Patent Application
20090163121
The present invention relates to a grinding machine for grinding a workpiece, comprising a machine frame, a bearing device provided on the machine frame and movable along guides, in which bearing device a cup-shaped grinding wheel is borne, drivable in a rotating way about a grinding wheel axis and electrically insulated, which wheel is constructed of electrically conductive material and has a first grinding region with an annular abrasive surface and second grinding regions with abrasive surfaces in the shape of the lateral area, in each case consisting of an electrically conductive bonding material and abrasive grit embedded therein, which grinding wheel is electrically connected to a generator, means for holding the workpiece to be ground, a device for conditioning the grinding wheel with at least one movable electrode, which is likewise connected to the generator, and means for supply of a cooling lubricant to the electrode and to the workpiece.
Grinding machines of this type are known. With such grinding machines, indexable inserts, for example, are able to be ground, which procedure has to take place with high precision, for which purpose the grinding wheel must also be kept in optimal condition with respect to precision and sharpness. To ensure this quality of the grinding wheel, the wheel must be prepared and conditioned accordingly. Essentially three steps are hereby carried out, i.e. profile dressing, sharpening and cleaning of the grinding wheel.
The profile dressing operation, by which the grinding wheel is brought into the desired shape, is usually carried out for each new grinding wheel. A profile dressing operation is also carried out when the grinding wheel has been in use for a longer period of time. In a known way, such a profile dressing operation is executed with a silicon carbide wheel that can be brought into contact with the grinding wheel in the grinding machine or with which the grinding wheel in the grinding machine can be brought into contact. Besides grinding wheel material, also silicon carbide from the dressing wheel is thereby also removed. This silicon carbide ends up in the cooling lubricant loop, and must be removed from the coolant lubricant medium as quickly as possible since this material is very aggressive. To do this, suitable and costly apparatus are necessary.
During the step of sharpening a grinding wheel, the bonding material of the abrasive surface is reduced to improve the height of the grinding grains projecting over the bonding material. It is known to carry out the step of sharpening of the grinding wheel for metal-bonded grinding wheels by means of electrochemical methods in which an electrochemical stripping of the conductive bonding material of the abrasive surface of the grinding wheel takes place by means of an electrode and an applied electrolyte. The stripped material must then be filtered, in a complex and time-consuming way, out of the electrolytic medium, acting as cooling lubricant, for which purpose expensive devices are needed.
The cleaning of the grinding wheel, by which the swarf which has been created by the grinding operation and which has accumulated in the irregularities of the abrasive surface, is removed, can be carried out in a known way with a white corundum wheel. It can also be carried out, however, using the previously described electrochemical method, whereby the aforementioned drawbacks arise with both methods.
An object of the present invention is therefore to design a grinding machine for grinding a workpiece in such a way that the profile dressing, the sharpening as well as the cleaning of the grinding wheel can be carried out in a simple way using a single tool, and the cleaning of the cooling lubricant can be accomplished in a simple way.
This object is achieved according to the invention in that the device for profile dressing, sharpening and cleaning is configured as a single tool designed as cup-shaped electrode that is provided with at least one annular machining surface, which cup-shaped electrode is borne on a slide in a way drivable in rotation about its central axis, by means of which slide a working gap existing between the respective machining surface of the cup-shaped electrode and the respective abrasive surface is adjustable, in which gap a spark-erosive discharge occurs with application of a spark voltage by the generator.
By means of this electroerosive discharge in the working gap, the bonding material is eroded, depending upon how wide the working gap is, how great the discharge energy is, and which discharge frequency is used. The grinding wheel can thereby be shaped, sharpened and cleaned, which can be achieved in a very simple way by the single electrode, employed here for this purpose. The sharpening and the cleaning of the grinding wheel can be carried out in a problem-free way during the machining by grinding of a workpiece, the efficiency of the machining operations being optimal, since there is no interruption. Ensured moreover is that the grinding wheel constantly has an optimal abrasive quality. The efficiency will also be thereby increased. The machining of the workpieces is very precise. The material removed by the spark-erosive discharge is carried away by the cooling lubricant conducted into the working gap; a cleaning of this cooling lubricant is possible in a simple way, as is also carried out with corresponding spark erosion machines.
