« METHOD AND APPARATUS FOR CHECKING THE WORKING POSITION OF A TOOL » Technical Field The present invention relates to a method for checking the working position of a tool, coupled to a tool-holder with a substantially conical coupling surface and a longitudinal symmetry axis, relative to a rotary support, for example a spindle of a machine tool, to which the tool-holder is firmly and removably coupled and that defines a rotation axis and a seat with a substantially conical surface that is symmetrical with respect to the rotation axis, the substantially conical surfaces of the seat and of the tool- holder being adapted for cooperating with each other in the working position of the tool, the tool-holder defining a first plane reference surface arranged substantially perpendicular to the longitudinal symmetry axis, the method including the checking of the arrangement of the first plane reference surface with respect to a second plane reference surface, perpendicular to the rotation axis, defined by the rotary support.

The invention also relates to an apparatus for implementing said method.

Background Art In the machine tools with at least a rotary spindle, for example milling machines and machining centers, it is important that the tools be coupled to the spindle with great accuracy, so that the axis of the spindle and that of the tool substantially coincide, and that the position of the cutting edge of the tool, with respect to the spindle, along the axis of rotation be exactly known. There are many types of tool-holders with precision fastening means

between the tool and the spindle, among these the most common one includes a shank with a conical surface integral with the tool (or"taper shank") and an associated conical seat integral with the spindle. The cooperation between the conical surfaces allows the accurate positioning of the tool and the aligning of the tool and spindle axes.

Moreover, the tool-holder-to which the tool is integrally coupled-has an annular transversal flange that in some cases has a groove on the external cylindrical surface utilized by handling devices for gripping the tool-holder in the course of the operations for the automatic picking up from a magazine and assembly in the spindle and vice versa. The former flange has plane transversal surfaces, one facing the spindle when the tool is in the working position. In some cases, when the shank has reached the correct coupling in the associated seat, said plane surface lies at a distance from a surface of the spindle. In other cases, the constructional features of the precision fastening means-more specifically, for example, the geometrical and/or manufacturing features of the shank and of the flange-allow a contact between the plane surface of the flange and a plane surface of the spindle to take place when the tool is in the working position, and this further contact between tool and spindle contributes to define and make more stable said working position. This occurs, for example, when the shank is hollow (as in the tool-holders of the"HSK"type, particularly suitable for applications in which the spindle rotates at high speeds) and undergoes deformations in the locking phase in the associated seat.

The accuracy of the working position of the tool can be jeopardized by dust, swarf or other debris between the tapered and/or plane coupling surfaces.

It is known to check the correct position of the tool in the spindle by means of sensors that detect the mutual arrangement of the plane surfaces of the flange (integral with the tool) and the spindle, such plane surfaces facing

each other, or being in mutual contact.

German patent application No. DE-A-4201013 discloses some embodiments in which the position of the surface of the flange is detected by a pair of mechanical switches or electromagnetic position detecting sensors connected to the rotary spindle. The information relating to the condition of the switches (or the position sensors) is transmitted by means of an inductive rotary coupling between the spindle and a stationary part.

The embodiments disclosed in the German patent application enable to detect deviations of a relatively large amount of the position of the tool, that are monitored, for example, by the closure of just one of the two switches.

Furthermore, as the sensors or switches and the associated transmission circuits rotate with the spindle, they undergo high accelerations owing to the high speeds of rotation at which the spindle rotates and risk possible damage.

In substance, the embodiments disclosed in DE-A-4201013 are not in general suitable for applications on spindles that rotate at high speeds (even higher than 15.000 rpm in the case of spindles of machining centers) and cannot detect relatively slight positioning errors (it is to be noted that, in high precision machining centers, errors of a few hundredths of a millimeter or even in the order of microns can negatively affect the machining).

Disclosure of Invention An object of the present invention is to provide a method for checking the working position of a rotary tool, in a particularly accurate and reliable way, without presenting the limits of the known solutions, and an apparatus for implementing said method.

This and other objects are achieved by a method according to claim 1 and an apparatus according to claim 14.

Among the advantages that the invention provides, there are the particular simplicity of the checking operations, the

possibility of avoiding complicated and delicate machinings for the accurate positioning of feelers and circuits in the rotary part and, as a consequence, the possibility of avoiding (contactless) transmission devices between the movable part (rotary spindle) and the stationary one.

