contain the retainer) , so that the stroke of the gauge would not be limited by the dimension of the retainer itself. In the embodiments illustrated in the figures 1-12, as well as in the embodiments with linear bearing with balls retainer, the antirotation elements can be connected to the guide devices (or to the support and protection structure) by means of calking or glueing, and even by means of idle coupling.

The gauges of the figures 5-12 can be modified in a known way for including a mechanical thrust element, for example a spring that thrusts the spindle (64; 94; 107) in rest or overstroke position. The spindle (64;94;107) is then led in the opposite - overstroke or rest, respectively - position, by means of the pneumatic thrust device which supplies the gauge with compressed air or sucks air up in

the gauge .

The hereinbefore described gauges can be advantageously employed in the dimensional checking of mechanical pieces. The embodiments illustrated in the figures 5-12 are advantageously employed in that checking wherein the feeler is required to exert a particularly low measuring force on the surface of the piece to be checked, for avoiding damages to the piece, and wherein the feeler is required to have a retracted rest position in order not to interfere with the positioning of the piece to be checked, so avoiding the risk of damaging the gauge. The typical case is that of the dimensional checking of glass parts, for example windshields, rear windows and sunroofs for the automotive industry.