EP2636483A1

EP2636483A1 - Hand-held machine tool for sanding, grinding or polishing a workpiece

Info

Publication number: EP2636483A1
Application number: EP12158590.5A
Authority: EP
Inventors: Guido Valentini
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-03-08
Filing date: 2012-03-08
Publication date: 2013-09-11

Abstract

The invention refers to a hand-held machine tool (1) for sanding, grinding or polishing a workpiece (10), the tool (1) comprising
- a motor with a motor shaft adapted for performing a rotational movement around an axis of rotation,
- a working element (2) adapted for performing an orbital movement (3), and
- means (21, 22, 23) for transforming the rotational movement of the shaft into the orbital movement (3) of the working element (2),
- the working element (2) comprising a planar base plate (6) made of a rigid material, a planar absorption plate (7) attached to the base plate (6) and made of a resilient material, and an abrasive or polishing sheet (8) attached to the absorption plate (7).

The planar base plate (6) seen in a plan view has at least three external corners (9) allowing sanding, grinding or polishing operation in a hard-to-reach internal corner (15) or along an inner edge (14) of the workpiece (10). In order to avoid a damage of the workpiece (10) during use of the tool (1), it is suggested that the planar absorption plate (7) has at least one bumper element (16) integrally formed therewith and externally covering the base plate (6) in the range of at least one of its external corners (9), in order to avoid the workpiece (10) being contacted and damaged by the base plate (6) during its orbital movement (3) while sanding, grinding or polishing the workpiece (10) in its hard-to-reach internal corner (15) or along its inner edge (14).

Description

The present invention refers to a hand-held machine tool for sanding, grinding or polishing a workpiece, the machine tool comprising

a motor with a motor shaft adapted for performing a rotational movement around an axis of rotation,
a working element adapted for performing an orbital movement, and
means for transforming the rotational movement of the shaft into the orbital movement of the working element.

