A device in a tool holding assembly for moving a rotatable shaft in the axial direction

Abstract: A device in a tool holding assembly for moving in the axial direction a rotatable shaft (1) that carries means arranged in a first end (4) for performing work during rotation of said shaft, comprising an electric motor (3) for said rotation and at least one bearing (5,6), which radially support the shaft (1) and permit movement of the shaft in the axial direction. According to the invention the shaft (1) has a free second end (7) and electro-magnetic means is arranged to affect said second end and draw said shaft in the axial direction from the first end (4) to the second end (7) against the effect of an pressure acting against said second end in the opposite axial direction. Means (15) is arranged to controll said electro-magnetic means in order to achieve said axial movement of the shaft (1) during rotation of the shaft.

Description: The present invention relates to a device in a tool holding assembly for moving in the axial direction a rotatable shaft that carries means arranged in a first end for performing work during rotation of said shaft, comprising a driving device, such as an electric motor, for said rotation and at least one bearing which radially support the shaft and permit movement of the shaft in the axial direction. In machines for grinding of holes it is for quality reasons advantageous if the grinding wheel during rotation work also can move back and forth in the axial direction. The quality, i.e. the degree of surface fineness and the straightness of the holes is improved compared to a non-oscillating shaft. The wear of the grinding wheel is also more uniform and less dressing is needed. In a conventional machine of this type the arrangement is such that the entire headstock with the spindle and the grinding wheel must move axially in order to displace the rotating shaft in its axial direction. As the object is to achieve very rapid and short axial movements these conventional machines are unsatisfactory. The entire mass of the headstock, the spindle and the grinding wheel must be displaced rapidly, which requires very stiff and clearance-free bearing arrangement as well as a powerfull driving motor. The wear on the headstock and driving mechanism is high which means that a lot of maintenance is necessary. In manufacturing process today the speed of production is high and the speed of rotation is often well above 100 000 r/min which means that in conventional machines the rotatable shaft cannot move or oscillate axially at a satisfactory high speed, i.e. the higher speed of production the higher speed of the axial motion is necessary in order to achieve a high quality of the performed work. The big mass that must be moved in conventional machines is in the size of 50-100 kg, which mass causes vibration and limits the speed of oscillation and thereby the speed of production. It exist since long time a high commercial demand for a satisfactory solution of the above described problem. Above the problem has been described in connection with grinding wheel. However, the same problem exist in connection with other machineries with a rotatable shaft that carries means for performing work during rotation of the shaft. One such example is drilling machines intended to perfom very small and fast axial movements, such as for use in manufacturing circuits cards. In the german patent publikation DE 31 23 199 A1 is shown a construction for axially oscillating a rotating shaft on which a working tool, such as a grinding wheel, is arranged. The oscillation is achieved with the aid of two springs arranged at the opposite side of a disk arranged on the shaft, whereby the springs act against each other and are brought into sympathetic vibration for oscillation of the shaft in the axial direction. The device in accordance with said german publication has several drawbacks. The mass that is brought into oscillation is rather big which as mentioned above is a serious problem. Another big disadvantage is that the speed of oscillation is restricted to the resonance frequency of the spring system. Another solution is described in the japanese patent publication 1-240266. The rotatable shaft is in this construction provided with a radially extending rotor and an electromagnet is arranged on each side of said rotor. The shaft is oscillated axially by controlling the magnitude of the current to each one of the electromagnets and thereby the magnetic forces on the rotor. One drawback with this construction is that the rotor arranged on the shaft is rather heavy and this makes the rotating axis heavier, which restricts speed of rotation of the shaft. Another drawback is that the construction with two electromagnets is rather expensive. A further serious drawback with the construction in accordance with said japanese publication is that the electromagnets and the rotor require a lot of space. With the present invention is achieved a device in which all the above mentioned problems are solved. The device in accordance with the invention is characterized in that the shaft has a free second end, that electro-magnetic means is arranged to affect said second end and draw said shaft in the axial direction from the first end to the second end against