The axis of the cup-shaped electrode is preferably aligned parallel to the grinding wheel axis and perpendicular to the machining surface of the grinding wheel. The machining surface is thereby conditioned in an ideal way, perfectly flat and perpendicular to the rotational axis of the grinding wheel.
The shaft of the cup-shaped electrode is preferably borne in the slide in an electrically insulated way, which slide is held on the bearing device via a linear guide, and is displaceable in a controlled way along the linear guide in direction of the axis. The cup-shaped electrode thereby allows itself in a simple and optimal way to be placed on and taken off the machining surface to be conditioned.
Another advantageous design for the invention consists in that the slide is designed as compound slide rest, so that the electrode is movable substantially axially and radially with respect to the grinding wheel, and the electrode is provided with a further machining surface having substantially the shape of the lateral area. Thus, with this electrode, not only the annular abrasive surface of the cup-shaped grinding wheel, but also an abrasive surface in the shape of the lateral area of this grinding wheel can be correspondingly conditioned, whereby the possible applications for grinding processes are increased.
The electrode's machining surface in the shape of the lateral area is preferably cylindrical, and the two slides, designed as compound slide rest, are pivotable toward one another about an axis situated perpendicular thereto. Thus, with this electrode, an abrasive surface in the shape of the lateral area can also be conditioned when the latter has a so-called clearance angle with respect to the annular abrasive surface.
The electrode's machining surface in the shape of the lateral area can also be designed frustoconical, whereby, by moving both slides at the same time, an abrasive surface in the shape of the lateral area can also be conditioned when the latter has a so-called clearance angle with respect to the annular abrasive surface.
The generator for creating the spark-erosive discharge is a spark generator with capacitive discharge, which makes possible an optimal spark-erosive discharge. It is disposed on the bearing device for the cup-shaped grinding wheel, resulting in the shortest possible discharge lines for the spark-erosive discharge, which has positive consequences for the latter.
The means of supply of the cooling lubricant preferably consist of jet nozzles disposed on the supply lines, via which jet nozzles the cooling lubricant is able to be conducted into the machining gap and to the workpiece, resulting in an optimal conditioning and an optimal cooling and lubrication.
Another preferred embodiment of the invention consists in the cooling lubricant being an oil-based dielectric fluid, whereby an optimal cooling and lubrication during the grinding procedure is achieved, and an optimal environment is obtained for the spark-erosive discharge for conditioning of the grinding wheel.
The electrode is preferably made of aluminum, whereby it can easily be put into shape, and moreover, in combination with the oil-based dielectric fluid, an optimal spark-erosive discharge is achievable.
A control device is preferably provided for control and regulation of the operational procedures, whereby these procedures can be optimally coordinated with the grinding steps to be carried out.
A further object of the invention consists in creating a method for conditioning a cup-shaped grinding wheel by means of which this grinding wheel is optimally shaped, sharpened and cleaned, which object is achieved according to the invention in that to condition the abrasive surfaces of the grinding wheel, a cooling lubricant is conducted into the working gap, a spark voltage is applied over the working gap via the generator, and the electrode is moved toward the grinding wheel at a feed rate until a predetermined threshold value of the average voltage, measured over the working gap, and/or of the average current flow, measured through the discharge lines, is exceeded; then the spark voltage over the working gap, the discharge energy, the discharge frequency and the feed rate are each set to a predetermined value for profile dressing, sharpening or cleaning of the grinding wheel, and the respective step is carried out by spark erosion discharge.
To profile dress the grinding wheel, a discharge energy of about 10 to 100 mJ and a discharge frequency of about 1 to 100 kHz are preferably selected, resulting in an optimal erosion rate.
The profile dressing operation is carried out until the average voltage, measured over the working gap, and/or the average current flow, measured through the discharge lines, is substantially constant, which indicates that the profile-dressed abrasive surface has an optimal shape.
