GB2439192A

GB2439192A - Drive device for a tool holder and method of driving it

Info

Publication number: GB2439192A
Application number: GB0711265A
Authority: GB
Inventors: Patrick Maillard; Jean-Francios Longchamp
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2006-06-14
Filing date: 2007-06-11
Publication date: 2007-12-19
Anticipated expiration: 2027-06-11
Also published as: DE102006027577A1; CN101088702A; GB0711265D0; GB2439192B; CN101088702B

Abstract

The invention concerns a drive device for a tool holder of a machine tool, particularly an electrical hand machine tool, preferably an electrical padsaw, with a drive and a movement device to move the tool holder backward and forward periodically. It is provided that the drive (18) is in the form of an oscillation exciting device (17) to excite the oscillations of an oscillation system (52). which form the backward and forward movement, the movement device (1) and tool holder (19) being parts of the oscillation system (52) or forming it, and the oscillation exciting device (17) exciting the oscillation system (52) with its natural frequency or approximately its natural frequency. The invention also concerns a corresponding method and a machine tool (16) with a corresponding drive device (15).

Description

Drive device for a tool holder and method of driving it The invention

concerns a drive device for a tool holder of a machine tool, particularly an electrical hand machine tool, preferably an electrical padsaw, with a drive and a movement device to move the tool holder backward and forward periodically.

Prior art

Such a drive device for a tool holder is known. A movement device generates a periodic backward and forward movement of a tool holder from a drive movement of a drive. In the case of a drive in the form of an electric motor, this drive movement is a rotational movement, which for instance is transmitted via a connecting rod of a movement device onto a linearly guided element, which is connected to the tool holder, of the movement device. With such a translation of a rotational movement into a linear movement, a driving force component which is perpendicular to the guidance direction cannot be used to generate the backward and forward movement. Simultaneously, additional energy is used to move the driven parts of the movement device, since in the case of a mechanical system of this kind, friction forces occur in the drive and at the joints and guide, so that electrical energy is converted into heat energy. This energy is not available for work on the workpiece by the tool. Such drive devices are therefore inefficient, and unsuitable for operation in a battery-operated hand machine tool.

Disclosure of the invention

Technical object The efficiency of the drive device is to be increased.

Technical solution To achieve this object, it is provided that the drive is in the form of an oscillation exciting device to excite the oscillations of an oscillation system which form the backward and forward movement, the movement device and tool holder being parts of the oscillation system or forming it, and the oscillation exciting device exciting the oscillation system with its natural frequency or approximately its natural frequency. The oscillation system is a mechanical oscillation system which oscillates autonomously, e.g. by deflecting a mechanically oscillating part of the oscillation system. Because the oscillation is damped, e.g. by friction forces, a maximum deflection (amplitude) of the oscillation system decreases more and more over time, so that the oscillation system finally reaches its static zero position. To counteract this damping, the oscillation system can be excited in phase with its natural frequency or approximately its natural frequency, so that the amplitude of the oscillation is held constant or increased. In the case of excitation with the natural frequency of the oscillation system, the energy which is necessary to maintain the oscillation amplitude, and is fed to the oscillation system by the oscillation exciting device, is only as great as is necessary to overcome the oscillation damping. An example of a mechanical system which is capable of oscillation is a so-called "heavy pendulum" with a body which has mass, which is carried so that it can be swivelled out of its centre of gravity, and which is deflected out of its static resting position, then to oscillate backward and forward around its static zero position at its natural frequency, driven by a component of the force of gravity opposed to the direction of movement of the mass.

Advantageous effects In particular, it is provided that the oscillation exciting device is in the form of a linear electric motor, particularly a voice coil motor. If a linear electric motor is used to excite the oscillations which form the backward and forward movement, the driving forces which occur are used completely or almost completely for conversion into the oscillation movement, if both movements are aligned in the same way. The voice coil motor consists of a permanent magnet with a rotationally symmetrical magnet yoke, which generates a radial magnetic field. In an annular air gap of the magnet yoke, an electrical coil moves axially backward and forward in the radial magnetic field, because of the periodically changing alignment of the current. In the case of the oscillation exciting device, this movement is used to excite the oscillation system, the coil being connected to an oscillating component of the oscillation system. Such a drive device has a low power consumption, and is therefore particularly suitable for an electrical hand machine tool which is powered from at least one electrical storage device, preferably a battery. Because such an electrical storage device can store only a finite quantity of electrical energy, efficient exploitation of this energy for working on the workpiece is specially important.