the effect of an pressure acting against said second end in the opposite axial direction, and that means is arranged to controll said electro-magnetic means in order to achieve said axial movement of the shaft during rotation of the shaft. A prefered embodiment in accordance with the invention is characterized in that said electro-magnetic means comprises a journal arranged with its free end adjacent said second end of the shaft and a magnetic coil arranged around said journal for generating a magnetic field in the axial direction of said journal, that said journal, magnetic coil and an end part of the shaft around the second end are encased in a housing arranged to guide said magnetic field in a closed loop including said end part and said journal and a gap between the second end of the shaft and the free end ofthe journal, whereby the magnetic field acts on said end part of the shaft with a force in the direction against the free end of the journal. In the device in accordance with the invention just the rotatable shaft is moved or oscillated in the axial direction. The small mass that moves (oscillates) in the device in accordance with the invention increase the possibility for high accelerations and an optimized pattern of movement. The axial movement of the shaft is therefor not restricted to sinus form. The invention will in the following be described in more detail with reference to exampels shown in the accompanying drawings. Figure 1 shows a schematic cross-sectional view of a first embodiment of the invention. Figure 2 shows a schematic cross-sectional view of a second embodiment. Figure 3 is a schematic view for illustrating the principle of one example of a landing bearing for taking up forces in the axial direction of a shaft. The embodiments of the present invention are disclosed in the context of providing axial movement and high precision positioning of a rotatable shaft 1 in a stationary machinery unit 2, such as a headstock of a grinding machine. An electric motor 3 is arranged to transfer power to the rotatable shaft for performing work during rotation. The shaft 1 is designed to carry a tool, such as a grinding wheel (not shown) in a first end 4 of the shaft. The shaft 1 is supported by two bearings, a rolling bearing 5, such as a rolling bearing sold under the trademark CARB, and a gas bearing 6. The bearings are arranged displacable in the axial direction, e.g. by axial bearing play. The bearings 5 and 6 may be hydrostatic or hydrodynamic bearings which have a sealing function and allow axial movements. The bearing 6 is non-magnetic or contain non-magnetic material to a certain thickness around the shaft in order to reduce radial forces on the shaft caused by the magnetic field. The shaft has a second end 7. A journal 8 is arranged with its free end adjacent the second end 7 of the shaft 1 and a magnetic coil 9 is arranged around said journal for generating a magnetic field in the axial direction ofsaid journal. The magnetic field has been designated with B in the figures. The journal 8, the magnetic coil 9 and an end part of the shaft 1 around the second end 7 are encased in a housing 10, which is arranged to guide the magnetic field generated by the coil 9 in closed loops including said end part, said journal 8 and a gap 11 between the second end of the shaft and the free end of the journal. The magnetic field acts on said end part of the shaft with a force in the direction against the free end of the journal 8. The housing 10 surrounds a space 12 including said gap 11 between the second end 7 of the shaft and the free end of the journal. Gas from the gas bearing 6 leaks into said space which is sealed so that an overpressure is created in said space. The magnitude of said overpressure is regulated with a valve (not shown). The shaft is drawn in the axial direction from the first end to the second end by the magnetic field against the effect of the overpressure acting against the second end 7 of the shaft. A position detecting means 13 is in the embodiment shown in figure 1 arranged in a bore hole 14 through the journal 8. The position detecting means is arranged to detect the axial position of the shaft 1 and emitting a corresponding signal to a control means 15. The control means 15 is arranged to control the current flowing in the electro-magnetic coil 9 in response to said signal from the position detecting means in order to control movements of the shaft 1. The control means is programmed to control the magnitude of the current in the electro-magnetic coil 9 in order to move or oscillate the shaft 1 and the grinding wheels in an optimized way for grinding with high quality with regards to surface roughness, bore straightness and with uniform wear and long intervals between dressing of the grinding wheel. Said overpressure regulating valve , as mentioned above, also serves as a safety means which is arranged to open to eliminate the overpressure when the magnetic field due to failure disappear. Preferably said safety means is a magnetic valve controlled by said control means 15. The big advantage with the device in accordance with the invention is that a minimal mass has to move for moving/oscillating the grinding wheel in the axial direction of the shaft. Only the shaft 1 and the grinding wheel