Abstract: An apparatus for adjusting the grip of a tubular chuck upon work, the chuck receivable in a bore formed by a holder, and comprising a rotatable connector member having first and second thread connections for exerting force transmitted to the chuck; the head connections characterized as differential, whereby as the member is rotated, the chuck closes toward the work, with high mechanical advantage; the chuck defining a first axis; the connector member defining a second axis generally transverse to the first axis, the axes being non-interconnecting.