The working element comprises from top to bottom a planar base plate made of a rigid material, a planar absorption plate attached to a bottom surface of the base plate and made of a resilient material, and an abrasive or polishing sheet attached to a bottom surface of the absorption plate. The planar base plate seen in a plan view has at least three external corners allowing sanding, grinding or polishing operation, in particular in a hard-to-reach internal corner or along an inner edge of the workpiece. Reference is made to a machine tool which, for example but not limited to, is an orbital sander, grinder or polisher in which the working element, comprising a planar absorption plate or a pad, respectively, capable of receiving an abrasive or polishing sheet, is eccentrically mounted in respect to the axis of rotation of the motor shaft. Rotation of the motor shaft is transformed into an orbital movement of the working element by means of transforming means. These can be some kind of a gear mechanism or, for example but not limited to, an eccentric attachment, eccentrically guiding an eccentric pin connected to the working element. Depending on how the working element is guided in the machine tool it performs an orbital or a rotary orbital or a random orbital movement. In the present invention only machine tools with an orbitally moving working element are considered. An orbital movement means that the working element moves two-dimensionally in its plane of extension, i.e. forward and backward as well as to both sides, whereby a rotation of the working element is inhibited, that is the rotational position or the orientation of the working element within its plane of extension remains essentially the same. Hand-held machine tools of the above identified kind are well known in the prior art. A problem arising from their use for sanding, grinding or polishing a working surface of a workpiece in internal corners and/or along inner edges of the workpiece is that the resilient absorption plate is laterally compressed by a part of the workpiece extending essentially perpendicular to the workpiece's working surface and that the rigid base plate may come into contact with the workpiece, in particular with the part of the workpiece extending essentially perpendicular to the workpiece's working surface. Due to the oscillating movement of the working element at a couple of 1,000 orbits per minute, there may be a vibrating contact between the rigid base plate and the workpiece. This can cause severe damages to the workpiece, in particular in the range of the part of the workpiece extending perpendicular to the working surface. The damages can be, for instance, scratches or dimples in the workpiece. If the working surface is a window frame and the part of the workpiece extending perpendicular to the working surface is a window pane made of glass, the damages can even result in a cracking or a breakage of the glass.
WO 02/098607 A1 discloses a machine tool of the above identified kind with a base plate having a protection element externally overlapping a lateral extension of the absorption plate along its entire circumference, in order to protect the resilient material of the absorption plate from wear. The protection element is necessarily made of rigid material in order to fulfill its protective purpose. Therefore, the protection element cannot prevent damage of the workpiece if the machine tool is used on a working surface of a workpiece in internal corners and/or along inner edges of the workpiece, because during operation of the machine tool the workpiece will be contacted by the protection element, that is by a rigid and hard part of the working element.
In view of the prior art the object of the present invention is to propose an orbital machine tool which can prevent damage of a workpiece when used for working the workpiece in internal corners and along inner edges of the workpiece.
In accordance with the present invention this object is achieved by a machine tool according to the preamble of claim 1, wherein the planar absorption plate has at least one bumper element integrally formed therewith and externally covering the base plate in the range of at least one of its external corners, in order to avoid the workpiece being contacted and damaged by the base plate during its orbital movement while sanding, grinding or polishing the workpiece in its hard-to-reach internal corner or along its inner edge.
The present invention is in particular directed to orbital sanders, grinders or polishers with a planar working element performing an orbital movement during operation of the tool. The working element comprises a planar base plate made of a rigid material having at least three external corners allowing sanding, grinding or polishing operation in a hard-to-reach internal corner or along an inner edge of the workpiece. In order to avoid that the rigid base plate, in particular one of its external corners, comes into contact with a part of the workpiece, the present invention suggests to externally cover at least one of the base plate's external corners by at least one bumper element integrally formed with the planar absorption plate. By integrally forming the at least one bumper element with the absorption plate the resilient material characteristics of the absorption plate can advantageously also be used for the damping and absorbtive characteristics of the bumper element when the working element comes into contact with a part of the workpiece during sanding, grinding or polishing operation in the hard-to-reach internal corner or the inner edge of the workpiece.
It is important that the protective bumper element covers at least one of the external corners of the rigid base plate because a user during sanding, grinding or polishing operation in the hard-to-reach internal corner or inner edge of the workpiece preferably enters the workpiece's internal corner and/or runs along the inner edge of the workpiece with the machine tool orientated such that one of the base plate's external corners is directed towards the workpiece's internal corner or its inner edge. Therefore, by protecting the rigid base plate with the protective bumper element in the range of at least one of its external corners, the usability of the machine tool is significantly enhanced by reliably preventing damages to the workpiece during sanding, grinding or polishing operation in the hard-to-reach internal corner or along the inner edge of the workpiece.
Of course, it is also possible to provide further bumper elements also integrally formed with the planar absorption plate and externally covering a lateral surface of the base plate between the base plate's external corners, preferably at least a part of the base plate's lateral surfaces. These further bumper elements provided along the lateral extension of the base plate's side surfaces can additionally enhance the usability of the machine tool, in particular when the user sands, grinds or polishes a workpiece along an inner edge of the workpiece, whereby the external lateral surface of the base plate or the working element, respectively, is guided along the workpiece's edge. The further bumper element impedes a direct contact between the lateral surface of the rigid base plate with a part of the workpiece.