For preliminary sharpening of the grinding wheel, a discharge energy of about 0.1 to 5 mJ and a discharge frequency of about 10 kHz to 1 MHz are selected. A corresponding discharge energy and discharge frequency are also selected for sharpening and cleaning of the grinding wheel, it being possible for the sharpening and cleaning of the grinding wheel to be carried out during the grinding of a workpiece.
After a grinding operation, it can be necessary to re-establish the optimal state of sharpness of the abrasive surface being used by means of an additional re-sharpening operation. This re-sharpening operation lasts a predetermined amount of time during which grinding does not take place, and works with parameters similar to those for the sharpening and cleaning of the grinding wheel during the grinding of a workpiece.
An optimal operation of conditioning of the grinding wheel by the electrode is then achieved when the feed rate of the electrode is set within a selectable range by a regulator, disposed in the control system, based on the average voltage, measured over the working gap, and on the average current flow, measured through the discharge lines.
The discharge energy and the discharge frequency during the sharpening and cleaning of the grinding wheel are preferably set within a selectable range, using an optimization algorithm stored in the control system, based on the maximal contact pressure, the average contact pressure during spark-out, the ratio of the power output of the drive motor to the contact pressure and to the wheel attrition, measured during the preceding and completed grinding operations. The process flow is thereby made easier.
For re-sharpening of the grinding wheel between two grinding operations, a discharge energy of about 0.1 to 5 mJ and a discharge frequency of about 10 kHz to 1 MHz are preferably selected, and this re-sharpening operation is carried out during a selectable re-sharpening time, whereby a high degree of process stability is achieved.
A further simplification of the handling is achieved in that the discharge energy and the discharge frequency during the re-sharpening of the grinding wheel and also the re-sharpening time are set within a selectable range, using an optimization algorithm stored in the control system, based on the maximal contact pressure, the average contact pressure during spark-out, the ratio of the power output of the drive motor to the contact pressure and the wheel attrition, measured during the preceding and completed grinding operations.
An embodiment of the device according to the invention and of the method according to the invention for conditioning a grinding wheel will be explained more closely in the following, by way of example, with reference to the attached drawing:
In
The cup-shaped grinding wheel 2 consists of a grinding wheel cup 5, on which an abrasive ring 6 is placed having an annular abrasive surface 7 and an abrasive surface 8 in the shape of the lateral area. With this grinding wheel 2, a workpiece 9 can be ground, for example an indexable insert, which is held in a known way via means 10 for holding the workpiece 9 to be ground, disposed in the grinding machine.
Provided for conditioning the abrasive surfaces 7, 8 of the cup-shaped grinding wheel 2 is a device 11 having a cup-shaped electrode 12, borne in a way rotatable about its central axis 13 in a slide 14, which is held on the bearing device 1 in a way displaceable in direction of the central axis 13. The displacement of the slide 14 on the bearing device 1 takes place via a ball screw drive 15, the drive motor 16 of which is mounted on the bearing device 1.
Likewise disposed on the bearing device 1 is a generator 17. This generator 17 is connected to the power supply for the grinding machine via lines 18. The generator 17 is connected to the cup-shaped grinding wheel 2 via a discharge line 19 and to the cup-shaped electrode 12 via a further discharge line 20, as will be seen in the following. The communication with the known machine control system (not shown) takes place via the line 35, which can fulfill the most diverse specifications, such as Ethernet, PROFIBUS or RS 232.
Provided in a known way in the region of the workpiece 9 to be ground is a jet nozzle 21, which is connected to a supply line (not shown), via which a cooling lubricant can be brought into the grinding region. A further jet nozzle 22 is disposed in a known way in the region of the electrode, via which jet nozzle, by means of a supply line (not shown), the cooling lubricant is able to be brought into the working gap 23 between the cup-shaped electrode 12 and the abrasive ring 6 of the grinding wheel 2.
