Patent Publication Number: US-7223161-B2

Title: Cutting apparatus with dust discharging

Description:
FIELD OF THE INVENTION 
   The present invention relates to a cutting apparatus used for at least one of cutting and grinding of an object to be cut, such as concrete blocks and stone works, and particularly to hand-held cutters for cutting an object and to a hand-held sander for grinding an object. 
   BACKGROUND OF THE INVENTION 
   Cutting apparatuses including a circular blade (disk cutter) for cutting objects and sanders for grinding a surface of an object such as concrete blocks are known. Japanese Laid-open patent application publication No. 2000-210866 discloses such a sander (see pages 1 to 2, FIGS. 1–2). This conventional sander is used in such a manner that one side of rotating cutting edges of a circular blade is pressed on the surface of the object. The sander mainly includes a body having a driving power source for a circular blade (disc cutter), a cutting unit, mounted on the body, having the circular blade, a dust cover (chip guard) for covering the circular blade, and a dust discharging unit (chip discharging unit) attached to the cutting unit for sucking chips (dust) in the dust cover and externally discharging the dust. 
   More specifically, in this prior art, the dust cover is fixed to a lower part of the body with screws or the like, and an output shaft of a drive power source protrudes into the dust cover to provide a mounting structure for detachably mounting the circular blade on output shaft within the dust cover. The dust cover is at an opening edge thereof provided with a brush. When edges of the circular blade are pressed to the surface of an object, the brush closes an open space between the surface of the object and the brush. This prevents chips (dust) from being scattered. Thus, the chips are discharged to a predetermined dust collecting bag (filter) by the dust discharging unit without scattering. 
   However, in the structure of the sander disclosed in Japanese Laid-open patent application publication No. 2000-210866, because the pulley and the flat belt is exposed to the inside of the dust cover, there is a problem that the fine chips (powder dust) within the dust cover may stick to these members, which can cause slippage therebetween. 
   Further, since the bodies and the circular blades are general members, there are various types of products having various configurations on market. On the other hand, due to a lack of compatibility, the dust cover is typically specially designed for each type of the sander. 
   In the dust cover disclosed in Japanese Laid-open patent application publication No. 2000-210866, there is a problem that the heights of the dust covers from the body to the edge portion are the same, so that there is no compatibility with other circular blades (disk cutters) having different sizes. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention provides a cutting apparatus comprising a dust cover and a dust discharging unit for sucking chips (dust particles) within the dust cover (chip guard), further comprising an inner cover for covering a pulley and belt for transmitting power at an output shaft, connectable to a circular blade, of the cutting apparatus to the dust discharging unit attached. Further, a gap between the output shaft side and a through hole allowing the output shaft or the like to protrude from the inner cover is sealed with a seal member comprising felt. 
   This structure can prevent fine chips (powder dust) from sticking to the flat belt or the pulley. The sealing member is made of felt with (1) a higher flexibility than the case, such as rubber or plastic, so that it has a superior sealing characteristic because a contact part with a peripheral surface of the output shaft or the rotation member tends to be fit to a configuration of this peripheral surface. Further, this structure provides (2) heat resistance against rotation friction, and (3) resistance to wear against the rotation friction. 
   According to the present invention, sticking of dust to the flat belt or the pulley within the dust cover can be prevented. This can also provide a high durability to the seal member. 
   A further aspect of the present invention provides a hand-held cutting apparatus comprising: a body including a driving power source having an output shaft coupled to the driving power source; a cutting unit comprising; mounting means for detachably mounting a circular blade (disk cutter) on the output shaft; and a dust cover including an opening edge facing an object of cuts for covering the circular blade and the mounting means; a dust discharging unit attached to the cutting unit for sucking dust particles within the dust cover and externally discharging the dust particles; and an adjusting mechanism including a contact edge member contactable with an object of cuts around an opening edge of the dust cover for making the contact edge member automatically movable along an axial direction of the output shaft in accordance with a pushing reaction force received from the object as a result of pushing the cutting apparatus on the object. 
   According to the sander with the structure mentioned above as a cutting apparatus, preferably, the adjusting mechanism automatically absorbs the difference in height between the dust cover side and the circular blade side. Thus, the dust cover having a specific design can fit bodies and circular blades having a plurality of different heights. 
   The adjusting mechanism may comprise a surface touching member, including the contact edge, movable along an axial direction of the output shaft and a pushing mechanism for always pushing the grounding member toward a side of the object. The pushing member may comprise a structure reciprocally movable in accordance with a relation in magnitude between a pushing force of the pushing member and the pushing reaction force from the object. 
   According to the sander with this structure, the adjusting mechanism has a simple structure, so that a low cost sander is provided because of ease in assembling. 