Integrally forming the protective bumper element with the planar absorption plate has the advantage that the bumper element can be manufactured and provided along at least part of the lateral extension of the base plate without any additional production, manufacturing or assembling steps. In particular, the protective bumper element can be manufactured together with the absorption plate in a single injection die moulding step. Of course, it is possible that the planar absorption plate is made of a multi-component material, in particular of a two-component material. This makes it possible to optimise the characteristics of the planar absorption plate and the integral bumper elements. For example, a centre or middle part of the absorption plate could be made of a first material whereas the outer marginal parts or the bumper elements of the absorption plate are made of a second material. The first material can be stiffer or less flexible than the second material. Alternatively, it is also possible that the planar absorption plate is made of a first resilient material and the at least one bumper element integrally formed therewith is formed of a different second material.
The planar absorption plate is preferably made of some kind of artificial or plastic material, in particular of a foamed polyurethane. Further, the planar base plate is preferably made of a rigid material, like artificial or plastic material, for example an aliphatic polyamide, a glass fibre reinforced aliphatic polyamide, polycaprolactam (PA6), glass fibre reinforced plycaprolactam, hexamethylene diamine and adipic acid (PA66), glass fibre reinforced hexamethylene diamine and adipic acid, IXEF®, glass fibre, or metal, for example aluminium. Of course, the various materials explicitly mentioned above for the rigid material of the planar base plate are merely examples and by no means intended to restrict the variety of materials which can be possibly used. The base plate's external corners made of rigid material can provoke serious damages to the workpiece upon contact, in particular if the contacted parts of the workpiece are made of glass or similarly sensitive or delicate material. Therefore, the present invention with the protective bumper elements integrally formed with the absorption plate is particularly advantageous in connection with a base plate made of rigid material.
According to another preferred embodiment of the present invention, the working element seen in a plan view has the form of a triangle, a rectangle or a square. In particular, it is suggested that the working element has a so-called delta shape seen in a plan view having the form of a triangle with the lateral surfaces between the base plate's external corners arcuated to the outside. Usually, a machine tool with a triangle or delta shape has one of its external corners directed to the front when the machine tool is held by a user in the intended operation position in respect to the workpiece, whereby the working element's external corner pointing forward projects beyond the machine tool's housing, thereby allowing the user to conveniently reach an inner edge or an internal corner of the workpiece during sanding, grinding or polishing operation in the hard-to-reach workpiece sections. Hence, preferably the base plate's external corner pointing forward is externally covered by a protective bumper element integrally formed with the absorption plate. Preferably, the planar absorption plate has a larger bottom surface than the bottom surface of the planar base plate. With other words, it is preferred that the plane dimensions of the resilient absorption plate exceed the plane dimensions of the rigid base plate. In that case it is possible to design the protective bumper elements and the further bumper elements in such a way that they have the form of a wall element extending essentially vertically upwards from the external boundary areas of the planar absorption plate. The wall element has essentially the same thickness along its entire vertical extension. This has the advantage that the protective bumper elements and further bumper elements have essentially the same damping or absorption characteristics along their entire vertical extension. This is important in order to assure the same protective characteristics of the bumper elements or the further bumper elements for their entire height.
Further, it is suggested that the planar absorption plate is releasably attached to the planar base plate. Similarly, the abrasive or polishing sheet is releasably attached to the planar absorption plate. The planar absorption plate can be releasably attached to the planar base plate and/or the abrasive or polishing sheet can be releasably attached to the planar absorption plate in many different ways including, for example, a glue type or adhesive type connection. Preferably, the planar absorption plate is releasably attached to the planar base plate and/or the abrasive or polishing sheet is releasably attached to the planar absorption plate by means of a hook and loop system fixed to at least part of the contact surfaces between the absorption plate and the base plate and/or the abrasive or polishing sheet and the absorption plate. A hook and loop system is also known as a Velcro® fastener type connection. Such a releasable connection allows a safe and reliable releasable connection and attachment of the various parts of the working element even in humid or dusty environments.
In an idle state and with no load applied to the working element, the working element has an orbital movement speed ranging from approximately 5,000 to approximately 15,000 orbits per minute. In the idle state of the working element this essentially corresponds to the rotational speed of the actuating motor or its rotating motor shaft, respectively. Hence, the machine tool according to the present invention has no breaking means acting on the working element. This rather high orbiting speed of up to 15,000 orbits per minute results in rather high-frequency vibrations which would lead to severe damages of the workpiece if the rigid base plate was not protected by the bumper elements and/or the further bumper elements as suggested by the present invention. Of course, other orbital movement speeds of the working element outside the range of 5,000 to 15,000 orbits per minute are possible, too.
Further characteristics and advantages of the present invention are described hereinafter with reference to the accompanying drawings. The drawings show a preferred embodiment of the present invention without, however, limiting the invention to the described embodiment. Rather, there are many possible alternative embodiments of the present invention besides the embodiment explicitly described hereinafter and shown in the figures. The figures show:

