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Hatra Ballista Jr: Scapus Frame
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There wasn't much to go on with historical references for the Hatra Ballista, just a meta spring frame covering, and an artist's rendition in the book Greek and Roman Artillery 399 BC-AD 363. As such, I opted to go with a mix of what we did with Mista Ballista in metal, or Baby Hatra Ballista.

First we did a loose layup of the lower tree. This is the part of the lower frame that will handle the brunt of the pullback forces, so we need it to be strong.


We cut angled tenons onto the angled beams. The tenon needed to come off straight on one edge, and angled on the other end.


Cutting the mortise was more difficult since one side of the hole came in at a 45 degree angle. Using a thick hand-chisel made short work of the pine.


Here is the tree after all the mortises and tenons have been cut. The tree is independent of the spring frame. Eventually the beam at the top of the tree will be attached to the spring frame via a hinge. You'll can read about how that works later.


For the center beam, two hardwood dowel pins are glued into the stock, an slide into holes in the top of the tree. You can also see in this picture how the various joints fit together.


Heavy duty hinges are added to the top of the tree, and attach directly to the rear lower beam of the spring frame. In this way, when we need to tighten the rope bundles, we'll pull off the upper regulea frame and the scapus, and the bundles will fold down into easy tighten position. Getting ready to fire again will involve pushing the bundles back into place, and reinstalling the scapus.


To make putting the scapus back into place an easy task, I fabricated these slide-lock brackets out of old bedframe. A 1/2" diameter steel rod will be put into the tree, and these lock brackets will let us securely attach the scapus to the tree without bolts or nuts.


Because the slide-lock pin would need to be so close to one edge of the pine beam, we made steel C brackets that will prevent the pin from ever pulling out. Once bent into this shape, holes were drilled into the key locations where the slide brackets are to be installed.


With the bracket painted, and the pin installed, you can now see how the sliding bracket will hold the scapus down onto the tree. We will have a set of brackets in two spots at both ends of the tree which should be more than enough to hold this little guy together.


Work continues in 2007

We missed our chance at the 2006 chunk. In 2007, after varnishing most of the parts on Hatra Jr, we re-assembled what we had.


One of the parts my Dad had made up before had was the scapus (bottom) and transverse regulea (top.) At the very back of the scapus an integrated winch-mount frame was built.


We bolted the tree to the spring frame, and then fit the transverse regulea (top) and scapus into the correct positions. We then placed our slide-lock brackets into the correct locations, and clamped everything together. Using small clamps, jigs were fitted to the bottom of the scapus to let us reposition the brackets.


Using the jigs to hold things in place, we drilled out pilot holes, and bolted the brackets in place. You can see on the right that the bolt heads will fit into slots cut into the tree.


The front brackets will have mostly down forces on it, so they are held on with lag bolts. The rear brackets will have some up forces, so we opted to use bolts that go all the way through the scapus. Here my Dad's wife Margaret holds the wrench inside the scapus housing while the bolts are tightened up with the drill.


We then did a test fit with the new slide-lock brackets. A perfect fit! One person can now add or remove the scapus in less than 30 seconds.


Top Bracket

Now that all the parts were assembled, it was time for me to build all the metal brackets that would be both strong, and quick to allow the tail of the machine to go on and off.


The transverse regulea needed it's main framework strengthened up, and a way to hold itself up during assembly. I took a sheet of 20 gage steel, and cut out all the right slots, then started bending.


Getting all the different parts bent around the various wooden areas was a bit of a challenge. I couldn't use
Dave's cool sheet metal break to do it since it won't bend only part of a sheet of metal.


Trevor helped to drill all the holes for the lag bolts that would hold the thing together.


A quick coat of paint, and it was bolted on. It how holds the regulea up and looks sharp too. The next part that was needed is a way to keep it from sliding around against the sprint frame.


The bracket was a custom C clamp we made out of 1/8" thick steel bar. We stuck it into Dave's metal brake, and I got the boys down to help get it bent up.


Add two holes, and here you can see how the addition of a bolt willclamp the regulea onto the spring frame. I left a small gap so I could add a wooden buffer between the bolt and the sprint frame.


Winch Mount

A special mount was needed to mount the winch onto the machine. I wanted to make sure this was both strong and easy to remove. Since I'll be moving most of these parts around by hand, having them be light is important.


First up was to make a metal bracket for the back of the scapus, with holes matching were some lag bolts were a holding the wooden parts of the scapus together. A rod was stuck all the way through which will eventually be the part that locks the winch into place. Since the rod is so close to the end of the wood, the metal will be used to prevent it from breaking the wood.


A coat of paint was added to the scapus reenforcement, and then some T stock was fitted to the uprights. The T stock wraps the back side of the wood, and a small notch was cut into the part of th T that sticks out backward.


Some old bedframe was cut to length, and holes drilled (slightly oversized) for the steel rod. A bit of 1/8" thick plate was cut to about the same size as the back of the scapus. The plate fits into the slots in the T bar, and rests on notches cut into the angle. With everything clamped in place, I tack-welded the plate frame. Welding over all the pretty and varnished wood meant draping wet towels, and setting up shields all around the work piece.


A couple of holes for the winch, and a coat of paint and tada! One winch mounted at the right height that can be assembled or removed in mere seconds.


Regulea to Scapus bracket

The regulea to scapus mount point also needed to be strong and quick to assemble, so yet more custom metal brackets where needed.


Starting with some paper templates, more bits of 20 gage steel was cut and bent up in
Dave's metal brake for the tail of the transverse regulea.


The shape of the top brace was a bit complex, so the paper template gave me the general size. I then did the first couple bends, and fit it directly onto the wood.


Dave's metal break was invaluable for these complex operations. This is a finger break, so you can adjust the width of the metal you will bend, and it can force tight right-angle corners as well. Very nifty!


Add some holes for lag-bolts, some paint, and a rod for holding everything in place, and a nice-tight and strong joint is formed at right near the winch.


Additional Pages for Hatra Jr
Hatra Ballista Jr After building and testing Baby Hatra Ballista, it was clear this design was worth pursuing. Eric decided we needed a mid-sized model which we could then enter into the World Championship Punkin Chunk in the 10 and under category.
Hatra Ballista Jr: Spring Frame A Hatra Ballista Spring frame is a bit different from a Straight or V spring ballista.
Hatra Ballista Jr: Scapus Frame
(This Page)
There wasn't much to go on with historical references for the Hatra Ballista, just a meta spring frame covering, and an artist's rendition in the book Greek and Roman Artillery 399 BC-AD 363. As such, I opted to go with a mix of what we did with Mista Ballista in metal, or Baby Hatra Ballista.
Hatra Ballista Jr: Torsion The load bearing parts for Hatra Ballista Jr are very similar to what I made for Onager Jr, though I've made several changes to help assist in the operation and use of the machine.
Hatra Ballista Jr: Arms and Bowline A hatra ballista has inward swinging arms that allows for 90 degree's of rotation during the throw. Our model has 3 foot long arms. For our first year, these arms were made particularly strong.
Hatra Ballista Jr: Deployment Hatra Jr is easy to deploy. That's one of the reasons it took so much extra time to build the thing.
Hatra Ballista Jr: Shooting Various pictures and stories of launching Hatra Ballista Jr.


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Twas' brillig, and the slithy toves did gyre and gimble in the wabe...
Last Modified: 09/17/10