Alternatively to using a linear electric motor, in particular a linear pneumatic motor or linear hydraulic motor can be used.

According to a further development of the invention, it is provided that the movement device has a guidance device to guide the tool holder along a movement path. Because of a guidance device, which guides the tool holder, in particular when work is being done on the workpiece, transverse forces which are introduced via the tool can be absorbed by this guidance device.

It is also provided that the oscillation system has at least one resetting element to reset or support the resetting of the oscillation system into an equilibrium position (zero position) . To achieve a sufficiently large resetting force, the oscillation system has at least one resetting element, which guides the oscillation system back into the equilibrium position.

In particular, it is provided that the resetting element is in the form of a flexible spring arm. A flexible spring arm is characterized by high carrying power and simple construction. In particular, if the flexible spring arm is in the form of a spring arm of leaf spring type, it demonstrates its elastic properties only in a preferred direction, whereas in the other directions it has supporting properties.

According to a further development of the invention, it is provided that the spring arm is aligned with its longitudinal axis perpendicular or almost perpendicular to the movement path. Because of a perpendicular or almost perpendicular alignment of the spring arm, the latter is aligned so that a preferred resetting direction runs along the movement path of the tool holder.

It is also provided that the guidance device has the resetting element to guide the tool holder. The resetting element is aligned within the guidance device so that a preferred resetting direction is aligned along the movement path of the tool holder.

According to a further development of the invention, it is provided that the guidance device is in the form of a parallel guidance device. In particular, by a parallel guidance device, a guidance device which has the resetting element also as a guidance element can be implemented. For instance, the parallel guidance can be formed of multiple resetting elements which are aligned in parallel and of the same form, these resetting elements being preferably flexible spring arms.

In particular, it is provided that the parallel guidance device is formed in one piece, particularly of steel. Steel is characterized by high strength and relatively low weight, and is therefore specially suitable for forming spring arms which are characterized by high elasticity in the guidance direction and high rigidity in the transverse direction. Because of the one-piece form of the parallel guidance device,the weight of the oscillation system can be reduced so much that the parallel guidance device can carry out an oscillation with a guidance stroke (oscillation amplitude of the tool holder) of about 14 mm at a frequency -e.g. 50 Hz 2 Hz -and also ensures sufficient resetting forces.

It is also provided that the tool is a part of the oscillation system and contributes to determining its natural frequency. Because the tool is also moved and has a mass, the tool contributes to determining the oscillation properties of the oscillation system.

According to a further development of the invention, it is provided that when work is being done on a workpiece using the tool, friction occurs and damps the oscillations of the oscillation system. Whereas in idle running, i.e. in the absence of reciprocal effect of the tool and a workpiece, the oscillation system is mainly damped by air friction, when work is being done on the workpiece using the tool it is mainly the friction between tool and workpiece which determines the damping.

The invention also concerns a machine tool, particularly an electrical hand machine tool, preferably an electrical padsaw, with a drive device according to the corresponding claims. The machine tool is suitable for tools of which the working movement is a backward and forward movement.

The invention also concerns a method of driving a tool holder, with a movement device for moving the tool holder backward and forward periodically. It is provided that the movement device and tool holder are parts of an oscillation system or form the oscillation system, which is excited by an oscillation exciting device to an oscillation, which forms the backward and forward movement of the tool holder with its natural frequency or approximately its natural frequency. The oscillation system is a weakly damped oscillation system while avoiding unnecessary damping, and its oscillation amplitude (maximum deflection of the oscillation system) can in particular be kept constant by excitation by means of an oscillation exciting device. The energy which the oscillation exciting device supplies to maintain the backward and forward movement in the case of idle running with no loading because of friction between tool and workpiece is relatively low. The necessary energy for working on a workpiece can be very high, depending on the material and the kind of work.

