Tiny HCW Desktop Trebuchet plans

The plans presented below should be used in conjunction with this text.

The Frame

You will need 18 sticks for the frame sides.  Start by squaring off the ends of 6 sticks.  2 sticks will form the uprights (part 1), and 4 sticks will form the base fore and aft of the uprights (parts 2 and 3).  For each side take 1 ordinary stick (part 4), and mark the centre.  Glue part 1 to this to form an upside down T, the square end of part 1 at the junction.  Take part 2 and part 3, and butt the square ends against the sides of part 1, extending the bottom of the upside down T sideways.  Glue another part 4 over this three way junction to sandwich the butted joints.  Use pegs to make sure that the sticks are clamped together while drying.

After drying, glue a part 4 either side each frame.  The first pair should sandwich parts 1 and 2, with the bottom of the parts 4 touching the ends of the parts 4 sandwiching the joint.  This is the front end of the machine.  A second pair of parts 4 should then be used to sandwich parts 1 and 3, with the top of the parts 4 butting against the underside of the top of the parts 4 at the front end, attached near the top of the uprights.  The bottoms of the parts 4 should then find a natural place to be attached to part 3.

When each part is dry I put a 2mm hole through the ends of the parts 4 supporting the uprights, and the parts 4 and the bottom of the uprights, see the diagram for placement.  I also put a 2mm hole through the tops of the parts 4 at the top of the uprights.  Through the holes in the bottom of the frame I glued 48mm long dowels, ensuring that the frame was square before allowing the glue to set.  I achieved this by not cutting the 9 * 4mm holes for the axle until later.  A 2mm pilot hole was drilled in readiness for the larger hole, but was used in this stage to hold a dowel joining the tops of the frams sides.  In the holes near the top of the uprights I glued 8.5mm pins to aid the mechanical strength of the joints.

Once everything was dry I cut some short lollipop sticks to go inside the frame between the parts 4 forming the upside down T.  These were glued in place either side of the middle dowel, spaced 1 lollipop sticks apart and were used to support the stabilizing legs and track later.  Another piece was glued in front of the rear dowel to support the track.  Under the front and rear dowels I glued short lengths of lollipop sticks to serve as feet, and under the middle dowel, using the short pieces glued earlier, I glued 88mm lengths to serve as stabilizing legs.  These were used to support a 45 degree buttress, in turn supporting the top of upright to prevent sideways movement.  See piccy below if in any doubt.

Before actually using the treb, cut the 9 * 4 mm ovals, and glue a small piece of stick across the bottom of each hole, ensuring that no glue fouls up the hole.  This will allow you to remove the arm, should you need to, by picking it up and moving it sideways so that one end comes free from the frame.

The Counter Weight

I cheated a little on this.  I reasoned that to have any chance of performing the counterweight needed to be as square as I could possibly make it.  I started by drilling three holes in parts 1, not 1 hole.  The extra two holes, one in the middle and one at the opposite end from the hole that the weight is suspended, were used to ensure the two parts 1 were square to each other by gluing 2mm dowel into them.  The dowel was long enough such that parts 2 and 3 could be glued in place accurately.  Parts 3 were glued to the side of their respective part 1, allowing 20mm of part 1 to extend past the truncated top of part 3.  When dry parts 2 were added, and were actually cut and sanded to fit since the structure would have to support 100 grams of lead.

When parts 1, 2 and 3 were dry I removed the dowels holding parts 1 together, and cut the lead sheet.  5 pieces were cut, each slightly smaller than part 3, and 4 triangles were cut to fill the gaps between the lead and parts 3 caused by the width of part 1.  All of the lead was glued in place, and the weight left upside down while the glue dried.  In the meantime, part 4 was steamed into shape, and left to dry while pinned to a board.  After it dried it was removed and kept it's shape.  It was trimmed to fit the gap under the weight and glued in place, with a little packing to help prevent the lead moving.

