Forging curved field-frame bars Edit
NOTE: It is very likely that the insides of the curves were formed cold for greatest possible precision, and that this process is not correct nor desirable. That said, this method can be used if the curved area is left fairly wide to allow post-assembly adjustments.
The curved field-frame bars are relatively easy to forge from round steel bar. If you're making a cheiroballistra, a 14mm thick bar is about right. Cut two longer-than-necessary pieces from the bar and forge the center of both into a curve:
Next make sure their curvature is about the same:
Once the curvature is correct, flatten the bars with a heavy hammer, until they are of correct width for their whole length:
Next make sure that both the thin and thick sides of the bars are completely straight, except for the curve at the center, of course. Finally cut the bars into the correct length, remembering to leave some extra for the tenons unless you intend to weld the bars into the field-frame rings.
Forging straight field-frame bars Edit
Unless you want to forge for forging's sake, I suggest simply cutting the straight field-frame bars from flat steel and leaving some extra length in case tenons are used.
Forming tenons at the ends of the bars Edit
Forming the tenons is trivial using a hacksaw and a small, flat file. Make sure that the tenons are long enough to be riveted to the field-frame rings.