are moved. This small movable mass makes it possible to programme the control means to guide the shaft to perform an optimized pattern of movement. The movement is not restricted to sinus form and high acceleration of the shaft 1 with the grinding wheel is possible in the axial direction of the shaft. The magnetic force between the second end 7 of the shaft 1 and the free end of the journal 8 increase when the distance between said ends is reduced. According to a preferred embodiment a landing bearing is arranged on the end surface of the free end of the shaft and/or on an end surface of the free end of the journal. In the embodiment shown in figure 1 the landing bearing includes a graphite layer or coating 16 applied on the end surface of the free end of the journal to prevent the second end of the shaft from comming in direct contact with the free end of the journal if the magnetic control should malfunction. Said graphite layer must have a certain thickness or must be applied on a layer or a washer of a non-magnetic material (not shown) so that the combined thickness is sufficiently high. A direct contact of said ends or a too short distance between said ends during work rotation would lead to that the spindle is destroyed if the regulation system fails. The graphite layer serves as a wear resistant surface and are arranged possibly in combination with other non-magnetic material to limit the magnetic force. With the graphite layer and possibly in combination with a further non-magnetic layer or washer said two ends can come in contact at least for a short while without risk for that the spindle is damaged. The landing bearing could instead of the graphite layer or coating be a washer formed of graphite. Other suitable material for the layer, coating or the washer are non-magnetic material with low friction, such as synthetic diamond. Further examples of non-magnetic material is a layer of air or a layer formed of ceramic balls. In the embodiment according to figure 2 a gas bearing 17 is arranged between the second end 7 of the shaft 1 and the journal 8. The gas bearing is arranged to act against the second end of the shaft and a piston 18 is arranged to transfer force from the spring 19 via the gas bearing to the shaft. This embodiment is suitable when high axial forces is needed such as when the invention is used in a drilling-machine. In the embodiment according to figure 2 the radial bearing close to the second end is a cylindrical bearing 20. High axial forces can be tranferred without the aid of a spring as shown in figure 2 if an air piston arrangement (not shown) is used. In the embodiment in accordance with figure 2 it must be possible to quickly decrease the axial force if the electro-magnetic means which generates the magnetic field B fails or stop working. If the axial forces , acting against the forces generated by said magnetic field is caused by a spring as shown in figure 2 the action of the spring can be controlled by a magnetic means, which bring the spring to an inactive position when said electro-magnetic means fails or stop working. If an air piston is used instead of a spring a magnetic valve can be arranged to decrease the air pressure when the electro-magnetic means fails or stop working. The landing bearing on the end surface of the free second end of the shaft and/or on an end surface of the free end of the journal need not to be a coating or a washer as described above. Other suitable examples of landing bearings are gas bearings, aerostatic bearings or aerodynamic bearings. An aerodynamic bearing could for example be acieved by arranging spirale groves 21 in the end surface of the second end 7 of the shaft 1 as shown in figure 3. An air pressure is then achieved outside the end surface when the shaft 1 rotates. The aerodynamic bearing, for instance in the form of spirale groves as shown in figure 3, could also be used to create the pressure acting against the second free end 7 of the shaft. The groves can of course have other forms than spirale groves, for instance they can have the form of herringbone. When the whole spindle assembly is moved toward a work piece for instance for grinding a hole in the work piece the spindle would in conventional devices be destroyed if the spindle with high speed missed the hole and crashed against the end surface of the work piece. In a prefered embodiment means, such as the detection means 13 in the embodiment shown in figure 1, are arranged to detect when the actual axial position of the shaft during movement of the spindle assembly toward a work piece differs from a reference position which deviation from the reference position indicates that unexpected forces have acted against the shaft. As the shaft in the device according to the invention can move axially a distance in relation to the spindle there is time for a signal to be sent from the detection means to the control means to stop the advancement of the spindle before it crash in stiff condition, i.e. with the free end of the shaft lying directly aginst the free end of the journal possibly with a landing bearing in between, towards the work piece. The present invention is not restricted to the above described embodiments but a number of modifications are possible within the scope of the following claims.

