Abstract: The application discloses an assembly comprising an elongated knife in association with a heating unit together with means to mount the assembly so as substantially to eliminate all warping or other distortion of the knife due to heat. Specifically, the knife is mounted directly against the sheath of an electric resistance type heating unit. The heating unit and knife are mounted from a supporting frame in such fashion that they may expand or "grow" axially, thus substantially to eliminate distortion of the heating unit and knife. Also associated with the supporting framework for the heating unit and knife is means slightly to tension the sheet to be cut on either side of the knife, while the cutting takes place, thus to prevent after-cutting contact of the sheet edges with the knife.