1.  Field of the invention
This invention relates to a sander, and more particularly to a sander having a pad for mounting an abrasive sheet which performs a dual motion including an orbital motion and a rotational motion. 
2.  Description of the prior art
In a typical sander, there is provided a mechanism for a pad to perform a dual motion including an orbital motion and a rotational motion.  Such sanders are disclosed, for example, in Japanese Utility Model Publication No.  58-41084 and Japanese Laid-Open patent publication No.  62-297066.  In such construction, the pad is eccentrically mounted on a rotary shaft of a motor through a bearing.  The pad can freely rotate relative to the rotary shaft through its orbital motion.  When the pad is rotated without load, its rotational speed increases until substantially the same speed as that of the rotary shaft or the motor. 
In order to obtain an excellent finished surface of a work, the motor may be rotated at high speed so that the pad may be rotated also at high speed.  However, when the pad is rotated at high speed, abrasive sheet such as sandpaper may be scattered by a centrifugal force.  Further, since the pad is idled at high speed from the beginning, the work is abruptly abraded when the pad is pressed thereon, so that the finished surface becomes rather rough. 
Further in the typical sander, the bearing is eccentrically mounted on a rotary shaft of a motor, while a shaft rotatably supported by the bearing is fixed to the pad.  In this arrangement, however, it is necessary to include another shaft other than the rotary shaft.  Further, a bearing support member of large diameter having a recess for mounting the bearing has to be integrally formed with or fixed to the rotary shaft.  Therefore, the number of parts of the sander increases, and construction around the rotary shaft becomes large.  Thus, the whole construction of the sander becomes large and an operation for assembling the parts is complicated.