A mandrel 10 for producing a thin-walled bent tube having a bending portion with high strength and a small radius of curvature by rotary draw bending without either cracks in an outside of bend or wrinkles and buckling in an inside of bend occurring includes a shank 14, a connection mechanism 15 and a mandrel ball 16. In a cross-section orthogonal to an axial direction of the mandrel ball 16 at a central position in the axial direction of the mandrel ball 16, the mandrel ball 16 has a first position 19 and a second position 20 at which a first straight line m that passes through a mandrel ball center 17 meets an outer periphery 21 of the mandrel ball. Further, a ratio (L.sub.1/L.sub.2) between a dimension L.sub.1 from the mandrel ball center 17 to the first position 19 and a dimension L.sub.2 from the mandrel ball center 17 to the second position 20 is in a range of 0.915 to 0.976.

A mandrel 10 for producing a thin-walled bent tube having a bending portion with high strength and a small radius of curvature by rotary draw bending without either cracks in an outside of bend or wrinkles and buckling in an inside of bend occurring includes a shank 14, a connection mechanism 15 and a mandrel ball 16. In a cross-section orthogonal to an axial direction of the mandrel ball 16 at a central position in the axial direction of the mandrel ball 16, the mandrel ball 16 has a first position 19 and a second position 20 at which a first straight line m that passes through a mandrel ball center 17 meets an outer periphery 21 of the mandrel ball. Further, a ratio (L.sub.1/L.sub.2) between a dimension L.sub.1 from the mandrel ball center 17 to the first position 19 and a dimension L.sub.2 from the mandrel ball center 17 to the second position 20 is in a range of 0.915 to 0.976.

A tie bar tensioning system that allows a bending machine operator to removably couple an upper end of a bend die post of the bending machine to a base of the bending machine via a tie bar, and to lock the tie bar in a tensioned position. The tensioning system includes a stationary member and a rotatable member, each with aligned tie bar passages for receiving the one end of the tie bar. Both the stationary member and the rotatable member include respective engagement surfaces, with the rotatable member engagement surface being rotatable relative to the stationary member engagement surface between a released position and a tensioned position whereat the rotatable member is selectively lockable. The rotation of the rotatable member with the stationary engagement surface in engagement with the stationary engagement surface longitudinally displacing the rotatable member relative to the stationary member.

A pipe bend die unit comprises a clamp member having a first groove part of half-circular cross section on an outer peripheral surface thereof with a fitting recess extending in a peripheral direction by a first predetermined length on a planar surface perpendicular to a rotary axis, and a counter pressure member having a second groove part of half-circular cross section formed on an outer peripheral surface thereof, and a fitting protrusion extending in a peripheral direction by a second predetermined length from a tip end portion of the second groove part. The fitting protrusion is positioned in the fitting recess so that the first and second groove parts are combined to form a pipe-receiving groove of half-circular cross section. The counter pressure member and the clamp member are hingedly connected to one another so as to be rotatable relative to each other about the rotary axis.

Provided is a mandrel which is inserted into a pipe material when the pipe material is bent, the mandrel including: a core member having a rod-shaped main body portion and a small-diameter portion which is provided on one end side of the main body portion and which is thinner than the main body portion; and a front end member which is provided on one end side of the core member and which is formed integrally with the core member so as to cover the small-diameter portion.