METHOD OF ELECTROCHEMICAL MACHINING AND BEARING MANUFACTURED WITH SAID METHOD

Abstract: The invention relates to electrochemical machining of metal pieces, and in particular rings of bearings, in a controlled way. To achieve this process, parameters, such as current, are actively controlled. This allows the raceway to be completed sooner and with a smoother, more circular surface. In addition the active control makes the formation of more complex forms possible and is suitable for the cheap manufacture of trilobed raceways.

Description: Method of electrochemical machining and bearing manufactured with said method The present invention relates to a method of electrochemically machining a metal piece, wherein a current is passed across a narrow gap formed by said metal piece and a adjustable electrode means, electrolyte being supplied to the gap in a direction substantially tangential to the surface of the metal piece. Such a method is known from GB 1 235 339. Here an electrode means is rotated around an axis and an electrolyte film is formed on said electrode means causing the electrolyte to be supplied to the gap between the piece and the electrode means in a direction substantially tangential to the surface of the metal piece. It is an object of the present invention to provide a method of electrochemical machining in which a fast and precise control of the machined surface is obtained. To this end the method according to the invention is characterized in that the metal piece is rotated around an axis and material from the metal piece is locally removed in a controlled way such that the amount of material removed is in a functional relationship with the angular position of the metal piece. According to the state of the art a constant voltage is applied over the gap which width will vary with the angular position of the metal piece as it rotates around the axis, in accordance with irregularities on the surface of the metal piece as well as any out-of-roundness. The change in gap width corresponds to a change in the resistance and consequently in the current, thus removing more material when the gap is narrow and less when the gap is wide. In this way elevations are removed and the surface of the metal piece is smoothed. Now, according to the invention the material is removed in a controlled way to speed up this process, i.e. when the gap is narrow the electrochemical removal of material from the metal piece is enhanced by adjusting relevant process parameters. With this active control it is possible to have more control over the final form of the metal piece and obtain this form faster. For example, it is possible to increase the voltage when the gap is narrow due to an elevation on the metal piece, resulting in additionally increased current, which in turn enhances the removal rate. It is also possible to stop or slow down the rotation of the metal piece around the axis such that the elevation is situated under the electrode means, which causes in particular the elevation to be subjected to the electrochemical treatment. Thus, with the active control according to the invention there is more ' control over the final form of the metal piece and this form can be achieved faster. Thus, it is possible to obtain metal pieces with improved circular form or with more complex forms. According to a preferred embodiment, the method according to the invention is characterized in that the metal piece is a cylindrical roller outer ring and material is removed from the raceway of the ring to result in a trilobed raceway by superimposing a harmonic variation on the current at three times the frequency of the ring's rotational speed. In this way it is possible to obtain trilobed rings cheaply, which allows a more widespread use of the superior bearings containing these rings. The invention also relates to bearings comprising rings manufactured with the method according to the invention. The invention will now be elucidated with the aid of the drawing, illustrating the invention by way of example. Fig. 1 is a schematic view of the key components suitable for controlled rounding off a ring. Fig. 2 is a schematic view of the key components suitable for controlled manufacture of a ring with a trilobed raceway. Fig. 1 shows a bearing ring 1 as a metal piece to be machined, said ring 1 being provided with a raceway which has to be treated electrochemically. Raceways may have out-of- roundness and elevations of up to ten μm, resulting from previous operations such as grinding. Removing these irregularities results in raceways with a circular geometry and consequently with reduced noise levels and increased operational life. An electrode means 2 includes a channel 3 for delivering electrolyte A through said channel 3 to an orifice 4. The electrode means 2 comprises an electrode 5 which is electrically isolated from the electrolyte A, apart from the end of the electrode 5 which forms a narrow gap 6 with the ring 1. Electrolyte A is passed, through the orifice 4, tangentially to the ring 1 through the gap 6 at a uniform velocity. The pressure with which the electrolyte A is passed through the gap is not very critical but higher pressures are unfavourable due to turbulence of the electrolyte resulting in uneven machining. The rotational direction of the ring 1 and the tangential electrolyte flow are preferably opposite to each other as this was found to give the best results. A DC power supply 7 maintains the ring 1 via a slip ring 8 at a positive voltage relative to the electrode 5. For the controlled removal of the amount of material from a particular part of the ring 1 data is obtained on where and how much material should be removed. The data for controlled removal of material can be obtained using the ring I, the electrode 5 and a volt-ampere meter (not shown), wherein both the voltage and current are measured to derive the required data. The data is then used to adjust the voltage applied over the gap. Fig. 1 shows a schematic electric circuit comprising a shunt 9, bandpass filter 10 and amplifier II. The current through the shunt 9 is used to the determine the level of feed-back to the power supply 7. Use of the electrode 2 and the ring 1 to control the electrochemical machining brings with it the advantage that the place where material is to be removed and the place where data is obtained are the same and data processing is near instantaneous, and is the simplest possible, as no time or angular position dependent information has to be processed for the proper place of the ring 1 to be machined. Using a constant angular velocity of the ring and controlling the amount of material removed by varying the voltage applied it proved to be possible to reduce the out-of-roundness of a ring 1 from 8 μm to 0.5 μm, compared to 2-3 μm with state of the art electrochemical machining procedures. It is also possible to use means (not shown) separated from the electrode 5, said means being capable of sensing the surface of the metal piece, to supply data. The means may comprise a contact sensor, such as displacement sensor, or non-contact sensors such as optical sensors. The data obtained comprises information on the deviations of the roundness of the ring 1 and this data is used to control the local removal of material, for example by changing the voltage applied as described above or by changing the length of time during which that particular part of the ring 1 is left under the electrode end 5. Thus the time needed to obtain a raceway with the desired quality is shortened. It is remarked that changing the voltage is usually easier to accomplish and gives more accurate results than changing the length of time. However, the latter possibility may have advantages during the first stages of the electrochemical machining operation. The method according to the invention offers the possibility to control the form of a raceway and allows the cheap manufacturing of a trilobed raceway, for example by superimposing a harmonic variation on the current at three times the frequency of the ring's rotation. To this end the angular position of the ring 1, which is usually an inner bearing ring, is continuously monitored, for example by position registration means 12 as shown in Fig. 2. The position registration means 12 controls a wave form generator 13 to modify the DC power supply 7. The electrochemical machining process can be speeded up by using a group of electrodes 5. The electrodes of a group are preferably in an equally spaced relationship to each other. For the manufacture of a trilobed ring, it is preferred to use one electrode for control over the form of the bearing ring while the others are used to improve the smoothness of the raceway. It is obvious for a person skilled in the art that numerous embodiments are suitable to create a functional relationship between the angular position of the ring and the part of the ring 1 where material is to be removed. For example the axis around which the bearing 1 turns can be used to control the power supply 7 to obtain an angular position dependent controlled removal rate. The electrode end 5 can be moved towards the axis of the ring 1 to adjust the width of the gap 6 as required.