   A further aspect of the present invention provides a cutting apparatus comprising: a pulley fixed to the output shaft engaged with a flat belt for transmitting power to the dust discharging unit; and an adjuster ring (cylinder) fixed to the output shaft for adjusting a height of the circular blade. The adjuster ring has a diameter larger than that of the pulley, wherein a difference in the diameter between the adjuster ring and the pulley functions as a guard for preventing the flat belt from detaching from the pulley. 
   According to the sander having this structure, a simple sleeve member without the guard can sufficiently operate as a pulley having a guard. For example, if the pulley is formed as a metal member subjected to a cutting process, a guard-making process is unnecessary, and thus the manufacturing process can be simplified because the adjuster ring has a diameter larger than that of the pulley. 
   A further aspect of the present invention provides the cutting apparatus further comprising a spacer provided between the body and the dust cover to prevent interference between the part of the dust discharging unit and the body. A part of the dust discharging unit may be arranged adjacent to a side of the body. Thus, the position of the body in height can be made different from the position of the part of the dust discharging unit, which can prevent interference therebetween. In general cutting processes, it is not common to change the body with respect to the same dust cover. Thus, the structure with the spacer does not largely affect the efficiency of the cutting process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object and features of the present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a perspective view, partially a cutaway view, of a sander according to the present invention; 
       FIG. 2  is a perspective view of the sander in a de-assembled condition according to the present invention; 
       FIG. 3  is a side sectional view, taken along line A 1 —A 1  in  FIG. 1 , mainly illustrating a body and a cutting unit; 
       FIG. 4  is a side sectional drawing viewed from B in  FIG. 1 , mainly illustrating a dust discharging unit; 
       FIG. 5  is a partial perspective drawing viewed from a side of an opening of a dust cover of the sander according to the present invention, illustrating a first power transmission section; 
       FIG. 6  is an enlarged view of an adjusting mechanism shown in  FIG. 2 ; 
       FIG. 7A  is a side sectional view illustrating the adjusting mechanism of the sander according to the present invention before contact with workpiece; 
       FIG. 7B  is a side sectional view illustrating the adjusting mechanism of the sander according to the present invention during touching the workpiece; 
       FIG. 8  is a perspective view of a cutter according to the present invention; and 
       FIG. 9  is a perspective view illustrating a structure inside the dust cover of the cutter according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Prior to describing embodiments of the present invention, the prior art described earlier will be further explained. 
   In the sander having the structure described previously, it is desired that a dust cover having one set of design dimensions (the same height) can fit bodies or circular blades having a plurality of specifications in dimension in consideration of management of products and costs. In other words, in a sander, its dust cover is required to fit bodies or circular blades manufactured by other makers. However, actually, in the conventional sanders, the length of the output shaft of a driving source may be different from those of other sanders, and the height (dimension along the axial direction of the driving source) of the circular blade of one sander may be different from those of other sanders. More specifically, if there is a difference in height between the tips of the circular blade and the opening edge (an edge of the opening) of the dust cover of the sander, there is a problem that the difference cannot be readily adjusted. 
   In this case, a countermeasure may be considered, in which a spacer for adjusting the height is provided around the output shaft. However, in this case, the structure around the mounting member of the disk cutter becomes complicated and the corresponding space is required. Further, for example, every exchange between circular blades having different heights along the output shaft correspondingly requires a spacer replacement operation. This decreases the cutting operation efficiency. 
   The present invention is provided to resolve the above-mentioned problems and to provide a sander capable of fitting cutters having different heights in the axial direction of the output shaft of a driving source. More specifically, there is provided a hand-held cutting apparatus comprising an adjusting mechanism comprising a contact edge member contactable with an object of cuts around an opening edge of the dust cover for making the contact edge member movable along an axial direction of the output shaft in accordance with a reaction force received from the object in accordance with pushing force of the cutting apparatus onto the object. 
   In the structure of the sander disclosed in Japanese Laid-open patent application publication No. 2000-210866, the pulley and the flat belt are exposed to the inside of the dust cover, chip particles (powder dust) within the dust cover may stick to these members and thus causes slippage between the pulley and the flat belt. To overcome this problem, for example, a structure may be considered in which another dust cover is separately provided for covering only the pulley and the flat belt. 
   However, in this structure, the pulley is also fixed to the output shaft for supporting the circular blade and thus, to cover the pulley, a through hole should be formed to allow the output shaft or a rotation member fixed to the output shaft to protrude from the cover. This structure is problematic because a gap between the dust cover and the output shaft may allow the powder dust to enter the side of the pulley. 
   The present invention is provided to solve such a problem. More specifically, the present invention provides, in a cutting apparatus comprising a pulley mounted on an output shaft within the dust cover and a flat belt partially wrapped around the pulley, a cutting unit capable of efficiently preventing the powder dust from sticking to these members. 
