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The Bungeelista

Or, How to Build a Surgical-Tubing Ballista


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For information on how to make ballista bolts, and other information on artillery design matters, follow this link.

A ballista that uses surgical tubing is a bit easier to make than one that uses real torsion skeins. There are fewer parts, and the only moving parts are the tubing and the catch in the release mechanism. I know some people will argue about calling this thing a ''ballista'' because it's powered by the elasticity of latex tubing instead of torsion. That's OK, I just want it to work. And it does work; the ones I've made according to the ideas expressed here will throw an unstuffed GT bolt more than 80 yards when the wind is calm. And it does look sort of like a ballista from 20 yards. Hopefully the recipient of its projecticles won't last long enough to be offended by its appearance.

Pictures of the ballista under construction.

The pictures were taken while the weapon was being tested, before I settled on how I wanted to rig the tubing. And I am using a different mount design now. Click on a thumbnail to see the full-size image. (Please be patient. The full-size pictures had to be put on another server because they are BIG files.)

View from left rear View from right rear View from left View from front

WARNING:If you try this, remember that a ballista IS a weapon, made to hurt people, and it can hurt you if you're not careful. There's a lot of force involved when it's fired. Remember that it will require periodic maintenance. Take care of it and check it regularly for any signs of wear or damage.

Materials Needed:

2x4, 10 feet long (2 of these)
Gate latch
1-1/4" oak dowels, 36" long (2 of these)
Wood molding, 9 feet
    (You can use any molding that is rounded out at the top; look at the figure farther down)
3/8" sisal rope, 100 feet
Welded metal ring, round or square, inside opening at least 1" across
Wood or drywall screws, 3" long (1 box)
Copper rivets and collars (2 each)
Bolts, 1/2" by 4", coarse thread (3)
Washers, 1/2" (3)
Nuts, 1/2", coarse thread (3)
1/8" cable ties (1 bag)
    (You only need 4 to build the weapon, but keep the rest for repairs later)
Carpenter's glue
Surgical tubing, 3/16" inner diameter, 3/32" wall thickness
    (You need 4 feet of tubing for each strand used on the weapon; I normally use 4 strands)

By the way, the parts list above does NOT include parts for the wheeled mount described at the bottom of this article. That is covered separately.

Wood: Get a good grade of lumber, as straight and as free of knots or other flaws as possible. Remember some parts of this thing will be under serious strain. DON'T use plain white pine--it won't hold up. Get yellow pine at least. Fir or hardwood would be great but probably too expensive. Try to make sure the wood is good and dry. If you suspect it is still a little moist and you have the time, leave it sit in the shop for a couple of weeks before starting work, to make sure it doesn't warp as it dries.

Tools Needed:

Drill, preferrably variable speed
Phillips bit for the drill
Drill bit the size to match the screw shanks
A drill press may or may not be needed, depending on how you decide to build the thing
Power saw
Heavy stapler
Razor blades or other implement for cutting leather
Leather punch the right size for the rivets
Ball peen hammer
Rivet setting anvil

Pieces to Cut:

  • 2x4 by 3 feet long -- 2 pieces (these will be the top and bottom bars of the prod box)
  • 2x4 by 8 inches long -- 6 pieces (these will be the uprights in the prod box)
  • 2x4 by 5 feet long -- 1 piece (this will be the tiller)
  • 2x4 by 4 feet long -- 1 piece (this will be the keel)
  • 2x4 by 3 inches long -- 1 piece (this will be the backing block for attaching the release mechanism)
  • 1 and 1/4" dowels -- cut 6 inches from each. Use the long pieces for the arms, and save the small pieces for use in making pivot points for your mount.

Don't cut the tubing yet.

Surface Treatments:

If you want to stain the pieces, it would be easier to do that before putting them together. Especially if you use use glue; the glue will interfere with staining. It would also be a good idea to give the wood a good coat of varnish or other waterproof sealer, to prevent it from absorbing moisture and warping.

Here are some figures representing the finished weapon (the mount is shown below).

The keel butts against the prod box, but is not attached to it.


Means of Connecting the Pieces

I personally favor using both glue and wood screws to connect the pieces. I don't trust screws by them selves, especially on the release mechanism, but using screws is a lot easier than drilling all the way through and using bolts. Adding glue makes the joins a lot stronger.

