Putting threads on a rod (bolt) is an easy matter with a die, lag bolts are a bit different in that the end is tapered to a point and threaded all the way through the taper to the point.

What does the blacksmith do in his shop to arrive at this hand made finished product without visiting the bolt rack at the local hardware store?

Stumped. Dan :)

It depends what you are trying to accomplish. Some smiths, when they want to forge hooks with lag bolts on the other end, purchase unplated lag bolts and re-forge the end that is not pointy into a hook. Hardware without plating/heavy metal can be acquired from fastener makers such as Fastenal.com.

A photograph or description of the finished product would make providing appropriate information much easier.

Howsabout forming a long square taper, then twisting it?

I would assume it would take some skillful file work.

Forming a lag Bolt is easy, forge the end as you wish and using a die thread the rest.

Making a lag Screw is more problematical but certainly doable.

After forming the head as you wish forge the shank to a long flat blade-like taper and twist.

Visualize twisting a dirk.

Frosty

Ok I'll bite. What is a dirk? -grant

Dirk is a Scots word for a short dagger; sometimes a cut-down sword blade mounted on a dagger hilt, rather than a knife blade.

The IForgeiron Gallery Search produces these dirks by sf Duck.

Colonial Williamsburg once had (maybe still has?) dies for making wood screws. The originals were probably made from a filed and hardened master that was sunk into each die half (while they were hot). The work was then put into the dies, which were hit once with a sledge. Since the work could not be rotated without fouling the thread, the forging was removed and any flash filed away. Sounds slow but was probably much faster than filing each screw or trying to cut by hand on a lathe.

Just forge a taper on square stock, then twist, instead threaded lag screw:D.