   In other words, there is provided a hand-held cutting apparatus comprising a dust cover, a dust discharging unit, a pulley fixed to the output shaft engaged with a flat belt for transmitting power to the dust discharging unit. An inner cover includes a through hole allowing at least one of the output shaft and a rotation member fixed to the output shaft to protrude from the inner cover through the though hole, arranged in the dust cover, for contactlessly covering the flat belt and the pulley. A seal comprises felt for sealing a gap between at least one of the output shaft and the rotation member and the through hole, to prevent the dust particles from entering the side of the pulley. 
   Two embodiments will be described. A first embodiment describes the cutting apparatus as a sander mainly used for sanding a surface of an object such as a concrete surface. The second embodiment describes the cutting apparatus mainly used as a cutter for cutting an object, such as a concrete block, to have a groove therein. 
     FIGS. 1 to 7  are provided to describe the first embodiment.  FIG. 1  is a perspective view, partially a cutaway view, of a sander.  FIG. 2  is a perspective view of the sander in a de-assembled condition.  FIG. 3  is a side sectional view, taken along line A 1 —A 1  in  FIG. 1 , mainly illustrating a body and a cutting unit.  FIG. 4  is a side sectional drawing viewed from B in  FIG. 1 , mainly illustrating a dust discharging unit.  FIG. 5  is a partial perspective drawing viewed from a side of opening of a dust cover of the sander according to the present invention, illustrating a first power transmission section  16 A.  FIG. 6  is an enlarged view of an adjusting mechanism  31  shown in  FIG. 2 .  FIG. 7A  is a side sectional view illustrating the adjusting mechanism  31  of the sander according to the present invention before touching an object.  FIG. 7B  is a side sectional view illustrating the adjusting mechanism  31  of the sander according to the present invention during contact with the object. 
   First Embodiment 
   In  FIG. 1 , the hand-held sander A comprises a body  1 , a cutting unit  2  mounted on an under part of one side of the body  1 , and a dust discharging unit  3  attached or connected to the cutting unit  2  and arranged adjacent to a side of the body  1 . 
   The body  1  is provided with a handle  4  to be gripped by an operator at the opposite side to the side of the body  1 . The dust discharging unit  3  comprises a discharging sleeve  5  connected to a dust collecting bag (not shown). The body  1  comprises an electric motor  6  as a driving power source of the cutting unit  2  and an output shaft  7  connected or coupled to the electric motor  6 . 
   Cutting Unit  2   
   Cutting edges  8   b  comprise carbide tools, grindstones, a diamond wheel, or the like. At a center of a top part of a circular blade (disk cutter)  8 , a mounting hole  8   a  is formed for mounting the circular blade  8  on the output shaft  7 . 
   The configuration of the circular blade  8  is not limited to that shown in the drawing. For example, the circular blade  8  may comprise an entirely flat plate. The dust cover  10  comprises a circumferential wall  10   b  upwardly extending from a circular opening edge  10   a , a ceiling section  10   c  connected to an upper edge of the circumferential wall section  10   b , formed in a conical configuration, and a protruding section  10   d , protruding from the ceiling section  10   c , extending from a peripheral of the center of the ceiling section  10   c  to a part of a peripheral edge of the ceiling section  10   c . The circumferential wall  10   b , the ceiling section  10   c , and the protruding section  10   d  are integrally formed to provide the dust cover  10  by molding, for example, by casting aluminum alloy. 
   Around the center of the ceiling section  10   c , on the upper surface of the protruding section  10   d , a through hole  10   e  is formed to allow the output shaft  7  to enter the inside of the dust cover  10  through the through hole  10   e  when the dust cover  10  is fixed to an under part of the body  1  with bolts (not shown). 
   The output shaft  7  is processed to have an external thread to which a pulley  17 , forming the first power transmission section  16 A, is fixed with a nut  11 . 
   Next, a mounting member  9  will be described, mainly referring to  FIG. 3 . The mounting member  9  comprises an adjuster ring (cylinder)  12  and a fastening member  13 . The adjuster ring  12  is formed to have a through hole  12   a  extending vertically in the drawing. The adjuster ring  12  serves to adjust the position of the circular blade  8  in height relative to the dust cover  10  and is in contact with the lower end of the pulley  17 . The fastening member  13  is inserted into the through hole  12   a  of the adjuster ring  12  from the lower side and comprises a sleeve section  13   a  provided at an upper end side with an internal thread and a bolt head  13   b  formed at the lower end of the sleeve section  13   a.    
   An example of steps of installing the circular blade  8  on the output shaft  7  is as follows: 
   First, the upper end surface of the adjuster ring  12  is placed to come into contact with the lower end of the pulley  17 . Then, the circular blade  8  is placed to come into contact with the lower end surface of the adjuster ring  12  with alignment between the mounting hole  8   a  with the through hole  12   a . The sleeve section  13   a  of the fastening member  13  is inserted into the mounting hole  8   a  from the lower side of the mounting hole  8   a  to couple an internal screw to the external screw of the output shaft  7 . Then, the bolt head  13   b  is rotated by a predetermined tool to be fixed to the output shaft  7 , so that the circular blade  8  is pinched between the lower surface of the adjuster ring  12  and the bolt head  13   b.    
