Building Blocks Repeating Catapult

I have no design or engineering experience and cannot guarantee the safety of or recommend constructing this project. This repeating catapult was designed for my nieces and nephews so that they could learn hard skills and test out theories while building a working mechanism. It is part of a larger project I am working on where easily produced building blocks would allow for a wide array of mechanisms and structures to be accessible for those who may be interested. I am making this project public in hopes that a wider community can be formed and humbly ask for any feedback you would be willing to provide. Thank you.  

The system of building blocks has three main components. 

1) 3D printed blocks that I find are best printed using “0.15mm Quality” setting in PrusaSlicer. This produces acceptable tapped holes that will accept ¼"-20 bolts. Generic PLA is the filament that I use and it has more than met my needs. For strength purposes I use 4 perimeters and a 10% gyroid infill with 5 layers on top and bottom. Each file's name will have a number in brackets. This number represents the total of a part needed.

2) ¼"-20 plastic bolts and nuts from McMaster-Carr. Though any ¼"-20 bolt would work, I would not trust children with metal bolts and the plastic ones from McMaster-Carr work well and are the most affordable option I could find. 

3) MDF planks that are 1 inch wide, 7/32 inch or ½ inch thick and variable lengths. Unfortunately, all the ¼ inch material options like plywood and MDF have a thickness of 7/32 inch. This is not the biggest problem as a 1/32 inch washer works “OK” when lining up parts but access to true ¼ inch material would be preferable. The planks have a 9/32 inch hole drilled on the midline starting half an inch in and spaced 1 inch apart. For every 1 inch in length, each plank would have one hole. The planks could be 3D printed however the cost in time seemed too much. A jig for drilling holes in the planks will be released as a separate project. Ripping MDF on a table saw into strips with width 127/128 inches worked well for me. 

Plank Lengths needed are:

(7/32 inch thick) 4 x 2 inches, 2 x 3 inches, 1 x 4 inches, 2 x 6 inches, 8 x 7 inches, 2 x 10 inches, 4 x 12 inches, 2 x 13 inches, 3 x 14 inches.

(½ inch thick) 5 x 4 inches, 3 x 6 inches, 2 x 10 inches, 3 x 16 inches. 

The only other items needed for this project are some form of elastic, metal washes for a weight and ½ inch ball bearings. I found the elastic bands used in clothes making worked best and could be purchased at dollar stores in my area. Any type of elastic material should work, but too strong an elastic will no permit the throwing arm of the catapult to accept a plastic ball from the magazine. This likely is a error on my part in this first design. The metal washers are to be secured to a bolt that sticks out of the blue forked piece of the magazine. This will act as a counter weight when one plastic ball is delivered to the throwing cup each cycle. ½ inch plastic ball bearings can be purchased from amazon. They are a little on the expensive end but work very well. Alternatively I have attached files for split ball bearings that can be printed using PrusaSlicer 0.07 ULTRADETAIL settings. A cube holds the two split halves together. It is an imperfect option but one that will work. 

A booklet is included. Unfortunately, there are errors and ways I could have made it better. That said, I believe it is serviceable in demonstrating this first project. Other projects will have booklets that are better and any recommendations on how to achieve this would be appreciated. 

To anyone interested in the project, I thank you for your time and any suggestions or feedback you may provide. There are dangers associated with this project such as choking hazards and moving parts that can cause injury or at the very least discomfort. Again, I cannot guarantee the safety and proper functioning of the project should you decide to construct it. I am releasing the files as a way to bring an idea of mine out into the public and not to encourage you to actually print and construct the repeating catapult. It is more a theory like some would present in a book or paper and I simply do not possess skill in the written word to facilitate such an undertaking.

Many of the blocks that are used in this project are used in other mechanisms that I am currently developing. Examples of these are a crane capable of loads up to 5 pounds, a tensegrity table, mixing machine and others.