DESCRIPTION

«HEAD FOR CHECKING LINEAR DIMENSIONS OF MECHANICAL PIECES»

Technical Field

The present invention relates to a head for checking the linear dimensions of mechanical pieces, including a support element that defines a longitudinal geometrical axis, a movable armset with an arm, movable with respect to the support element and carrying a feeler, a fulcrum, coupled to the arm and to the support element for enabling limited rotational displacements of the arm with respect to the support element, a retraction device, including a piston movable with respect to the support element and sealing elements, the piston being adapted for cooperating with the movable arm to bring the movable arm to a definite inoperative position, and a transducer for providing signals depending on the position of the arm with respect to the support element.

Background Art

There are known many types of gauging or measuring heads, including such features, for the inspection of mechanical pieces in benches, transfer lines or in so-called "in- process" applications in the course of the machining in machine tools.

U.S. patent US-B1-6269546 refers to this kind of heads. The U.S. patent discloses, among a number of features, two different systems for realizing the so-called feeler/-s retraction, i.e. for displacing the feeler/-s to a position which is far from the surface of the workpiece to be checked, in order to avoid accidental impacts and/or undesired slidings of the feeler/-s in the course of displacements between head and workpiece towards/away from each other.

The first system, shown in simplified form in figure 2 of the U.S. patent, includes a device with a piston coupled to a pneumatic circuit. When the device is activated, the piston urges the arm to an inoperative position, that is defined by appropriate limitation abutments. As the head has reached the checking position, the air supply is interrupted: the piston takes back the former position thanks to the action of a spring, and the feeler can freely displace towards the surface to be checked. In the second system, to which figure 9 of the mentioned U.S. patent refers, the piston is replaced by a bellows - also fed by a pneumatic circuit - including one end which is open and coupled to the casing, and the other end which is closed and free to urge the arm to a retracted position. The first system, known for a long time, presents some problems especially in the case of damping liquid inside the casing. In fact, sealing gaskets between the piston and the internal surface of the associated cylinder must enable the suitable mutual displacement between the two parts, e.g. avoiding to hinder the correct return of the piston to the initial position. As a consequence, the wet seal action is only partial and fluid is progressively released through the ducts of the pneumatic circuit. Thus, it is difficult to combine sealing action and friction limitation at the same time, to ensure correct operation and reliability in time .

The device with the bellows shown in figure 9 of patent US- Bl-6269546 enables remarkable advantages in terms of simplicity and friction absence. The bellows is very stressed and is subject to great deformations. For this reason, choosing materials that can assure the solution reliability is often difficult.

Disclosure of the invention

Object of the present invention is to provide a head for

checking linear dimensions of mechanical pieces including a retraction device that is simple and assures an increased reliability with respect to the already known solutions. This object is achieved by a head according to claim 1. Among the advantages that the solution according to the present invention provides, there is the following remarkable advantage. In the known retraction piston devices, in the course of sliding displacements, small amounts of air inevitably leak between the gaskets and the cylinder wall and, flowing among the movable parts (for example in piston seats that are only partially filled by gaskets) interferes with the correct operation of the device. In order to solve this problem, the known devices provide appropriates ducts for discharging the leaked air through a drainage pipe. A known solution is shown in simplified form in figure 1. In addition to electrical and pneumatic connections of the head, there is the drainage pipe that must be correctly located to prevent fluids present in the machine from being sucked. The solution according to the present invention enables to eliminate the drainage pipe, as it will be hereinafter illustrated.

Brief Description of the Drawings

The invention is now described in more detail with reference to the enclosed sheets of drawings, given by way of non limiting example, wherein: figure 1 is a longitudinal cross-sectional view of a head, according to the prior art, with some details shown in front view for the sake of simplicity and clarity; and figure 2 is a partial longitudinal cross-sectional view, in enlarged scale, of a head according to the present invention, showing a retraction device.

Figure 1 shows a gauging head of a known type, substantially of the same type of heads outlined in the mentioned patent US-B1-6269546, to which reference is made for a more detailed description.