   Discharging Unit  3   
   As shown in  FIGS. 2 and 4 , a discharging hole  10   f  is formed in the ceiling section  10   c  of the dust cover  10  to which the dust discharging unit  3  is attached with bolts or the like (not shown) so as to face the discharging hole  10   f.    
   As shown in  FIG. 4 , the dust discharging section  3  is provided with a ventilation hole  5   a  facing the discharging hole  10   f  and comprises the discharging sleeve  5  fixed to the dust cover  10  and a fan  14  within the discharging sleeve  5  located just above the discharging hole  10   f.    
   The fan  14  provides dust discharging means for sucking powder dust or dust particles within the dust cover  10  by rotation thereof to the discharging sleeve  5  through the discharge hole  10   f  and the ventilation hole  5   a  to discharge the powder dust or exhaust a air flow containing dust particles to the dust collecting bag. The fan  14  is at its rotation shaft  14   a  rotatably supported by the discharging sleeve  5  via a bearing  15 . 
   Here, there may be a case that a concrete piece having a size of, for example, about several millimeters may exist within the dust cover  10 . When this piece is sucked into the dust sleeve  5 , it may generate a large noise because of hitting the fan  14  or may stop the rotation of the fan  14  due to the piece becoming pinched between the main wall of the discharging sleeve  5  and the fan  14 . Then, at least one of the discharging hole  10   f  and the ventilation hole  5   a  is provided with a dust net to prevent the large dust pieces from entering the dust discharging unit  3 . 
   In this embodiment, as shown in  FIG. 6 , a dust net  10   k  having a grid mesh is attached to the discharging hole  10   f . The material of the dust net  10   k  is made of aluminum alloy, iron, or the like. 
   Making the size of the mesh of the dust net  10   k  too small may damage the original dust discharging function or may cause clogging at the dust net  10   k . In this embodiment, as an example of the size, the mesh is made small down to about 3 mm square to such an extent that a dust particle having a dimension possible to stop the rotation of the fan  14  cannot pass therethrough. 
   Further, in accordance with other embodiments, instead of the structure in which the dust net  10   k  is mounted, at least one of the discharging hole  10   f  and the ventilation hole  5   a  may be formed to have a plurality of small holes to provide a function equivalent to the mesh of the dust net  10   k.    
   Further, the dust cover  10  may be occasionally provided with an intake hole  10   g  as shown in  FIG. 2 . The intake hole  10   g  is a hole for preventing the dust cover  10  from sticking on a surface of the workpiece due to a negative pressure inside the dust cover  10 . A flow of the air through the intake hole  10   g  suppresses generation of an excessive negative pressure. 
   Power Transmission Section 
   In this embodiment, the rotation force of the output shaft  7  is used as a rotation driving source for the fan  14 . Hereinafter, the power transmission section  16  from the output shaft  7  to a rotation shaft  14   a  of the fan  14  will be described. 
   As shown in  FIG. 3 , the power transmission means  16  comprises the first power transmission section  16 A inside the dust cover  10 , i.e., arranged inside the protrusion section  10   d  and a second power transmission section  16 B arranged above and outside the dust cover  10  as shown in  FIG. 4 . Both power transmission sections  16 A and  16 B comprise mechanisms including combinations of flat belts and pulleys for the flat belts. 
   The first power transmission section  16 A will be described. As shown in  FIG. 3 , the pulley  17  is fixed to the output shaft  7  with the nut  11 . A bearing member  18  is mounted on the upper surface of the protruding section  10   d  of the dust cover  10 , and rotatably supports a vertically arranged transmission shaft  19  through bearings  20 . The lower part of the transmission shaft  19  extends into the inside of the protruding part  10   d  and is provided with a pulley  21  at the lower end in which an endless flat belt  22  is wrapped between the pulley  21  and the pulley  17  at the side of the output shaft  7 . 
   A diameter of the upper end surface of the adjuster ring  12  that will be in contact with the lower end surface of the pulley  17  is set to have a larger diameter than that of the pulley  17 . According to this structure, the difference in the diameter from the adjuster ring  12  to the pulley  17 , i.e., a bump therebetween, functions as a guard for preventing dislocation or detachment of the endless belt  22 . 
   As described above, in the structure comprising the pulley  17  engaged with the flat belt  22  for transmitting power to the dust discharging unit, i.e., the fan  14  (see  FIG. 4 ) and the adjuster ring  12  for adjusting a position in height of the circular blade  8 , the larger diameter of the adjuster ring  12  than that of the pulley  17  as a guard for preventing dislocation of the endless belt  22 , allows the pulley  17  to have a simple sleeve member without an additional guard for the pulley  17 . 
