Catapult on a card

Introduction

This is my attempt at a printed catapult design.  I needed axel pivots to hold the parts in place so decided to use 1/8” (3mm) dowels instead of printing axels.  So, while the card is printing, go down to the local hardware store and purchase a dowel (hardwood if possible, next best, is to pick out one from a tree’s heartwood).

Cut four lengths of dowel. Mark the lengths then roll the dowel with a knife blade on the marks to score all around.  After a couple rolls the sections can be snapped apart. You’ll need two lengths 63mm long for the wheel axels, one that’s 40mm long for the arm pivot, and one 20mm long for the trigger release.

Printing

I printed using ABS with a 0.5mm nozzle and a 0.25mm Z-step.  There are two horizontal/side shells and four layers top and bottom.  I used 15% hexagonal fill and no supports.

Assembly

Separate the pieces from the card.  If needed, run a 1/8” (3.2mm) drill through the trigger pivot and axel holes in the bed, the throwing arm axel also the wheel axel holes in the side braces. This is for dowel axel clearances since your printer may slightly over extrude when printing.

Snap the upright beams onto the base.  This was difficult with parts printed in ABS but they snapped into place without damage. You could trim off part of the bottom catch to make this step easier.

Start the 20mm dowel into the holes for the release catch then push it through the catch release hole (noting the proper orientation).  The hole in the catch is a tight fit like the wheels. 

Slide the axel dowels through the bed holes and slide the side braces onto the axel dowels.  Push (maybe drive) the axel into a wheel (flat side toward the catapult).  This should be a tight fit.  If the wheels are too tight you can drill out the hole and glue the wheel onto the axel with CA (super glue). 

Slide the dowel into place that hold the throwing arm.  Loop a rubber band around the uprights and the throwing arm.  Note: the range can be adjusted by the number of rubber bands that pull on the throwing arm.

If needed, a drop of glue on the ends of the dowels helps to hold things in place.