Claims: A tool holding assembly comprising a device for moving in the axial direction a rotatable shaft (1) that carries means arranged in a first end (4) for performing work during rotation of said shaft, comprising a driving device, such as an electric motor (3), for said rotation and at least one bearing (5,6), which radially support the shaft (1) and permit movement of the shaft in the axial direction, characterized in that the shaft (1) has a free second end (7), that electro-magnetic means is arranged to affect said second end and draw said shaft in the axial direction from the first end (4) to the second end (7) against the effect of a pressure acting against said second end in the opposite axial direction, and that means (15) is arranged to control said electro-magnetic means in order to achieve said axial movement of the shaft (1) during rotation of the shaft. A tool holding assembly according to claim 1, characterized in that said electro-magnetic means comprises a journal (8) arranged with its free end adjacent said second end (7) of the shaft (1) and a magnetic coil (9) arranged around said journal (8) for generating a magnetic field (B) in the axial direction of said journal (8), that said journal (8), magnetic coil (9) and an end part of the shaft (1) around the second end (7) are encased in a housing (10) arranged to guide said magnetic field (B) in closed loops including said end part and said journal (8) and a gap (11) between the second end of the shaft (1) and the free end of the journal (8), whereby the magnetic field acts on said end part of the shaft (1) with a force in the direction against the free end of the journal (8). A tool holding assembly according to claim 2, characterized in that said housing (10) surrounds a space (12) including said gap (11) between the second end (7) of the shaft (1) and the free end of the journal (8), and that means (6) are arranged to create in said space (12) said pressure acting against the second end (7) of the shaft (1). A tool holding assembly according to claim 3, characterized in that a bearing (6) is arranged to support said end part of the rotateable shaft (1), that said bearing includes non-magnetic material of a certain thickness around said shaft (1) to limit radial magnetic forces on said end part of the rotateable shaft (1). A tool holding assembly according to claim 4, characterized in that said bearing (6) at the end part of the shaft (1) is a gas bearing and that said pressure is achieved by using gas leaking from said gas bearing. A tool holding assembly according to any of the claims 2-5, characterized in that for safety a landing bearing (16 or 17) is arranged on an end surface of the second end of the rotateable shaft (1) and/or on an end surface of the free end of the journal (8). A tool holding assembly according to claim 6, characterized in that said landing bearing (16 or 17) comprises a wear resistant washer or coating with low friction, said washer or coating is formed of graphite or other non-magnetic material with low friction, such as synthetic diamond. A tool holding assembly according to claim 6, characterized in that said landing bearing is a gas bearing (17). A tool holding assembly according to claim 6, characterized in that said landing bearing (16 or 17) is an aerostatic bearing. A tool holding assembly according to claim 6, characterized in that said landing bearing (16 or 17) is an aerodynamic bearing. A tool holding assembly according to claim 10, characterized in that said aerodynamic bearing is arranged in the end surface of the second end (7) of the shaft (1) or on the end surface ofthe journal in the form of radially extending spirale groves (21) or herringbone groves in said end surface, whereby said groves (21) create an increased air pressure outside the end surface of the second end (7) of the shaft when the shaft (1) rotates. A tool holding assembly according to claim 10, characterized in that said aerodynamic bearing is arranged to also create the pressure acting against the second end (7) of the shaft (1). A tool holding assembly according to any of the preceeding claims, characterized in that position detecting means (13) is arranged for detecting at least the axial position of said shaft (1) and emitting a corresponding signal to said control means (15) for controlling the electro-magnetic means, that said control means (15) is arranged to control the current flowing in the electro-magnetic means in response to said signal from the position detecting means (13) in order to control the movement of the shaft (1). A tool holding assembly according to any of claims 1-5, characterized in that a gas bearing or a hydrostatic bearing (17) is arranged between the second end (7) of the shaft (1) and the journal (8), that said gas bearing or hydrostatic bearing (17) is arranged to act against the second end (7) of the shaft (1), and that means (19) is arranged to apply a force against said second end via said bearing (17) for generating said pressure which acts against said second end. A tool holding assembly according to claim 14, characterized in that said means (19) is a spring or an air piston. A tool holding assembly according to any of the preceeding claims, characterized in that safety means is arranged, when the electro-magnetic means fails or stop to work, to inactivate said means (19) or said pressure which creates a force against said second end (7). A tool holding assembly according to any of the preceeding claims, characterized in that means (13) are arranged to detect when the actual axial position of the shaft (1) during movement of the spindle assembly towards a workpiece differs from a reference position, which deviation from the reference position indicates that unexpected forces act on the shaft (1) and that said control means is arranged to stop the advancement of the spindle assembly towards the work piece when said deviation occurs.

Inventors: GOERANSSON BO, GOERANSSON, BO, Göransson, Bo
Applicants: SKF NOVA AB
Application year: 2000
Main classification: B24B47
IPC codes: B24B47, B23Q1, B23Q5, B24B5, B24B41, B23B47