   Thus, for example, if the pulley  17  is provided as a metal member subject to a cutting process, this structure will eliminate a step in the cutting process, which simplifies the manufacturing process. 
   As shown in  FIG. 4 , in the second power transmission section  16 B, a pulley  23  is fixed to an upper end of the transmission shaft  19 , wherein an endless belt  25  of a flat belt is wrapped between the pulley  23  and the pulley  24  rotatably fixed to the upper end of the rotation shaft  14   a . According to this structure, rotation force of the output shaft  7  is transmitted to the rotation shaft  14   a  of the fan  14  via the pulley  17  (see  FIG. 3 ), the endless belt  22 , the pulley  21 , the transmission shaft  19 , the pulley  23 , the endless belt  25 , and the pulley  24 . 
   As mentioned above, in using the rotation force of the output shaft  7  as rotation power for the fan  14  in the dust discharging unit, the use of the pulleys  17 ,  21 ,  23 , and  24  and the endless belts  22  and  25  for power transmission as the power transmission means  16  provides a simple structure to the sander A at a low cost. 
   Further, in this embodiment, as shown in  FIGS. 2 and 3 , around the opening edge  10   a  of the dust cover  10 , an adjusting mechanism  31  is provided to make a contact edge member movable in an axial direction of the output shaft in accordance with the pressure reaction force from the object. 
   As shown in  FIGS. 6 and 7 , the adjusting mechanism  31  in this embodiment comprises a surface touching member (a brush holder  33 ) formed along the opening edge  10   a  of the dust cover  10  movable in an axial direction of the output shaft  7  with a brush  32  (contact edge member) and energizing means  34  for always pressing the brush holder  33  toward the object side (spring-loading). 
   In this embodiment, the opening edge  10   a  is formed to have a circle edge and thus, the brush holder  33  is also made in a ring form to fit to this by molding with plastic or the like. 
   The lower end of the brush holder  33  is provided with the brush  32  at its lower edge. The brush  32  provides a function of sealing for preventing dust from being externally scattered when an operator presses the sander A to a concrete surface during cutting operation and smoothes the movement of the sander itself along the concrete surface. 
   The brush holder  33  comprises a guide wall  33   a  formed along the circumferential wall  10   b  of the dust cover  10  and vertically extending. The brush holder  33  can move in a sliding manner along the axial direction of the output shaft  7  (see  FIG. 7 ) with guidance by the circumferential wall  10   b  of the dust cover  10 . In the structure as mentioned above, there are cases that the brush holder  33  is fit inside the circumferential wall  10   b  of the dust cover  10  and outside the circumferential wall  10   b , respectively. The former allows powder dust within the dust cover  10  generated during cutting to enter a gap (sliding part) between the circumferential wall  10   b  and the guide wall  33   a . Thus, there is a possibility of plugging in the gap which may disable the brush holder  33  from smoothly sliding. On the other hand, the latter can more reduce the entrance of the powder dust into the gap than that of the former case because the gap exists outside the dust cover  10 .  FIGS. 6 and 7  show the structure of the latter. 
   At the lower edge on the inner peripheral surface side of the guide wall  33   a , a stepwise wall section  33   b  is annularly formed. The upper surface of the stepwise wall section  33   b  limits the upward movement of the brush holder  33  to the dust cover  10  by contact with the opening edge  10   a  of the dust cover  10 . 
   The stepwise wall  33   b  is locally provided with protrusion sections  33   c  extending toward the center of the brush holder  33 . The protrusion sections  33   c  are provided at a suitable interval in a circumferential direction of the brush holder  33 . In this embodiment, the protrusion section  33   c  are arranged at four places, each being provided with a spring containing section  33   d  formed to have a hollow part upwardly opening. 
   On the other hand, on the inner surface of the circumferential wall  10   b  of the dust cover  10 , protrusions  10   h  are formed at locations corresponding to the protrusions  33   c , respectively, in which hollow spring containing sections  10   i  downwardly opening are formed, respectively. 
   The energizing means  34  of this embodiment comprises a helical compression spring  35  at its lower end internally fit into and supported by the spring containing section  33   d , and the upper end is internally fit into the hollow spring containing section  10   i.    
   A holding mechanism  36  for holding the brush holder  33  with respect to the dust cover  10  will be described. In the guide wall  33   a  of the brush holder  33 , ellipses having major axes extending in the direction of the axis of the output shaft  7  (see  FIG. 7 ) are formed with a suitable equi-distant interval in the circumferential direction ( FIG. 6  illustrates a case that they are provided at three places). 
