EP0293673B1 - Method and apparatus for mechanical grinding of workpieces by electric conductive worktools - Google Patents

Description

The invention relates to a method for the mechanical grinding of workpieces, wherein the grinding tool is probed against the workpiece or a measuring probe by applying an electrical spark voltage between them and the test position being determined by the spark current, and then from this touch position certain infeed and / or feed movements for the mechanical grinding process are carried out, as well as a device for carrying out such a method.

In mechanical grinding processes with which the present invention is concerned, the grinding tool has normally been moved from the operator to the point of contact with the workpiece, ie when the position of the surface to be ground in space and / or the grinding wheel diameter are not precisely specified it is touched, and then the intended infeed and feed movements are carried out according to a specific control program. The probing is thus manually controlled, with the operator depending on the noise that occurs when touch is made and the extent of the formation of grinding sparks. If, for example, a large number of differently worn tool cutting edges are to be reground in a clamping of a workpiece or a group of workpieces, For example, the teeth of a saw blade, the cutting edges of a milling cutter or a number of turning tools inserted together in a clamping device must be manually touched again before each cutting edge is re-sharpened. You cannot automatically edit all cutting edges one after the other. The same applies to grinding processes in the manufacturing process if the prefabricated workpieces have different starting dimensions, there are differences in workpiece clamping or the grinding wheel diameter is not constantly checked.

In the case of electroerosive machining processes, also known as spark grinding or erosion grinding according to EP-A1-0076 997, for example, in which the material is removed essentially without contact due to the sparks flashing between the tool and the workpiece, a certain spark gap is used at least at the beginning, compliance with which is stated in Dependence on the spark current is regulated. Even if workpieces containing electrically non-conductive materials, e.g. diamond, in an electrically conductive metallic matrix also undergo mechanical removal of the electrically non-conductive components, this only occurs after the metal matrix has been erosionally eroded, in which the electrically non-conductive components are largely exposed are so that they can be easily knocked off by the EDM tool. The control circuit, which is controlled as a function of the spark current, has so far basically had the function of providing the necessary distance (= spark gap) to the workpiece for the contactless spark-erosive removal of the electrically conductive material. In the case of spark erosion processes, there is initially no intentional contact with mainly mechanical material removal.

The invention is based on the method described in DE-A1-24 22 940 and the corresponding device. According to this, it is known to embed conductors insulated at the wheel periphery in a grinding wheel made of electrically non-conductive material and to apply such a high voltage between them and the workpiece that when the grinding wheel approaches the workpiece in rapid traverse, a spark discharge occurs at a predetermined distance, which can be used to switch off the tool infeed or to switch from rapid traverse to creep speed, which can be deactivated in the same way after the high voltage has been reduced to a lower voltage.

Apart from the fact that the embedding of cables in grinding wheels jeopardizes their strength and durability at high speeds, the known control method is not a probing that takes place during a certain period of time, which follows the described manual probing after the noise that increasingly occurs when touching begins and corresponds to the extent of the formation of grinding sparks, but to a sudden shutdown of the delivery as soon as a spark discharge occurs at a certain distance. The distance must be dimensioned according to the caster due to inertia. Only after switching off does the uncontrolled pressure of the grinding tool against the workpiece occur. In practice, inaccuracies are unavoidable, because even a punctiform geometric irregularity or a drop of water are sufficient to trigger the spark discharge and thus the cut-off of the infeed prematurely. Finally, a major disadvantage of the known device is that a complex distance measuring device and switching device are used solely for the purpose will switch off the feed movement of the mechanical grinding tool at a certain distance from the workpiece.

The object of the invention is to improve the mechanical grinding method mentioned at the outset in such a way that it can be carried out very precisely in a simple manner in connection with a spark erosion or erosion grinding process.

The above object is achieved in that an existing grinding tool made of electrically conductive material is used, the workpiece is machined in the same setting first with a first tool spark erosion or erosion grinding and then with a grinding wheel brought into the contact position by means of spark current, the spark current for probing is generated by the spark generator used in the EDM or EDM grinding of the workpiece with the associated control circuit, and after probing for the subsequent grinding process the spark voltage is switched off or set to such a value that essentially mechanical material removal occurs.

Great importance is attached to the method according to the invention of spark current-controlled probing prior to material-removing processing, the control circuit with spark generator previously used in another removal method for another purpose, namely distance control during electroerosive processing, being used with advantage. However, the spark generator does not need to demand a working current for material removal, since the spark current only serves to control a certain contact position while the material is being removed essentially mechanically.

The electrically non-conductive grain of the grinding wheel also ensures a certain distance between the workpiece and the metallic matrix of the grinding wheel during the contact and the mechanical removal of electrically conductive material of a workpiece, so that according to the invention there is also the possibility of continuing after the electrically controlled probing to allow a spark voltage to be applied between the tool and the workpiece and, depending on a weak spark current, which is of no importance for metal removal, to continuously regulate a specific contact position of the grinding wheel for mechanical removal of the electrically conductive material of the workpiece.