If you use glue, use actual carpenter's wood glue; plain Elmer's and other types of white glue aren't strong enough for this. Smear a thin layer on one of the connecting surfaces. You don't want a thick layer; you want a layer just thick enough that pressure makes the glue seep into the wood fibers to bond them together. A very thick layer of dry glue will crystalize and break under stress.

For screws, make sure you drill out pilot holes. Don't try to drive in screws without pilot holes because that will split the grain. Use a drill bit that is the diameter of the screw body, slightly smaller than the outer diameter of the threads.


Order of Work:

  1. Cut out all the pieces.
  2. Connect the tiller to the keel.
  3. Put the rails on the tiller.
  4. Connect the outer uprights, and the upper and lower bars of the prod box.
  5. Connect the tiller to the prod box.
  6. Drill the inner uprights, and glue and screw the arm ends to them.
  7. Attach the inner uprights to the prod box.
  8. Attach the middle uprights to the prod box.
  9. Lash the arms to the middle uprights.
  10. Attach the release mechanism to the tiller.
  11. Attach the tubing to the arms.
  12. Drill a hole in the keel for the pivot device on the mount. This step has to be last because you need all the parts to be in place before you can find the center of balance.

The Rails:

The guide rails must be straight or the shots will go wild.

The front ends of the rails should end at the point where the tiller touches the prod box, to give clearance for the bolts that attach the tiller to the box. But they must still be at least 3 or 4 inches in front of the tubing when the tubing is at rest. When the weapon is fired, the tubing will continue forward of its normal rest position for a few inches before it bounces back. If the rails are not long enough, the tubing will catch on the end of the rails with each shot and it will take more time to free it and draw it again. It is also a good idea to cut the forward ends of the rails on a slant, to prevent the ends of the rails from damaging the leather of the slide.

If you use molding for the rails, you must make sure it lays on the tiller properly:

  • Cut the molding to length.
  • Draw a straight line down the center of the top of the tiller.
  • Draw two more lines on top of the tiller, parallel to the center line you just drew and about 1/2" away from the center line.
  • Lay the molding pieces on top of the tiller, one on each side, with the inner edge of each molding piece on one of the outer lines. Do NOT tack down at this point.
  • Use tape to hold the molding in place.
  • Lay a bolt in the rails and make sure the rails are aligned properly and do not interfere with the fins - move the rails as needed.
  • ONCE THE RAIL ALIGNMENT IS CORRECT, for each rail lightly tack down one end and then put a nail (lightly) every few inches, bending the molding as needed to make sure it goes on straight.
  • When you have tacked down the entire length of the rails and are sure they are straight, then drive the nails all the way down.
  • Remove the tape.

Relation of the Arms to the Rails:

This is important when drilling the inner uprights for the prod box: the centers of the holes must be the right distance from the bottoms of the uprights. I can't give an exact distance because the material you use for rails may vary, and that changes the distance, but here's how to calculate that distance: From the bottoms of the inner uprights, measure up the actual thickness of the tiller (1.5), plus the height of the rails, plus one-half the diameter of a golf tube (0.5 inch). Or: 1.5" + 0.5" + rail height.

The idea is to put the arms in such a position that a line drawn from the center of one arm end to the center of the other is the same height above the top of the rail as the centerline of the golf tube when a bolt is loaded in the weapon. This means when the weapon is fired, there is no difference in the line of travel of the tubing, and the centerline of the bolt. That gives the most efficient transfer of energy from the tubing to the bolt, and the greatest range.

Unlike a crossbow, you don't want the points where the tubing anchors on the arms to be lower than the top of the rails. That would cause greater friction between the slide and the rails, and would reduce the energy that goes into throwing the bolt. Crossbow ammunition is much lighter and more aerodynamic than golf tube shot, so a crossbow can afford to waste some energy in friction. Golf tube shot are heavy and have bad flight characteristics, so they need all the energy on launch they can get.

The Arms:

The arms are made of 1-1/4" oak dowels. DON'T use any kind of soft wood. Connect the arms to the inner uprights before you put the uprights in the prod box.

If you drill the inner uprights at an angle, remember the axis of the arm must be perpendicular to the long sides of the upright, but parallel to the line of the bottom of the upright. You want the arms to be angled back from the front, but you don't want them skewed below or above the level of the launch rails.

