Non-obvious solution
So, I was able to figure out the problem with bolts flying with the butt-end pointing to the right. The first thing I tried, and which did not help,was leveling the uneven and protruding surface of the top of the crosspiece. I did this because I noticed that the T-clamps were slightly off at one side, and then noticed that the field frame was attached to the case in a slight angle which make the bowstring rise up from the slider at the right. The crosspiece in its original shape:
And after fixing it:
Now the little ladder beams, not the crosspiece, rest against the case. This corrected the angle somewhat, but not fully.
Now, what did help was non-obvious: rotating the washers more. It appears that with low pretension any differences in linear …
Still not shooting straight
In my previous blog post I triumphantly declared that I managed to fix the "bolts leave the ballista butt-end pointing to the right" problem. But that turned out not to be the case. So I spent several hours yesterday trying to figure out what was wrong:
- Rotated one of the torsion springs 180 degrees to ensure that both halves act the same. Did not help.
- Switched the arms to the other torsion springs. If one of the arms had had a problem this should have reversed the problem, i.e. bolts should have left the ballista butt-end pointing to the left. But no, butt end was still pointing right.
- Double the number of holes in the top washers to double the precision of washer rotation.
Before the draw the bowstring is exactly at the center of the slider…
Chasing red herrings
I've been working on the cheiroballistra again after a long pause. Why or how, you may ask? This is only possible because family is out of town and I'm on vacation. Basically all that was missing was a new pair of arms. The previous incarnation of arms was "elliptically tillered" whose lack of flexibility caused them to waste quite a bit of the energy (10-20%) fed to them. In addition the crushing of the springs had eventually destroyed all the previous versions of arms with which I was able to do full-power tests.
The new pair of arms, which is now in operation, attempts to fix all these shortcomings:
- On the belly side a strip of reindeer horn is sunk into the wood to prevent the wood from crushing
- On the back side the bar prevents crushing …
Print version of my Journal of the Society of Archer-Antiquaries article
My article is now available on GitHub (28MB):
- Seppänen, Samuli 2018 - The Cheiroballistra - a reconstruction - screen quality.pdf
I tried to upload it to academia.edu which gave a most unhelpful message "Something went wrong". I may need to contact their support to understand what the problem was. Meanwhile please use the GitHub link above.
My new cheiroballistra article
It has been a while since the latest blog post. Work has kept me fully occupied so there has been little progress since late last summer. However, my article in Journal of the Socienty of Archer-Antiquaries volume 61 (2018) was published recently, so I'll publish it here. Unlike (almost?) all ballista articles mine is focused on practical aspects of the reconstruction. Thanks to Rex Harpham for bugging me about writing an article to Journal of the Society of Archer-Antiquaries and to Arthur Credland for actually making it happen.
The quality of this "copy" is not particularly good, but I'll fix that in a couple of weeks when I get access to a scanner. But this version is at least readable.