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  • Samuli.seppanen

    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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  • Samuli.seppanen

    Today I was writing an email to a fellow ballista reconstructor when I realized I should probably make the email a blog post. So here are two of the most impressive ballista reconstructions/reconstructors right now as far a I know.


    Their reconstructions are very interesting as they don't use sinew at all. Nor nylon. They've tested various oiled animal and plant fibres and achieved quite impressive results: bolt velocity up to 86 m/s, range 480+ meters. Their most interesting hypothesis is this: they believe sinew was not typically used as spring cord material, at least not in inswingers. This is because it simply is not elastic enough for springs where

    • The spring is relatively thick compared to its length
    • The spring needs to be rotated a lot (…

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  • Samuli.seppanen

    The renovations in our apartment have finally come to a phase where I have been able to resume work on my cheiroballistra.

    A few days back I finished attaching the triggering mechanism part to the composite slider which I created a few months ago. The composite construction can be seen in this image:


    The back-end of the slider is of elm for hardness and splitting resistance, whereas the frontend is of pine to keep the weight down. The two pieces are joined together with a half lap splice joint, glue and two vertical wooden dowels. This type of joint should work well, as the slider has to endure mostly compressive forces. The slider is slightly wider than previously, which gives it additional strength. The angle of the dovetail has been adjus…


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  • Samuli.seppanen

    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 s…


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  • Samuli.seppanen

    New parts

    June 29, 2016 by Samuli.seppanen

    It has been quite a while since I actually published pictures of new cheiroballistra parts, so here it goes. First the new little ladder forged from 6mm spring steel coupled with a better and stronger little arch forged from concrete reinforcement steel. Note how the tenons of the rungs simply project above the beams, instead of being riveted or otherwise attached to the beams. This allow one to bend the little ladder beams outwards when inserting their tenons into the pi-brackets in the field frame bars.



    As can be seen from the picture below, the little arch's retaining pins are significantly stronger (5mm) than the previous ones (3mm):


    Because the little ladder tenons are larger than previously, the wedge system needs to be adapted slightl…




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  • Samuli.seppanen

    In the recent weeks I've been working on several new parts for the cheiroballistra.

    The new case, which is made of pine, now has the crescent-shaped from the old cheiroballistra. I originally intended to make the new crescent-shaped piece from the crooked piece of spruce I had lying around, but that would have slowed down the project for basically cosmetic reasons. The female dovetails are basically ready, but they have not been glued to the case because the slider is missing.

    The new slider is of composite construction, with the front part composed of two vertical layers of pine, and the short rear part made from a single piece of elm. The lamination step was not really necessary, but it reduces the change of the slider bending and getting …

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  • Samuli.seppanen

    Routledge is having an open access week. This is of particular interest because there are some ballista articles in there. In particular the Performance of Greek–Roman Artillery by Rossi et al 2015 is extremely interesting. After the open access week is gone, that article probably goes back behind a paywall.

    Because this is a blog, and in blogs one is allowed to rant, I will take this oppurtunity to rant about these paywall-erecting publishers. Take the article "The Reorganization of the Defences of Romano-British Towns in the Fourth Century" for example. It contains the word "ballista" in it, because it shows up in the site search. However, there is probably no way to know if the article is actually relevant to ballista research in any way…

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  • Samuli.seppanen

    In my blog post from yesterday I talked about the attachment of the little ladder rungs. I'll go a bit more depth here (as if yesterday's discussion was not esoteric enough).

    It is a bit puzzling why the cross-piece and rungs had different names, because their descriptions are identical:

    "And let the cross-piece(1) ΤΥ be made, and having length of 3 dactyls not counting the tenons, and [having] width of 2½ dactyls. And there shall be rungs(2) ΦΧΨΩ, having length of 3 dactyls not counting the tenons, and [having] width of 2½ dactyls."