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 partly cut away schematic cross- sectional view of an apparatus according to the present invention in a checking phase with some details shown in front view; figure 2 is a partly cut away side view of the apparatus of figure 1, according to arrow II in figure 1, with some parts cross-sectioned and others omitted ; figure 3 is a block diagram of a checking method according to the present invention, and figure 4 is a graph showing the trend of an electric signal acquired and utilized in the course of the method of figure 3.

Best Mode for Carrying Out the Invention In the application shown, in an extremely simplified and partly cut away form, in figures 1 and 2, a rotary support 1, more specifically a spindle in a machine tool (for example a machining centre) is coupled to a stationary part 3 of the machine by means of bearings 5 (only one is shown in figures 1 and 2) and can rotate with respect to the stationary part 3 about a longitudinal rotation axis 7. The spindle 1 has a seat 9 with a substantially tapered, or conical, surface 11 centered about the longitudinal rotation axis 7 and symmetrical with respect to it.

A tool 13, shown in figures 1 and 2 coupled to the spindle

1 in the working position, includes a cutting portion for machining a mechanical piece, and is rigidly coupled to a tool-holder 15. The tool-holder 15 includes a shank 17 and a flange 19 for the clamping, in a removable way, to the spindle 1. The shank 17 defines a substantially conical or tapered surface 21 and a longitudinal symmetry axis 8, and the flange 19 defines a first annular reference and abutment plane surface 23, the axes 7 and 8 being substantially coincident in the proper working position of the tool 13. The tool-holder 15, shown in simplified form in the cross-sectional view of figure 1, is of the"HSK" type, in which the shank 17 is hollow and has limited length and taper ratio close to 1.

The position of shank 17 within seat 9 is locked by clamping devices of a known type identified by reference 25, that also determine contact between the first plane annular surface 23 of the flange 19 and a plane abutment surface 27 of spindle 1 near seat 9. The working position of the tool 13 is defined by the cooperation between the conical surface 21 of shank 17 and the conical surface 11 of seat 9, that are centered along the longitudinal rotation axis 7, and by contact between the plane annular abutment surfaces 23 and 27. There is a second plane annular reference surface 29 in spindle 1, defined by an annular recess on the edge of the spindle 1 and, when the tool 13 is in the working position, it faces surface 23 of flange 19, at a predetermined distance from it. The surface 29, the facing portion of surface 23 and a cylindrical surface of the spindle 1 achieve a groove 31.

It should be realized that the dimension representing the distance separating the surfaces 23 and 29, i. e. the width of groove 31, is exaggerated in figures 1 and 2 for the sake of clarity and simplicity, and that in practice said distance is of a few tenths of a millimeter (for example 0.3mm).

An emitter device with a laser generator, shown in simplified form in figure 1 only and identified by

reference number 33, is arranged on a stationary part of the machine tool, belongs to an apparatus according to the present invention and emits a collimated light beam 35, with larger width as compared to that of groove 31, in a direction perpendicular to the longitudinal rotation axis 7. The beam 35 is sent towards the groove 31 and part of it passes through groove 31 and reaches a receiver 37 (shown in figure 1 only) that includes, for example, a photodiode and sends a substantially continuous signal to a memorizing, processing and control unit 39, for example a voltage V indicative of the luminous intensity of the received beam. The dimensions of beam 35, too, is exaggerated in the figures for the sake of clarity (a typical value of the maximum width of the beam is approximately 1 mm).

A checking method according to the invention is now described with reference to the block diagram shown in figure 3.

When the tool-holder 15, carrying tool 13, has been inserted and locked in seat 9, spindle 1 starts to rotate and device 33 emits the light beam 35 that is partially intercepted by elements of the flange 19 and of the spindle 1 and, by passing through groove 31, reaches photodiode 37 (block 40). As a consequence unit 39 receives a measurement signal V (a) that, suitably filtered in a known way, is indicative of the trend, in the course of the rotation of the spindle 1 (angle a), of the luminous intensity of the beam 35 output by groove 31 that reaches photodiode 37 (block 42). The value of a zero signal Vz previously detected in a calibration phase, stored in unit 39, is compared with the above mentioned measurement signal V (a) (block 44): if the value of the latter is greater than the former by a prefixed entity t-in other words if V (a) > Vz + t-the incorrect arrangement of the first reference surface 23 and, consequently, the improper positioning of the tool-holder 15 is signalled (block 46) and the procedure ends (block 52). On the contrary, if the

measurement signal V (a) does not exceed the value Vz + t and the spindle 1 has accomplished a complete rotation (a = 360°) with respect to the stationary part 3 of the machine (block 48), the correct working position of the tool-holder 15, and consequently that of the tool 13, is signalled (block 50) at the end of the checking procedure (block 52).

