The cheiroballistra project has stalled for quite a while as we've been renovating our apartment thoroughly since July. Fortunately after Christmas I could spend two days at the forge.
The results of the ballista-related work are these:
On the left there is an old ring for the cone and below it is an old 3mm thick pi-bracket. On the right side, from top, two spare 5mm pi-brackets, a new washer, three thick rings for the cones and eight thick pi-brackets with partially formed tenons. They attempt to fix a few mildly annoying issues with the cheiroballistra:
- The 3mm thick pi-brackets were a bit flimsy, even though they had survived earlier fairly high power tests.
- One of the old washers could not be easily rotated. This was probably caused by some asymmetry in the washer, as none of the others had that problem. All the contact surfaces were polished to a high degree and oiled, so that was not the issue. In any case, increasing cord tension was out of the question with the old washer.
- The rings in the cones were somewhat thin (~6mm). While the spring cords had not slipped over them as long as cord tension was high enough, I decided that thicker (~8mm) rings would be even safer. The impact on the performance of the ballista should be minimal, as the rings are right next to the torsion spring bundle and thus their rotational speed / residual energy is very low.
The field frame rings may still have to be fixed, as in my infinent wisdom I decided to make them quite thin. This caused the sides, which are not supported by the field frame bars, to bend slightly downwards during repeated washer rotation. If I have to redo the field frame rings, I will probably end up making them about the same thickness as the bars, and round, instead of oval. The oval rings seemed like a smart idea to help get a tight fit with the washer core, but the idea turned out not to work all that well. I now think that the horizontal ring forging template I used would also allow forging round rings of quite exact dimensions.
Finally a few additional pictures of the new parts. First a close-up of an old 3mm pi-bracket:
And here's one of a new 5mm pi-bracket:
All that remains is finishing the tenons with a file and hacksaw, and they can be riveted to the field frame bars. If I manage to mess up a few of them I have two spares at hand. To speed up marking of the tenons I made a simple pi-bracket tenon marker:
This tool removed the need to measure the location of the tenons on the pi-brackets.