    The most reasonable explanation seems to be that the components were basically identical, but had different names to make the manuscript less ambiguous, as the cross-piece is referred to in several places:

    "...an…

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  • Samuli.seppanen

    The new little ladder made from 6mm (unhardened) spring steel is shaping up nicely. Today I finished sawing and filing the slots into the ends of the tenons:



    The field frames snapped into the slots beautifully, and when there, the little ladder beams bend outwards only very slightly. I now consider the "little ladder beams should be 3 dactyls from each other"-mystery solved in an elegant fashion.

    I had played with the idea of using a fancy wedge system to bind the crosspiece and the little ladder beams together. The idea was to allow easy dismantling the entire little ladder, which could prove useful if/when I create a second set of field frames for the sinew torsion springs. In the end I came to my senses and just riveted the thing together…



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  • Samuli.seppanen

    A few weeks back I finished translating the cheiroballistra manuscript, first into English, then into my native language, Finnish. I did the Finnish translation to see how well Greek translates in Finnish, and indeed, Finnish was much easier to work with than English, because of richer grammatical rules of Finnish. The second reason was to give the text a second go and see if anything changes. A few corrections were made to the English-language version during the Finnish translation round, but nothing really important changed.

    Anyways, here are the translations:

    • Translation of Cheiroballistra
    • Translation of Cheiroballistra into Finnish

    In retrospect the cheiroballistra manuscript was fairly easy to translate even with only two basic Greek cour…

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  • Samuli.seppanen

    After a hectic January at work it seems things have finally calmed down a bit, and I can resume my project. First a few thoughts about recontructing ballistas, or any ancient weapons for that matter.

    When I've talked about the cheiroballistra reconstruction project with other people of various backgrounds, it seems that putting oneself into the shoes of the Romans who actually used the cheiroballistras is quite difficult. After pondering about this a bit, I think one needs to approach this problem by asking these two questions:

    1. What was the purpose of the cheiroballistra?
    2. What properties did it need to fulfill this purpose?

    The answer is clear in my opinion: the cheiroballistra was a weapon designed to kill (possibly) armored people at long ra…

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  • Samuli.seppanen

    NOTE: This blog posting was converted into a real article with complete illustrations.

    A few days back I had to really think about how the little ladder and little arch are stressed when the arms are rotated in the torsion springs. The way I reasoned it, there are two main forces involved. The first force is caused by the field frames trying to rotate as the arm is rotated. In an inswinger this means the following:

    • The little arch starts bending towards the shooter, the stress being concentrated at the middle (i.e. at the center curve)
    • The longer little ladder beam (at front) is squeezed (compressed) fairly directly along the longitudinal axis
    • The shorter little ladder beam (at back) is pulled (stretched) fairly directly along the longitudinal…
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  • Samuli.seppanen

    During Christmas holidays I was at the forge making replacements for various ballista parts. As I mentioned earlier, my original little arch, and thus the little ladder, was too long, which caused all sorts of unnecessary fuss and misinterpretations. So those needed to be replaced. Here's a comparison of the length of old and new little ladder beams - the outer beams are the old ones:


    As can be seen, the old beams are way too long. The new beams are forged so that tenons' one side is in-line with the outer surface of the beam:


    This forging technique, when coupled with the notch attachment mechanism shown below, elegantly solves the problem of crosspieces and rungs being too close (3 dactyls) to each other and preventing insertion of the ten…



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  • Samuli.seppanen

    I and Nick Watts have been having many good discussions on his talk page lately. Nick is currently engaged in making a new ballista based on the Elenovo find (see Kayumov & Minchev 2010), and while doing that he thought of combining ideas from Philon's wedge machine and incorporating them into a "standard" ballista. The basic idea is that the top surface of the washer bars is flat, and a pair of iron wedges is mounted on top of each one. Spring cord is then wound into the torsion springs, and each cord is individually pretensioned using a stretcher. So in the end we have a torsion spring frame full of cords which pass over the crossbars and the wedges on top of them. This idea is pretty ingenious, because it solves two very real and seriou…

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  • Samuli.seppanen

    Earlier this week I finished revising my old translation of the case, slider and the crescent-shaped piece in the cheiroballistra manuscript. Then, when I continued on to the part describing the triggering mechanism, I was caught by surprise. Here's the part:

