Abstract: A treatment head of a tool machine and a member are relatively angularly positioned by an angularly indexable mount having a first coupling having first and second elements displaceable relative to each other, each formed with a respective array of a respective predetermined number of teeth, and respectively connected to the member and the treatment head. The number of teeth of the first-coupling first element varies by more than one from the number of teeth of the first-coupling second element. A second coupling meshable with the first coupling has first and second elements engageable with the first and second elements of the first coupling, fixed relative to each other, and each formed with a respective array of a respective predetermined number of teeth. The number of teeth of the second-coupling first element varies by more than one from the number of teeth of the second-coupling second element.