TECHNICAL FIELD The present invention concerns an operating head for a machine tool with a spindle rotating on a vertical axis, in particular for the processing of packs of printed circuit boards, and a machine tool fitted with such operating head.

BACKGROUND ART In the known machine tools, the spindle is axially movable with respect to a support member, by a servomotor, both to bring the tool to engage the pack to be processed, as well as cause the feed of the tool during processing. Normally the spindle is relatively heavy, so the shifting servomotor of the spindle must be relatively powerful, and is achieved by a rotating electric motor, joined to a screw/nut screw pair.

The known operating heads thus have the drawback of proving to be of considerable weight and relatively expensive and complicated to manufacture. Moreover, for the shifting of the spindle as regards a transversely movable carriage, both the carriage as well as the relative guides and operation means prove to be of considerable weight and require suitably robust structures to ensure rapidity and precision in the positioning of the spindle.

DISCLOSURE OF INVENTION One aim of the invention is to realize an operating head that requires a servomotor of reduced power, and which ensures the maximum spindle positioning precision, proving to be of limited cost, and eliminating the drawbacks of known operating heads.

Another aim of the invention is to eliminate the transmission means of the motion between the servomotor and the operating

head.

According to the invention, the first aim is reached by an operating head for a machine tool with a spindle rotating on a vertical axis, said spindle being movable axially as regards a support member by a servomotor, characterized by balancing means of the weight of said spindle, said balancing means being disposed between said spindle and said support member to allow a reduction of the power required by said servomotor.

In particular the balancing means include a pneumatic cylinder collaborating with a piston, which engages a stop member, said cylinder and said stop member being connected, one to the spindle and the other to the support member.

The second aim is achieved by moving the operating head by means of a servomotor formed by an electric linear motor and arranging some automatically operated locking means to lock the spindle on the support member when the balancing means are inactive, or when the electric linear motor is not energized.

BRIEF DESCRIPTION OF THE DRAWINGS In order to better comprehend the invention, herein described are some preferred embodiments, done by way of example with the help'of the enclosed drawings, in which: Figure 1 is a partial perspective view of a machine tool with two operating heads according to a first embodiment of the invention; Figure 2 is a perspective view of a detail of a variant of the operating heads of Figure 1, in enlarged scale and with the spindle removed; Figure 3 is a perspective view of the detail of Figure 2, sectioned along the plane of trace III-III ; Figure 4 is a vertical section of a part of Figure 3, in subsequently enlarged scale; Figure 5 is a perspective view of the detail of Figure 2,

viewed from another angle; Figure 6 is a perspective view of the detail of Figure 5, sectioned along the plane of trace VI-VI; Figure 7 is a diagram of the electric circuit and of the pneumatic circuit of the operating head; Figure 8 is an aerial view, partially sectioned, of an operating head according to another embodiment of the invention; Figure 9 is a section according to the line IX-IX of Figure 8; Figure 10 is a section according to the line X-X of Figure 8.

BEST MODE FOR CARRYING OUT THE INVENTION With reference to Figure 1, an operating head for a machine tool, is generically denoted by 5, in particular for the processing of packs of printed circuit boards. The head 5 includes a spindle 6 rotating on a vertical axis, which is able to lock a tool 7, e. g. a drill bit. The spindle 6 is fitted with an electric motor, not visible in the drawings, to turn the tool 7 at high speed.

As is known, the printed circuit boards are perforated with a series of small diameter holes, disposed according to a row and column matrix, so the tools 7 are generally of very small diameter. The spindle 6 is axially movable as regards a support'member 8, along a direction, generally indicated as the Z axis, both to bring the tool 7 into contact with the pack of boards to be perforated, as well as to make the tool 7 advance during perforation. This shifting is performed by a servomotor that will be better seen as follows.

According to the invention, the spindle 6 has a basically cylindrical external shape and can be assembled in any angular position on a member, or spindle-holder body, generically denoted by 11. The body 11 includes a plate 12 of basically rectangular shape, which is vertically disposed and carries en bloc at least one ring 13. The spindle 6 is assembled on the

body 11 in a way that it is easily removable, inserting it in the ring 13, against which it rests, by means of a stop part, formed by a ring-shaped shoulder 14.

In particular, the plate 12 carries en bloc two axially spaced rings 13 (Figures 2 and 5), each of which is connected to the plate 12 by two reinforcement unions 16. Each union 16 is besides fitted with two parallel and rectilinear ribs 17. At one of the two unions 16, each ring 13 is fitted with a notch 18, that defines a portion 19 of the ring 13. The portion 19 is able to bend elastically, varying the width of the notch 18 by means of a corresponding screw 21, preferably disposed in a hole between the corresponding ribs 17.