    "Γεγοέτω ἐξ ὕλης σιδηρᾶς χειρολάβη ἡ ΑΒΓΔ"

    Translated to English:

    "Handle ΑΒΓΔ shall be made of wood as strong as iron"

    Here's an alternative translation:

    "Handle ΑΒΓΔ shall be made of iron material"

    And an "obfuscated" translation used by most scholar:

    "Handle ΑΒΓΔ shall be made of iron"

    I call this approach obfuscated, because the word "ὕλης" has just been ignored. If "iron handle" was what Heron (or P.H.) ment, then the way he said it in Greek is very odd, and that alone would warrant a comme…

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  • Samuli.seppanen

    In the past I've talked about the problem of having "too few holes" in the archaeological all-metal kambestria (field frame). The problem is that there are 6 holes in the washers (at least in the Lyon specimen) and four holes in the kambestrion. This does not seem to be granular enough for adjusting the limbs (in nylon torsion springs) so that they are perfectly in sync during pullback.

    A week or so ago I stumbled on the "Pre­tensioning applied to ancient torsion­-powered ballistae" article (download link, Bibliography), in which the researchers claim that sinew is considerably more elastic than nylon. If this is the case, it would mean a lot of things:

    1. Rotating a washer N degrees would increase pretension less in a sinew spring than in a n…
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  • Samuli.seppanen

    A few months ago I was asked by a friend to write an article about the cheiroballistra to a small archaeological publication. Naturally I said yes, even though at that point I was not sure how much power I would manage to squeeze out of the thing before the deadline, and writing about a peashooter would have been humiliating. Fortunately I managed to produce a reasonable amount power just before the final editing of the publication. Most importantly the previous tests showed that the cheiroballistra is capable of generating Firefly level performance, once all the components are strong enough to take the strain.

    The article is called "The Cheiroballistra - Producing a viable weapon based on historical manuscripts and archaeological finds" and…

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  • Samuli.seppanen

    Today I updated the Cheiroballistra arms article with the latest information, and most importantly, with new CAD drawing of elliptically tillered 11 dactyl arms:


    The design shown in the drawing incorporates all what I've learned about making cheiroballistra arms. It is highly unlikely the design will significantly change after this, because all the "bugs" have been squashed already. The fact that the cones are now short again helps me a lot, because I have tons of elm pieces of just the right size stored away.

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  • Samuli.seppanen

    A few weeks ago I pretensioned the spring cord in one field frame to 350hz. This should give about 20% increase in power with the same amount of washer rotation. While the field frame bars are slightly more bent now, I don't think the bend is excessive. Thus it seems that I don't have to make new field frames to reach "full" power, which obviously saves me a lot of time and effort.

    Before restringing the other field frame I'll do practical to compare how much more resistance the 350hz frame offers compared to the 320hz. I have some old otherwise useless limbs which I can use for this testing. Hopefully I have some results of this tomorrow.

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  • Samuli.seppanen

    Stretching to 350hz

    October 29, 2015 by Samuli.seppanen

    Today I started pretensioning one of the spring bundles to 350hz. This should yield 1.2 times the power of the previous 320hz setup, and 1.96 times the power of the original 250hz springs. I suspect the field frames can take this power increase just fine. This time I'm using unmodified locking pliers, which work just fine with the "tighten with a winch first, then release pliers" (a.k.a. "snap in place") technique. Smooth jaws would only be required if the winch was actually used to forcefully pull the cord from between the jaws, which is what I did when I used the clip of my own construction.

    While the torsion spring was dismantled, I noticed that one of the field frame rings had bent inwards on the side. This had happen earlier with the 2…

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  • Samuli.seppanen

    While at the forge I started wondering how to make field frame rings of the type found in the archaeological record. So far I've successfully used two techniques:

    • Making steel field-frame rings
    • Forging rings using a horizontal ring forging template

    Neither of these produce the exact kind of rings as in Lyon (Baatz 1981), Orsova (Baatz 1974; 1978) or Elenovo (Kayumov & Minchev 2010) field frames. The Gornea (Baatz 1974; 1978) field frames are round, so the method I used for forging washer rims would work for those. A few potential techniques for making truly oval rings came to mind after a bit of pondering:

    1. Cut (e.g. hot-chisel) the ring from steel plate and finish the form with a file.
    2. Start from a flat steel bar. Cut a longitunal slot at the …
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  • Samuli.seppanen

    I started the work on new ballista the previous weekend at the forge, focusing on the little arch and the little ladder, which were too wide.