Finally, it is provided that the oscillation exciting device is in the form of a linear electric motor, particularly a voice coil motor, which excites the oscillation system by its drive movement in the direction of the oscillation or approximately in the direction of the oscillation. If the excitation takes place in the direction of the oscillation, the excitation energy is efficiently transmitted to the oscillation system.

Brief description of the drawings

The invention is explained in more detail on the basis of the figures, in which: Fig. 1 shows a schematic representation of a drive device which is not according to the invention, Fig. 2 shows a schematic representation of a drive device according to the invention for a tool holder, Fig. 3 shows a block diagram of a mechanical oscillation system, Fig. 4 shows a diagram of necessary driving forces of an oscillation exciting device over excitation frequencies, Fig. 5 shows a first embodiment of a drive device according to the invention, and Figs. 6 to 11 show further embodiments of a drive device according to the invention.

Embodiment(s) of the invention Fig. 1 shows, in a schematic representation, the structure of a movement device 1 which is not according to the invention, for converting a rotational movement (arrow 2) of a drive (not shown) into a backward and forward movement (arrow 3) of a tool holder (not shown) . On the drive shaft 4 of the drive (not shown), a first end 5 of a lever 6 is arranged, so that the lever 6 is driven by the drive so that it rotates in the movement direction (arrow 2) around the drive shaft 4. At an end 7 of the lever 6 opposite the end 5, the lever 6 is connected articulatedly to an end 8 of a connecting rod 9, the end 10 of which opposite the end 8 is guided on a linear guide 11. In this way the rotational movement (arrow 2) of the movement device 1 is translated into the linear movement (arrow 3) . The driving force (arrow 12) which the drive (not shown) supplies is determined by the torque of the drive (not shown) and the length and alignment of the lever 6. However, when the rotational movement (arrow 2) is converted into the linear movement (arrow 3), only the component (arrow 13) of the driving force (arrow 12) which is aligned parallel to the linear guide 11 and resulting linear movement (arrow 3) is used. A transverse component (arrow 14) which is aligned perpendicularly to this component (arrow 13) of the driving force (arrow 12) is aligned perpendicularly to the linear guide 11, and is not used to generate the linear movement (arrow 3) Fig. 2 shows, in a schematic representation, a drive device according to the invention of a machine tool 16, with a drive 18 in the form of an oscillation exciting device 17, and a movement device 1 to move a tool holder 19 backward and forward periodically. In this schematic representation, the machine tool 16 shown in Fig. 2 has a supporting element 21 in the form of an angle 20. On the first arm 22 of the angle 20, the oscillation exciting device 17 in the form of a voice coil motor 23 is arranged. The voice coil motor 23 is a linear electric motor 67, and consists of a rotationally symmetri.cal magnet yoke 24, with a permanent magnet which is arranged magnetically axially. In an annular air gap of the magnet yoke, an electrical coil 25 moves axially backward and forward in the radial magnetic field, because of the periodically changing alignment of the current. The longitudinal axis 26 of the coil 25 coincides with the axis of the magnet yoke 24. The electrical coil 25 is carried so that it can move along its longitudinal axis 26. An end area 27 of the electrical coil extending out of the magnet yoke 24 is connected to an end 28 of a displacement element 29, which has the tool holder 19 at an end 30 opposite the end 28. The tool holder 19 receives a tool 32 in the form of a padsaw 31. Between a mounting area 47 which is arranged on the second arm 33 of the supporting element 21 in the form of an angle 20 and the displacement element 29 which is arranged in parallel, there are two resetting elements 36, 37, which are arranged in parallel and in the form of flexible spring arms 34, 35 (particularly spring arms in the form of leaf springs) . The spring arms 34, 35 are arranged and rigidly fixed at their first ends 38, 39 to the ends 28, 30 of the displacement element 29 and perpendicularly to it. The second ends 40, 41 of the spring arms 34, 35, opposite the first ends 38, 39, are fixed rigidly to the mounting area 47 (or directly to the second arm 33 of the angle 20), the spring arms 34, being arranged perpendicularly to the longitudinal alignment of the second arm 33. In the area of the displacement element 29, there is a position capturing device 42, which is connected rigidly to the supporting element 21. The position capturing device 42 is connected via an electrical line 43 to a controller 44 of a control unit 45, said controller 44 determining the power of an amplifier 46 of the control unit 45. This amplifier 46 is connected to the coil 25 of the voice coil motor 23 via electrical lines 43. The mounting area 47, the resetting elements 36, 37 in the form of spring arms 34, 35, and the displacement element 29 form a guidance device 49 in the form of a parallel guidance device 48.

