Abstract:
A power grinder, in particular an oscillating grinder ( 10 ), includes a platelike grinding sheet holder ( 14 ), on whose working face ( 15 ) a grinding sheet ( 16 ) can be braced. Secure clamping of the grinding sheet and easy changing of the grinding sheet are provided, because the grinding sheet ( 16 ) can be inserted between the clamping jaw ( 22 ) and the grinding sheet holder ( 14 )—counter to the elastic prestressing of the clamping jaw ( 22 )—and is firmly held increasingly strongly counter to the insertion direction in proportion to the tensile force engaging it.

Description:
BACKGROUND OF INVENTION 
   The invention relates to a power grinder, in particular oscillating grinder with a platelike grinding sheet holder. 
   From British Patent GB 23 22 582, a pivotable pincer with a grinding plate for holding grinding sheets is known; the grinding sheet is braced with their back against the underside of the grinding plate and can be firmly clamped to its top. 
   For firmly clamping the grinding sheet, the user has to use both hands, and if at all possible the power tool should be placed on a firm support. One grinding sheet end has to be introduced with one hand into a slot between an opened clamping jaw and the top side of the grinding sheet holder, while the clamping jaw has to be kept open in the release position with the other hand until the grinding sheet end has been introduced. Once the clamping jaw is let go, it then closes by spring force and assumes its clamping position, in which it firmly clamps the grinding sheet end, on a skewed support face on the top of the grinding sheet holder. In the process, the grinding sheet is automatically retightened to a certain extent, because the rotatably suspended clamping jaw rolls outward along the skewed support face and carries the grinding sheet end along with it. 
   With the other grinding sheet end, the procedure until now was the same, but the clamping force and the retightening force were limited. 
   SUMMARY OF THE INVENTION 
   The power grinder of the invention has the advantage over the prior art that equipping it with new grinding sheets can be accomplished with only one hand, more easily and faster than in the tools known before. Also, because the clamping means allows the grinding sheet to enter in a preferential direction but does not release it in the opposite direction unless particular action is taken, it is possible for the grinding sheet to be clamped to the grinding sheet holder solidly and without play using only one h and. One-handed operation here means In fact only a single hand has to be active in order to insert and fix the grinding sheet. The other hand can rest in the meantime or fix the power tool, for instance pressing against a support. 
   Because the outer contour of the clamping jaw curves outward with an increasing radius of curvature, and in particular within the range of self-locking, so that the clamping force increases in proportion to the tensile force that tends to loosen the grinding sheet, the clamped grinding sheet is securely held firmly regardless of its thickness; that is, both thin and thick grinding sheets are held equally securely. Because of the firm clamping of the grinding sheet, the play between it and the grinding sheet holder is limited. Thus the oscillating grinding motion of the grinding sheet holder is also transferred virtually without slip to the grinding sheet, so that the grinding sheet can effectively engage a workpiece so as to grind it. 
   Because the clamping jaw has an outer contour whose greatest spacing from the pivot axis is less than the spacing between the pivot axis and the top side of the grinding sheet holder, and because the clamping jaw, toward the edge of the grinding sheet holder, braces itself in elastically rotationally prestressed fashion on one side of the grinding sheet holder, a high clamping force between the clamping jaw and the top side of the grinding sheet holder is established automatically, with a simple design of the clamping means, and a servo-clamping effect, which means that with increasing tensile force on the sandpaper counter to the insertion direction, the clamping force becomes greater and grater. As a result, the sandpaper can be clamped firmly practically up to the tearing limit without being able to come loose by itself. 
   Because the clamping jaw, to release the fastened grinding sheet, is movable counter to the clamping direction by means of a pushbutton, it is especially simple to release the grinding sheet end from the power grinder. 
   Because the clamping jaw comprises elastic, rubberlike material, the servo effect is reinforced for increasing clamping of the grinding sheet. 
   The servo effect is furthermore reinforced by the provision that the outer contour of the clamping jaw is progressively curved in saber-like fashion. 
   A further advantage is that after the grinding sheet has been clamped, its tensing or tautening can be increased simply by pushing it onward into the clamping slot; once a grinding sheet end has been inserted, the clamping device does not let it go again even if great force counter to the insertion direction is exerted, unless a release switch is actuated first. The farther the grinding sheet end is slipped under the clamping jaw, the greater the clamping surface area and thus the more forceful the clamping. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One exemplary embodiment of the invention is explained in further detail in the ensuing description in conjunction with the drawing. 
     Shown are 
       FIG. 1 , a schematic side view of the power grinder of the invention; 
       FIG. 2 , a plan view from above on the clamping means of the power grinder; 
       FIG. 3 , the detail of a single clamping jaw from above; 
       FIG. 4 , the saber-like outer and inner contours of the clamping jaw; 
       FIG. 5 , a plan view on the grinding sheet holder; 
       FIG. 6 , a side view of  FIG. 5 ; 
       FIGS. 7   a, b, c  and  8   a ,  8   b , the side view of the clamping means upon insertion and release, respectively, of the grinding sheet; and 
       FIG. 9 , a basic sketch of a clamping jaw, which can be actuated with a switch, in the power grinder of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a schematic side view of the power grinder  10  with a rectangular housing  12  that at the same time serves as a grip for holding and guiding the power grinder  10 . 