    Instead of just shortening the little arch I decided to make the curved part shallower to increase its strength, because I suspected it might have been crushed a little. However, doing that while keeping the straight parts correctly aligned proved to very difficult. So I had to let go of the old little arch completely and start planning on making an entirely new one from fresh, round stock. Forging the shape first and then flattening is something I've done plenty and which works very well. On the bright side I did manage to forge nice U-shaped forks with sharp, 90 degree corners for the little arch wi…

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  • Samuli.seppanen

    Total rebuild

    October 13, 2015 by Samuli.seppanen

    Today, after documenting the stretching process, I inspected the damage on the cheiroballistra caused by the previous dry-fire. One of the strong screws that kept the little ladder beams attached to the rungs had become partially loose, which is probably enough to explain the dry-fire. I suspect the hole for the screw was drilled a bit too large, so that the threads had not sunk deep enough: the other three screws had remained perfectly static. The field-frame on this weakened side had also canted slightly backwards, even though the reason is not entirely clear. The T-clamp that also came off during the dry-fire also came from this side, but it is not clear whether that had contributed to, or was the result of the dry-fire. In any case the …

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  • Samuli.seppanen

    Today I finished the first round of shooting with springs tuned to 320hz. The results were really good and what even more important, showed that lots of improvement is still possible using this spring configuration. These tests were performed with a new 48.85 gram test bolt:


    The new bolt is significantly heavier than the bolt used for testing the 250hz springs, which skews the results in favor of today's shooting session. With this new bolt the 250hz springs could probably have yielded about 90-95 joules of energy as opposed to 85 joules.

    I did a fairly extensive battery of shots with the 48.85 gram bolt, progressively increasing washer rotation. Washer rotation angles were not measured exactly, but should be roughly correct:


    Washer rotation…

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  • Samuli.seppanen

    In my earlier blog post I planned on tuning my torsion spring cords to 350hz from the original 250hz. That change would have roughly doubled (1.96x) the force required to pull back the arms, given the same amount of washer rotation.

    Later I peddled back and decided to settle for 320hz, which should increase energy to ~1.64 times the original. There were several reasons for the decision. First, I was afraid that I could not rotate the washers at all, because they're not that symmetrical. Second, I did not want to bend the field-frame bars for no good reason. Third, based on the amount of cord in Nick's Firefly and in my cheiroballistra, the "Firefly" level of performance would be 150 joules. My previous best with 250hz was about 85 joules wit…

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  • Samuli.seppanen

    Today, when I was about to start arming one of my field-frames, I noticed that all of the washer bars had bent:




    The bend was not much, around 1mm, but it was obvious that doubling the pressure of the springs would not be possible using these washer bars.

    So, I naturally made stronger washer bars that had a shoulder and all. The total height of the new bars including the 4mm shoulders is 24mm. The old bars were 14-15mm high, so we're speaking of a very significant increase in strength. The new bars are probably an overkill, but I just want the damn thing to stay in one piece for a while, plus the washer bar's height has absolutely no effect on any other aspects of the machine.

    I know the text gives 2/3 dactyls (~1.33 cm) for the height of the ba…




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  • Samuli.seppanen

    Little arch too long

    September 28, 2015 by Samuli.seppanen

    Today, while measuring my Cheiroballistra parts for updated CAD pictures I noticed that I had (unintentionally) made the little arch sigificantly wider than it was supposed to be:


    Little arch Mine Manuscript Difference
    Base length (ΓΕ) 53.5cm 47cm (23.5d) 6.5cm
    Longer side 67.5cm 63.0cm (31.5d) 4.5cm
    Shorter side 63.5cm 55.5cm (27.5d) 8cm

    As can be seen, the error had occurred when measuring the base length (ΓΕ) of the little arch. I'm not entirely sure how an error of this magnitude managed to slip in.

