Abstract: A device for clamping a work piece or a tool in a well defined position comprises a first coupling element assigned to the work piece or tool to be clamped and a second, stationary coupling element. The first coupling element is provided with grooves that cooperate with aligning members provided on the second coupling element to effect a precise positioning of the first coupling element with respect to the second coupling element in X- and Y-directions. A clamping pin member required for locking the two coupling elements to each other is located on the second coupling element. To protect the parts and element essential for the fine positioning, the first coupling element is provided with a supporting member towering above the aforementioned parts and elements.