1. A method for assessing accuracy of positioning of a multi-axis numerically controlled machine, the machine being of the type having a tool-holding spindle mounted on a ram that is movable along each of at least two mutually orthogonal translational axes, the machine having a numerical control unit for controlling positioning of the spindle, the method comprising: mounting a target on the machine adjacent the spindle and mounting a measuring instrument in a fixed position proximate the machine, the measuring instrument being operable to measure position coordinates of the target; creating a master frame of reference relative to the translational axes of the machine by: moving the machine along a first of the translational axes and using the measuring instrument to determine the coordinates of the target at each of a plurality of spaced-apart points along the first translational axis; moving the machine along a second of the translational axes orthogonal to the first translational axis and using the measuring instrument to determine the coordinates of the target at each of a plurality of spaced-apart points along the second translational axis; and calculating a linear fit of the determined coordinates so as to create a set of mutually orthogonal axes defining the master frame of reference; and checking accuracy of positioning of the machine in the master frame of reference by moving the machine so as to position the spindle at each of a plurality of locations, measuring the coordinates of the target in the measuring instrument for each said location, and determining errors in positioning of the machine by comparing the measured coordinates with machine-determined coordinates determined within the numerical control unit of the machine.