1. A method for estimating the rotational speed of a tool mounted on a rotating spindle of a machine tool by using a visual system which acquires images of the tool while it is rotating; the method including the following steps: defining a first interval of preselected speed values of rotational speed that is substantially centred on a nominal speed value of rotational speed of the spindle; for each of said preselected speed values, determining a respective image acquisition period such that it is equal to a multiple of one rotational period of the spindle as calculated for that preselected speed value and compatible with said visual system; rotating the spindle at the nominal speed value and positioning the tool in the visual field of said visual system; obtaining, by means of said visual system, representative couples of images of the tool which are, within each representative couple, temporally spaced apart from each other of the image acquisition period corresponding to one of said preselected speed values; and determining an estimated speed value to be associated with said nominal speed value as that preselected speed value, selected among all the preselected speed values, to which the representative couple of images that are the most similar to each other on the basis of a similarity rule corresponds.