A device for mating strips joined at an angle for forming rectangular frames having a L-shaped element (200), acting as movable abutment for the strips, has a symmetrical hinged connection to a pair of linkages (201), pivotally connected to a free end to the supporting element (202) fixed to the platform (102), and an arm protruding outside the L-shaped element along the extension of its bisecting line and undergoing elastic centering. The element (200) and linkages (201) are accommodated in the supporting element (202) in two mutually aligned seats (203) which lie transversely to the slot (104) of the platform, and are each constituted by a bush (204) from which a slider (205) protrudes; the slider is pushed, until it rests against an abutment of the bush, by a cylindrical spring (206) which acts between the slider and the screw-type cap (207) used also to adjust the preloading of the spring.