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In brief, the head shown in figure 1 includes a support element with a casing 1, that is made of steel and substantially parallelepipedon shaped, and defines a longitudinal geometrical axis. A movable armset includes an arm 4 arranged along a direction substantially parallel to the longitudinal geometrical axis of the casing 1. The arm 4 includes an internal portion, located inside casing 1, and an external portion, located outside casing 1, carrying a feeler 7. With reference to the arrangement shown in figure 1, the feeler 7 is intended for touching a mechanical surface arranged in the upper part of the figure. Through appropriated openings, an inside hollow of casing 1 can be filled with damping fluid, such as oil with particular viscosity features in order to decrease the speed of the arm displacement. This feature is particularly important in the case of checking of grooved surfaces. A fulcrum 10 is coupled to surfaces of the casing 1 and of the arm 4 and includes a deformable element. The fulcrum 10 enables the arm 4 to perform limited but accurate rotational displacements about an axis perpendicular to the longitudinal geometrical axis of the casing 1. These displacements are limited by means of mechanical limiting devices obtained in a known way. For example, figure 1 shows a screw 12, passing through a hole of the arm 4 and screwed into the casing 1. The abutment of an area of the internal portion of the arm 4 against the head of such screw 12 limits the upward rotation of the arm 4, in a counterclockwise direction in figure 1, that corresponds to a displacement of the feeler 7 away from the surface of the workpiece to be checked. Other mechanical elements, not visible in the cross-section of figure 1, cooperate for limiting rotations of the arm 4 in the opposite direction. A biasing device for urging the feeler 7 against the surface of a mechanical piece to be checked includes a return spring 14 that is inserted in an appropriated seat and has one end coupled to the arm 4 and the other end coupled to a setting element, which is, in turn, coupled to

the cas ing 1 .

A pneumatically-operated retraction device 18 for bringing the arm 4 to a definite inoperative position, that is generally defined by the mechanical limit stop of the screw 12, is made as follows.

In a casing seat there is located a cylinder 20. A piston 22 can slide inside cylinder 20 and includes two circular seats 24 and 26 for housing sealing elements with gaskets 25 and 27, that are intended for realizing the sealing between the piston 22 and the inside wall of the cylinder 20. More specifically, the gasket 25 belongs to the "lip seal" type and provides a prevailing sealing action in one of the two displacement directions of the piston 22. The gasket 27 is substantially symmetric, for example it is a so-called "0-ring" having the shape of an annulus . The piston has a central hollow area 28 and an axially arranged ball 30, which defines a thrust surface.

The ends of a compression spring 33 are coupled to the piston 22 and the cylinder 20, respectively. An axial opening 35 at an end portion of the cylinder 20 protruding from the casing 1 is coupled to a tube 37 for the inlet of air coming from a known pneumatic circuit, which includes a source of compressed air (not shown in figure 1) . An air drainage system includes an interspace or chamber 40, defined in the wall of the cylinder 20. Chamber 40 communicates on one side with the internal surface of the cylinder 20 by means of an internal opening facing the wall of the piston 22 between the two gaskets 25 and 27, on the other side with a drainage duct 44 coupled to an associated discharge pipe, not shown in figure 1. The drainage system is necessary to exhaust the air that inevitably leaks in the spaces between cylinder 20 and piston 22 in the course of mutual displacements. It is to be noted that air mostly leaks from the hollow of cylinder 20 on the side of the gasket 25, since the latter can not be too compressed in order to enable correct sliding displacements. A locating stem 46 has one end coupled in an adjustable way

to the arm 4, whereas the other end is free and defines a substantially plane abutment surface 48 facing the piston 22, more specifically the thrust surface of the ball 30. A differential inductive transducer 50 of a known type includes windings fixed to the casing 1 and a core made of ferromagnetic material, coupled to the arm 4 by means of a stem.

Under normal working conditions, before moving the head and a workpiece to be checked towards each other to a checking mutual arrangement in a known way, in order to prevent the feeler 7 from impacting against the workpiece or other obstacles during such movement, the arm 4 is brought to a definite inoperative position in which the feeler 7 is far from the checking position. To this purpose, the retraction device 18 is activated by making compressed air flow inside through the tube 37. The pressure that the air applies against the piston 22 urges the latter to displace upwards (with reference to the arrangement of figure 1) in opposition to the thrust of spring 33. When the thrust surface of the ball 30 contacts the abutment surface 48 of the locating stem 46, the arm 4 is urged to move in a counterclockwise sense (with reference to figure 1) until the internal portion of the arm 4 is in abutment against the head of the screw 12, so defining the inoperative position of the arm 4.

