Once the nylon cheiroballistra is powerful enough - hopefully in the current configuration - I will move on to comparing authentic spring materials like sinew and horsehair with nylon. My plan is to have a "mini-onager", probably in 1:2 scale, except that it probably would not have a sling like the real one. The nylon version will act as a reference point: it will tuned to store (and release) the same amount of energy as the cheiroballistra in proportion, that is, (1/2)^3 * 1/2 = 0,0625 times that of the cheiroballistra. This translates to about 6-9 joules, assuming the cheiroballistra's energy output is 100-150 joules
After calibration the other spring cord materials will be tested to see if they can produce comparable energy levels (6-9 …Read more >
After yesterday's good day of work the cheiroballistra is now almost ready for "final" round of shots:
- Both torsion springs have been tuned to ~350hz. In practice the initial frequency of the cords was around 360-375hz. To prevent excessive damage to the cords they were not pulled through the jaws of the (unmodified) locking pliers using the winch. Instead the locking pliers were released after a reasonable, probably around 200hz+ tension was applied to the cord. On release of the pliers the cord snapped into its place. This may have had a minor effect on the amount of pretension in the previous cord - how much remains to be seen.
- The slider was scraped and ground slightly make it slide more easily in the female dovetail, assuming both parts are …
The cheiroballistra is now truly close to being "finished". The biggest job was making the new field frames which I touched upon in my previous blog post. The field frames are now ready:
Some additional work is required to adapt the field frames to the little ladder and little arch. I've also modified two of the old 14 dactyl cones to make new 11 dactyl cones; only assembly of the throwing arms is still work in progress. I had to replace one of the old washers as its welding seams looked suspicious; fortunately I had made a replacement the last time I visited the forge. The slider is a bit too thick right now - even when it and the case have been stored in a dry place the fit is a bit too tight. A tiny amount of planing and sanding should s…
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During my work on the cheiroballistra I've ran into a bunch of manufacturing techniques and technical solutions that seemed reasonable at the time, but proved to not work properly in the end. This blog post outlines some of them, and why they failed.
At one point I thought about making the tenons of the little ladder beams from a separate pieces of steel:
There is no particular reason why this would not work. It just turned out that when the little ladder is made from thick enough (~6mm) steel there's no reason not to just forge the tenons from the beams themselves. It the beams were made from thinner steel then separate tenons would make more sense.
I've also tested various ways to make the field frame rings. Two such experiments, which fail…
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Yesterday I made good progress. The case is now finished and the slider was finished earlier:
I've reused the old crescent-shaped piece for pragmatic reasons, even though it is not 100% match with the text as it's attached with screws, not with a rectangular wooden tenon.
The attachment points for the T-clamps had to be reinforced with brass screws - something I'm not overly happy with:
This is largely because of the pine which I used for the case. While the pine is high-quality it still splits easily. However, this tendency is offset by much smaller weight of the case.
I also drilled holes to the new field frame bars:
The straight field frame bars are slightly wider at the middle to increase strength. This is another thing I'm not 100% happy w…
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