After everything had dried 2 2mm holes were drilled all the way through the counter weight, ensuring that both holes went through parts 1, and 2 lengths of dowel were used to pin the whole weight together.  See piccy below for a nice little shot of the weight attached to the arm.

Here the weight can be seen attached to the short end of the arm.  You can clearly see the pins holding the lead in place.  You can also see the sling pouch and the loose end of the sling.  The fixed end of the sling loops around a small piece of dowel behind the tip of the upper arm.

The Throwing Arm

The arm was built very carefully, as it had to be straight and square.  Care was taken to select the best lollipop sticks.  These were glued together in overlapping layers, and pinned after gluing.  The central layer is an uncut stick, sandwiched by two long parts and two short parts.  These in turn were sandwiched to cover the joint, and the whole thing pinned in 4 places, 3 pins in the upper part of the arm, and 1 pin in the lower part of the arm between the two axles.  The axle the arm pivots on is 3mm dowel (probably 1/8", I haven't measured).  The pins and the counter weight axle are 2mm dowel.

The small piece of wood between the tips of the upper arm was selected for it's strength.  After it was glued in place and allowed to dry I drilled a hole large enough to accept a 18mm length of paper clip.  The hole itself was less than 13mm deep, so I was left with a piece some 5 to 7mm long to accept one end of the sling.  I drilled a 2mm hole 16mm from the tip and put a small length of 2mm dowel in it.  This was not glued, as it would allow me to change the fixed end of the sling more easily.

The Sling

The sling is made from a piece of plastic carrier bag 65mm long by 30mm wide.  Each end was cut into a V shape 20mm from each end, so that the sling had six sides.  The overall shape resembles a stretched hexagon, with the centre part 25mm long and 30mm wide, with the V shaped sides tapering 20mm from the end.  The sling string was gleaned from a plentiful supply of kite line that I have, so was very strong and very light.  The line was stitched through the pouch with a loose running stitch, starting from the tip of one V and pausing half way around to ensure a large loop was left.  The loop had a knot placed 65mm from the end of the loop, this loop formed the fixed end of the sling.

Next, the pouch was moved up to meet the knot and another knot tied on the other side of the pouch, placed to gather in the plastic and form the pouch itself.  A small loop was tied off on to form the loose end of the sling, with the end of the loop some 70mm from the knot forming the pouch.  You can see the sling in the picture above.

The sling is attached the fixed end using a lark's head knot.  Pass the loop through the two sticks forming the arm, between the dowel and the tip.  Now pass the loop through the two sticks on the other side of the dowel, then pass the whole sling through the bit of loop you've just moved around the dowel and pull tight.  To remove the fixed end just pull out the dowel.

Now the fun part

Assembly of the frame, arm and counter weight should be straight forward.  Choose a projectile such as a small wooden dice, or large bead, or at a push (and limited range) a marble.  Hold the arm down with the sling attached and ease the chosen projectile into the pouch.  When ready remove your finger from the tip of the arm, and measure how far your projectile flew.

It didn't fly!

This could be due to several things.  I've found that I get best results using a larger light object so that the pouch doesn't swallow up the projectile.  Something too small and heavy seems to stay in the pouch.  When loaded you could try smoothing out the underside of the pouch, which should be attached to the loose end of the sling.  If all else fails, experiment.

It didn't go far enough, or it went too far

The angle of the paper clip determines when the projectile is released.  When the arm is horizontal, bending the clip up will release the projectile later, and bending it down will release it sooner.  I've found that lighter projectiles need to be released sooner, and heavier objects need to be released later.  Generally every time I try throwing something I need to bend the clip a little to get the throw right.

The End

I hope you enjoyed building a desktop trebuchet, and that you are inspired to build bigger and better machines.  If you do build a treb, I'd love to see pictures of it.

Other tiny trebs

Karen mailed me in May 2005, to tell me about the tiny treb that she built for a school project.  She gave me some super photos, and some good background, here it is.