When the checking mutual arrangement is reached and a workpiece is present in the checking position, the retraction device 18 is excluded, by cutting off the air flow and discharging the pressure by means of known appropriate devices, and the spring 33 draws the piston 22 back down. Then, the tractive force that spring 14 applies to arm 4 urges the feeler 7 in contact with the workpiece to be checked. The position that the feeler 7 and consequently the arm 4 undertake further to such contact is detected, in a known way, by the transducer 50 which transmits associated signals to a display and processing unit, not shown in figure 1.

Figure 2 illustrates, in different scale, a retraction device 68 that substitutes the device 18, shown in figure 1, in a head according to the present invention. Likewise the device 18, the retraction device 68 is pneumatically-operated and includes a cylinder 70 coupled to the casing 1 at an appropriate seat. A piston 72 can slide inside the cylinder 70 and sealing elements between the piston 72 and the cylinder 70 are provided. In particular a sliding gasket or "lip seal" gasket 75 is housed in a circular seat 74 of the piston 72. The piston 72 has a central hollow area 78, and a ball 80, axially arranged, that defines a thrust surface. A return compression spring 83 has its ends coupled to the piston 72 and to the cylinder 70, respectively. An axial opening 85 at an ending area of the cylinder 70 protruding from the casing 1 is coupled to a tube 87 for the inlet of air coming from a known pneumatic circuit that includes a source of compressed air (not shown in figure 2) . The sealing elements further include a bellows gasket 88, which is fixed (for example glued) to surfaces of the cylinder 70 and of the piston 72 facing each other, and housed in the cylinder 70, partially embraced in the central hollow area 78 of the piston 72. In this way, the ends of the sealing gasket 88 are stationary with respect to the casing 1 (to which the cylinder is coupled) and to the piston 72.

The head according to the present invention operates as described with reference to figure 1, also insofar as the retraction phase of the arm 4 achieved by means of the device 68 is concerned.

The presence and the arrangement of the bellows gasket 88 provide various advantages with respect to the known solution of figure 1. First of all, it enables to eliminate one of the two sliding sealing gaskets (the "0-ring" 27 shown in figure 1), so considerably limiting the negative effects of friction and the consequently difficult compromise between a high degree of sealing and a low

interference with the correct displacement between piston and cylinder. Moreover, the bellows gasket 88 separates the area of cylinder 70 which contacts the internal hollow of the casing 1 from the sliding area between piston 72 and cylinder 70, substantially preventing damping fluids, located in the head, from leaking outside.

In addition, as the only gasket, the "lip seal" gasket 75, is substantially unidirectional (i.e. it provides a prevailing sealing action along one of the two displacement directions of the piston, or, in other words, ensures the sealing only along one displacement direction) , the drainage circuit, that is necessary in the embodiment of figure 1, can be omitted. In fact, the air that could leak through the gasket 75 when the piston 72 displaces for providing the retraction of the arm 4 (upwards with reference to the figures) is held by the bellows gasket 88 and can flow back to the opening 85, and consequently to the tube 87 through the same gasket 75, when the air supply is cut off and the circuit is in course to discharge in a known way.

Thus, the solution according to the present invention has a further remarkable advantage: one of the couplings communicating with the exterior of the casing may be dispensed with, enabling costs reduction and more simplicity both of the head and associated applications.

The embodiment according to figure 2 has a further advantage with respect to the arrangement illustrated in figure 9 of the mentioned patent US-B1-6269546, wherein the retraction device is simply achieved by means of a bellows, which is closed at one end and is activated by compressed air. In fact, in the case of the present invention, even thought problems owing to frictions between displacing parts are considerably limited, the bellows gasket embraced in the cylinder 70 is definitely more protected and less stressed, ensuring, as a consequence, more reliability and extended life. Preferably, the bellows gasket 88 can be made of rubber, or

plastic materials, such as polyurethane, or metal. Thrust surface (ball 80) and abutment surface 48 are arranged in such a way that they mutually contact substantially at, or near, the longitudinal plane that includes the rotation axis defined by the fulcrum 10. In this way transversal slidings between stem 48 and ball 80 are substantially avoided.

A head according to the present invention can differ with respect to what is described and illustrated herein so far. For example, the thrust surface of the piston 72 can include a convex portion different from the portion of the ball 80, or can be substantially plane. The abutment surface 48 can further include a spherical portion or, generally, a convex one. In a head according to the present invention, the head features, that have been briefly illustrated with reference to figure 1, can be realized in different ways. Such features include, for example, nature, shape and coupling of the casing 1, of the arm 4, of the fulcrum 10, of the biasing device (spring 14) , of the mechanical limiting devices (screw 12) , of the transducer 50 and other components .