To disassemble the spindle 6 from the body 11, the two screws 21 are unscrewed and the portions 19 of the two rings 13 are bent, lastly extracting the spindle 6 from the rings 13. To assemble the spindle 6 on the body 11, with the screws 21 sufficiently unscrewed, the spindle 6 is inserted in the rings 13, letting the shoulder 14 rest against the upper ring 13, with the interposition of a gauged spacer ring 22 if necessary. Lastly, the two screws 21 are tightened, so the spindle 6 stays rigidly fixed on the body 11.

The support member 8 is essentially made up of a rectangular plate 23 with two vertical edges 24 and 26, and is fitted with two vertical prismatic guides 27, next to the two edges 24 and 26. The plate 12 is fitted with two pairs of ball circulation sliding blocks, joined to the two prismatic guides 27, so each pair of ball circulation sliding blocks 28 slide engages the corresponding guide 27. The drive servomotor of the vertical shifting of the plate 12, as regards the support member 8, includes an electric linear motor, generically denoted by 29, which is able to be checked in a known way by a position indicator 31.

In particular, the linear motor 29 includes a stator formed by a series of permanent magnets 32 carried by a structure 33 formed by a C-shaped profile 34 on whose wings are fixed two flat bars 35, by means of screws 36. The structure 33 is fixed on the edge 24 of the plate 23, by means of other screws 37.

The permanent magnets 32 are disposed on two vertical rows and are fixed on the two bars 36, e. g. with a 20mm pitch. The linear motor 29 besides includes a set of three electrical windings 38, only two of which are partially visible in Figure 2.

The windings 38 are carried by a support 39, fixed by means of a spacer 40 on a basically prismatic block 41, so as to arrange the electrical windings 38 between the two rows of permanent magnets 32. The block 41 is disposed in contact with the two unions 16, left-hand side in Figure 2, of reinforcement of the two rings 13, and is fitted with two grooves 42, each able to engage one of the two ribs 17 of such unions 16. In turn the block 41 is fixed, by means of two screws 43, on a vertical wall 44 (Figure 3) of the body 11, opposite the guides 27.

The position indicator 32 includes a scale 46 (see also Figure 5) formed by a thin strip with a series of optical signs, which is fixed on an edge 47 of the plate 12, disposed on the opposite side of the block 41. The signs of the scale 46 are read by a position transducer 48, carried by a box appendix 49 of a support member 51, which is fixed on the plate 23.

The operating head 5 includes some balancing means, generically denoted by 52 (Figure 2), disposed between the spindle 6 and the support member 8, and that can be automatically operated. The balancing means 52 are to balance the weight of the spindle 6 during processing, so one can reduce the power required by the linear motor 29. The balancing means 52 include a vertical axis pneumatic cylinder

53 and integral, or en bloc, with the body 11, and a piston 54 able to slide in the cylinder 53. A pneumostatic bushing 55 is disposed in the cylinder 53, in which the piston 54 slides and whose function will be seen better afterwards. The bushing 55 is fitted with a pair of compressed airtight gaskets 57, which are axially spaced. The bushing 55 engages a portion 60 of the cylinder 53, with an oversize internal diameter, so as to form a ring-shaped shoulder 56.

The cylinder 53 has an aperture 58 at the bottom, from which a rounded end 59 of the piston 54 comes out. The end 59 collaborates with a stop member, formed by a bracket 62, fixed by means of a screw 63 on the plate 23 of the support member 8. The bracket 62 is fitted with a slot in which is inserted a stop part (61) gauged so as to accurately fix the relative position of the body 11 as regards the support member 8.

Between the upper wall of the cylinder 53 and the upper end of the piston 54 is created a cylindrical chamber 64, into which a hole 66 leads (see also Figure 4). The hole 66 is fitted with a union 67 able to be connected, by means of a flexible external tube 68 (see also Figure 7), an adjustment valve 69, and a solenoid valve 70, to a compressed air tank 71. The valve 69 can be adjusted by hand, or in any other known way, depending on the weight of the spindle 6, so as to partially or totally discharge the weight of the spindle 6 on the support member 8. In fact, the air pressure in the chamber 64 keeps the end 59 of the piston 54 rested against the stop part 61, and tends to move the cylinder 53 upwards, together with the body 11 and the spindle 6.

Between the piston 54 and the cylinder 53, the two gaskets 57 of the bushing 55 create a ring-shaped chamber 72, into which leads a tube 73 made in the cylinder 53 itself. The tube 73 terminates in a union 74 able to be connected, by means of an external tube 76 and'the solenoid valve 70, to the compressed

air tank 71. The bushing 55 is fitted with two ring-shaped rows of radial holes 75, which put the ring-shaped chamber 72 into contact with the interspace between the piston 54 and the bushing 55. Operating the solenoid valve 70, the compressed air puts the ring-shaped chamber 72 under pressure and, through the holes 75 puts the interspace under pressure between the piston 54 and the bushing 55, which acts therefore as a sliding block of pneumostatic support, hugely reducing the friction between the piston 54 and the bushing 55 or the cylinder 53.

