Monday, 7 April 2014

Handheld ballista ... ...

Last year me and the kids built a couple of siege engines out of wood,we made a Trebuchet and an Onager,ever since then I've been wanting to have a go at making a Ballista,I've had to scale things down a bit,I'd really like to have a go at making a large siege engine at some point,but our garden is a bit small for such a device,so I built a hand held Ballista,like a crossbow ... sort of.


Here it is -


I'm quite pleased with it.

It's made from lengths of tile batten and has a working trigger mechanism,which whilst it works quite well it's proved to be the weak point of the weapon.

To be honest I mainly made this to test out the trigger,it works but there's room for improvement as always.

The basic body of it is made from three lengths of tile batten fixed together which I shaped so that I could fit the trigger gubbins in.

The frame work that holds both torsion bars is also made from tile batten,and again I used copper tube to strengthen the wood a little as I did with the Onager.


Here's the section that holds the trigger mechanism - 


It's all held in place with wooden pegs.

I left the middle piece of batten slightly longer so that the latch (the part that holds the string and bolt) doesn't go all the way down,otherwise it would have been tricky to get the latch back up again,I also made a groove down the entire length of the middle piece of batten,this acts as a guide for the bolt to slide down.

The trigger mechanism consists of three parts,the latch that the string hooks onto,the release lever (the bit you squeeze to fire it) and a block with two springs in it.


Here are the parts - 


Simple,but quite effective.

Basically when it's loaded the latch,which has a groove in the bottom is forced onto the end of the release lever, there's a plate on the bottom that stops the lever being forced too far down and firing the Ballista.

The block with the springs is to make it easier to load,basically to load it you pull the string back and push the latch back at the same time,the springs help keep it from firing accidentally,a kind of safety feature if you will.


Here's a picture of it set up and loaded - 


The two pins in the latch help with loading.

As I said it's held together with pegs,the pegs run all the way through the body of the Ballista,and smaller pegs stop them from coming out,this means that the trigger mechanism can be removed in pieces easily should any part fail.

The two torsion arms are what provide the power to actually fire a bolt,they are basically the same as the version I made for our Onager,all be it it slightly smaller and obviously there's two of them,one on either side with a string between the two.


Like so - 


Another simple system.

Front view - 


Like a double sided Onager.

Here are some more pictures of the torsion arms,as I mentioned earlier I've used copper to give a bit more strength to the wood,I've used steel pins to stop the torsion bundles from unwinding and because of the amount of tension in each bundle the pins stopping the bundles from unwinding won't fall out.


Reinforcements - 


The copper stops the wood from getting crushed.

Another picture - 


Each torsion bundle can take more twists.
The arms are made from two bits of batten glued (and screwed) together,then rounded off on my lathe,in all honesty I could have left them square,then I wrapped them in string to stop them getting damaged,and to make the thing a little quieter when it fires.


And another - 


A simple knot holds the string in place in each arm.

It's not perfect,and should I make another there are things I'll change,for a start the amount of power I can get it to generate,I can add more twists to each torsion bundle quite easily,the problem is the trigger mechanism won't hold it,so I need to refine that.

I've already started roughing out a crossbow type weapon,that will use elastic rather than torsion bundles,and I'm working on different triggers,I'm looking to use something similar to the crossbows found on the terracotta warriors,but I need to find a way to fabricate them as I don't have a forge.

The bolts were made from a short length of dowel that I attached a thin piece of plywood to to act as fletchings (flights) they aren't the best thought out bolts but they work well enough,I added a bit of metal bar to each one to give them a bit more weight which helps them go a bit further.


The bolts - 


Crude,but they do the job.

I did have to add a nock to each bolt, so that I could get them to fit on the string.


End of one of the bolts - 


Okay,very crude,but they do work.

And to prove it works here's a short video,it's not great as I was holding the Ballista with one hand and my phone with the other, luckily it only weighs in at 1kg so it's actually quite easy to hold one handed,there is a slow motion part,which is not that good,mainly because I'm still experimenting with the software that helped me make the slow motion video.


Firing - 






You can find details on the Trebuchet we made - here (opens in new window)
And the Onager we made - here (opens in new window)

Thanks for reading.

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