Tool holder

A tool holder comprising a substantially cylindrical body portion having an axial bore formed therethrough and mounting means formed at one end thereof, ways formed through said body in a direction transverse to said axial bore, the axis of said ways located normal to the axis of said body and intersecting the axis of said body, a pair of separate but mating anvil pieces having exterior surfaces formed to slidingly engage respective internal surfaces of said ways and gripping surfaces disposed at an angle .alpha. to a plane parallel to said axis of said body and normal to the axis of said ways, said angle .alpha. being an angle in the range of 15.degree. to 75.degree., said pair of mating anvil pieces having formed thereon internal threads of a first pitch, a differential screw element having at one end external threads of a first pitch for engagement with said internal threads on said pair of mating anvil pieces, said differential screw element having formed on the opposite end thereof and coaxial with said external threads of a first pitch external threads of a second pitch, nut means adjustably fixed relative to said body means and having formed therein internal threads of a second pitch for engagement with said external threads of a second pitch formed on said differential screw element, said differential screw element having jaw face means formed on said one end, said threads of a first and second pitch and said angle .alpha. being related by the formula:

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