Since the operating head 5 is moved vertically by the electric linear motor 29, when the electrical windings 38 (Figure 2) of the linear motor 29 are not powered, they are free to move along the stator 32,33, so the body 11 and the spindle 6 tend to move into the lowest position due to gravity. Similarly, when the compressed air is not connected with the chamber 64, the upper wall of the cylinder 53 tends to go against the piston 54, so the body 11 and the spindle 6 tend to move downwards due to gravity. If the electrical power supply of the linear motor 29, or the supply of the usual compressor of the compressed air, is interrupted accidentally, the shifting of the head 5 due to gravity may occur abruptly, damaging the spindle 6 or the pack of boards being processed.

To eliminate the risk due to the interruption of the electrical power supply, or the lack of compressed air, the operating head 5 is fitted with locking means, generically denoted by 78 (Figure 6), which are able to automatically and instantly lock the spindle 6 on the support member 8 in the current position. The locking means 78 include a serrated member 79 and a locking element 81, one carried by the support member 8 and the other by the body 11. In particular, the serrated member is formed by a rack 79, preferably saw- toothed, which is fixed in a hollow 82 of the plate 23, by means of a pair of screws 83.

The locking element of the rack 79 is formed by one end 81 of a pneumatic piston 84 able to slide in a horizontal axis pneumatic cylinder 86.. The end 81 is fitted with a tooth which is able to engage itself between the teeth of the rack 79. The cylinder 86 is fixed, by means of two screws 87, on a rear portion 88 of the body 11, between the two pairs of unions 16.

A'chamber 89 is situated between the cylinder 86 and the piston 84, into which a hole 91 leads fitted with a union 92.

This union 92, by means of another external tube 93 (see also Figure 7), opens on another solenoid valve 77 connected with the outlet of the solenoid valve 70. Lastly, between the piston 84 and a portion 88 a compression spring 90 is disposed, able to bias the piston 84 towards the rack 79.

When the operating head 5 is not operated, the spring 90 biases the piston 84 towards the plate 23, so the end 81 of the piston 84 is kept engaged with the rack 79 and the body 11 is locked on the support member 8. When the windings 38 of the linear motor 29 are electrically powered, the solenoid valves 70 and 77 are also electrically powered. On the one hand, the compressed air arrives in the ring-shaped chamber 72 and thus in the cylindrical chamber 64 of the cylinder 53, so the balancing of the spindle 6 comes into operation. On the other hand, the compressed air arrives in the chamber 89 of the cylinder 86 overcoming the action of the spring 90, so the end 81 disengages from the rack 79, and the head 5 is operated by the usual control unit according to the processing programme.

If, for any reason, the electricity supply of the linear motor 29 is lacking, also the usual compressor of the compressed air and the solenoid valves 70 and 77 remain de-energized. Then the compressed air no longer arrives in the chamber 89 of the cylinder 86 and the spring 90 automatically and instantly releases the piston 84 towards the rack 79, locking the body 11 in the current position as regards the support member 8.

The spring 90 controls the locking of the body 11 also in the event in which, although the linear motor 29 is electrically powered, for some reason the compressed air does not reach the chamber 89 of the cylinder 86.

The support member 8 of the operating head 5 can be carried by a fixed upright as regards the machine tool, or can be made up of a carriage 8 able to slide along the usual X axis, on a crosspiece 101 (Figure 1) of the machine tool, with a prismatic hollow 100. In this case the plate 23 of the carriage 8 is fitted with two pairs of ball circulation sliding blocks 102 (see also Figures 3 and 5), pairs which engage two prismatic guides 103 envisaged on the crosspiece 101.

The carriage 8 is moved along the crosspiece 101 by another electric linear motor 104, under the control of a position indicator 109 (Figure 1) of the operating head 5 along the usual X axis. The linear motor 104 includes a stator with a series of permanent magnets 105 disposed on two rows on two flat bars 106, fixed in the hollow 100 of the crosspiece 101, in a similar way to that seen for the stator 32,33 of the linear motor 29. The linear motor 104 also includes a set of three electrical windings 107, which are carried by a support member 108, fixed on the plate 23, so as to arrange the electrical windings 107 between the two rows of permanent magnets 105.

