Best made in a paddock with plenty of room for hurling and a tractor with a loader will make it easy to get the throwing arm onto the support frame.
Step 1: Prepare Materials
Detatch roots from trunk
We used a couple of blue gums, a fast growing eucalyptus (Eucalyptus globulus) approx 10 years old.
We cut the following:
Main arm 10.5m long
Legs 2 x 3m long
2 x 3.5m long
cross arm (pivot) 2m long
For cross bracing we used 1 by 5m length and 2 x 2.5m lengths.
(multiply by 3 for approx lengths in ft)
(note the chaps, face mask and safety helmet used when felling tree are omitted by model pretending to fell tree)
After cutting timber to length, find a good location for building the treb and set out timbers. Starting with legs, lash the tops of each 3m leg to a 3.5m leg leaving enough overhang at the top for the cross arm (pivot) to sit in. I had some help from my kids who had practised lashing at scouts with this step.
Step 2: Set Up Frame
Outrigger style cross bracing is a better solution. Lay a long pole (approx 5m) on the ground in front of the legs and fix to legs by lashing or heavy duty coach bolt. Then lash or bolt cross braces between the pole and the legs.
Step 3: Throwing Arm
Lift the arm and set the cross arm in the v at the top of the legs. We used a tractor so this was quick and easy. If you don't have access to a tractor, place the cross arm (which is light enogh to carry up a ladder) in position first and then use a cum along winch or block and tackle to lift the main arm and lash in place.
Screw a coach screw into the end of the throwing arm (long end) at an angle that is slightly above horizontal when the arm is pulled down to the ground. Leave the unthreaded portion of the bolt exposed. Cut the hex end off the bolt, leaving a smooth pin.
At the short (butt) end, drill a hole right through the log, close to the end at right angles to the arm and parallel to the cross arm to suit a long bolt or rod, about 16mm. Insert a rod through this hole to pivot a counterweight from.
Step 4: Counterweight
Step 5: Throw Something
1. Every good treb needs a name. This was a job for the kids, and they dubbed her "Pain Bringer"
2. Tie a rope to the long end of the throwing arm that you can use to pull the arm back down for firing.
3. Keep people, buildings, cars etc away from the treb, particularly in front and behind. Yes it is possible to fire the projectile backwards
4. Always assume that a rope or a joint will fail. Place yourself and anyone you don't want to send to hospital in a place where they will not be hit if something fails.
What to throw? Plastic bottles filled with water are great, make a nice wet splat. Strictly a single use item. Small heavy thing go further, eg the hammer that we were using for the build.
Update - In the video we are throwing bottles of water 2 - 3 kg about 30m or 90ft. In order to pull the arm back by one person pulling on a rope, we were restricted on the counterweight size we could use. We recently increased the counterweight by only about 25% and saw the throwing distance increase fairly dramatically to 65m or 200ft. Why so much from a small(ish) increase in counterweight size? I think this is because the weight of the arm is also quite heavy and this offsets the counterweight, so increasing the counterweight by a relatively small percentage increases the difference in weight between the throwing arm and the counterweight by a much greater amount.