   At the positions corresponding to the guide holes  33  the circumference wall  10   b  of the dust cover  10  has external thread holes  10   j . The brush holder  33  is fit to the outside of the circumferential wall  10  so as to overlap the guide holes  33   e  with the external thread holes  10   j . In this condition, screwing external threads  37  into the internal thread holes  10   j  causes existence of tip sides of the external threads  37  within the guide holes  33   e , so that the brush holder  33  is movably supported by the dust cover  10  without detaching. 
   In other words, the external threads  37  serve as engagement pins preventing the dust cover  10  from detaching from the dust cover  10 . Further, the guide holes  33   e  and the external threads  37  provide a function for guiding the sliding movement of the brush holder  33  with respect to the dust cover  10  and a function for limiting movement in the circumferential movement. 
   Hereinafter, operation of the adjusting mechanism  32  will be described with reference to  FIGS. 7A and 7B . 
   In  FIGS. 7A and 7B , for convenience, the adjusting mechanism  31  is shown at the left of the dust cover in the drawing, and the holding mechanism  36 , at the right. 
     FIG. 7A  shows a condition prior to cutting operation in which the brush holder  33  is downwardly (in the drawing) energized by energizing force of the helical compression spring  35  (spring-loaded). Thus, the brush holder  33  is downwardly positioned apart from the dust cover  10 . 
   During this operation, a position of the brush holder  33  is determined by contact of the external threads  37  with the upper end of the guide holes  33 . 
   Inside the dust cover  10 , the cutting edges  8   b  of the circular blade  8  locate at an upper position than that of the lower end of the brush  32 .  FIG. 7A  shows the case that the tips of the cutting edges  8   b  locate at substantially the same height as the lower end of the brush holder  33 . In this case, in the conventional sander, because a distance in height between the lower end of the brush  32  and the tips of the cutting edges  8   b  is too large, so that the cutting operation is substantially impossible in this condition. 
   Here, to adjust the height a spacer might be provided between the adjuster ring  12  and the circular blade  8  or there might be provided adjuster rings  12  having different heights designed to have a function of the spacers. However, when a circular blade  8  is replaced with another circular blade  8  having a different height, the corresponding spacer or adjuster ring having the function of the spacer should be replaced, which is inconvenient. 
   On the other hand, according to the present invention, as shown in  FIG. 7B , when the brush  32  is pressed onto a surface of concrete or the like during the cutting operation, a reaction force from the surface of concrete against the pressure increases gradually, so that the brush  32  (brush holder  33 ) slides upwardly in accordance with the reaction force against the pressure. 
   More specifically, the lower end of the brush  32  moves automatically along the axial direction of the output shaft  7  in accordance with the reaction force against the pressure received from the surface of concrete. This provides the same height of the lower end of the brush  32  as the cutting edges  8   b  of the circular blade  8 . This enables cutting by the cutting edges  8   b  and prevents powder dust from scattering outside the dust cover  10  with the brush  32 . 
   Further,  FIG. 7B  shows the condition that the energizing force or spring load force generated by the helical compression spring  35  equals to the reaction force against the pressure. In this condition, when an operator lifts the sander, the energizing force generated by the helical compression spring  35  becomes larger than the reaction force against the pressure, which causes the brush holder  33  to move downwardly. 
     FIGS. 7A and 7B  show conditions where the circumference wall  10   b  of the dust cover  10  and the guide wall  33   a  of the brush holder  33  are in contact with each other. However, in fact, a slight gap for relief is provided therebetween. Thus, the brush holder  33  can be slightly inclined to the dust cover  10  by the distance of the gap. 
   Further, since a plurality of the helical compression springs  35  are provided in the circumference direction of circumference wall  10   b , when the sander is pressed to a concrete surface at an angle, the brush holder  33  inclines so as to follow the inclination. 
   Therefore, no partial gap is formed between the concrete surface and the dust cover  10 , which prevents powder dust from being externally scattered and provides air-tightness regarding the sucking force generated by the fan  14 . 
   As mentioned above, the adjusting mechanism  31  is provided around the opening edge  10   a  of the dust cover  10  to enable the contact edge (the lower end of the brush  32 ) to touch a workpiece such as a concrete surface to move the brush holder  33  automatically along the axial direction of the output shaft  7  in accordance with a reaction force against the pressure received from the object. Thus, the dust cover  10  having only one set of design sizes allows the bodies  1  having different lengths in the output shaft  7  and the circular blades  8  having different heights to be used. 
   Further, the adjusting mechanism  31  automatically absorbs the difference in height between the side of the brush  32  (the side of the dust cover  10 ) and the side of the disk cutter  8 . This eliminates operation of replacing the spacer. 
   Further, the adjusting mechanism  31  is provided with the surface touching member (brush holder  33 ) having the brush  32  movable (slidable) along the axial direction of the output shaft  7 , and the energizing means  34  always energizing the surface touching member  33  toward the object side, in which the surface touching member reciprocally shifts in accordance with the magnitude relation between the energizing force by the energizing means  34  and the reaction force against the pressure. This provides the adjusting mechanism  31  with a simple structure. 
