This article describes one way to make and attach metal hoops to the arms. The technique described here uses an open hoop that is riveted close. The hoop is only used to prevent the wooden cone from splitting and the bar from rising from it's channel. If the hoop is also used to prevent the arm from sliding forward, a shallow groove in the wooden part of the arm is required.
The hoop should be placed so that it stops the arm from slipping through the torsion spring, and so that it is in contact with the curved field-frame bar when the arm is at rest.
NOTE: this article is obsolete and kept here only for reference. You should refer to the Cheiroballistra arms article for updated information.
The process step by step Edit
Mark the location of the hoop on the arm. Then take a long piece of steel about 2mm thick for the hoop:
File a shallow groove to the bar. The hoop will be squeezed into this groove to prevent it from moving towards the tip of the cone:
Polish the to-be hoop, if necessary:
Start rounding the hoop in a partially open vise using a cross-pein hammer:
Stop when the hoop resembles a semicircle:
Continue bending the hoop carefully into a tighter curve on top of an anvil:
Squeeze ends of the hoop against each other in a vise, taking care not to make the hoop too small:
If the hoop is too oval, flatten it slightly before removing it from the vise:
Now push the hoop to it's correct position in the arm and squeeze the ends tightly together in a vise:
After this the hoop and the arm should look like this:
Mark the places for rivet holes. Place them in a row in the overlapping part of the hoop right next to the cone:
Drill or punch the rivet holes:
Rivet the hoop so that it tightens:
Finally cut away excess steel and you're done.