NOTE: This article is not yet complete, and some information in Gudea & Baatz 1974 and Baatz 1978 articles may still be missing.
Unfinished field-frames? Edit
When looking at the pictures of the Gornea field-frames (Gudea & Baatz 1974: 51-53) it's fairly obvious that the field-frames were never completed, as the inner sides of the curves in the bars have not yet been formed. If, for the sake of argument, we suppose that the field-frames were indeed complete, then we have no reasonable explanation for the bulges in the field-frame bars, as a straight bar would have been simpler to make, lighter and as strong.
The field-frame #3 is especially interesting, as one large pi-bracket is at the top and the other at the bottom. This makes little sense for a number of reasons:
- Small pi-brackets work best at the top, as the thin little arch is passed through it
- In all other archaeological field-frames the bottom pi-brackets are larger than those on the top, possibly to accommodate a wooden little ladder.
It is likely that the artificer had simply made a mistake during assembly of field-frame #3 and never had time to fix it.
Manufacturing technique Edit
The curved field-frame bars in the Gornea field-frames were finished after assembling the entire field-frame. This suggests that it was necessary to adjust the depth and shape of the curves in the field-frame bars after assembling the field-frames, possibly after arming the field-frame with spring cord and inserting the arms. This was probably done to allow precisely defining the point at which the hoops in the arms contact the curved bar. If the exact form of the curve had not been important, the curve could have been forged before assembly like described here.
Depending on the piece of steel the blacksmith started with, there are several possible ways to forge the Gornea-style curved field-frame bars. If starting from thin round rod, the center can be upset until it has become thick enough. If one starts from thick, round rod then the center is left alone and the ends are drawn away from it to form the straight part of the bar. In either case, the bar is then flattened, after which the curve is forced to one side of the bar. Alternatively the curve can be a separate piece of steel forge-welded to a straight bar after the bar has been flattened.
Once the Gornea field-frames were assembled, the insides of the curves were probably finished cold (e.g. using a file) to prevent the field-frames from bending and twisting due to repeated heating, chiseling and hammering. Cold-forming would also have been a more precise than hot-forming, and could be done while the field-frame had spring cord in it.
The easiest way to determine the correct depth and form of the curve is to arm the field-frames, but only use a small amount of whatever cord that is available, and skip the linear pretensioning (stretching) process altogether; for testing purposes a sufficient amount of tension can be obtained by rotating the washers. This allows assembling the ballista and testing arm, bowstring and field-frame bar interactions to determine the correct depth of the curve. If the washer is not full of cord, there's pelnty of space to file the curves. Additionally the spring cord can be removed and reapplied easily for major adjustments.