The position indicator 109 includes a scale 110 and a position transducer 111 (see also Figure 5), respectively similar to the scale 46 and the position transducer 48. The scale 101 is fixed on the crosspiece 101, while the transducer 111 is fixed on the support member 51. In Figure 1, are shown two operating heads 5,5'carried by two carriages 8,8'able to slide independently of each other along the crosspiece 101. The two guides 103 are then in common to the two carriages 8,8', the

two linear motors 104 have the stator 105,106 in common and the position indicators 109 have the scale 110 in common.

Advantageously the two operating heads 5,5'and the two carriages 8,8'can mirror each other, so as to reduce the minimum distance, in which the two heads 5,5'can work simultaneously.

Figures 8-10 contain another embodiment of the invention, in which the operating head 5 is fitted with adjustment means, generically denoted by 94,. which can be operated to adjust the position of the spindle 6 as regards the support member 8 along a horizontal direction, or Y axis. In the Figures 8-10, the parts that are similar or identical to those of the embodiment of the Figures 2-6 are denoted by the same reference numbers, and will not be described hereafter.

In the embodiment of Figures 8-10, the support member or carriage 8, includes two plates 95 and 96 of rectangular shape, respectively a rear plate 95 and a front plate 96, which are basically disposed parallel to each other. The rear plate 95 carries the support member 108 of the windings of the linear motor 104, and the sliding blocks 102 that engage the guides 103 of the X axis. The support member 108 and the sliding blocks 102 are denoted by dotted lines in Figure 9.

The front plate 96 instead carries the two guides 27 of the Z axis, the stator 32,33 of the linear motor 29, and the rack 79 of the locking device 78.

The two plates 95 and 96 are rigidly fixed to each other, on the left-hand side in Figure 8, by means of a pair of fastening screws 97. The adjustment means 94 include a pair of adjusting screws 98 able to connect the two plates 95 and 96 in an adjustable way, on the opposite side, i. e. the right- hand side in Figure 8. The adjustment means also include a pair of specular Belleville washers 99 disposed between the two plates 95 and 96.-The plate 96 is thus fixed on the left-

hand side and is able to undergo a certain elastic deformation towards the right-hand side.

Lastly, the adjustment means 94 include a monitoring unit, formed by a permanent magnet 80 and an analogue sensor 85 for such magnet 80. In particular, the permanent magnet 80 is carried by the front plate 96, while the sensor 85 is carried by the rear plate 95, and is able to send some indicative signals, to the usual control unit, of the distance from the magnet 80. In reply, for the operator, the control unit monitors, in a known way, the position of the body 11 along the Y axis with an accuracy of the order of microns.

In the setting-up of the operating head 5, turning the adjusting screws 98 in one direction, allows the Belleville washers 99 to bend the right-hand part of the plate 96, that moves away from the plate 95. Instead turning the screws 98 in the opposite direction overcomes the action of the Belleville washers 99, so the adjacent part of the plate 96 draws near the plate 95. All the movements of the right-hand part of the plate are monitored according to the signals of the sensor 85. The axis of the rings 13 is thus moved along the Y axis, performing the adjustment of the head 5.

In the event in which the crosspiece 101 is fitted with two or more operating heads 5,5', as in Figure 1, the adjustment of the position of one of the two heads 5,5'along the Y axis, as regards the other head, takes on enormous importance, since it enables the two heads 5,5'to work simultaneously on different areas of a row of holes of the pack of boards.

From that seen above the advantages of the operating head according to the invention appear clear with respect to known operating heads. First of all,-the balancing means 53,54 of the spindle 6 require reduced power of the linear motor 29 for their operation. Moreover, the linear motor 29 greatly

simplifies the construction of the operating head. Lastly, the locking means 78 of the operating head as regards the support member 8, acting automatically, prevent any damage in the event of an interruption in the power supply, or the compressed air, being caused to the operating head itself.

Naturally, various modifications and improvements can be made to the operating head described, without leading away from the scope of the claims.

For example, the shape of the body 11, of the spindle 6 and of the relative fastening means can be varied. In addition, the support member 8 can be varied and the head 5 can be designed for a machine tool in which the movements between the pack of boards and the spindle 6 are carried out exclusively by moving the worktable.

In the case of an operating head 95 fitted with adjustment means 94 (Figure 8) along the Y axis, two monitoring units 80, 85 can be envisaged, disposed at the two adjusting screws 98.

Also the two plates 95 and 96 can be replaced by a single body with a vertical notch, so as to create an elastically pliable portion due to the action of the adjusting screws 98.

The monitoring unit 80,85 can also include a position sensor of the proximity of an element different to the magnetic one, e. g. a capacitive component. Finally, in a machine tool with two operating heads 5, 5' (Figure 1), the adjustment means 94 (Figure 8) can be envisaged in only one of the two heads, e. g. on the head 5.