Fig. 1: a conventional orbital sander having a delta-shaped working element;
Fig. 2: the front part of the working element of the orbital sander of figure 1 in detail;
Fig. 3: an exemplary use of the orbital sander of figure 1 in a sanding operation in a hard-to-reach inner edge or internal corner of a workpiece;
Fig. 4: major problems during use of a conventional orbital sander according to figure 1 in the employment shown in figure 3;
Fig. 5: a sectional view of a working element of an orbital sander according to the present invention provided with a protective bumper element;
Fig. 6: a sectional view of a working element of an orbital sander according to the present invention provided with a further protective bumper element;
Fig. 7: the advantages of the orbital sander according to the present invention during use in the exemplary employment of figure 3; and
Fig. 8: a sectional view of part of an orbital sander according to the present invention.

With reference to the attached figures 1 to 4, and in particular to figure 1, a conventional hand-held machine tool 1 is shown. The tool 1 comprises a working element 2 adapted to perform an orbital movement indicated by circular arrows 3. An orbital movement 3 of the working element 2 means that it moves in two directions extending perpendicular to one another and defining the planar extension of the working element 2. The machine tool 1 is designed as an orbital sander but, of course, can be designed as any hand-held machine tool 1 comprising a working element 2 adapted to perform an orbital movement like, for example, an orbital grinder or an orbital polisher.
The machine tool 1 comprises a housing 4, preferably made of an artificial material, in particular a plastic material. The working element 2 is moveably connected to the housing 4 in order to allow an orbital movement 3 of the working element 2 in respect to the machine tool's housing 4. The housing 4 comprises in its inside a motor (not shown) for actuating the machine tool 1. The motor has a motor shaft adapted to perform a rotational movement around its axis of rotation. Furthermore, means (not shown) for transforming the rotational movement of the motor shaft into the orbital movement 3 of the working element 2 are provided inside the housing 4. The means for transforming the movement can be designed as any type of gear mechanism, including an eccentrical mechanism for transforming the motor shaft's rotational movement into the working element's 2 orbital movement 3. The working element 2 is guided in respect to the housing 4 such that it can only perform the orbital movement 3 into the directions essentially extending along the working element's 2 plane and not, for example, a rotating orbital or a random orbital movement around a working element's axis of rotation.
The housing 4 is formed such that it can be conveniently grasped by a user of the machine tool 1 with one or two hands. Hence, the housing 4 also serves as a grip or handle of the machine tool 1 for safely guiding the machine tool 1 into internal corners and/or along inner edges of a workpiece with a high precision.
The motor for actuating the machine tool 1 can be designed as any kind of electrical motor or a pneumatic motor, running by means of an air flow supplied to the machine tool 1 through an air-pressure conduit (not shown) from an air-pressure generator.
The working element 2 has a so-called delta-shape, which seen in the plan view of figure 1 corresponds to a triangle-shape with the edges arcuated to the outside. This kind of working element 2 is preferably used for working workpieces in hard-to-reach internal corners and/or along inner edges of the workpiece. The working element 2 is shown in detail in figure 2. It comprises from top to bottom a planar base plate 6 made of a rigid material like a hard plastic material or metal, in particular aluminium. The planar base plate 6 is a structural part of the tool 1. Therefore, it is made of a hard, rigid and strong material. Further, the working element 2 comprises a planar absorption plate 7 attached to a bottom surface of the base plate 6 and made of a resilient material, in particular of a flexible and soft plastic material, like foamed polyurethane. Finally, the working element 2 also comprises an abrasive or polishing sheet 8 attached to a bottom surface of the absorption plate 7. The sheets 8 are preferably releasably attached to the absorption plate 7, in order to allow for their easy replacement if they are consumed or worn out. Alternatively, instead of directly attaching the abrasive or polishing sheet 8 to the bottom surface of the absorption plate 7, it is possible to attach some kind of accessory thereto. For example, an accessory may be an attachment interface consisting of a thick polyurethane layer placed between a top layer of plush and a lower layer of Velcro®-fasteners. The top layer of plush serves for attaching the interface to the absorption plate 7, and the Velcro®-fasteners serve for removably attaching the abrasive or polishing sheet 8 for consumption to the interface.