If surfaces that are arranged relative to one another, for example cutting a milling cutter, are to be ground, in a preferred practical embodiment of the invention, one of the surfaces can be touched by means of a spark current before and / or after grinding, and the other surfaces can be corresponding to this touch position ground or infeed movements to compensate for grinding wheel wear. Instead of using a specific surface on a workpiece as a reference for the automatic grinding of other surfaces, a model surface or a sensor can be touched with the grinding tool and used as a reference for all surfaces to be ground.

A device for carrying out the method according to the invention consists of a grinding tool which can be driven in rotation, a workpiece clamping device and a controllable movement drive for changing the relative position between the grinding tool and workpiece, the grinding tool and the workpiece or a sensor being connected to a spark generator and the movement drive when probing is controlled by a control circuit influenced by the spark current, and is characterized in that the grinding tool and a spark-erosive tool can be used in the same workpiece clamping, the grinding tool consists of an electrically conductive material, the spark generator and the control circuit are connected to the spark-erosive tool and the controllable movement drive is controlled during the mechanical grinding process by a program memory and / or a spark current which is only dimensioned as a control current.

The new device can be implemented in a particularly simple and cost-effective manner in that both tools are rotatingly drivable tools arranged on the same drive shaft.

The invention is explained below with reference to the drawing.

The drawing schematically shows a workpiece 10, e.g. a saw blade which is clamped on a cross slide 12 and can thus be moved in the direction of two mutually perpendicular coordinates x and y. A rotary drive 14, whose angle of rotation α can be controlled, is used for adjusting the angle of rotation or for advancing the saw blade from one cutting edge to be machined to the next.

Two tools 16 and 18 are provided for machining the workpiece 10. The tool 16 is a disk which can be driven in rotation, for example made of graphite, copper or another electrically conductive material, if appropriate also with enclosed abrasive grains made of electrically non-conductive material, for example diamond grain. This tool is used to machine the saw blade 10 or another workpiece, for example a milling cutter to be machined on its cutting edges, which may be made of hard metal or polycrystalline diamond, for example, or another similar tool.

The second tool 18 is an electrically conductive, i.e. e.g. metal-bonded grinding wheel with diamond grit, for example. Both tools 16 and 18 are seated on the same drive shaft 20 and can be moved by a tool slide 22 in the direction of a z coordinate perpendicular to the x and y coordinates. In addition, the angular velocity ω of the shaft 20 driven by a drive motor 24 can be controlled.

A spark generator 26 is connected via lines 28 and 30 to the workpiece 10 and the respective tool 16 and 18, respectively. In a conventional manner, a dielectric is flushed in between the workpiece 10 and the respective tool 16 or 18, so that they are insulated from one another and can jump sparks between them if the erosion tool 16 is in the working position with the intermediate distance of the spark gap required for spark erosion or when touching the workpiece 10 with the grinding tool 18.

With the aid of a control circuit 32, the infeeds, feed movements and setpoint settings provided in the individual case can be carried out. The specifications for the control circuit can be entered into the control circuit either manually using a keyboard (not shown) or through a program memory 34. This can then have the cross slide 12 execute a movement in the direction of the x coordinate via a line 36 and a signal converter 38, which, for example, influences the spark gap between workpiece 10 and tool 16. To control and readjust a specific spark gap, the changes in the electrical voltage Us at the spark gap can be tapped via a measuring device 40 and, alternatively or at the same time, a gap current change Is can be tapped via a measuring device 42 and fed to the control circuit 32 for evaluation. Parallel to the spark gap between the workpiece and the tool, a step-by-step can be set Capacity 44 available, as is known from metalworking, to make adjustments.

The infeed of the tool can be regulated depending on the current Is measured at 42. The value proportional to a current to the size of the spark gap can be fed to a comparator, in which a comparison is made with an adjustable setpoint.

The movement of the cross slide 12 in the direction of the y coordinate is controlled via a control line 46 and a signal converter 48. This can e.g. a reciprocating movement of the workpiece when machining straight cutting edges. The extent of the to-and-fro movement can be predetermined by the program memory 34 or by a simple reversal by means of limit switches.

The tool slide 22 is controlled in the direction of the z coordinate via a line 50 and a signal converter 52 in order to e.g. make a height adjustment of the tool 16 or 18.

The speed of the drive motor 24 is controlled by the control circuit 32 via a control line 54 and a signal converter 56. In order to finally bring the workpiece 10 into a different angle of rotation position after each machining stage with a rotary switching movement for machining another tooth, a corresponding signal is sent from the control circuit 32 via a control line 58 and a signal converter 60 to the rotary drive 14 of the workpiece 10 . In another machining task, the rotary drive 14 can also be a constantly rotating motor if a workpiece is to be machined as a rotating body while rotating about its axis during machining.

