DOUBLE-SHAFTED ARBOR

The present invention relates to a double-shafted arbor in accordance with the introduction to the main claim. Machines for mechanical chip removal operations comprise revolving turrets in which numerous milling arbors and receding head arbors are mounted. These latter have the special function of executing turning operations while maintaining the workpiece at rest. In this respect, in contrast to the classical lathe, the tools mounted on the rotating head of the arbor themselves perform the chip removal function. The tools can also vary their diametrical position by means of a worm device, mounted inside the rotary head, in order to execute turning operations on different diameters.

To be able to simultaneously rotate the head and operate the tool positioning device, the arbors have two rotary shafts one mounted inside the other. When the two shafts rotate rigid with each other (at the same r.p.m.), the tools maintain a fixed diametrical position. When the r.p.m. of the inner shaft (tool control) is varied relative to the outer shaft (arbor rotation) a relative movement is generated which operates the tool positioning device. The diametrical position of the tools can be increased or decreased by making the speed of the inner shaft greater or less than the speed of the outer shaft. To operate milling arbors and receding head arbors the known art uses a main motor which transmits movement simultaneously to the main arbor shaft (usually the outer shaft) and to a differential unit via belt and/or gear systems. The differential unit comprises its own auxiliary motor and is also connected to the inner shaft.

The use of a differential unit is necessary to drive the inner receding head shaft, as a difference in r.p.m. has to be created between the two shafts in order to operate the tool diametrical positioning device.

The main motor transmits movement to the arbor shaft which operates the differential unit to rotate the inner tool control shaft. In this condition the two shafts rotate at the same r.p.m.

To create the r.p.m. difference between the two shafts, the auxiliary motor is operated so that the shafts rotate at different speeds.

A disadvantage of this arrangement is that the differential unit is always engaged to ensure that the two receding head shafts rotate rigidly.

This is a heavily penalizing condition when milling arbors are operated, because the presence of the engaged differential unit limits rotational speed to a maximum of about 3000 r.p.m., so reducing the productive potentiality of milling arbors, which could operate at higher rotational speeds (up to 12,000 r.p.m.).

The continuous operation of the differential unit can also result in its rapid deterioration. An object of the invention is therefore to provide a double-shafted arbor which is improved compared with the known art.

A further object of the invention is to provide a double-shafted arbor which can operate at much higher speeds than traditional arbors.

Another object of the invention is to provide a double-shafted arbor which is reliable and durable with time.

These and further objects are attained by a double-shafted arbor in accordance with the technical teachings of the accompanying claims.

Further characteristics and advantages of the invention will be apparent from the description of a preferred but non-exclusive embodiment of the double- shafted arbor illustrated by way of non-limiting example in the accompanying drawings, in which:

Figure 1 is a partly sectional schematic view of the-double-shafted arbor of the present invention while cooperating with a receding head;

Figure 2 is a partly sectional enlarged schematic view of the double-shafted arbor of Figure 1 , showing the inner shafts and outer shafts coupled together;

Figure 2a is an enlarged detail of Figure 2;

Figure 3 is a partly sectional enlarged view of the arbor of Figure 1 , showing the shafts uncoupled and independent, while a differential unit is engaged to rotate the inner shaft and outer shaft at different speeds; and

Figure is a perspective view of the device of the present invention.

Said figures show a double-shafted arbor indicated overall by 1.

The double-shafted arbor 1 faces a coupling device 2 by which it can be coupled to a receding head 3 mounted on a rotary selection device 5. These devices are entirely conventional, with the exception of the arbor 1 , and will therefore not be further described hereinafter.

The arbor 1 comprises a housing 8 to which a first series of bearings 9 are fixed, locked by a flange 12, and on which a hollow outer shaft 6 is mounted. The inner shaft 7 is mounted inside the shaft 6 on a second series of bearings

10 separated by suitable spacers 11 and locked by a catch 150. It should be noted that the bearings 10 are in contact with the inner surface of the outer shaft 6.

The outer surface of the outer shaft 6 comprises rotor windings 13 cooperating magnetically with suitable windings 14 of a stator fixed to the housing 8 in known manner.

The outer shaft 6 is hence the rotor of a main motor 15 of the arbor 1.

A first end portion 6b of the outer shaft 6 presents conventional means for engaging the coupling device 2, while a second end portion 6a presents a fixed flange 16 rigid with the outer shaft 6 and a movable flange 18 axially slidable on the outer shaft 6. The fixed flange 16 and movable flange form part of a device

70 for blocking independent-rotation of the inner shaft 7 and outer shaft 6.

The fixed flange 16 and movable flange 18 are interconnected by pins 19 on

which springs 20 are mounted. These springs maintain the slidable flange 18 abutting against suitable stops 21 provided on the pins.

The slidable flange 18 presents a surface facing the inner shaft 7 and mounted thereon, at which four grooves 22 are present, shaped to present in cross- section a first step 22a and a second step 22b which are spaced apart by a compartment 51.

An end portion 7b of the inner shaft presents known means for its engagement with the coupling device 2, while a second portion 7a presents, torsionally rigid therewith, four keys 23 which in cross-section present a first tooth 23a and a second tooth 23b disposed radially and separated by a compartment 52.

When the movable flange 18 rests, against the stops 21 the teeth 23a and-23b of the key are engaged respectively with the first step 22a and second step 22b of the groove, consequently the two shafts are torsionally locked together and rotate rigidly with each other. Consequently the movable flange 18 acts as the device 70 for locking together the two shafts, i.e. the inner and outer.

A clutch unit 30 faces the second end portions 6a, 7a of the inner shaft 7 and outer shaft 6.