   Further, as the energizing means  34 , in addition to the helical compression spring  35 , for example, a bellows structure having elasticity, or a similar configuration can used. 
   As shown in  FIG. 1 , in the sander A according to this embodiment, the dust discharging sleeve  5  which is a part of the dust discharging unit  3  is arranged adjacent to the side of the body  1 , and a part containing the fan  14  is located on the upper part of the dust cover  10 . Thus, the dust discharging sleeve  5  tends to interfer with the body  1 . Then, as shown in  FIG. 2 , between the body  1  and the dust cover  10 , a spacer  38  is provided to prevent the physical interference between the body  1  and the dust discharging sleeve. This differentiates the height of the body  1  along the output shaft  7  from that of the discharging sleeve  5 , which prevents the interference therebetween. 
   In a cutting operation, replacement of the body  1  is less frequent than the case that the circular blade  8  is replaced with respect to the dust cover  10 . Therefore, the structure including the spacer  38  does not affect the cutting operation. 
   As shown in  FIG. 3  or  5 , inside the dust cover  10 , there is provided an inner cover  28  covering the pulley  17  fixed to the output shaft  7 , the endless belt  22  engaged with the pulley  17  and having a through hole  28   a  allowing the output shaft  7  or a rotation member fixed to the output shaft  7  to protrude from the inner cover  28  therethrough. In this embodiment, the inner cover  28  is fit into the protruding section  10   d  so as to cover also the pulley  21  to seal the whole of the first power transmission section  16 A and detachably fixed to the dust cover  10  with a screw  29  (see  FIG. 5 ). The edge of the opening in the inner cover  28  touches the inner surface of the protruding section  10   d.    
   Further, in this embodiment, as mentioned above, the output shaft  7  is provided with the adjuster ring  12  (corresponding to the above-described rotation member), and thus the through hole  28   a  allows the adjuster ring  12  to protrude from the inner cover  28  therethrough. Thus, the inner diameter of the through hole  28   a  is made larger than that of the outer diameter of the adjuster ring  12 . 
   Here, in the case that the output shaft  7  protrudes through the through hole  28   a  itself (without the adjuster ring  12 ), the inner diameter of the through hole  28   a  has such a size that the output shaft  7  can be inserted. 
   The inner cover  28  has a function of blocking power dust filled inside the dust cover  10  during cutting. This prevents powder dust from sticking to the pulleys  17  and  21  and the endless belt  22 . 
   However, in fact, a gap around the adjuster ring  12  tends to allow powder dust to enter the inside the inner cover  28 . 
   According to the present invention, the gap around the adjuster ring  12  is sealed or closed with a seal member comprising felt to prevent powder dust to enter the inside through the gap. The felt is made by densification of wool to have a sheet configuration as described, for example, in the JIS (Japanese Industrial Standard) industrial term dictionary. 
   As shown in  FIG. 5 , an annular seal member  30  made of felt is mounted on the inner edge of the through hole  28   a . The inner circumferential edge of the seal member  30  touches an outer circumferential surface of the adjuster ring  12 . This prevents powder dust from entering the inside of the inner cover  28  by sealing the gap in the through hole around the adjuster ring  12 . 
   The seal member  30 , partially covering the through hole  28   a , fixed to the inner cover  28  around the through hole  28   a , is a member touching the circumferential surface of the rotation member rotating such as the output shaft  7  and the adjuster ring  12  or the like. Here, the seal member  30  comprises felt, which provides a higher flexibility than the case that the seal member was formed with rubber or plastic. More specifically, the felt has a low friction against a metal surface (a surface of the output shaft  7  or the adjuster ring  12 ), but rather higher friction against dust because of fibers in the felt. Further, the felt is suitably deformable to fit a surface configuration of the output shaft. Further, in addition to felt, a block comprising fibers can be used. Thus, the contact part of the seal member  30  comprising felt with the outer circumferential surface of the adjuster ring  12  or the like fits to the configuration of the circumferential surface. This provides a superior sealing characteristic, a superior heat resistivity against rotation friction, and a superior wear resistance against the rotation friction. 
   The seal member  30  is fixed to circumference of the through hole  28   a  with an adhesive or a structure in which a groove is formed around the inner circumference of the through hole  28   a , and the seal member  30  is fit into the groove. However, the seal can be fixed to the side of the output shaft  7 . 
   Second Embodiment 
     FIGS. 8 and 9  illustrate a second embodiment of a cutter.  FIG. 8  is a perspective view of the cutter, and  FIG. 9  is a partial perspective view illustrating a de-assembled condition, wherein a second cover, mentioned later, is removed. 
   In this embodiment, the same members as those described in the first embodiment are designated with the same references. Descriptions about portions other than the main portion will be omitted in the following description. 