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The grinding wheel cup 5 of the cup-shaped grinding wheel 2 is made of an electrically conductive material. The abrasive ring 6 put on the grinding wheel cup 5 consists of a base body of aluminum, bronze or steel. Provided on this base body are the abrasive surfaces 7, 8 consisting of a compound in which the abrasive grains are embedded. The bonding material is composed of a metal alloy, of synthetic resin or of ceramic, which are likewise able to conduct electricity. Embedded in a known way in this electrically conductive bonding material are the abrasive grains, which can be made of diamond or of another correspondingly suitable material.
The cup-shaped electrode 12 likewise consists of an electrically conductive material, preferably aluminum. This cup-shaped electrode 12 can also be made of copper, graphite or another electrically conductive material, however.
Used as cooling lubricant is preferably an oil-based dielectric fluid, for instance the cooling lubricant marketed under the designation “lonogrind” by the company Oelheld GmbH, Stuttgart, Germany. The generator 17 used here is a spark generator, such as described, for example, in U.S. Pat. No. 4,710,603 of the company Fanuc Ltd.
For conditioning the abrasive surfaces 7, 8 of the cup-shaped grinding wheel 2, a spark voltage is applied by the generator 17 over the working gap 23, whereby an ion channel is formed in the dielectric cooling lubricant between the cup-shaped electrode 12 and the cup-shaped grinding wheel 2, and a discharge occurs. The working gap 23 must be sufficiently large so that the disengaged bonding material as well as the disengaged abrasive grains are able to be flushed away without the cup-shaped electrode 12 or the abrasive surfaces 7, 8 of the cup-shaped grinding wheel 2 being damaged. For a metal-bonded diamond grinding wheel with 25 micrometer grit, the working gap 23, i.e. the spacing between the bottom of the bonding material of the abrasive surface of the grinding wheel 2 and the cup-shaped electrode 12 should measure between 50 and 100 micrometers. To be able to achieve this, a spark voltage over the working gap 23 is required of 300 to 500 volts, preferably 400 volts. With lesser spark voltage there is the risk that the working gap is too small, and the disengagement of the bonding material and of the abrasive grains damages the surface of the cup-shaped electrode 12.
As has already been mentioned, the generator 17 is disposed on the bearing device 1, which means that the electrical discharge lines 19 and 20 (
This also corresponds to the speed of the grinding wheel for grinding a workpiece. The cup-shaped electrode rotates about the central axis 13 at a lower speed. Through the rotation of the electrode 12, a very precise evenness of the electrode 12 and of the abrasive surface 7 is achieved.
Before the conditioning step with spark-erosive discharge may be carried out, the cup-shaped electrode 12 must be brought into the correctly spaced-apart position with respect to the abrasive surfaces 7, 8 to be conditioned. The conditioning steps described in the following are carried out with a cup-shaped grinding wheel having a diameter of 400 mm, a surface cover layer of 10 mm and a granularity of 25 micrometers. The discharge energy at the generator is set; the cup-shaped electrode 12 is moved along the central axis 13 towards the grinding wheel 2 via the slide 14, whereby the speed can amount to 10 to 100 micrometers per minute. As soon as the average voltage over the working gap 23, which is measured in a known way, and/or the average current flowing through the electrical discharge lines 19 and 20 (
The profile dressing operation is then ended when the average voltage over the working gap and/or the average current flowing through the discharge lines remains substantially constant, i.e. does not vary more than 10% during a revolution of the grinding wheel 2 or respectively of the electrode 12. With this profile dressing operation, an absolutely flat annular abrasive surface 7 is obtained, which lies in a plane perpendicular to the grinding wheel axis 3. It would also be conceivable to chamfer the annular machining surface 32 of the electrode in a beveled way and not to align the central axis 13 parallel to the grinding wheel axis 3; one would then obtain an annular abrasive surface 7 which would be at an angle with respect to the plane situated perpendicular to the grinding wheel axis 3.
The profile dressing operation can be shortened in that the grinding wheel 2 with the corresponding abrasive surface 7, 8 and the electrode 12 with the corresponding surface abut; the generator 17 remains switched off. The grinding wheel 2 and the electrode 12 are driven in a rotating way. The grinding wheel 2, which is usually delivered in a relatively precisely profiled state, thereby dresses the electrode 12 through a grinding operation. The profile dressing operation can then be brought to an end through the previously described dressing procedure.