   A motor  11  disposed in the housing  12  serves to drive a platelike grinding sheet holder  14 , which is disposed on the underside of the housing  12 , to reciprocate. On its underside  15  serving as a working face, the grinding sheet holder  14  carries a grinding sheet  16 , which is braced with its back side on the grinding sheet holder  14 , or on its grinding pad  18 . 
   Clamping means embodied as two-armed clamping levers  20  are disposed on opposite sides on the top  13  of the grinding sheet holder  14 . The clamping levers  20  have a swinging arm  21  above a pivot shaft  24  and a clamping jaw  22  below, which latter clamping jaw is braced with a saber-shaped curved outer contour  27  on a detent face  23  on the top  13  of the grinding sheet holder  14 . The pivot shaft  24  is seated on a bearing block  28  on the top of the grinding sheet holder  14 . 
   Between the top of each swinging arm  21  and an abutment, not identified by reference numeral, in the housing  12 , a compression spring  26  is braced; it tends to pivot the respective clamping levers  20  outward, presses the clamping jaw  22  against the detent face  23 , and thus firmly clamps the grinding sheet end  17 ,  19 . 
   The swinging arm  21  also carries a protruding angle bracket, acting as a toggle switch  121 , with which it can be pivoted by finger pressure—counter to the compression spring  26 —in such a way that the clamping jaw  22  lifts from the detent face  23 , in the course of which the gap between the outer contour  27  of the clamping jaw  22  and the detent face  23  enlarges, and the grinding sheet  16  can be readily removed. 
   In both clamping levers  20 , the spacing  30  between the pivot shaft  24  and the detent face  23  is less than the spacing  31  between the pivot shaft  24  and the outermost point of the outer contour  27 , so that in the position prestressed by the spring  26 , the tensing jaw  21  comes to a stop—up to the range of self-locking—against the detent face  23  on the top  13  of the grinding sheet holder  14 . In a servo-effectlike way, this reinforces the clamping force on the grinding sheet  16  counter to the insertion direction for corresponding displacement forces that tend to release the grinding sheet  16 . 
   On the other hand, the compression spring  26  is prestressed only slightly, such that the grinding sheet  16  from outside, by slight pressure against the outer contour  27  of the clamping jaw  22 , displaces the clamping jaw counter to the compression spring  26  and counter to the tensing direction, creating the gap required for the insertion on its own, and can easily be displaced inward and onward easily with one hand. The clamping jaw  22  has at least one outer layer of elastic, rubberlike material with a high coefficient of friction, which largely restricts a relative motion between the grinding sheet  16  and the clamping jaw  22 . 
   For equipping the power grinder  10  with a grinding sheet, the tool must be held firmly with one hand on the housing  12 . With the other hand, one grinding sheet end  17  should be inserted first at the first clamping lever  20  in the manner described and then locked. Then the second grinding sheet end  19  can be introduced at the other clamping lever  20  in the same way and locked, and by sweeping the hand along the grinding sheet  16 , it can be made taut, and its ends  17 ,  19  can be inserted farther between the clamping jaws  22  and the detent faces  23 , without the ends emerging again if let go and without any letup in the tension on the grinding sheet  16 . As a result, with comparatively little effort, taut and firm clamping of the grinding sheet  16  to the pad  18  or to the grinding sheet holder  14  is possible. 
     FIG. 2  shows a plan view on the power grinder  10  with the details of the clamping means  20 , and  FIGS. 3-6  show the clamping means, which are described above in conjunction with  FIG. 1 , in the form of a detail. 
     FIG. 7   a  shows the phase of motion of the clamping means before the insertion;  FIG. 7   b  shows it upon insertion of the grinding sheet end  19 ; and  FIG. 7   c  shows it after the insertion, in the end position of the grinding sheet once the clamping action has developed. It can be seen from this that for inserting the grinding sheet  16 , the grinding sheet end  19  need merely be pressed laterally slightly against the outer contour  27  of the clamping jaw  22 , so that a gap that the sheet enters is formed between the clamping jaw  22  and the detent face  23 . The clamping lever  20  need not be actuated separately by hand. 
     FIGS. 8   a  and  8   b  show the phases of motion of the clamping lever  20  upon removal of the grinding sheet  16 ; the clamping lever  20  should then be moved clockwise about the pivot shaft  24  by hand, that is, by finger pressure on the switch  121 . The clamping jaw  22  then lifts with its outer contour from the grinding sheet  16 , or from the detent face  23 , in such a way that from the enlarging gap, the grinding sheet end  17 ,  19  can easily be pulled out, and thus the grinding sheet  16  can easily be released. 
     FIG. 9  shows an enlarged view of a clamping lever  20 , whose shaft  24  is disposed on a bearing block  228  on the grinding sheet holder  14 . The clamping lever  20  has a clamping jaw  22 , which can be braced against the detent face  23 , and a tension spring  226  engaging the outermost end of the swinging arm  21  tends to pull the clamping jaw  22  toward the detent face  23 . On the outermost end, the swinging arm  21  has a ball-type socket  222 , which is engaged by a link pin  322  of a switch  221 . 
   The switch  221  can be pivoted in the direction of the directional arrow  333  by finger pressure about a shaft  224  retained on the bearing block  228  and can thus move the clamping lever  20  into its opened position so that a clamped grinding sheet can be removed. To return the switch  221  to its outset position, a compression spring  227  is disposed between the back side of the switch  221  and the housing  12  of the power grinder  10 .