When we use rebar mechanical splice technology to connect steel bar cage, there are two ways, one is cold extrusion press, the other is adding length thread connection, the cost of the coupler for the first way is higher, so normally we adopt adding length thread connection in steel bar cage.
Adding length thread connection is the application of upset forging parallel thread technology in special condition, especially when two rebars can not be moved, such as those in cage.

Principle:
First, process two rebars with upset forging parallel thread machines, the length of thread on one rebar's end is equal to coupler, the length of thread on another rebar's end is standard;
Second, screw coupler on the rebar whose thread length is longer;
Third, screw coupler from one rebar to the other rebar when end-to-end join the two cages.