Turning – Process of turning
Patent
1988-08-31
1990-05-29
Schmidt, Frederick R.
Turning
Process of turning
82147, 310 905, F16C 3906
Patent
active
049285610
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an apparatus for machining typical and atypical ultra-precise surfaces of revolution, the apparatus comprising tool guide means displaceable in translation along a predetermined direction or in rotation about a predetermined axis of rotation in order to position a tool, a spindle slide for guiding a workpiece-carrier spindle assembly along an axis, first and second sensors for measuring displacement respectively of the spindle slide and of the tool guide means, and means for displacing the tool guide means at a determined speed corresponding to a given machining frequency.
High precision machine tools for machining complex surfaces using cartesian co-ordinates or polar co-ordinates make use of a workpiece-carrier spindle assembly mounted on a slide and generally under the control of 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 confer a high degree of stiffness to the spindle.
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 number 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.
The present invention seeks to remedy the above drawbacks and to make it possible to perform a variety of 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, using a machine which is very flexible in use and capable of being adapted to performing various high quality special tasks.
These objects are achieved by means of an ultraprecise machining apparatus of the type mentioned at the beginning of the description, characterized in that the workpiece-carrier assembly comprises a stator fixed to the spindle slide and a spindle mounted in the stator by means of an active magnetic suspension having five servo-controlled axes and comprising at least two radial bearings and an axial bearing, in that the radial and axial magnetic bearings are servo-controlled at natural frequencies which are less than about 80 Hz, in that means are provided for rotating the spindle of the assembly at a predetermined speed less that the natural frequencies and lying between about 20 Hz and about 75 Hz, and in that it further comprises means for applying variable reference voltages to the servo-control circuits of the radial and axial magnetic bearings, which reference voltages are varied as a function of machining control signals for the purpose of selectively modifying the radial and axial positions of the spindle of the workpiece-carrier assembly about a nominal position, and means for authorizing displacement of the spindle slide only when said control signals exceed predetermined values.
The invention also provides a method of machining an ultra-precise surface of revolution on a workpiece held in position by a spindle which is mounted via a five-axis act
REFERENCES:
patent: 4114960 (1978-09-01), Habermann
patent: 4180946 (1980-01-01), Heijkenskjold
patent: 4794290 (1988-12-01), Nagasaka
patent: 4839550 (1989-06-01), Mizuno
S.A.: Societe Europeenne de Propulsion
Schmidt Frederick R.
Shideler Blynn
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