The following function of the drive device 15 shown in Fig. 2 results: the voice coil motor 23, by supplying current periodically to the electrical coil 25, causes a periodic backward and forward movement (double arrow 50), and thus drives the displacement element 29. The displacement element 29, together with the tool holder 19, the tool 32 and partly the spring arms 34, 35, has the moved mass M of an oscillation system 52. The moved mass N is guided by the guidance device 49 on a movement path 51. The movement path 51. is a mainly linear up and down movement which is specified by the flexibility of the spring arms 34, 35, and which has a sideways component depending on the ratio of the length of the spring arms 34, 35 to the oscillation amplitude of the oscillation system 52 in the area of the reversal points of the backward and forward movement. The backward and forward movement is therefore a movement consisting of a linear backward and forward movement and a backward and forward swivelling movement. In this embodiment, the displacement element 29, the tool holder 19 and the tool 32 are always guided parallel to the second arm 33 of the supporting element 21. Feeding back a signal of the position capturing device 42 to the control unit 45 results in a servo loop, in which the controller 44, from the signal of the position capturing device 42, by comparison with a setpoint value, outputs a control signal for the amplifier 46, and thus determines the driving force of the voice coil motor 23. By this regulation, in particular the amplitude of the oscillation of the oscillation system 52 can be regulated to be constant, irrespective of how strongly the oscillation system 52 is damped. The damping of the oscillation system 52 consists of the friction damping of the air friction, and -in the case of work loading -the friction in the reciprocal effect of the tool 32 and a workpiece (not shown). If it is only the very small air friction which determines the damping of the oscillation system 52, only a little energy must be applied to maintain the oscillation amplitude in idle running. To limit the maximum deflection of the oscillation system 52, the first arm 22 has a stopper 53. A transverse element 54 of the displacement element 29 extends into a recess 55 of the stopper 53. An upper limit stop 56 and a lower limit stop 57 of the recess 55 limit the movement path 51 of the oscillation system 52 and/or of the tool 32.

Fig. 3 shows a block diagram of an oscillation system for damped constrained oscillation. The oscillation system consists of an oscillating mass body 58 of mass M, a spring 59 with spring constant K, and an oscillation damper 60, the friction occurring with a constant friction force with damping constant X. The mass body 58, or its centre of gravity 61, oscillates backwards and forwards with reference to a fixed element 62 (double arrow 63) . In the shown situation, the centre of gravity 61 is shifted in the direction of the fixed element 62 by the path length s relative to the equilibrium position (zero position with s = 0, drawn dashed) of the centre of gravity 61. The oscillation system 64 is excited to oscillate by a driving force (arrow 65), which is applied, for instance, to the mass body 58, so that a damped constrained oscillation occurs. The alignment of a driving force (arrow 65), which is applied, for instance, by an oscillation exciting device (linear electric motor 67), is aligned parallel to the oscillation alignment (double arrow 63) . In this schematic drawing, the oscillation damper 60 is shown as a gas or liquid damper, in which a piston displaces a medium, so that friction occurs.

The oscillation properties of the real oscillation system 52 can be determined through the simplified oscillation system 64. The oscillation system 64, consisting of the mass body 58, the spring 59 and the oscillation damper 60, has a natural frequency f0, or a corresponding angular frequency (*)o, where = 2rvfo, where w02 = K/M Because of the damping by the oscillation damper 60, the oscillation system 64 carries out a damped (harmonic) oscillation, the amplitude of this oscillation -i.e. the maximum deflection -decreasing exponentially over time.

The driving force (arrow 65), which is necessary to generate a constrained oscillation with the frequency f at which the oscillation amplitude of the oscillation system 64 does not decrease but remains constant, depends on the frequency f of the excitation and the natural frequency fo.