The clutch unit 30 couples/decouples a first shaft 33 and a second shaft 34 of a differential unit 31 , driven by its own motor 32, to/from the outer shaft 6 and inner shaft 7 respectively.

The clutch unit comprises a guide member 35 connected in known manner to the housing 8, where a cylindrical cavity 37 is provided. A movable element 36 presenting a flange 36a housed in the cylindrical cavity 37 is slidable within the guide member 35. The engagement between the guide member 35 and movable member 36 is sealed. The flange 36a sealedly divides the cylindrical chamber into a first chamber 37a and second chamber 37b. Pressurized fluid can be fed into the chambers through suitable passageways

(not shown), the pressure difference between one chamber and the other enabling the movable element 36 to move between a first position (Figure 3) in which the differential unit 31 is engaged with the inner shaft 7 and outer shaft 6, and a second position (Figures 2, 2a) in which the differential unit is disengaged.

The movable element 36 presents bearings 38, suitably fixed and spaced apart, which connect to the movable element 36 an outer intermediate shaft 39, connected by suitable bearings 41 to an inner intermediate shaft 40. The outer intermediate shaft 39 presents two seats 42 to receive frontal teeth 43 of the outer shaft 6. These frontal teeth pass into suitable slots 60 present in the flange 18. The intermediate shaft 39 also presents a splined profile 46 allowing it to torsionally engage the first shaft 33 of the differential unit 31. By cooperating with a corresponding splined profile 47 on the first shaft 33, the splined profile 46 allows the outer intermediate shaft 39 to slide on the first shaft 33 of the differential unit.

The inner intermediate shaft 40 presents frontal teeth 44 facing corresponding teeth 45 provided on the inner shaft 7; it also presents a splined profile 48 enabling it to torsionally engage the second shaft 34 of the differential unit 31. As in the previous case the splined profile 48, by cooperating with a corresponding splined profile 49 on the second shaft 34, allows the inner intermediate shaft 40 to slide axially within the second shaft 34 of the differential unit.

To complete the description it should be noted that the outer intermediate shaft presents pins 50 for cooperating with suitable holes 510 provided in the casing of the differential unit. The outer shaft 6 also presents, associated therewith, a first encoder 520 for determining its position, the second shaft 34 of the differential unit presenting a second encoder (not shown) which determines its position.

The encoders are associated with a control system for the main and auxiliary motor.

The invention operates essentially in the following manner.

The rotary selection device 5 rotates and brings the arbor into its working position with the receding head 3. The coupling device 2 connects the arbor 1 to the receding head 3.

The movable flange 18 is rested against the stops 21 , the teeth 23a and 23b of the key being engaged with the first step 22a and second step 22b of the groove 22 respectively, The outer shaft 6 and inner shaft 7 are hence torsionally locked together and rotate rigid with each other.

The encoder 520 reads the angular position of the (mutually rigid) shafts, and on the basis of these data are orientated in their coupling position.

The outer intermediate shaft 39 is already in its coupling position as it is locked by the pins 50 inserted into the casing of the differential unit 31.

The encoder associated with the second shaft 34 of the differential unit reads its position and the motor 32 brings the inner intermediate shaft 40 into its coupling position.

At this point pressurized fluid is fed into the chamber 37a, bringing the movable element 36 into the position of Figure 3.

The frontal teeth 44 of the inner intermediate shaft 40 engage the teeth 45 of the inner shaft 7, to hence torsionally lock the inner intermediate shaft 40 to the inner shaft 7.

Simultaneously the frontal teeth 43 of the outer shaft 6 become inserted into the seats 42 of the outer intermediate shaft 39, hence also torsionally locking the outer intermediate shaft 39 to the outer shaft 6.

The outer intermediate shaft 39 also urges the flange 18, which moves axially against the spring 20 to reach the position of Figure 3, hence making the

rotation of the inner shaft 7 and outer shaft 6 mutually independent.

In this respect, the axial movement of the flange 18 disengages the teeth 23a and 23b of the key 23 respectively from the first step 22a and second step 22b of the grooves 22. The compartment 51 becomes positioned above the tooth 23a to enable it to rotate, while the compartment 52 receives the steps 22b, hence not interfering with their rotation.

When in this condition (Figure 3) the outer shaft 6 is driven by its associated motor 15, the differential unit 31 , with its auxiliary motor at rest, then imposing on the inner shaft a rotation at the same angular speed as the outer shaft.

Operation of the motor 32. of the differential unit results in a difference in rotational speed between the inner shaft 7 and the outer shaft 6, necessary to control the receding head 3.

After use of the receding head, the motor 15 controlled by the control system positions (via the readings of the encoder 520) the outer shaft 6 such that the pins 50 are aligned with the holes 510.

The motor of the differential unit is then operated to align the keys 23 with the seats 22.

The chamber 37b is then pressurized and the movable element retracts to the position of Figure 2, hence decoupling the differential unit from the inner shaft 7 and outer shaft 6.

The springs 20 simultaneously return the flange 18 to its locked position so that the inner shaft and outer shaft become torsionally rigid.

The pins 50 become inserted into the holes 510 to lock the outer intermediate shaft 39 to the body of the differential unit 31. This operation is necessary in order not to lose the angular position of the shaft for a subsequent engagement of the differential unit 31.

The receding head 3 is then replaced by a milling head which is driven by the

motor 15 alone.

Advantageously as the differential unit 31 is disconnected from the inner shaft 7 and outer shaft 6, the motor 15 can raise the rotational speed to the maximum speed allowed by the milling tool. Moreover the differential unit is engaged only when a receding head is used, consequently it lasts longer.