Due to the delta-shaped working element 2, the planar base plate 6 seen in a plan view has three external corners 9 allowing sanding operation in a hard-to-reach internal corner or along an inner edge of a workpiece. Of course, the working element 2 and the planar base plate 6, respectively, can have any other desired shape besides the delta-shape shown, in particular a rectangular or a square shape. In that case the planar absorption plate 7 and the abrasive or polishing sheet 8 would be shaped essentially correspondingly.
The planar absorption plate 7 is preferably releasably attached to the planar base plate 6. Further, the abrasive or polishing sheet 8 is preferably releasably attached to the planar absorption plate 7. The abrasive or polishing sheet 8 is releasably attached to the planar absorption plate 7 preferably by means of a hook-and-loop system (Velcro®-fastener) fixed to at least part of the contact surfaces between the abrasive or polishing sheet 8 and the absorption plate 7.
The orbital sander's 1 working element 2 has an orbital movement 3 of up to approximately 15,000 orbits per minute or even more. This orbital movement 3 associated to the abrasive sheet 8 attached to the bottom surface of the absorption plate 7 makes the sanding operation. The movement of up to 15,000 orbits per minute is achieved during an idle state of the machine tool 1, that is with no load applied to the working element 2 by pressing the tool 1 onto a working surface 13. In the tool's 1 idle state the orbital movement of up to 15,000 orbits per minute essentially corresponds to the rotational speed of the tool's 1 motor and its motor shaft, respectively. Preferably, the machine tool 1 has no breaking means acting on the working element 2.
Figure 3 shows a preferred use of the machine tool 1 of figures 1 and 2. Due to the fact that the machine tool 1 has a working element 2 and a planar base plate 6, respectively, with several external corners 9 it can be advantageously used for sanding operation in a hard-to-reach internal corner and/or along a hard-to-reach inner edge of the workpiece. In figure 3 the workpiece is a window 10 comprising a glass pane 11 and a window frame 12 surrounding and holding the glass pane 11. The window frame 12 can be made of wood, plastic or metal. The machine tool 1 can be used for sanding the window frame 12 in particular in its reveal 13. The reveal 13 of a window frame 12 comprises the surfaces extending essentially perpendicular to the planar extension of the glass pane 11. A window 10 mounted into an appropriate opening of a building usually has an inner reveal 13' at the inside of the building and an external reveal 13" at the outside of the building.
The window 10 comprises hard-to-reach inner edges composed of the reveal 13 abutting against the glass pane 11 and/or by a first (horizontal) reveal 13 abutting against an adjacent (vertical) reveal 13. These edges are indicated with reference signs 14' and 14", respectively. Furthermore, the window 10 comprises hard-to-reach internal corners, which are composed, for example, by the glass pane 11 and two adjacent reveals 13. In figure 3 an example for such an internal corner is indicated with reference sign 15. In particular, the internal corner 15 is defined by an intersection of a horizontal edge 14', a vertical edge 14' and another horizontal edge 14". The inner edges 14 and the internal corners 15 of the window 10 are preferably worked with a machine tool, like the orbiting delta sander 1 shown in figures 1 to 4.
A problem of the conventional orbital sanders 1 of figures 1 to 4 is explained hereinafter in detail with reference to figure 4. When the machine tool 1 is guided along an inner edge 14 or into an internal corner 15 of the window 10 during sanding operation of the reveal 13 of the window 10, the working element 2 abuts against the glass pane 11 extending essentially perpendicularly in respect to the worked surface, i. e. the reveal 13. Due to the fact that the absorption plate 7 has a larger planar dimension than the base plate 6 with a border of the absorption plate 7 laterally extending beyond the base plate 6, a first slight contact of the working element 2 with the glass pane 11 is not critical, because the exceeding border of the absorption plate 7 acts as a damping member. However, due to the soft and resilient characteristics of the material of the absorption plate 7, it may well be and in practice often happens when the machine tool 1 is pressed against the workpiece 10 with a slightly higher pressure that the border of the absorption plate 7 is deformed and compressed so far that an outer lateral surface 19 of the base plate 6 comes into contact with the glass pane 11. This can cause severe damages to the glass pane 11 like scratches, cracks or other damages. Hence, with the conventional orbital sanders 1 it is difficult to provide for a complete sanding of the entire reveal 13 surface right up to the glass pane 11 without damaging the glass pane 11. The collission between the base plate 6 and the glass pane 11 and the base plate's 6 impact onto the glass pane 11 is indicated with reference sign 27.