It should be realized that the previously mentioned prefixed entity t represents the tolerance value that corresponds to the admissible position error. A method according to the present invention enables to detect very small position errors (values of t that correspond to distances of a few micron between the reference surfaces 23 and 29).

As far as the zero signal Vz is concerned, it is detected in a calibration procedure performed in a predetermined condition of correct positioning of the tool-holder 15 in spindle 1, a condition that is achieved for example by manually assembling the former (15) in the latter (1) after the conical surfaces (21 and 11) and the plane surfaces (23 and 27) intended for matching have been accurately cleaned for the purpose of removing debris and any other potentially present foreign matter.

The graph of figure 4 shows the trend of the measurement signal V (a) relating to a checking in the course of which there is detected an error in the working position of the tool 15.

The above description clearly evidences the simplicity and the reliability of the apparatus and the checking method, according to the present invention, wherein it is not necessary to assemble sensors and associated circuits in the rotary spindle, by performing complicated and delicate operations, and consequently it is not necessary to achieve equally complicated coupling. (with or without contact) between the rotating part and the stationary part for transmitting the signal of the sensors. Moreover, the absence of sensors and circuits in the movable part avoids

potential problems caused by possible damage of components including such sensors/circuits due to high rotation speeds of the spindle and consequently high accelerations.

In addition to simplicity and reliability, the described method and apparatus feature a remarkable high resolution that, as previously outlined, enables to reveal even very small positioning errors by detecting distance variations of a few micron between the plane annular surfaces.

The method according to the diagram of figure 3 is obviously just an example, as there are many possible variants in any single steps.

The collimated laser beam 35 has been described as a preferred embodiment-insofar as the application simplicity, the low costs and the performance are concerned-of the light beam employed to perform a method according to the invention. Other alternatives are possible, as the use of a non-collimated (focalized) laser beam or a light beam of different nature. In general, the optoelectronic device that, in the illustrated example, includes the emitter 33 and the receiver 37 can be implemented in a different way and include, for example, an LED that emits radiations in the infra-red range, optical collimation components for generating the light beam and a receiver with a linear array of photodiodes (CCD). The range of a similar optoelectronic device is definitely broader than that of the illustrated and formerly described device, in other words the light beam is definitely broader in width with respect to the laser beam 35 (for example some tens of millimeters). In this case, the feature that is checked in the beam output from groove 31 is not the luminous intensity-as in the illustrated embodiment-but the width of the beam detected by means of the CCD receiver, according to the so-called"shadow casting" technique.

Although the arrangement of the beam 35 in a plane perpendicular to the longitudinal axis is preferable in consideration of the substantial parallelism between the

direction of the rays of such light beam 35 and the plane surfaces 23 and 29 that define the groove 31, this is not a limiting aspect for the embodiment of the invention. In fact, beam 35 can be arranged in a direction sloping with respect to the plane surfaces 23 and 29 and consequently undergo reflections on the surfaces, reflections that can be taken into account in the calibration phase.

A method and an apparatus according to the present invention can obviously be utilized for carrying out checkings of the position of tool-holders in applications including grooves defined in a different way with respect to the described and illustrated groove 31. For example, a similar groove can be defined by the abutment surface 27 of spindle 1 and a different reference surface on a recess of the tool-holder 15, at a small distance (a few tens of a millimeter) from the abutment surface 23. Furthermore, the groove can be defined by two plane reference surfaces, in the tool-holder 15 and in the spindle 1, both separated from their associated abutment surfaces 23 and 27 and at a small distance from them.

A method and an apparatus according to the present invention can also be utilized for carrying out checkings in the case of couplings in which plane surfaces of flange 19 and of spindle 1 do not contact each other, but face each other at a limited mutual distance (for example in the order of 1 mm). This occurs, for example, in tool-holders of the known type with a substantially solid and non- deformable taper shank (tool-holders of the"BT"type). In this case, in consideration of the greater distance existing between the plane reference surfaces, a distance that can also be greater than the width of the light beam 35, a method that utilizes the apparatus illustrated in figures 1 and 2 includes two checking steps-carried out in sequence-in which the light beam is partially intercepted once by the flange of the tool-holder, the other time by the edge of the spindle, and a further step to process together the so-achieved signals. The carrying out of two checking steps may not be necessary in the event there be utilized the above-mentioned alternative optoelectronic device including the CCD receiver, that has a definitely broader range.