Rick and Carl Build A Ballista
The 'plans' I mention here, were for the 'Spruce Moose', which used to have a page at: www.austintx.net/users/dmiller/ballista.html; but that link seems to be defunct now.
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This is the bow frame itself. The first part of construction. Note the nylon cord wrapped around the arm stops, instead of the rubber sheet originally called for in the plans. This looks much better, and probably works about as well, considering that they should not take much impact under normal use. We also considered wrapping the stacks of blocks in the frame, with cord as well (jute would look best); to hide my sloppy carpentry.
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Here we have added the stock (more properly the "tiller", I believe); to the frame. |
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Here is the (mostly) finished item; with arms, skeins, string, and release added.
The release mechanisim is my own idea; based on some descriptions of peasant crossbows. It uses a block, which the string is hooked behind. Pulling on the release string, pulls up on the end of a hinge underneath the stock, which pushes a 1" dowel peg up through the stock, and pushes the string up and off the block. The original plans call for a "gate latch"; but i decided to go for a somewhat simpler design, in the name of expediency. |
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PVC javelins quickly turned out to be far too heavy; at least for the draw weight we had. We were using the 8 strands(4 loops) of nylon clothesline for the skeins, which was indicated in the plans; but this turned out to be too weak. We later went to 10 strands of rope for each of the skeins; and gave them a lot more initial slack; but even this turned out to be too weak.
It does shoot golf-tube arrows reasonably well, tho. |
Don't burn drillbits, especially when drilling through the metal plates. Let off drilling for a few seconds every once in a while; to let the bit cool down. Otherwise it loses its temper, and becomes soft.
Measure better than we did. We did a somewhat sloppy job of measuring, and it shows. Of course, we didn't have a proper workbench, sawhorses, or even a square. We built it on the deck behind my house, using the steps and railing as work areas.
Get a long 3/8" bit, to drill through all the blocks in the bow at once. This will make them line up better. Clamp the blocks in place as you do this.
If substituting ready-rod for long bolts, be sure to make it long enough. You should add 1/4" to 1/2" to the recommended bolt size. We tried using ready-rod instead of the 8" bolts that hold the bow together. Probably would have been easier to just get the bolts.
The bolts that hold the tensioners for the skeins should be 1/4". It's not clearly stated in the instuctions, and we drilled 3/8" holes in the plates by mistake.
The designs call for 10 tensioning bolt holes. You need them. We tried it with 8, and it wasn't as good. It will work; but you won't be able to balance the torsion on each side quite as well. 12 holes would be even better. Next time, i will try 12 holes, in 2 rings, one ring of holes slightly farther out than the other (if i can make it fit).
The designs say to angle the tensioning bolt holes. This really isn't necessary. I screwed up, and drilled some of them angled in the wrong direction. They still work, tho. Probably better to avoid that hassle. It's easier to drill consistent vertical holes.
We used 34" balusters for the arms. there were 36" ones at the store as well; so next time we'll try those, to get a slightly longer draw.
If you can find washers bigger than 5/8" (we could find up to 1"), make the skein holes as big as you can. the box covers use 3/4" holes (or slightly larger, in our case), so that might be your limit; unless you drill those out, too.
Make sure the holes you drill in the plates, to hold the tensioning rod in place, are far enough out to clear the washers you use. Something we learned the hard way.
Clamps are a good thing; have several types and sizes on hand.
We wound rope around the arm stops, instead of rubber sheet. if you do this, make sure you wind it tightly. We used 2 layers, to make it a tad softer.
We bought 100' of nylon clothesline. used most of it, too. (broken skeins, and other things in the process of doing it the first time).
The bolts that hold the stock to the bow itself, should be carriage bolts with hex nuts. The wing nuts must be tightened with pliers (IMHE), in order to get them tight enough; so you might as well use real nuts, and avoid mangling the threads with the pliers.
Use 1" PVC if you want to try PVC javelins/bolts. we tried 4'x1 1/4" PVC, and it was far too heavy. our bow drew 27 1/2", so they don't really need to be much longer than 30".
We originally tried PVC javelins, because there were no golf tubes to be found in this city. Eventually, we did find some, and made a quantity of golf tube arrows. Zip ties work wonders for fastening the tennis balls on the end; much more secure than the string or twine suggested in the manual.
The launch rails we made are reversed from the ones in the original plans. We have the vertical sides of the molding on the inside, so the bolt simply rides on the top edges. This vastly reduces friction, for a smoother launch.
Make sure the launch rails are long enough. they must go several inches past the bowstring, so the string doesn't catch itself in front of them.
We did without the pieces of 1 x 1 1/2" stock called for in the plans. We simply substituted the 1 x 2" stock.
Tennis balls for ammunition can be found cheap at used-sporting-goods stores. I got mine for 25¢ apeice.