Therefore, according to the present invention and as shown in figure 5, it is suggested that the planar absorption plate 7 has at least one bumper element 16 integrally formed therewith and externally covering the base plate 6 in the range of at least one of its external corners 9. This avoids the workpiece 10 from being directly contacted and damaged by the base plate 6 during its orbital movement while sanding the workpiece 10 in its hard-to-reach internal corners 15 and/or along its inner edges 14. Now, those parts of the rigid base plate 6 which during sanding, grinding or polishing operation in the hard-to-reach internal corners 15 or along the inner edges 14 of the workpiece 10 run the risk of directly contacting and damaging the workpiece 10 are provided with shock absorbing means in form of the protective bumper element 16. By virtue of the bumper element 16 the external corners 9 of the base plate 6 as well as lateral surfaces 19 of the base plate 6 directly connecting to the corner 9 are prevented from directly contacting and damaging the workpiece 10, for example from contacting and damaging a glass pane 11 of a window 10 when working on a surface 13 of the window frame 12. The protective bumper element 16 is preferably made of a resilient, flexible and soft material which can damp and absorb oscillating vibrating movements of the working element 2 and the base plate 6, respectively, while laterally in contact with the workpiece 10.
Preferably, the bumper element 16 covers that external corner 9 of the working element 2 and the base plate 6, respectively, which during operation and intended use of the machine tool 1 is directed forwards, that is into the inner edge 14 and/or the internal corner 15 of the workpiece 10. Of course, one or more of the other corners 9 of the working element 2 and the base plate 6, respectively, could also be covered by a bumper element 16 correspondingly.
Further, the invention is not limited to one or more bumper elements 16 covering one or more external corners 9 of the base element 6. Additionally, there may be one or more further bumper elements like further bumper element 17 shown in figure 6. The further bumper element 17 is integrally formed with the absorption plate 7 and covers externally the base plate 6 in the range of at least one of its edges 18 or lateral surfaces 19 between two of its corners 9. The further bumper elements 17 can avoid the workpiece 10 being contacted and damaged by the lateral surfaces 19 of the base plate 6 during its orbital movement 3 while sanding the workpiece 10 in its hart-to-reach internal corner 15 or along its inner edge 14. Of course, the further protective bumper element 17 can have any longitudinal extension and can be located at any desired position along the base plate's 6 edges 18 and/or lateral surfaces 19. For the sake of a better clearness, in figure 6 the bumper element 16 covering an external corner 9 of the base plate 6 of figure 5 is not shown. Of course, the machine tool 1 according to the present invention and the working element 2, respectively, can be provided with both one or more bumper elements 16 covering one or more corners 9 of the base plate 6 as well as with one or more further bumper elements 17 covering edges 18 and/or lateral surfaces 19 of the base plate 6.
The planar absorption plate 7 can be made of a multi-component material, in particular of a two-component material. It is possible that the at least one bumper element 16 and/or the at least one further bumper element 17 is made of a different material than the rest of the planar absorption plate 7.
Preferably, the planar absorption plate 7 has a larger bottom surface than the bottom surface of the planar base plate 6. This allows a special construction of the bumper elements 16 and the further bumper elements 17 in that they have the form of a wall element extending essentially vertically upwards from the laterally protruding border portions of the planar absorption plate 7 and having essentially the same thickness along their entire vertical extension. In particular, that part of the bumper element 16 and/or the further bumper element 17, which is located on the outside of the base plate's 6 corners 9 and/or lateral surfaces 19, has a constant thickness along its entire vertical extension. This means that the damping and/or absorption characteristics of the bumper element 16 and/or the further bumper element 17 are the same for the entire lateral external surfaces 19 and the external corners 9 of the base plate 6.