Whether angled or straight through, drill the uprights and set the arms in them, but don't secure the arms yet. Put the uprights in position in the prod box and check that the line of the arms is in line with the rails. If you find that the line of the arms relative to the rails is a little skewed, there is a chance to recover. If the line of the arms is off by just a little, you can try trimming the top and bottom of the uprights at a slight angle to correct the cant of the arms. This can be done only if the angle is off by only a few degrees. If the angle is off more than 4 or 5 degrees, cut a new set of inner uprights and drill them.

Once the arms are properly aligned, glue the ends of the arms in place in the inner uprights, and put one screw in each arm end. Trim the ends of the arms flush with the inner surfaces of the uprights, so they don't interfere with the bolt fins. For this trimming, a manual saw would probably be better than a power saw.

As shown in the figure below, there are a couple of ways the arms can be connected to the other parts. In the original design, the rope lashings that hold the arms to the middle uprights are actually an important element, not just camouflage. In any case, the arms should actually be secured against the middle uprights. The weapon's power comes entirely from the tubing, so the other parts should remain as close to immobile as you can make them. Place some padding between the uprights and the arms. Otherwise, vibration when the weapon is fired could eventually cause wear at the contact points.

For a discussion of arm length, go here.

When you assemble the prod box, first connect the upper and lower bars and the outer uprights. Then set the inner uprights in place and screw them to the upper and lower bars. You may need to plane or sand the ends of the inner uprights, but be careful not to reduce them too much. You want a snug fit between the uprights and the rest of the box.

The same comments apply to the middle uprights. Put them in the prod box last. You may need to trim the areas where they contact the arms, so you may have to try fitting them several times before they are right.

DON'T nail or screw the arms to the middle uprights. That would disrupt the grain of the wood at the point where it is under the greatest strain, and that would probably cause the arms to break when you draw the tubing.

You could probably secure the arms to the middle uprights with pipe clamps, in which case the rope becomes only decorative, to hide the clamps. I haven't tried that approach, but you can give it a shot if you want. Remember to pad the inside of the clamps.

The amount that the arms slant back from the front is mostly a matter of looks. I set mine at 15 degrees from the centerline of the prod box. I don't know if using a different angle has any advantages or not. However, an angle that is too shallow would make it impossible to attach the tubing where it is far enough from the rail ends.


The Rope:

The rope lashings do not have to be really tight, but they must be neat. You will see them loosen a little with use, as the tension causes the rope to stretch. You will have to remove them and rewind them from time to time.

The ends of the rope must be secured. The beginning end can be stapled to the upright.

Below are some illustrations on one way of winding the rope. What you see doing it in person won't be exactly like what you see here, but this is as close as I can get it without photographs (on their way).

Begin by securing one end to the upright:

Wind the rope up until it is about one rope thickness from the arm.

Pass the rope around the arm. Double it back and pass it around the other side. You can repeat this step.

Cross the rope over the arms a couple of times. You can repeat this step.

Now finish winding the rope to the top of the upright, pass it under itself, and secure the working end to the upright. Again, you can use staples for this.

Attaching the Tiller to the Prod Box:


The Release Mechanism:


The Slide:

The slide helps transfer force smoothly from the tubing to the bolt by gathering all the tubing and making sure it all works together.

Use a welded ring; rings with the ends butted together will pull open under the strain. The inner opening of the ring must be large enough for the hook on the latch to fit between the ring and the leather of the slide.

The pattern for the slide is shown above. Remember that one end of the slide piece has to be longer, to overlap the other when it's folded.



The Tubing:

The tubing must be attached to the arms so that when it is at rest, it is 3 or 4 inches short of the ends of the rails. Beyond that concern, positioning the tubing relative to the ends of the arms is not exact.

5 feet of tubing per strand may seem like too much, but it comes in handy now. You don't want to cut the tubing to any particular lengths till you experiment and find exactly where you want to attach it. Your range will be affected by the length of the tubing, and by how tight you pull it when you put it on the arms. Clamp it in position through the slide and around the arms, and experiment with shooting for a while. Try stretching the tubing and moving the anchor points, to see how your range is affected by the tubing placement. When you are satisfied that everything is right, then attach stops to the arms.

I use 4 strands of latex surgical tubing, 3/16" inner diameter with a wall thickness of 3/32". I have found that using many strands of small-size tubing works better than using a few strands of large-size tubing. Apparently the thinner tubing contracts faster when it is stretched and released, so the bolts come off the rails faster. When I switched from two strands of 3/8" tubing to three strands of 3/16", I added about 12 yards to my range. I added a fourth strand later just for more range.