Claims: CLAIMS 1. Method of electrochemically machining a metal piece (1) , wherein a current is passed across a narrow gap (6) formed by said metal piece (1) and a adjustable electrode means (2) , electrolyte A being supplied to the gap (6) in a direction substantially tangential to the surface of the metal piece (1) , characterized in that the metal piece (1) is rotated around an axis and material from the metal piece (1) is locally removed in a controlled way such that the amount of material removed is in a functional relationship with the angular position of the metal piece (1) . 2. Method according to claim 1, characterized in that the angular position of the metal piece (1) is continuously monitored. 3. Method according to claim 1, characterized in that the metal piece (1) is rotated on the axis at a constant angular velocity and the amount of material removed is controlled by varying the voltage applied. 4. Method according to any of the preceding claims, characterized in that data from means capable of sensing the surface of the metal piece (1) is used to control the local removal of material . 5. Method according claim 4, characterized in that the sensing means comprise the metal piece (1) , the adjustable electrode means (2) and a volt-ampere meter, the voltage and current being measured to derive the data for controlled removal of material. 6. Method according to any of the preceding claims, characterized in that the rotational direction of the metal piece (1) and the tangential electrolyte A flow are opposite to each other. 7. Method according to any of the preceding claims, characterized in that a group of electrodes (5) is used. 8. Method according to claim 7, characterized in that the electrodes (5) of a group are in an equally spaced relationship to each other. 9. Method according to any of the preceding claims, characterized in that a ring (1) is used as the metal piece (1) to be machined. 10. Method according to claim 9, characterized in that material is removed from the raceway of the ring (1) to result in a circular raceway (1) . 11. Method according to claim 9, characterized in that the metal piece (1) is a cylindrical roller bearing outer ring (1) and material is removed from the raceway of the ring (1) to result in a trilobed raceway by superposing a harmonic variation on the current at three times the frequency of the rings rotational speed. 12. Bearing comprising a ring (1) , characterized in that the ring (1) is manufactured according to any of the claims 9 or 11.

Inventors: WARDLE FRANK PETER, WARDLE, FRANK, PETER
Applicants: SKF IND TRADING & DEV, WARDLE FRANK PETER, SKF INDUSTRIAL TRADING & DEVELOPMENT COMPANY B.V., WARDLE, FRANK, PETER
Application year: 1996
Main classification: B23H3
IPC codes: B23H3, B23H9, F16C33