During use of the machine tool 1 dust is created by the sanding, grinding or polishing operation. The bumper element 16 of figure 5 comprises dust suction ducts 20 through which dust can be aspired from the working surface 13 of the workpiece 10. Of course, the further bumper element 17 could be provided with one or more corresponding dust suction ducts 20, too, although not shown in figure 6.
The use of the machine tool 1 according to the present invention during a sanding operation in a hard-to-reach internal corner 15 and/or along an inner edge 14 of a workpiece 10 is shown in detail in figure 7. However, the invention is not limited to sanding operation. The explanations apply for grinding and polishing just the same. Again, the workpiece 10 is exemplary shown as a window comprising a window frame 12 and a glass pane 11. It can be well seen in figure 7 that during the intended use of the machine tool 1 according to the present invention the working element 2 at the most abuts against the glass pane 11 with the resilient bumper element 16. Accordingly, if an edge 18 and/or a lateral surface 19 of the base plate 6 would be directed towards the glass pane 11 during sanding operation along an inner edge 14 of the workpiece 10, at the most the working element 2 would come into contact with the glass pane 11 with one or more of the resilient further bumper elements 17 disposed laterally along the lateral surface 19 of the working element's 2 base element 6 but not shown in figure 7. Hence, the bumper elements 16 and/or the further bumper elements 17 can safely and reliably avoid direct contact between the base plate 6 and certain parts of the workpiece 10, for example a glass pane 11, during working operation in a hard-to-reach internal corner 15 and/or along an inner edge 14 of the workpiece 10.
Finally, figure 8 shows a sectional view through part of the machine tool 1 according to the present invention. It can be clearly seen that approximately at the center of the rigid base plate 6 there is a ball bearing 23 for rotatably holding a part 21 of the eccentrical mechanism. Due to the bearing 23, part 21 can be rotated with low friction in respect to the base plate 6. Part 21 comprises an opening 22, into which an eccenter shaft (or eccentric pin) can be introduced and fixed in such a way, that a rotation of the eccenter shaft in respect to part 21 is not possible or only possible with a very high friction force. The eccenter shaft is preferably located with its longitudinal axis spaced apart from the rotational axis of the motor shaft and rotating eccentrically around the motor shaft's rotation axis. The eccenter shaft makes part of the eccentrical mechanism, which transforms the rotational movement of the motor shaft around its rotational axis into the orbital movement 3 of the working element 2and the base plate 6, respectively. A low-friction rotation of the eccenter shaft in respect to the base plate 6 is achieved by the bearing 23.
The absorption plate 7 comprises in its inside rigid support members 24, which give the resilient, flexible and soft material of the absorption plate 7 a certain degree of stiffness and which allow a better and safer fixing of the absorption plate 7 to the base plate 6, for example by means of screws or other fastening elements. The support members 24 can be made of an artificial material, in particular a plastic material, being less flexible (having a higher rigidity) than the material of the remaining part of the absorption plate 7 surrounding the support members 24. The entire absorption plate 7 comprising the two different material components can be manufactured in a single injection die moulding step.
Further, it can be well seen in figure 8 that the bumper element 16 is located externally from an external corner 9 of the base plate 6, as described above. Similarly, although not shown in figure 8, the further bumper element 17 is located along at least part of a lateral external surface 19 of the base plate 6, as described above.
Furthermore, one or more suction holes 25 are provided in the absorption plate 7, which open into corresponding parts of a suction channel 26 designed in the base plate 6. The suction holes 25 and the suction channels 26 serve for absorbing dust from the working surface 13 created during the sanding, grinding or polishing operation of the machine tool 1. The absorbed dust is aspired by the tool's 1 means for dust extraction including a rotating fan with a plurality of fins. The aspired dust is discharged through a vent 5 and conveyed to a dust collection or filtering unit by means of an extraction hose (not shown).

