Quick-change holder

A quick-change holder comprising:

• a cylindrical body of sleeved shape having one end fixed to the spindle of a machine tool and formed with both a plurality of radial through-holes, which are arranged in a circumferential direction thereon, and a center bore which extends in the axial direction thereof;
• a plurality of balls loosely fitted in said radial through-holes, respectively, and having a larger diameter than the radial thickness of said cylindrical body at the circumferential portion where said radial through-holes are arranged;
• a nut insertable into the center bore of said cylindrical body at the other end thereof opposite to said spindle for attachment of an adaptor coupled in a driving power transmitting relationship to said spindle, said nut having its outer circumference formed with a circumferential groove, with and from which said balls can engage and disengage, so that said balls are brought into engagement with said circumferential groove when said nut is inserted into said circumferential body;
• and
• a sleeve mounted on said cylindrical body, said sleeve enclosing said cylindrical body, being movable in the axial direction and being urged uni-directionally in the axial direction by means of a spring, said sleeve having its inner circumference engageable with said balls and composed of portions having larger and smaller diameters, said sleeve being axially moved against the action of said spring so that said balls can be moved outwardly in the radial direction from the smaller diameter portion to the larger diameter portion of said inner circumference thereby bringing said balls into and out of free engagement with said circumferential groove whereby said nut can be mounted on and demounted from said cylindrical body,
• wherein the improvement resides in that the portion of said cylindrical body which merges into said radial through-holes and which is located at the side of said spindle is formed with a conical inner circumference which diverges to the outside;
• and in that a spring having free wire ends with a gap therebetween so as to be compressible inwardly in the radial direction and brought into abutment engagement with the conical inner circumference of said cylindrical body is in contact with said conical inner circumference, so that said second-named spring made engageable with said conical inner circumference is brought into abutment engagement, when said nut is not inserted, with those portions of said balls, which protrude inwardly from said radial through-holes, to push said balls outwardly in the radial direction and is thrusted, when said nut is inserted, by the leading end of said nut facing the spindle so that it is released from engagement with said balls until it is positioned on the inner conical circumference of said cylindrical body.