    This means a couple of things:

    • The new, longer 14 dactyl cones ("the text must be corrupted") are too long to steer clear of the case. The manuscript's number for the cones, 11 dactyls (22cm), is just fine.
    • I will need to make a new little arch an…




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  • Samuli.seppanen

    Replacement parts

    September 21, 2015 by Samuli.seppanen

    Three days at the forge/workshop allowed me to get lots of things done.

    Reinforced old little ladder beams

    I welded small wedge-shaped pieces to the tenons in the old little ladder beams:



    This should allow me to continue shooting with high-power without the need to (immediately) finish the new little ladder.

    Reinforced the curved field-frame bars

    Small pieces of steel were welded to the inside of the curves:





    The curves are now much stronger and can certainly handle the spring cords tuned to 350 hz.

    Forged parts for a new little ladder

    The main differences to the old ones are:

    • Beams are made from 5mm, not 4mm steel
    • The beam tenons are slightly narrower (9mm vs. 11mm)
    • The ends of the bars (tenons) have been forged flat, not folded
    • T-clamps were forged …






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  • Samuli.seppanen

    I've been discussing torsion spring stacking with Nicholas Watts. For those who have been hiding under rock, he is the maker of the Firefly - an extremely powerful reconstruction of the Orsova ballista.

    As mentioned earlier, I believe the only way forward (=more power) is by applying more linear pretensioning. My theory is that in previous tests the washers needed to be rotated too much (up to 270 degrees), which caused the torsion springs to stack too early in the draw. This theory is based on simple geometrics: with zero washer rotation the spring cords are vertical and look like an "I" if you will. Rotating the arm displaces the cords easily, because the cord needs to stretch only a little. As washers are rotated more and more, the cords …

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  • Samuli.seppanen

    Hard won joules

    September 12, 2015 by Samuli.seppanen

    Today I assembled the cheiroballistra and shot quite a few bolts through the chronograph. I could easily get to 67m/s with 30 gram bolts (66 joules) with 225 degree washer rotation and a draw of 59 cm. I tried rotating the washers 270 degrees, but the stacking at around 10cm to full draw was pretty extreme. Drawing the thing may have been physically possible given enough courage, which I lacked. Having gotten scared, I reduced the washer rotation to about 250 degrees. At this rotation I only managed to pull of one successful shot, from which I can roughly estimate velocities for two different bolt sizes:



    Bolt weight (g) Velocity (m/s) Energy (J) Real shot?
    29.50 75.65 84.41 Yes
    20.58 83.44 71.65 Estimated
    10.23 96.53 47.66 Estimated

    The estimat…






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  • Samuli.seppanen

    Back in business

    September 11, 2015 by Samuli.seppanen

    It has been a while (over 6 months) since I actively worked on the cheiroballistra. Some weeks ago I finished my new workshop and, after a few detours[1], was able to resume the work on the cheiroballistra. Over the last few days I've fixed a large number of minor issues in the cheiroballistra.

    Wooden shims were added between the little ladder tenons and the pi-brackets under the wedges:


    The shims should help with two things:

    • The slow loosening of the wedges due to little ladder tenon movement
    • Slipping of the field-frame bars from the notches in the tenons

    A pair of pins was added to each end of the little arch to rigidify the upper part of the field-frames. The holes in the Orsova little arch may be explained with similar pins:


    The elliptically…



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  • Samuli.seppanen

    In my practical experiments I've noticed that the little ladder tenons have a tendency to slip away from the field-frame bars, despite the fact the the tenons have deep notches for the bars:




    A while back I made the notches deeper, which helped, but apparently did not solve the issue. Now I finally bent the little latter beams to more proper angle. Originally the beams were set wide apart, which was ok until the notches were filed into the tenons:


    Now the beams are bent close to each other, which ensures a tighter fit between the notches and field-frame bars. To insert the tenons into the pi-brackets one now has to bend the little ladder beams apart. Thus there are now two safeguards against the tenon slippage. The new little ladder looks li…





