EP1103350B1 - Machine tool with electromagnet for mounting on a ferromagnetic workpiece - Google Patents

Description

Machine tool with an electromagnet for attachment to a ferromagnetic workpiece, with a switching device with at least two switching states, a working state in which the electromagnet and a drive device of the machine tool are activated, and a holding state in which the electromagnet, but not the drive device is activated, and means by which the current of the electromagnet in the holding state is reduced with respect to the current of the electromagnet in the working state.

Such a machine tool with an electromagnet is out of the US 3,371,257 known.

Such a machine tool is from the DE 31 00 933 C2 known.

In such machine tools are usually to power tools, in particular drills, which can be fixed by means of a magnetic stator to the workpiece to be machined from ferromagnetic material. Such machine tools are used, for example, when working on power pylons, in steel construction, when working on supports, etc., e.g. to create holes where you have to work with high torque.

The machine tool mentioned above has a base plate in which an electromagnetic coil is provided for fastening a drill stand when the coil is excited on the workpiece. The coil is driven via a switching arrangement, responsive to the magnetic flux of the coil, the power supply to the machine tool in the absence of a predetermined magnetic flux density interrupting switches are provided.

By such a safety device can indeed avoid that the machining process of the workpiece begins before the solenoid is activated, however, it has been found that the electromagnet can heat up significantly during prolonged work, especially when in the implementation of a variety of machining operations of the solenoid constantly remains switched on because the machine tool between the individual processing operations can not be removed or removed only with great effort. If a larger number of Machining operations is necessary, such machine tools often remain even several days or weeks on a workpiece, the electromagnet must remain constantly turned on during this time to ensure the fixation on the workpiece. This is associated with a significant energy consumption.

From the above US 3,371,257 For example, a motor control for an electromagnetic tool is known, in which a magnetizable base member can be switched to a small magnetic flux holding state and a high magnetic flux working state. In the hold state, a resistor limits the magnetic flux, whereas the resistance to the working state is shorted.

The invention has for its object to avoid the disadvantages of the prior art, in particular, the energy consumption should be reduced as possible.

This object is achieved in a machine tool according to the aforementioned type in that the electromagnet is fed from an AC voltage source in the hold state via a half-wave rectifier and that the electromagnet is energized in the working state via a full-wave rectifier.

By reducing the magnetic flux in the holding state of the energy consumption is reduced while counteracting heating of the electromagnet.

It has been shown that even with a reduced current flow through the electromagnet, the holding force is completely sufficient to ensure a secure positioning of the machine tool on the workpiece, in particular during work interruptions. The full holding torque of the electromagnet must be available only in working condition.

Even with reduced current flow through the electromagnet only a relatively small decrease in the holding force is recorded due to the magnetization characteristic.

In an advantageous embodiment of the invention, the electromagnet is fed from an AC voltage source in the hold state via a half-wave rectifier, while the electromagnet is energized in the working state via a full-wave rectifier.

As a result of the inductance of the electromagnet, the current through the electromagnet, even when using a half-wave rectifier, at no time to zero.

In this way, the current reduction is achieved by the electromagnet with particularly simple means, with a simple switch is usually sufficient to allow a switch between one-way and full-wave rectification.

In this way, the electromagnet in the hold state is driven only with a half-wave of the rectified AC voltage, while in the working state on the full-wave rectifier both half-waves are used to supply the electromagnet with pulsating DC voltage.

It has been shown that, due to the hysteresis of the electromagnet, despite the reduction of the effective voltage, approximately 75% of the holding force is still available.

This is quite sufficient to attach the machine tool to the workpiece to be machined and sufficiently precise to position. Only in the actual processing of the workpiece in the working state, the second half-wave is used to apply the full holding power as a safety reserve during this time.

In this embodiment, the rectifier is expediently designed as a bridge rectifier, wherein the electromagnet is connected between the positive pole and the negative pole of the bridge rectifier and in a branch of the bridge rectifier, a switch for switching between working position and holding position is provided.

According to a further embodiment of the invention, the electromagnet is fed from an AC voltage source via a Brükkengleichrichter, the bridge rectifier on the input side, a diode is connected upstream and a switch for bridging the diode is provided in the working state.

Such an embodiment is advantageous when a bridge rectifier is used as a complete component, so that the insertion of a switch for switching between working position and holding position in a branch of the bridge rectifier is not possible. In this case, only one half-wave can be used by connecting the diode, while both half-waves are used in bridging the diode.

