A method for assembling the rotor (R) connected with a shaft (4) and made by permanent magnets (2). The method includes providing an annular flange (9); forming a plurality of fixed linear guides on the annular flange (9); positioning another annular flange (90) on the guides; and locking the second flange (90) to the guides, so that the magnets (2) are clamped, inside the guides, by the first and the second flange (9, 90). The guides are formed by inserting a rod (13) through each of passing holes (91) in the first flange (9) and locking the rods (13) to the first flange (9); fitting, on the rods (13), a body (12) having a central portion with a passing hole (121) and side arms (122); fitting a spacer (11) on each rod (13); and fitting, on each rod, a body (12) having a passing hole (121) and two arms (122).