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  • Samuli.seppanen

    A few weeks ago I calculated the correct tiller for the arms. The calculation were based on a few facts:

    • The leverage reduces from 1.0 to 0.0 times maximum as we move towards the tips of the arms. This is not exactly correct, but close enough.
    • Reducing width by percent increases bend by percent (at any given point)
    • Halving thickness increases bend to eight-fold (2^3, i.e. to cube root)

    Here are the results:


    I already made templates for thickness and width tapering from thin bamboo strips. The manufacturing process will need to be changed:

    1. Plane the arms to rectangular form
    2. Tiller to correct thickness and width taper
    3. Round the corners

    EDIT: Content from this blog post as well as from others was copied to the Cheiroballistra arms Wiki page.


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  • Samuli.seppanen

    In my earlier blog post I outlined a why trying to fully draw the slider back does not make from a technical perspective. What I did not mention is that the end of the slider does not protrude from the case enough to allow cocking the weapon on soft ground. In fact, a hard surface is required if full draw is to be obtained. The usability of this exact type of cheiroballistra in battlefield warfare is a bit questionable: you'd like to be able to cock the weapon regardless of the type of ground you happen to be standing on. A few random thoughts about this matter:

    • Was the role of the handle to assist pullback, as I've assumed? Using the handle would defintely reduce the amount of pressure at the end of the slider, and thus the amount it sinks …
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  • Samuli.seppanen

    In my previous blog post I mentioned that cones start bending visibly when power levels are ramped up. Most of the bending in the cones is concentrated to the area halfway between the hook and the outer half of the torsion spring. What this means is that there's a stress concentration point in there caused by suboptimal tillering (=the way cones are tapered). For a thorough explanation on how tillering works please refer to the Bow design article in the Crossbow building wiki. In the cheiroballistra case the cones taper in all directions, not just in width, meaning that a linear taper will automatically create a stress concentration at the middle and reduce the durability of the arms[*]. What I need to do is make new cones where the taperi…

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  • Samuli.seppanen

    Getting closer

    December 21, 2014 by Samuli.seppanen

    Today I started gradually increasing the pretension in the torsion springs. I also started experimenting with different draw lengths with the 14 dactyl arms. The best results were with attained with draw that was 14cm less than maximum; all numbers are 2 shot averages.



    Bolt weight (g) Velocity (m/s) Energy (J)
    30.10 69.07 71.28
    20.58 76.685 60.51
    10.23 88.71 40.25

    This is not bad at all, even though there is lots of room for improvement. To get some idea what these still preliminary numbers mean here are some comparisons:

    • My wippe-operated ~300 pound medieval-style crossbow with 28.5cm powerstroke, a dynema string and a spring steel bow shoots the same 30 gram bolt at roughly equal velocity, 69.91m/s. A real medieval, forged prod with less opti…





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  • Samuli.seppanen

    About a week ago I finished the arms with 14 dactyl cones:


    Yesterday I finally had time to assemble the cheiroballistra, which now looks like this:


    After assembling I shot two low-power test shots:

    • 30 gram bolt
    • ~54.4 m/s velocity
    • ~45 joules of energy

    Everything now fits pretty perfectly together:

    • When the arms and the case are penpendicular, there's just enough clearance (~1.5 cm) between the case and the hooks, not an excessive amount like before (7.5 cm).
    • When the arms are at rest, the bowstring lays about 11 centimeters from the end of the slider. This seems enough for the groove in the slider to guide the bolt straight into it's target, even if the arms were slightly asynchronous and thus created a sideways motion to the bowstring.
    • The insurmo…


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  • Samuli.seppanen

    Work resumes

    November 8, 2014 by Samuli.seppanen

    It has been almost a year since my last post and almost as long since I worked on the Cheiroballistra the last time. The reason has not been lack of interest but lack of time due to buyng a house and having to build a new workshop from scratch. Now that the workshop is finally "good enough" and all the other mundane things have been taken care off, I'm resuming the cheiroballistra project where I left off. I really need to put it all together, because the project is already almost 5 years old.