   In the hand-held cutter C, a body  1  including the electronic motor  6  is so inclined that the axial direction of an output shaft  7  extends in the horizontal direction, and the tip of the output shaft  7  enters the inside of a dust cover  41  of the cutting unit  2 . 
   The dust cover  41  is provided with an opening edge  41   a  facing a workpiece. An outline configuration viewed from the axial direction of the output shaft  7  is substantially a semi-circle in which the opening edge  41   a  corresponds to the side of the semi-circle. 
   The dust cover  41  is dividable into a first cover  41 A and a second cover  41 B in the axial direction of the output shaft  7  which are combined with a plurality of bolts  42  as a unit of the dust cover  41 . The first cover  41 A and the second cover  41 B are molded with aluminum alloy by casting or the like. 
   The dust cover  41  contactlessly covers a circular blade  43  and a mounting member for detachably mounting the circular blade  43  on the output shaft  7 . The circular blade  43  has a flat disk configuration provided with a cutting edge  43   a  at the circumferential edge thereof, or a conical cap configuration having a relative low height which is mounted on the output shaft  7  and arranged vertically within the dust cover  41 , of which lower circumference protrudes from the opening edge  41   a.    
   Regarding a mounting member  9 , for example, the same structure as the mounting member of the first embodiment is sufficient, in which the circular blade  43  is mounted on the output shaft with an adjuster ring  12  by a fastening member  13 . 
   As shown in  FIG. 9 , inside the first cover  41 A, a side surface  41   b  of the first cover  41 A is provided with a hollow part  41   c  having a rectangular form. The hollow part  41   c  is provided with a hollow power transmission section containing compartment  41   d . The output shaft  7  is exposed in the power transmission section containing compartment  41   d  in which the pulley  17  is fixed to the output shaft by the nut  11 . 
   Further, the endless belt  22  is wrapped between the pulleys  17  and  21  to transmit the rotation force to the discharging unit section  3  (see  FIG. 8 ). This structure is the same as the structure as the first power transmission section  16 A shown in  FIG. 5 . 
   Further, in the side surface  41   b  of the first cover  41 A, a dust discharging hole  41   e  facing the dust discharging unit  3  is formed in which a dust net  41   f  is also mounted thereto for the same reason mentioned in the first embodiment to prevent larger dust particles from entering the dust discharging unit  3 . 
   The size of the mesh of the dust net  41   f  is determined so as to stop entrance of dust particles having such a size as to stop the rotation of the fan in the dust discharging unit  3  (not shown in  FIG. 8 ). 
   To cover the first power transmission section  16 A, an inner cover  44  having a flat square configuration is fit into the hollow section  41   c  and fixed to the first cover  41 A with a plurality of bolts  45 . Further, the seal member  30  comprising felt is provided to seal the gap around the adjuster ring  12  in the though hole  44   a.    
   The function of the seal member  30  is the same as that of the first embodiment. The advantageous effects mentioned above regarding the seal member  30  described in the first embodiment are also provided. 
   In  FIG. 9 , the inner cover  44  is used to cover the first power transmission section  16 A. However, the inner cover  28  may be used instead of the inner cover  44 . In this case, the inner cover  28  (see  FIG. 5 ) is fit into the power transmission section containing compartment  41   d  so as to cover the pulley  21  to seal the whole of the first power transmission section  16 A and detachably fixed to the dust cover  41  with the screw  29  (see  FIG. 5 ), and a corresponding internal screw hole (not shown) in the dust cover  41 . The edge of the opening in the inner cover  28  touches the bottom surface of the power transmission section containing compartment  41   d.    
   If the output shaft  7  protrudes through the through hole  28   a  with the adjuster ring  12 , the inner diameter of the through hole  28   a  has such a size that the output shaft  7  with the adjuster ring  12  can be inserted if the adjuster ring  12  is used. 
   The inner cover  28  has the function for blocking power dust filled inside the dust cover  41  during cutting. The seal member  30  fixed to the through hole  28   a  is a member touching the circumferential surface of the rotation member rotating such as the output shaft  7  and the adjuster ring  12  or the like. This prevents powder dust from sticking to the pulleys  17  and  21  and the endless belt  22 . The seal member  30  is fixed to the inner cover  28  around the through hole  28   a . In other words, the seal member  30  has a through hole therein allowing at least one of the output shaft  7  and the adjuster ring  12  to protrude therethrough and is fixed to the inner cover  28  around the through hole  28   a  of the inner cover  28 , the through hole in the seal member  30  being slidable relative to the output shaft  7 . 
   As mentioned above, preferred embodiments have been described. However, a layout, a shape, and the numbers of respective elements are not limited to those shown in the drawings. Particularly, as the pushing mechanism in the adjusting mechanism, plate springs, hinge springs, or rubber members are applicable.