The device shown in the drawing is fundamentally suitable for processing metallic workpieces, especially for the manufacture and regrinding of tools, in particular with very hard cutting edges, for example made of polycrystalline material. The cutting edges of the workpiece can first be machined using the tool 16 by spark erosion or by erosion grinding. The surfaces that are still relatively rough in this machining process can then be finished in the same clamping of the workpiece 10 and with the aid of the same control circuit by means of the grinding tool 18. It is possible to either process tooth by tooth or edge by edge of the workpiece 10 in succession first with the eroding tool 16 and then with the grinding tool 18, or first all teeth or cutting with the eroding tool 16 and then again all teeth or cutting one after the other with the grinding tool 18. In the second processing stage using the grinding tool 18, the spark current Is, unlike in the previous eroding process, is no longer used to remove material, but only to control the relative position between the workpiece 10 and the grinding tool 18. The workpiece 10 can thus be moved quickly and automatically very precisely up to a specific distance or until the abrasive grains come into contact with the grinding tool 18 - or conversely, the workpiece 10 - in order to touch it. The desired starting position for the grinding process, whether in contact or with a very specific intermediate distance between the workpiece and the tool, can be set very precisely in the manner described, because each spark gap between the tool and the workpiece is a specific one even in the case of the electrically conductive grinding tool 18 Spark current Is is assigned, the value of which is passed to the control circuit 32 and is compared there with a specific target value for the starting position or contact position. Even during the mechanical During the grinding process by means of the grinding wheel 18, the spark voltage Us can be maintained in order to generate a spark current Is to the extent of a measuring or control current and in this way to control the mechanical grinding contact between the grinding wheel 18 and the workpiece 10. The eroding effect of the sparks is kept to a minimum in this processing step in order not to impair the smoothing effect of the mechanical grinding process.

Finally, the method according to the invention can also be used in conjunction with a control of the grinding wheel feed as a function of the torque or the rotational speed influenced thereby. Such a control ensures that whenever the pressure of the grinding wheel against the workpiece becomes too great, the grinding wheel is withdrawn from the workpiece. Then the workpiece is approached again. In this case, the invention opens up the possibility that when the grinding wheel is withdrawn from the workpiece, the electrical voltage between these two parts is switched on and the spark current is measured. This then becomes smaller as the distance between the grinding wheel and the workpiece increases, and at a certain limit value for the spark current, the retraction movement of the grinding wheel is stopped and switched back to feed. Then the spark voltage can be switched off again until the next time the grinding wheel presses too hard against the workpiece, its speed drops below a certain limit and it therefore has to be withdrawn again from the workpiece.

The method described last shows that the inventive touching of the grinding wheel to the workpiece by means of a spark current can take place not only when approaching but also during a retraction movement.

Claims (4)

1. A method for the mechanical grinding of workpieces, in which a grinding tool (18) is applied against the workpiece (10) or a reference element, an electric spark voltage is applied between them, and the contact position is determined by the spark current and subsequently, starting from this contact position, specific feed and/or advance movements are carried out for the mechanical grinding operation, characterised in that a grinding tool (18) consisting of electrically conductive material is used, the workpiece (10) is machined in the same setting firstly with a first tool (16) by spark erosion or erosion-grinding and is then mechanically machined with a grinding wheel (18) brought into the contact position by means of spark current, wherein the spark current for effecting contact is generated by the spark generator (26) with its associated control circuit (32) used during the spark erosion or erosion-grinding machining of the workpiece (10), and after contact for the subsequent grinding operation the spark voltage is cut off or adjusted to a value such that a substantially mechanical removal of material takes place.
2. A method according to Claim 1 for grinding surfaces arranged at specific relative positions to one another, characterised in that, before and/or after the grinding of one of the surfaces, contact is made with said surface by means of spark current and the other surfaces are ground as a function of this contact position.
3. An apparatus for carrying out the method according to Claim 1 or 2, comprising a grinding tool (18) which can be driven in rotating motion, a workpiece-clamping device and a controllable motion drive (12, 22) for varying the relative position between the grinding tool (18) and the workpiece (10), wherein the grinding tool (18) and the workpiece (10) or a reference element are connected to a spark generator (26) and while contact is effected the motion drive (12, 22) is controlled by a control circuit (32) controlled by the spark current, characterised in that the grinding tool (18) and a tool (16) acting in a spark-erosive manner can be used in the same workpiece setting, the grinding tool (18) consists of an electrically conductive material, the spark generator (26) and the control circuit (32) are connected to the tool (16) acting in a spark-erosive manner and, during the mechanical grinding operation, the controllable motion drive (12, 22) is controlled by a program memory (34) and/or a spark current measured only as control current.
4. An apparatus according to Claim 3, characterised in that the grinding tool (18) and the tool (16) acting in a spark-erosive manner are tools which can be driven in rotating motion and which are mounted on the same drive shaft (20).

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