    Today I started making new, 14 dactyl long elm cones to replace the 11 dactyl elm cones which were simply too short to provide adequate draw length. Fortunately I had "delayed commitment"[1] and had not cut all the bars and to-be cones to the "correct…

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  • Samuli.seppanen

    My ballista project has progressed very slowly during last few months, not only because of being quite busy with other things, but also because I've been quite unmotivated. The reason for my lack of motivation was that I was still unable to draw the bowstring fully - about 65 cm - no matter what I did. I've discussed this problem in more detail in many of my earlier blog posts:

    • Hunt for the 65 cm power stroke
    • Good news, bad news
    • More thoughts on the cheiroballistra draw length

    Yesterday, while I was making new cones it occurred to me that I had not even considered the option that the figure for the length of the cones might simply be corrupt. It is quite possible that the original manuscript in fact said ΙΔ (14) - a number which could easily be…

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  • Samuli.seppanen

    Originally this post was intended to be a part of the previous blog post, but it didn't make sense to put it there. Here's the outswinging cheiroballistra CAD drawing again:


    What is visible from this drawing is that the end of the slider is very far - 48 cm away - from the bowstring at rest when the slider is fully retracted. This seems odd, as this was not the case on any of the old wooden-framed outswingers I know of. At this point it's worth showing two mock images of the stomach cocking process, here shown at the final stages:



    As can be seen, when cocking the cheiroballistra by pushing it against a wall (or here, a door), the little ladder arrangement has to be placed quite far away from the end of the case, or the upper washers will pre…




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  • Samuli.seppanen

    This week I've been engaged in email discussions with Digby Stevenson, the only(?) person who has actually made real sinew springs for the cheiroballistra. I applaud him for that, knowing the amount of work is involved in making enough sinew spring cord for the cheiroballistra, even if it's a fairly small engine.

    Digby is a proponent of outswinging cheiroballistra, whereas I (in case you haven't noticed) strongly believe that constructing the cheiroballistra as an inswinger is the only reasonable option. Regardless, it's always good to question one's own assumptions, so I decided to flip my field-frames around and make my cheiroballistra an outswinger for a while. Unfortunately one of the arms is broken and some of the notches at the ends o…

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  • Samuli.seppanen

    Today I did some experimenting with spiral pretensioning. Once again I had to conclude that it does not make any sense, and now I believe to have figured out the root cause. The thing is that when arm is pulled, only one half of the spiral/spring tightens and stores energy whereas the other one actually loosens. More details are available here.

    So, this leaves me with the problem of breaking arms. I've come up with three possible remedies:

    Make the cross-section of the arms more rectangular

    So far I've used cones which have a near-round cross-section because I wanted to stick to the manuscript diagrams as closely as possible. This means that very narrow strips of wood at the surface have to withstand the enormous stretch and compression stre…

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  • Samuli.seppanen

    Speedbumps

    October 12, 2013 by Samuli.seppanen

    As I discussed earlier, my previous attempts to significantly improve the energy output of the cheiroballistra have failed. Today I tried again with fixed arms, improved claw, a new 73 cm bowstring and reduced ~58 cm draw. I was able to reach 55 joules with the 30 gram bolt before one of the cones broke under strain:



    The other cone did not break, but the cords did crush it's back pretty badly:


    What makes this incident especially annoying is that once again I couldn't get past the 55 joule limit, which I had reached quite a few times before. Also, the design of the arms is pretty much perfect, as is the material (elm) and workmanship. There is definitely a weak point where the bar is turned downwards inside the cone, but I'm not at all sure if…




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  • Samuli.seppanen

    In previous blog posts (here and here) I've talked about cheiroballistra's draw length, or more correctly it's power stroke. My initial assumptions were that

    1. When the cheiroballistra is cocked, the slider fills the entire female dovetail in the case. In other words, the slider is pulled as far as it goes.
    2. The location of the claw from the end of the slider (5 dactyls) given in the manuscript is correct.