With this dressing procedure there is the risk that the electrode is ground unnecessarily too intensely. To prevent this, the generator 17 can be switched on for carrying out the dressing procedure. A medium-sized voltage is applied. Then grinding wheel 2 and electrode 12 are driven toward each other until grinding wheel 2 and electrode 12 abut one another. A short circuit voltage results. The feed motion of the grinding wheel 2 or respectively of the electrode 12 is stopped. One can wait until an equilibrium is reached with respect to the spark-erosive discharge.
For preliminary sharpening of the annular abrasive surface 7 of the cup-shaped grinding wheel 2, a discharge energy of typically 0.1 to 5 mJ and a discharge frequency of typically 10 kHz to 1 MHz are selected. The feed motion of the cup-shaped electrode 12 is brought to a low speed of typically 0.1 to 0.4 micrometers per minute. The feed rate is optimally set within a certain range based on the measured average voltage over the working gap 23 and the average current flowing through the discharge lines, by means of a regulator in the control system. The preliminary sharpening operation can be considered ended when a feed distance is reached of 20 to 50 micrometers, this feed distance corresponding approximately to the grain diameter. Thermally stressed grains are thereby eliminated.
For sharpening and cleaning of the annular abrasive surface 7 of the cup-shaped grinding wheel 2 during the grinding operation (inprocess), the feed motion of the cup-shaped electrode 12 is set at a speed of maximally 0.4 micrometers per minute. Selected thereby are typically discharge energies of 0.1 to 5 mJ and discharge frequencies of 10 kHz to 1 MHz. By means of a regulator in the control system, the feed rate is optimally set within a particular range, based on the measured average voltage over the working gap 23 and the average current flowing through the discharge lines.
During the grinding of a workpiece 9, the contact pressure with which the workpiece 9 is pressed against the grinding wheel 2, and the output of the electromotor 4 for the grinding wheel can be measured in a known way. Calculated in particular are the maximal contact pressure, the average contact pressure during spark out and the ratio of the output of the electromotor to the contact pressure. At the end of each grinding operation, the wheel attrition is estimated in a known way. From these measurement values, or respectively from the data correspondingly processed in a computer and regulator device, the state of sharpness of the abrasive surfaces 7, 8 in use of the grinding wheel 2 allow themselves to be quantified in a known way.
The discharge energy and discharge frequency for sharpening and cleaning are preferably set within a certain range based on the state of sharpness of the abrasive surfaces 7, 8 of the grinding wheel 2 in use during the preceding and completed grinding operations.
To re-sharpen the annular abrasive surface 7 of the cup-shaped grinding wheel 2 between two grinding operations, the feed movement of the cup-shaped electrode 12 is set at a speed of maximally 0.4 micrometers per minute. Selected thereby are typically discharge energies of 01 to 5 mJ and discharge frequencies of 10 kHz to 1 MHz. By means of the regulator in the control system, the feed rate is optimally set within a certain range based on the measured average voltage over the working gap 23 and the average current flowing through the discharge lines. After a particular re-sharpening time, this procedure can be considered finished.
The discharge energy, the discharge frequency and the re-sharpening time are preferably set within a certain range based on the state of sharpness of the abrasive surface 7, 8 of the grinding wheel 2 in use during the preceding and completed grinding operations.
As already mentioned, the values described in the foregoing apply to the conditioning of a cup-shaped grinding wheel with a diameter of 400 mm and having a cover layer thickness of 10 mm and a granularity of 25 micrometers. With larger cover layer thicknesses, the feed rate must be correspondingly reduced, depending upon the removable volume per unit of time. With different granularity, other feed distances correspondingly apply.
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With this device according to the invention and the method according to the invention, a cup-shaped grinding wheel can be conditioned in an optimal way, the sharpening and cleaning operations can be carried out without any problem also during the grinding of workpieces. The grinding wheel always has an optimal state, and the efficiency is thereby increased.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07123579.0 | Dec 2007 | EP | regional |
