A mechanism for clamping a tool (e.g. a probe) and a shank together comprises a clamping pin (144), which bears against an annular lip (142) in the rear face of the probe. A shaft (148) of the pin (144) extends into the shank (150), and two diametrically opposite, radially extending clamping bolts (160) supported by the shank (150), bear against a frusto-conical clamping surface (158). The action of the bolts (160) on the surfaces (158) urges the probe and shank together. The probe and shank are adjusted laterally relative to each other by four adjusting bolts (166) supported in the annular lip (142) which bear against a flange (152). The flange (152) is provided on the shank (150) and extends into the probe (140).
