Building a magnetic catapult / trebuchet is a fun and educational project.
Step 1: Prototype..
This prototype is a straight forward build. Took me an hour or so to make.
Base is a 24 cm x 9 cm x 1,2 cm wooden board
throwing arm lenght 25 cm + one popsicle stick at the end with a matchbox card board added as a cup for the projectile. 1:3,75 ratio in throwing arm gives the most bang for your buck (the pivot being the divider)
Threaded rod is at height of 10 cm from the base, I used popsicle sticks to carry the axle. one stick is 147 mm long and 17 mm wide and 1,5 mm thick. (three in each beam for added strength)
There is magnets on the throwing arm and magnets in the base, attracting each other.
Neodymium magnets are brittle, so I used a standard plastic bottle cap as a cushion between the attracting magnets. I thought I would destroy a lot of magnets doing this project, but so far I haven't broke any. about 2mm cushion is adequate
I recommend countersunk ring magnets, they are good because when a screw keeps them together reloading becomes a whole lot easier.
The trigger system is a stick through couple of eye bolts holding the throwing arm back.
Step 2: Larger Scale + Repelling Magnet
So, if you are serious about this project, you definitely want also use repelling magnets on the top for a lot of more punch
Throwing arm measurements, total length 38 cm pivot point is at 8 cm / 30 cm
4 mm diameter threaded rod is located at height of 9 cm from the base
take into consideration:
The repelling magnets at the top, don't want to come together, so any sideways movement of the throwing arm becomes a problem. To avoid the sideways movement The structure needs to be bulky and where the repelling magnets meet, the crossbeams are located close to the magnets so that they don't allow the sideways movement.
The magnets I used were:
2 pieces of grade N35, 60mm x 18 mm, 100 kg pull force ring magnets NdFeb
2 pieces of grade N52, 35mm x 10mm, 57 kg pull force ring magnets NdFeb 20 pieces of grade N42, 20mm x 3 mm, 17,7 kg pull force disc magnets, Ndfeb
Releasing trigger a wooden stick that goes through the thwoing arm and a block of wood that has a slot for it.
Step 3: The Sling Attachment
There's two strings tied to a piece of leather. The other end of the string is tied to the end of the throwing arm and the other string has a nut tied to it. The nut goes into a stick that is in a slight angle in respect of the throwing arm. The angle of that stick defines the releasing point of the sling. more angle means later point of release.
The lenght of the sling should be same as the lenght from the end of the throwing arm to the pivot point in other words if the throwing arm is in 1:3,75 ratio sling should be that 3,75 too.
The sling requires more than 90 degrees of angular throwing arm movement in order to function properly. I compensated the inadequate movement of the throwing arm with a shorter sling. I would recommend something like 120 degree angular movement of the throwing arm
I'm happy to answer any questions