Surgical tubing is available in either tan or black, at just about any medical supply store. Almost any place that sells medical oxygen will also have tubing. The 3/16" stuff costs about $1.95 a foot in the Bryn Gwlad area.

The most important thing to remember about surgical tubing is that you MUST take care of it. Keep it out of direct sunlight, and don't let it contact petroleum products. Both of them will cause the tubing to break down faster. When you're not actually firing, take the tubing off and keep it covered. And don't let it get nicked; with the stress it's put through, even a small cut can quickly open and cause the entire strand to fail in use. If you take good care of it, it should last a year of steady use.

When you clamp the tubing, pull it a bit so it tightens down around the arm a little. It should be snug around the arm enough that you have to work it on and off rather than sliding it. When the tubing is in place, it should be under just a little tension.

As pointed out in one of the first illustrations, you should attach stops to the arms to keep the tubing from being pulled in. The stops have to hold against only the tension left at the end of a shot, so they aren't under a lot of strain. Gluing a block of thick leather to the fronts of the arms works fine.



Making a Mount

    In addition to a mount, the weapon must have a traversing/elevation mechanism. This is the part that connects the draw works to the mount itself, and allows the weapon to be aimed. I favor making the mechanism out of wood becuase it is much easier to work than metal, and I can use the leftover oak dowel pieces for the axles needed for rotation.

    The design below gives a mechanism that is more or less permanently attached to the draw works. It can be pulled loose from the mount so the weapon can be moved in two packages.

    To actually attach the traverse/elevation mechanism to the draw works:

    • With the draw works fully assembled, find its center of balance. Place the edge of the keel on the edge of a thin board and move it back and forth until it balances. Mark that spot.
    • Drill a hole in the keel of the draw works about an inch forward of the balance point. Make that hole 1-and-1/4 inches in diameter, and put the hole center 2 inches up from the bottom of the keel. Make sure the axis of this hole is perpendicular to the plane of the keel.
    • Put the traverse/elevation mechanism in place so the holes in its uprights line up with the hole in the keel.
    • Insert a piece of oak dowel through all 3 holes.
    • Glue and screw the dowel to the uprights of the traverse/elevation mechanism.

    Now you can make the mount itself. For the design I now favor, shown below, you need 2 2x4's, both 10 feet long, and a 7-inch piece of 1-and-1/4 inch oak dowel. You also need 2 bolts 1/2-inch by 4 inches, 2 bolts 3/8-inch by 5.5 inches, nuts and washers for those bolts, and a heavy-duty hinge with a removable pin. Carlyle came up with this design, and it is relatively light and is very easy to store and move.

    Cut these pieces:

      2x4, 4 feet long (3)
      2x4, 5.5 feet long (1)
      2x4, 9 inches long (2)

    Place two of the 4-foot pieces so the end of one is on top of the center of the other, in an upside-down "T" shape (make sure they meet at a right angle). Drill through both and bolt them together with the 1/2-inch bolts.

    Connect the bottom of the upright 4-foot piece of the "T" to the end of the 5.5-foot piece, using the hinge.

    Drill 3/8-inch holes through the flat of the last 4-foot piece, about 3 inches in from each end. Place the 4-foot piece against the standing pieces of the mount, in the position shown in the image above and mark the hole positions on the edges of the standing 2x4's. Drill 3/8-inch holes all the way through the width of the standing 2x4's. (Make sure the holes are centered.) Bolt the pieces together with the 3/8 inch bolts.

    For both 9-inch pieces of wood: Mark the front edge 7 inches down from the top, and mark the back edge 8.5 inches down from the top. Draw a line between those marks and cut the pieces on those lines. Glue and screw the slanted wood pieces together. Make sure the screws do NOT go through the center of the resulting block.

    Place the slanted block on the 5.5-foot piece, so its front edge is about 4 or 5 inches down from the hinge. Check the fit between the slanted surfaces of the block and the top of the mount leg; you may need to rasp and sand the slant to get a good fit. When the pieces fit well enough, glue the slanted end of the block to the mount leg, and screw it securely into place, with screws coming up from the bottom of the mount leg.

    Drill a 1-and-1/4 inch hole down the center of the slanted block. Seat the dowel piece in the hole, and fix it in place. This is the pivot pin.

    It would be a good idea to rasp and sandpaper the end of the pivot pin until the edge is rounded and smoothed. That makes it easier to guide the traverse/elevation mechanism into place when the draw works are placed on the mount.

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