    With these assumptions the power stroke of the cheiroballistra is about 65 cm. As discussed earlier (here and here) reaching full 65 cm draw is fairly tricky, and any modifications to arm length, distance between field-frames or location of the little ladder will not make technical sense and/or violates the measurements in the manuscript. In f…

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  • Samuli.seppanen

    Forged new parts

    September 1, 2013 by Samuli.seppanen

    Today I forged a few new parts for the cheiroballistra. Most importantly, I simplified the process of forging the claw significantly - the new process is described here. I also forged some reinforcement parts for the two rightmost (pulling) handles in the picture below:


    In my earlier tests the fairly thin handle steel was too prone to bending.

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  • Samuli.seppanen

    Good news, bad news

    September 1, 2013 by Samuli.seppanen

    On Friday I filed the curves in the field-frame bars deeper to increase the bowstring length. The end result was that the bases of the hooks are now 80.5 cm apart when the arms are at rest. I also made a 78 cm long bowstring to replace the old one which is 68 cm long.

    Using the longer bowstring I was able to draw the slider as far as it could go, but as there initially wasn't sufficient washer rotation, the arms managed to squeeze themselves partially out of the bundles after the first shot. After adding some washer rotation one of the hooks apparently gave in at near full draw, and the ballista dry fired in a fairly nasty fashion. Left arm's hook hit a horizontal doorpost and bent the whole bar into an interesting shape:




    Interestingly left …




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  • Samuli.seppanen

    Integrity checks

    August 28, 2013 by Samuli.seppanen

    As I mentioned in my previous blog post I need to deepen the curves in the field-frame bars to allow a longer string to avoid excessive stack at the end of the draw. I started filing the curves deeper today, but even unhardened spring still is very tough, and it takes time, and there are no convenient shortcuts I could take. Lots of patience and a few hours of time is all that it takes.

    Anyways, once I had dismantled the ballista I noticed that the spring cords had squeezed noticeable dents into the elm cones:



    I recall Nick Watts publishing similar pictures where he showed his (hickory?) arms having similar dents. I think these small dents are actually beneficial, as they reduce the risk of cord slippage without jeopardizing the integrity of…



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  • Samuli.seppanen

    In last few weeks I've been making various small enhancements to the cheiroballistra. I made the triggering mechanism much more robust by (temporarily) using a small nut and bolt as the trigger axle. In the final version I'll probably use a small rivet or maybe a strong nail. I also replaced the nut used as the claw axle with a more rigid axle made from uniformly round steel.

    My first crescent-shaped piece was way too narrow and rough for high-power tests - my abs were actually sore for several days after my previous shooting session last weekend. So, I made a new crescent-shaped piece by laminating several thin sheets of 10 cm wide ash together. The process was way too complex for my taste, and it included making a bending form, steaming, …

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  • Samuli.seppanen

    As discussed here, I believe the handle in the cheiroballistra was used for assisting slider pullback. So, the operator would both push (with his stomach) and pull (with his hands) when cocking the weapon.

    Last visit at the forge yielded two new types of handles:


    At the left the original, non-pullable handle of a "simplified Iriarte-style". The middle handle is designed to be compatible with the pin arrangements I created for the leftmost handle. The rightmost handle is probably the simplest possible pulling handle one can imagine, but it will need to be anchored to the case in a new fashion.

    I will be interested to see how much easier the pullback will be with these handles, and whether the wood at the end of slider can take it. Eventually, …


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  • Samuli.seppanen

    A day at the forge produced tons of parts for new set of arms:


    Today I combined the parts into what I hope will be my last bunch of cheiroballistra arms for a while:



    As can be seen, the arms are for all intents and purposes identical to the arms shown in the manuscript diagrams. The "socket" at the tip of the cone might have been made of metal, but besides that I'm almost certain this is what the Romans used. These are the lightest arms so far, and also have the best mass placement. They should thus improve performance even further. However, as this newest incarnation lacks metal shielding, I need to pad the field-frame bars somehow. I think I'll start with thick belt leather. Based on my tests with the earlier set of arms - which were funct…




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