Patent ID: 8192290

Claim:
A shrink-fit tool unit comprising a rotary tool having a cylindrical terminal area which is formed in a cylindrical shape having an axis and a tool holder having a chuck in which a hollow section is provided so that the cylindrical terminal area of the rotary tool can be inserted in, the rotary tool being maintained in the tool holder by shrink-fitting the cylindrical terminal area of the rotary tool into the chuck of the tool holder, wherein: the rotary tool is a thread forming tap including a cutting part provided at an end of the cylindrical terminal area and has a margin section projecting radially to form an internal thread by plastic-deforming a work piece; the cylindrical terminal area has a first engaging part which is formed on a side of the cylindrical terminal area and formed approximately rectangle as seen from the axis of the cylindrical terminal area; the first engaging part is disposed symmetrically relative to the axis of the cylindrical terminal area by matching a center thereof with the axis of the cylindrical terminal area, and each sides of the rectangle shape is lengthened to the direction which the margin projects; the tool holder has a second engaging part that is formed in the chuck to engage with the first engaging part where the second engaging part is formed approximately rectangle shape as seen from an axis of the chuck to engage with the first engaging part and is disposed symmetrically relative to the axis of the chuck; and the relative rotation of the thread forming tap held in the tool holder is restricted with respect to the tool holder, even when machining load receiving from a workpiece acts both in forward and reverse circumferential directions, by engaging the first engaging part and the second engaging part with each other when the cylindrical terminal area of the thread forming tap is inserted in the chuck.