Power drill with drill bit unit capable of providing intermittent axial impact

A power drill comprising:

• a handle assembly;
• a casing connected securely to said handle unit;
• a drill bit unit having a bit-carrying shaft provided with a front toothed member which includes a plurality of axially extending teeth that are arranged annularly on said shaft;
• a rear toothed member fixed on said casing and spaced apart from said front toothed member;
• a rotatable driving unit rotating said bit-carrying shaft;
• a speed reduction device interconnecting said driving unit and said shaft so that said shaft rotates at a speed smaller than that of said driving unit, said speed reduction device including a planetary gear train provided with an internal gear mounted in said casing;
• a coiled compression spring pressing said internal gear against an inner wall of said casing so as to fix said internal gear in said casing;
• and
• an adjustment member mounted rotatably on said casing in such a manner that axial movement of said adjustment member relative to said casing is prevented, said adjustment member including a circumferentially stepped portion which is arranged so as to position said compression spring between said internal gear and said stepped portion, said adjustment member being rotatable relative to said casing so as to reduce total length of said compression spring, thereby increasing push force of said compression spring toward said internal gear, and so as to interengage said front and rear toothed members, thereby urging said front toothed member and said shaft to move forward away from said rear toothed member, forward movement of said shaft away from said rear toothed member and from said casing being capable of providing an impact to a workpiece to be drilled, said compression spring providing a push action on said internal gear so as to enable rotation of said internal gear when said driving unit rotates and when said shaft cannot rotate, thereby avoiding damage to said drill.