Claims

Hand-held machine tool (1) for sanding, grinding or polishing a workpiece (10), the machine tool (1) comprising
- a motor with a motor shaft adapted for performing a rotational movement around an axis of rotation,

- a working element (2) adapted for performing an orbital movement (3), and

- means (21, 22, 23) for transforming the rotational movement of the shaft into the orbital movement (3) of the working element (2),

- the working element (2) comprising a planar base plate (6) made of a rigid material, a planar absorption plate (7) attached to a bottom surface of the base plate (6) and made of a resilient material, and an abrasive or polishing sheet (8) attached to a bottom surface of the absorption plate (7), wherein the planar base plate (6) seen in a plan view has at least three external corners (9) allowing sanding, grinding or polishing operation in a hard-to-reach internal corner (15) or along an inner edge (14) of the workpiece (10),
characterized in that the planar absorption plate (7) has at least one bumper element (16) integrally formed therewith and externally covering the base plate (6) in the range of at least one of its external corners (9), in order to avoid the workpiece (10) being contacted and damaged by the base plate (6) during its orbital movement (3) while sanding, grinding or polishing the workpiece (10) in its hard-to-reach internal corner (15) or along its inner edge (14).
Machine tool (1) according to claim 1, wherein the working element (2) seen in a plan view has the form of a triangle, a rectangle or a square.
Machine tool (1) according to claim 2, wherein the planar absorption plate (7) has at least one further bumper element (17) integrally formed therewith and externally covering the base plate (6) in the range of at least one of its lateral surfaces (19) between two of its external corners (9), in order to avoid the workpiece (10) being contacted and damaged by the base plate (6) during its orbital movement (3) while sanding, grinding or polishing the workpiece (10) in its hard-to-reach internal corner (15) or along its inner edge (14).
Machine tool (1) according to claim 2 or 3, wherein the working element (2) has a delta shape seen in a plan view having the form of a triangle with the lateral surfaces (19) arcuated to the outside.
Machine tool (1) according to one of the preceding claims, wherein the planar base plate (6) is made of metal, in particular aluminium.
Machine tool (1) according to one of the preceding claims 1 to 4, wherein the planar base plate (6) is made of an artificial or plastic material, in particular a glass fibre reinforced artificial or plastic material.
Machine tool (1) according to one of the preceding claims, wherein the planar absorption plate (7) is made of an artificial material, in particular of a foamed polyurethan.
Machine tool (1) according to one of the preceding claims, wherein the planar absorption plate (7) has a larger bottom surface than the bottom surface of the planar base plate (6).
Machine tool (1) according to claim 8, wherein the at least one bumper element (16) and/or the at least one further bumper element (17) has the form of a wall element extending essentially vertically upwards from the planar absorption plate (7), the wall element having essentially the same thickness along its vertical extension.
Machine tool (1) according to one of the preceding claims, wherein the planar absorption plate (7) is releasably attached to the planar base plate (6).
Machine tool (1) according to one of the preceding claims, wherein the abrasive or polishing sheet (8) is releasably attached to the planar absorption plate (7).
Machine tool (1) according to claim 10 or 11, wherein the planar absorption plate (7) is releasably attached to the planar base plate (6) and/or the abrasive or polishing sheet (8) is releasably attached to the planar absorption plate (7) by means of a hook and loop system fixed to at least part of the contact surfaces between the absorption plate (7) and the base plate (6) and/or the abrasive or polishing sheet (8) and the absorption plate (7).
Machine tool (1) according to one of the preceding claims, wherein the planar absorption plate (7) is made of a multi-component material, in particular of a two-component material.
Machine tool (1) according to claim 13, wherein the at least one bumper element (16) and/or the at least one further bumper element (17) is made of a different material than the rest of the planar absorption plate (7).
Machine tool (1) according to one of the preceding claims, wherein the at least one bumper element (16) and/or the at least one further bumper element (17) comprises at least one dust suction duct (20).