According to a further embodiment of the invention, the electromagnet consists of a plurality of individual magnets, wherein in the holding state, only a portion of the individual magnets is subjected to voltage which is smaller than the plurality of individual magnets, which are acted upon in the working state with voltage.

Also in this way, a reduction of the holding force and the power consumption in the holding state can be achieved by e.g. in this state, only a single one of two individual magnets is energized.

Another way to reduce the current through the electromagnet in the holding state is to apply the electromagnet in the hold state only via a series resistor with voltage.

In contrast, the series resistor can be bypassed in the working state.

Finally, according to a further embodiment of the invention, an electrical or electronic control element can be provided, via which the electromagnet is operated in the holding state with a reduced current.

For this purpose, numerous electrical or electronic controls, e.g. switched diodes, transistor switch or the like conceivable.

It is understood that the features mentioned above and those yet to be explained not only in the particular combination given, but also in other combinations or alone, without departing from the scope of the present invention.

Fig. 1

eine erste Schaltung zur erfindungsgemäßen Ansteuerung eines Elektromagneten, anhand derer das Prinzip der Erfindung verdeutlicht wird;

Fig. 2

die Schaltung einer erfindungsgemäßen Werkzeugmaschine in Prinzipdarstellung;

Fig. 3

eine abgewandelte Ausführung einer erfindungsgemäßen Werkzeugmaschine;

Fig. 4

eine weitere Abwandlung einer erfindungsgemäßen Werkzeugmaschine und

Fig. 5,6

zwei weitere Prinzipschaltungen von abgewandelten Ausführungen der Erfindung.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the drawings. Show it:

Fig. 1

a first circuit for driving an electromagnet according to the invention, based on which the principle of the invention is illustrated;

Fig. 2

the circuit of a machine tool according to the invention in a schematic representation;

Fig. 3

a modified embodiment of a machine tool according to the invention;

Fig. 4

a further modification of a machine tool according to the invention and

Fig. 5.6

two further principle circuits of modified embodiments of the invention.

In Fig. 1 is a circuit for an electromagnet according to the invention for attaching a machine tool to a ferromagnetic workpiece indicated generally by the numeral 10.

The electromagnet 30 is fed via a bridge rectifier 20 from an AC voltage source 12, 14.

The bridge rectifier 20 has four diodes 22, 24, 26, 28, which are connected in a known manner to the bridge rectifier circuit 20. The electromagnet 30 is connected at its two ends between the negative pole 32 and the positive pole 34 of the bridge rectifier 20. While one AC terminal of the bridge rectifier 20 is directly connected to the supply voltage pole 14, the other is AC power connection of the bridge rectifier 20 via an on / off switch 16 connected to the supply voltage pole 12. While the other terminal of the on / off switch 16 is connected directly to the anode of the diode 26, leading to the negative terminal 32 of the bridge rectifier 20 diode 22 is connected via a further on / off switch 18 to the on / off switch 16. Both switches 16, 18 together form a switching device, which is designated by the letter S.

The circuit 10 now works as follows:

The switching device S has a total of three switching states, which are due to the positions of the switches 16, 18. A first switching state of the switching device S is the off state, which is present when the switch 16 is in the OFF position, so that the bridge rectifier 20 and the electromagnet 30 are de-energized. A second switching state of the switching device S is the holding state, which is present when the switch 16 is in the ON position and the switch 18 is in the OFF position. In this case, there is a connection between the supply voltage pole 12 via the switch 16, the diode 26, the electromagnet 30 and the diode 24 to the supply voltage pole 14. In this case, it is a one-way rectification, i. that only a half-wave of the AC voltage is used.

A third switching state of the switching device S is the working state, which results when both switches 16, 18 are closed. In this case, the diode 22 of the bridge rectifier 20 is supplied with voltage, so that another branch of the bridge rectifier 20 is supplied with power, the is given by the path through the switches 16, 18, the diode 22, the electromagnet 30 and the diode 28, so that the bridge rectifier 20 is now fully utilized and the solenoid 30 is fed from a full-wave rectifier, ie that both half-waves of the supply voltage used become.

It is understood that the switch 18 can of course also be installed in another branch of the bridge rectifier 20.

Such a circuit 10 can be used for a machine tool, which is to be held with a magnetic base on a ferromagnetic workpiece (not shown). In the magnetic base, the electromagnet 30 is in a near-surface arrangement, so that when current through the electromagnet 30, the machine tool is attracted with its magnetic base to a ferromagnetic workpiece, so as to position the machine tool and hold in the working position.

