A rotary cutting tool includes a cutter body having a head, a shank, and a cavity extending along a central, longitudinal axis of the cutter body. The head includes at least one cutting insert mounted in a pocket adjacent to a chip groove. A stiffening device includes a plurality of slugs with high compressive strength disposed within the cavity, and a first compression screw threaded into a rearward end of the cutting tool. In another embodiment, a second compression screw is threaded into a forward end of the cutter body. In another embodiment, each slug includes a coolant hole, circumferential grooves and axial grooves on its outer diameter, and a plurality of radial grooves on each end, and the first compression screw has a coolant hole for allowing coolant to pass therethrough.