This required force F(f) is shown in a diagram of Fig. 4.

On the ordinate of the diagram, the force F is shown in newtons (N), and on the abscissa, the frequency f is shown in hertz (Hz). The curve F(f) shows the course of the required driving force to obtain a constant amplitude Z0 in a frequency range from 0 to 60 Hz. The curve F(f) shows a pronounced minimum at a frequency of about 42 Hz, this frequency corresponding to the natural frequency fo of the oscillation system 52, 64. At lower and higher frequencies f, the curve of the required driving force F(f) rises. The curve F(f) is given mathematically by the formula F(f) z42(1_f2/f + . (.fl which results from the movement equation Ms+As+ Ks = F(t) with mass M, viscosity X, spring constant K and deflection s, and their first and second derivative with respect to time.

At the natural frequency f0, the force F(f) is equal to the friction force which results from the damping constant 2 and the amplitude Zo of the medium in the oscillation damper, according to the equation F(f0) = Z0A0 The friction force is proportional to the velocity of the piston in the oscillation damper 60, and/or of the mass body 58. In the case of a voice coil motor 23, the required current is proportional to the obtained force, so that for the required power P(f) = (F(f)/E)2R where E is the force characteristic (force constant) of the voice coil motor in newton/ampere (N/A) units, and R is the electrical resistance. This required power P(f), like the exciting force F(f), has. a minimum at the natural frequency f0, where R RK 22 P(f0) = -ZO2ZTO = Zo and depending on the time t I \ R 2' / \\2 R 2 2 Pktf)=-.A. .0Z cospf) = _. , *v(t) Figs. 5 to 9 show several embodiments of the drive unit, essentially corresponding to the schematic representation of Fig. 2. Therefore, only the differences will be considered here. Fig. 5 shows a three-dimensional representation of the drive device 15 which is shown schematically in Fig. 2, but without the control unit 45, the position capturing device 42 and the stopper 53.

Fig. 6 shows an alternative embodiment of the drive device 15. The displacement element 29 is not arranged directly on the first ends 38, 39 of the flexible spring arms 34, 35.

Between the displacement element 29 and the first ends 38, 39 of the spring arms 34, 35, a fixing element 69 in the form of a T-shaped fixing element 68 is arranged. It is connected at the ends 70, 71 of the T transverse stroke 72 rigidly to the spring arms 34, 35, and at the end 73 of the T longitudinal stroke 74 rigidly to the displacement element 29, which is aligned transversely to the T longitudinal stroke 74. The spring arms 34, 35 each have, in a central section 75, 76 of their longitudinal extent, an opening 77, 78, through which the displacement element 29 passes without contact. By this arrangement, all components of the oscillation system 51 are arranged in one plane 79, so that possible transverse forces are minimised and/or compensated for by the large degree of symmetry relative to the plane 79.

The versions of the embodiment of Fig. 6 shown in Figs. 7 to 9 differ only in the form of the fixing element 69. The fixing element 69 shown in Fig. 7 is in the form of an L-shaped fixing element 80. The fixing element 69 shown in Fig. 8 is in the form of an F-shaped fixing element 81, and the fixing element 69 shown in Fig. 9 is in the form of an E-shaped fixing element 82. A feature of the spring arms 34, 35 of the embodiments of Figs. 6 to 9 is that the forces which occur act on the central sections 75, 76 of the spring arms 34, 35. In the case of stress, the spring arms 34, 35 have the lowest mechanical tensions in the centre of their longitudinal extent.

Fig. 1OA shows a further embodiment of the drive device 15 according to the invention. It corresponds to Fig. 5 apart from the form of the resetting elements 36, 37 of the drive device 15. The resetting elements 36, 37 are in the form of blade-like arms 83, 84 which are arranged in parallel and have at their ends 38, 39, 40, 41 flexible end areas 85, 86, 87, 88, which are used in pairs as resetting elements 36, 37. The cross-section of the flexible end areas 85, 86, 87, 88 can be in rectangular form as shown in Figs. lOA and lOB, or alternatively in cylindrical or elliptical form.

