A power tool comprising: a housing including a gear case; a motor driving a shaft; a drive spindle including an outer spindle rotatably driven by the shaft and an inner spindle disposed within the outer spindle, the outer spindle having an end portion defining an engagement surface for mounting a working accessory, the inner spindle being axially moveable along a longitudinal axis of the outer spindle; and a spring member arranged to apply a biasing force on the inner spindle in a first direction with respect to the outer spindle, wherein the outer spindle is axially fixed but rotationally moveable with respect to the gear case; and wherein securing the working accessory on the drive spindle against the engagement surface of the outer spindle applies an axial force on the inner spindle in a second direction opposite the first direction with respect to the outer spindle; wherein the outer spindle comprises an inner collar on an inner surface thereof, and the inner spindle comprises a top portion arranged to engage the inner collar; wherein the spring member is arranged to engage the inner collar opposite the top portion of the inner spindle; wherein the power tool further comprises a shoulder bolt member engaging the spring member opposite the inner collar, said shoulder bolt member having a lower portion secured to the top portion of the inner spindle through the inner collar; wherein it further comprises a depressor arranged to engage the shoulder bolt member opposite the spring member, wherein a depression of the depressor applies an axial force to the inner spindle in the second direction with respect to the outer spindle; the inner spindle being rotationally fixed to the outer spindle; wherein the inner spindle includes a lower portion extending out of the end portion of the outer spindle and having a threaded portion arranged to be received through an opening of the working accessory; wherein the shoulder bolt member has a head portion which engages the spring member opposite the inner collar; wherein the depressor is arranged to engage the head portion of the shoulder bolt member; wherein a rotation arrester is disposed around the depressor, the rotation arrestor keeping the depressor from rotating relative to the gear case; wherein a lower portion of the depressor includes an uneven surface; wherein the upper surface of the head portion has a corresponding uneven surface that mates with the lower surface of the depressor when the depressor is pressed against the head portion to rotationally lock the shoulder bolt member. a housing including a gear case; a motor driving a shaft; a drive spindle including an outer spindle rotatably driven by the shaft and an inner spindle disposed within the outer spindle, the outer spindle having an end portion defining an engagement surface for mounting a working accessory, the inner spindle being axially moveable along a longitudinal axis of the outer spindle; and a spring member arranged to apply a biasing force on the inner spindle in a first direction with respect to the outer spindle, wherein the outer spindle is axially fixed but rotationally moveable with respect to the gear case; and wherein securing the working accessory on the drive spindle against the engagement surface of the outer spindle applies an axial force on the inner spindle in a second direction opposite the first direction with respect to the outer spindle; wherein the outer spindle comprises an inner collar on an inner surface thereof, and the inner spindle comprises a top portion arranged to engage the inner collar; wherein the spring member is arranged to engage the inner collar opposite the top portion of the inner spindle; wherein the power tool further comprises a shoulder bolt member engaging the spring member opposite the inner collar, said shoulder bolt member having a lower portion secured to the top portion of the inner spindle through the inner collar; wherein it further comprises a depressor arranged to engage the shoulder bolt member opposite the spring member, wherein a depression of the depressor applies an axial force to the inner spindle in the second direction with respect to the outer spindle; the inner spindle being rotationally fixed to the outer spindle; wherein the inner spindle includes a lower portion extending out of the end portion of the outer spindle and having a threaded portion arranged to be received through an opening of the working accessory; wherein the shoulder bolt member has a head portion which engages the spring member opposite the inner collar; wherein the depressor is arranged to engage the head portion of the shoulder bolt member; wherein a rotation arrester is disposed around the depressor, the rotation arrestor keeping the depressor from rotating relative to the gear case; wherein a lower portion of the depressor includes an uneven surface; wherein the upper surface of the head portion has a corresponding uneven surface that mates with the lower surface of the depressor when the depressor is pressed against the head portion to rotationally lock the shoulder bolt member.