Threads

Cut threads are made with a cutting tool and tend to have sharp crests and roots whereas rolled threads are made with thread rollers and tend to be cleaner with rounded crests and roots.

bicycle spokes use rolled threads, no metal is removed just deformed. For this reason the outside diameter will be larger than the base or original material, with cut threads the outside diameter will be smaller than original.

One of the basic differences between the two is the strength! With a cut thread, the grain of the metal is cut! With a rolled thread, the grain is not cut but pushed into the form of a thread thus keeping the structure intact. A certain amount of work hardening is created when the thread is formed which also increases the strength of the thread! The initial part will have allowances for the method you undertake! A thread rolled part will increase in length and diameter whereas a cut thread will not! Also some threads are more easily produced using thread rolling such as the thread on a recirculating ball lead screw! Try to cut one of them and you would get into difficulties!

I have a question about increasing length, I suggest the forming rollers will direct and contain the metal deformation and will direct it to increase width but not length (aside from that little bulge on the end)

another point we both missed is that the cross-sectional area is larger for a rolled thread than a cut thread giving it a stronger tensile strength for that reason as well as the grain structure improvements you describe so well