Although it is fundamentally advantageous when the machine tool is designed as a power tool, it is also possible to use such a machine tool also with another drive, e.g. equipped with a pneumatic drive.

The circuit 10 according to the invention has the advantage that the power consumption of the electromagnet 30 is reduced in the holding state compared to the power consumption of the electromagnet 30 in the working state by about 50%, since only one half-wave of the AC voltage is used in the holding state, while both half-waves are used in the working state , This leads to a significantly lower heating of the electromagnet 30, which is particularly noticeable in continuous operation. Likewise results in a drastic reduction in energy consumption, since the power consumption in the hold state is reduced by about 50%. Nevertheless, even in the holding state due to the magnetization curve of the core material of the electromagnet 30, a holding force of about 75% of the holding force of the electromagnet 30 in the working state. This value in the hold state is sufficient to accurately position the machine tool in question before a work on the workpiece or hold between different operations on the workpiece. Only during the machining process itself, the full magnetic flux of the electromagnet 30 is necessary to have a sufficient safety margin against a release from the workpiece or against slipping due to the machining forces available.

In the FIGS. 2 to 4 Now the practical application of such a circuit for a machine tool 40, 50 and 60 is shown.

Corresponding reference numerals are used for corresponding parts.

In the machine tools 40, 50 and 60 are drilling machines, which are equipped with a magnetic base, by means of which the drill is fixed upon excitation of an electromagnet on a workpiece.

The machine tool 40 has a drive shaft which is driven by means of a universal motor, which in Fig. 2 indicated by the reference numeral 42.

For controlling the circuit, in turn, a switching device S is provided which comprises a two-pole main switch 16, 17 and a motor switch 18. The two-pole main switch 16, 17 can be connected to the two supply voltage poles 12, 14 via a power plug.

The two outputs of the two-pole main switch 16, 17 are in turn connected to a bridge rectifier 20, between the negative pole 32 and positive pole 34, an electromagnet 30 is connected. The bridge rectifier 20 again consists of four diodes 22, 24, 26, 28, which in a known manner, as in Fig. 1 explained, are connected. In this case, the diode 22 is connected to the output of the motor switch 18, which is designed as an on / off switch. To the output of the motor switch 18, the universal motor 42 is also connected, which is connected via a speed control 44 and the second pole 17 of the main switch 16, 17 to the other pole 14 of the supply voltage source.

The motor switch 18 is used to turn on / off the motor 42, at the same time hereby the switching of the electromagnet 30 between holding state and working state.

Another modification of the machine tool 40 according to Fig. 2 is in Fig. 3 represented and designated in total by the numeral 50. The only difference to the execution according to Fig. 2 is that instead of a single-pole motor switch 18 now a two-pole motor switch 18, 19 is provided. While the first pole 18 of the motor switch 18, 19 connects the motor 42 via the pole 16 of the main switch 16, 17 with the first pole 12 of the AC voltage source, the second pole connects 19 of the motor switch, which is coupled to the pole 18 of the motor switch for joint operation, the cathode of the diode 22 via the first pole 16 of the main switch 16, 17 with the pole 12 of the AC voltage source.

Incidentally, structure and operation of the machine tool 50 are completely identical to those of the machine tool 40 according to FIG Fig. 2 ,

Another modification of the previously based on Fig. 3 described machine tool 50 is in Fig. 4 represented and designated overall by the numeral 60. Unlike the version according to Fig. 3 Here, the bridge rectifier 62 does not consist of four individual diodes, but is designed as a complete component. Again, the electromagnet 30 is connected between the positive pole 34 and the negative pole 32 of the bridge rectifier 62. The bridge rectifier 62 is connected to an AC input at the output of the second pole 17 of the main switch 16, 17, which is connected to the pole 14 of the mains voltage, while the other AC input of the bridge rectifier 62 via the pole 16 of the main switch 16, 17 and a diode 64 at the other pole 12 of the mains voltage is connected. For switching on and off of the motor 42, which is connected via the speed control 44 via the second pole 17 of the main switch 16, 17 to the pole 14 of the mains voltage, serves a two-pole motor switch 18, 19. The motor 42 is above the first pole 18th of the motor switch connected to the output of the first pole 16 of the main switch 16, 17. The second pole 19 of the motor switch is connected on the input side to the first pole 18 and connected on the output side to the cathode of the diode 64, which is connected to the input side of the bridge rectifier 62.

