I have gradually modified the morphology of this tool through several versions. The basic idea is that it is hard to securely clamp a piece of round stock in a post vise because the area of the tangent surfaces of the round stock is small so the workpiece can easily rotate in the plane of the vise jaw faces.
I first encountered this problem when making some small decorative bolts from 1/4” stock. I had seen a tool made by a more experienced blacksmith which I copied. It could grip round stock from 1/8” up to about 1/2”.
Later, I made another version dedicated to stock about 1” in diameter but changed the design from a spring type linkage to a continuous loop linkage.
This week, I made an even larger version which I can use for stock up to about 3” and I don’t plan to work anything bigger than that.
This is the way I did it. There are probably better designs and I look forward to seeing what other smiths have to offer.
I cut 36” of 1/8” x 1/2” flat bar and pulled it into a circle and MIG welded the ends to close the circle. Then, I heated the rather ugly weld with a torch set to a reducing flame until the weld melted, just right for fusion, and hammered it to a nice finished joint.
Next, I cut two short pieces of angle and lightly tacked them together as shown, to keep them in alignment. I cut two more short pieces of angle which rest on the tops of the vise jaw.
It’s not very pretty, but it is amazingly effective. I hang it on the wall right beside the vise.