Abstract: An adaptor (18) for clamping an optical scale (20) on a tooling button (12). The adaptor (18) has two elongated rectangular plates (24, 60) pivotally engaged, one above the other, by a pair of oppositely disposed dowels (74, 76). One of the plates (24) has an opening (30) to receive a tooling button (12) therein. The plate (24) has a flat surface (54) to receive a scale (20) and a peripheral surface of the opening is cutaway (38) to permit the scale on the surface (54) to abut the tooling button. The scale is clamped in the proper tooling position by means of the upper plate (60) when it is pivoted with respect to the lower plate by means of a thumb screw (86).