The switching device S, which includes the main switch 16, 17 and the motor switch 18, 19, in turn, as previously with reference to Fig. 1 described, three switching states. In the first switching state, both the main switch 16, 17 and the motor switch 18, 19 are in the OFF position. If the main switch 16, 17 is turned on while the motor switch 18, 19 is in the OFF position, the bridge rectifier 62 is supplied on the input side with alternating voltage, but only one half cycle of the alternating voltage is used by the upstream diode 64. If now additionally the motor switch 18, 19 closed, so in this third switching state, the diode 64 is bridged by the second pole 19 of the motor switch 18, 19, so that the bridge rectifier 62 is now coupled to the mains voltage and thus both half-waves of the AC voltage for the Electromagnet 30 can be used.

In the FIGS. 5 and 6 two further circuit variants of the invention are shown schematically, with which the current can be reduced by the electromagnet 30 in the holding state against the current in the working state.

Again, corresponding reference numerals are used for corresponding parts.

In the circuit 70, a DC voltage source with the poles 72 and 74 is provided for supplying the electromagnet 30. The electromagnet 30 is connected via a series resistor 76 and a On / off switch 16 connected to both poles 72, 74 of the DC voltage source. Parallel to the series resistor 76, a single-pole on / off switch 78 is connected, through which the series resistor 76 can be bridged.

While with open switch 78, the electromagnet 30 can only be traversed by a smaller current due to the series resistor 76, the current is increased when the switch 78 is closed, so as to achieve an increased holding force of the electromagnet 30 in the working state.

A modification of the circuit 10 is shown in FIG Fig. 6 represented and designated by the numeral 80 in total. The electromagnet 30, which now consists of two individual magnets 82, 84 connected in parallel, is in turn connected between the negative pole 32 and the positive pole 34 of a bridge rectifier 20, whose input side is connected to the first pole 12 of an alternating voltage source via a single pole on / off switch 16 with its other input is directly connected to the second pole 14 of the AC voltage source.

While 82 of the individual magnets 82, 84 is connected directly between the negative pole 32 and the positive pole 34 of the bridge rectifier 20, the second single magnet 84 is connected via a single-pole on / off switch 86 between the positive pole 34 and the negative pole 32 of the bridge rectifier 20.

When the main switch 16 and the open switch 86 is closed, therefore, only the individual magnet 82 is traversed by current, while in additionally closed switch 86, both individual magnets 82, 84 are traversed by current.

It is understood that in addition many other variants are conceivable to reduce the current flow through the electromagnet in the holding state against the current flow in the working state, so as to reduce the heating and energy consumption. For example, resistor 76 and switch 78 could be in Fig. 5 be replaced by an electronic switch.

Claims (6)

1. Power tool comprising an electromagnet (30) for securing to a ferromagnetic workpiece, comprising a switching device (S) having at least two switching states, a working state, wherein the electromagnet (30) and a drive device (42) of the power tool (40, 50, 60) are activated, and a holding state, in which the electromagnet (30) is activated, but not the drive device (42), and further comprising means (18, 19, 64, 76, 78, 86), whereby the current of the electromagnet in the holding state is reduced when compared with the current of the electromagnet (30) in the holding state, characterized in that the electromagnet (30) is powered by an alternating power source (12, 14) in the holding state via a one-way rectifier (24, 26), and in that the electromagnet (30) is powered via a two-way rectifier (22, 24, 26, 28) when being in the working state.
2. The power tool of claim 1, characterized in that the rectifier is configured as a bridge rectifier (20), in that the electromagnet (30) is connected between the positive pole (34) and the negative pole (32) of the bridge rectifier (20), and in that a switch (18, 19) for switching between the working state and the holding state is provided within a path of the bridge rectifier (20).
3. The power tool of claim 1, characterized in that the rectifier is configured as a bridge rectifier (62) which on its input side is preceded by a diode (64), and in that a switch (19) is provided for shortening the diode (64) in the working state.
4. The power tool of any of the preceding claims, characterized in that the electromagnet (30) comprises a plurality (n) of single magnets (82, 84), and in that in the holding state only a part (m) of the single magnets (82, 84) is powered, the part being smaller than the magnitude (n) of the single magnets (82, 84) which are powered with voltage when being in the working state.
5. The power tool of any of the preceding claims, characterized in that the electromagnet (30) is powered with a voltage via a pre-resistance (76) when being in the holding state.
6. The power tool of any of the preceding claims, characterized in that an electric or electronic control element (78) is provided for controlling the electromagnet (30) in the holding state with a smaller power than in the working state.

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