A connecting device for connecting a shank (16) to a measuring probe (18) is disclosed, to enable the probe (18) to be incorporated on a machine tool for the purpose of tracing a contour of a model. The device includes first and second parts (20,22) connected to the shank and the probe respectively, retained together by a connecting bolt (50) which is spring-loaded by means of Belville washers (56). An arm (60) extends from the second part to engage the housing (14) of the spindle (10), and thereby prevent rotation of the spindle shaft (12). In the event of accidental spindle start-up the two parts (20,22) may rotate relative to each other (the shank rotating with the shaft 12); a thrust bearing (42) enabling sliding of the two parts.