Analogously to Figs. 6 to 9, a fixing element 49 can create the connection between ends 38, 39 of the arms 83, 84 with the displacement element 29. Fig. 108 shows the form of the flexible end area 87 in an enlarged section.

In Fig. 11, a drive device 15 of which the guidance device 49 is in the form of a parallel guidance device 48 is shown, the resetting element 89 being separate from the guidance device 49. The very rigid arms 90, 91 of the parallel guidance device 48 are of rigid form, and at their ends 38, 39, 40, 41 are connected articulatedly via the joints 92, 93, 94, 95 to the mounting area 47 and displacement element 29. The displacement element 29 has, at its end 28, a transversely arranged bracket 96, which runs parallel to the first arm 22 of the supporting element 21. Between the bracket 96 and the first arm 22, a resetting element 89 in the form of a helical spring 97 is arranged. The helical spring 97 is supported at one end on the underside 100 of the first arm 22, and at its second end 101 on an end area 102 of the bracket 96. The result is the following function: whereas the parallel guidance device 48 guides the displacement element 29 and thus the tool 32 on the movement path 51, the helical spring 97 is essentially used only as a resetting element 89 for the oscillation system 52. In the case of the embodiment of Fig. 11 too, the displacement element 29 -analogously to Figs. 6 to 9 -can be fixed on a fixing element 69.

Claims

Claims 1. Drive device for a tool holder of a machine tool,

particularly an electrical hand machine tool, preferably an electrical padsaw, with a drive and a movement device to move the tool holder backward and forward periodically, characterized in that the drive (18) is in the form of an oscillation exciting device (17) to excite the oscillations of an oscillation system (52) which form the backward and forward movement, the movement device (1) and tool holder (19) being parts of the oscillation system (52) or forming it, and the oscillation exciting device (17) exciting the oscillation system (52) with its natural frequency or approximately its natural frequency.

2. Drive device according to Claim 1, characterized in that the oscillation exciting device (17) is in the form of a linear electric motor (67), particularly a voice coil motor (23).

3. Drive device according to one of the preceding claims, characterized in that the movement device (1) has a guidance device (49) to guide the tool holder (19) along a movement path (51) 4. Drive device according to one of the preceding claims, characterized in that the oscillation system (52) has at least one resetting element (36, 37, 89) to reset or support the resetting of the oscillation system (52) into an equilibrium position.

5. Drive device according to one of the preceding claims, characterized in that the resetting element (36, 37) is in the form of a flexible spring arm (34, 35).

6. Drive device according to one of the preceding claims, characterized in that the spring arm (34, 35) is aligned with its longitudinal axis perpendicular or almost perpendicular to the movement path (51) 7. Drive device according to one of the preceding claims, characterized in that the guidance device (49) has the resetting element (36, 37) to guide the tool holder (19) 8. Drive device according to one of the preceding claims, characterized in that the guidance device (49) is in the form of a parallel guidance device (48).

9. Drive device according to one of the preceding claims, characterized in that the parallel guidance device (48) is formed in one piece, particularly of steel.

10. Drive device according to one of the preceding claims, characterized in that the tool (32) is a part of the oscillation system (52) and contributes to determining its natural frequency.

11. Drive device according to one of the preceding claims, characterized in that when work is being done on a workpiece using the tool (32), friction occurs and damps the oscillations of the oscillation system (52) 12. Machine tool, particularly an electrical hand machine tool, preferably an electrical padsaw, with a drive device according to one of the preceding claims.

13. Method of driving a tool holder, with a movement device to move the tool holder backward and forward periodically, characterized in that the movement device and tool holder are parts of an oscillation system or form the oscillation system, which is excited by an oscillation exciting device to an oscillation, which forms the backward and forward movement of the tool holder, with its natural frequency or approximately its natural frequency.

14. Method according to one of the preceding claims, characterized in that the oscillation exciting device is in the form of a linear electric motor, particularly a voice coil motor, which excites the oscillation system by its drive movement in the direction of the oscillation or approximately in the direction of the oscillation.

15. A drive device for a tool holder substantially as herein described with reference to the accompanying drawings.

16. A method of driving a tool holder substantially as herein described.