Turning – Lathe – Headstock
Patent
1988-08-31
1990-12-11
Schmidt, Frederick R.
Turning
Lathe
Headstock
310 905, B23B 1902, F16C 3204
Patent
active
049761773
DESCRIPTION:
BRIEF SUMMARY
The present invention provides a workpiece-carrier spindle assembly having active magnetic bearings for a very high precision machine tool, the assembly comprising a stator mounted on a slide for guidance along a predetermined direction, a spindle mounted inside the stator, a chuck fixed to the spindle, first and second radial active bearings supporting the spindle and controlled from radial detectors for detecting the position of the rotor, an axial abutment constituted by an axial active magnetic bearing co-operating with a disk fixed to the spindle and controlled from at least one axial detector for detecting the position of the spindle, and an electric motor for rotating the spindle.
The invention also relates to a device implementing such a spindle assembly for a very high precision machine tool in order to facilitate precise machining of complex surfaces using cartesian co-ordinates or polar co-ordinates.
High precision machine tools use a workpiece-carrier spindle assembly mounted on a slide which is generally controlled by a screw-and-nut system associated with a D.C. electric motor whose rotor is directly coupled to the slide drive screw. The spindle on which the workpiece is fixed is itself mounted in the spindle assembly by means of air bearings which give the spindle a high degree of stiffness.
The use of air bearings requires very small gaps between the spindle and its stator, said gaps being about 5 .mu.m across. However, this does not eliminate precision-limiting residual disturbance movements in the spindle, particularly since slide defects can never be totally eliminated.
Proposals have already been made to rectify workpieces mounted on active magnetic bearings in order to obtain a high degree of precision and stability in the control of workpiece rotation. However, this implies that removable magnetic bearings are available, which is applicable to making workpieces of certain shapes only.
U.S. Pat. No. 4,180,946 corresponding to French patent No. 75 30209 describes a tool-carrier spindle assembly, in particular for a rectifier, in which the spindle is mounted in active radial bearings and which includes an electric drive motor incorporated in the stator of the spindle assembly and situated between the two end radial bearings. Although such a tool-carrier spindle assembly provides a degree of flexibility in use for performing a certain number of machining operations, it is not suitable for being incorporated in the architecture of a very high precision machine tool. In addition, having an electric motor integrated in the stator of the spindle assembly and disposed between the two radial bearings increases the amount of heat given off and the amount of vibration in the magnetic suspension, and generates electrical and magnetic interference at the various detectors which are used for servo-controlling the magnetic bearings, such that control of tool movement remains limited in precision.
The present invention seeks to remedy the above drawbacks and to make it possible to perform very high precision machining work for the purpose of making surfaces which are typical or atypical and having the general shape of a surface of revolution, by eliminating or correcting the defects of the various mechanical assemblies working relative to one another in order to make complex surfaces in a system of axes which are cartesian or polar.
These objects are achieved by means of a workpiece-carrier spindle assembly of the type specified at the beginning of the description, and characterized in that the electric motor is external to the spindle assembly stator, is disposed behind the spindle and substantially in alignment therewith, in that the external motor has a smaller air gap than the air gap between the radial magnetic bearings and the spindle, in that the rotary drive communicated to the spindle by the external motor expressed in revolutions per second (Hz) lies outside the servo-control passband of the magnetic suspension system expressed in hertz, in that a flexible coupling device connects the outlet
REFERENCES:
patent: 4114960 (1978-09-01), Habermann
patent: 4180946 (1980-01-01), Heijkenskjoid
S.A.: Societe Europeenne de Propulsion
Schmidt Frederick R.
Shideler Blynn
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