Catapult for GraviTrax Junior

Take your game to new heights!

This catapult for GraviTrax Junior will add a new level to your marble runs. Just press the arm down to load the catapult, and it will automatically send your marble flying once it rolls in!

Adjust the force of the shot with a simple dial to make sure the marble lands exactly where you want it.
The catapult uses magnetic repulsion, so you don't have to worry about it weakening over time.

Watch it in action*:

A few examples of marble runs using the catapult (and other printed components*):

Needed to complete

• One M3 screw, length 6 mm.

• 5 round neodym magnets (diameter 5 mm, height 2mm) - one for the catapult arm, four for the force dial. The ones in the force dial are stacked, so you can also go with 2x 4 mm height, or 2 mm + 6 mm - you get the idea.
  You can also try the three-magnet variant of the force dial - it'll shoot a little less far, but it's still fun.

Printing

Material

• The release piece should be printed from a material that has some amount of flexibility and is wear-resistant. PCTG is perfect for the job; PETG should also work. PLA is too brittle.

• The catapult arm should also be printed from a wear-resistant material.

• The base, dial and other parts can be printed from PLA or any other material you like.

Recommendations

• A layer height of 0.15 mm is recommended to ensure a proper fit of the hinge and the thread of the force dial.

• The base will require paint-on supports around the hole for the force dial, regardless of the print orientation. I recommend printing it upside-down. Refer to the 3mf project for an example.

• When printing the release piece, the line width for the external perimeters shouldn't be too large to make sure it can properly hold the catapult arm down. A width of 0.4 mm usually works; 0.25 mm (on a corresponding nozzle) may even improve your results.

• I recommend printing multiple release pieces to have some spares in case they wear or snap.

• Depending on whether you want to use three or four magnets in the force dial, print the corresponding variant.

• Once the base and force dial are printed, check the fit of the thread. It should be effortless to turn the dial, but it shouldn't be too loose either. If it is, try re-printing scaled to 101% (only in the X and Y direction!).

Assembling

1. Press one of the magnets into the hole on the underside of the catapult arm. It should press-fit in place; if not, apply some glue. Once it's in, it should be flush with the underside of the piece.

2. Insert a stack of four (or three) magnets into the force dial. 
   IMPORTANT:The magnets need to be oriented so they repel the catapult arm when they are moved close to its underside!
   This is easiest to get right by placing the catapult arm flat on a table and snapping the magnet stack onto its upper side, then pushing the force dial onto the stack until at least the first magnet is inserted.
   Again, the stack of magnets should press-fit into place and sit flush with the upper side of the force dial.

3. Place the catapult arm in the base. Push a piece of filament all the way through the hole in the side of the base and arm to create a hinge. Ensure that the arm can move freely without any resistance.
   PLEASE NOTE: The movement of the catapult arm is intentionally limited to an angle of ~45°, to get trajectories that work well.
   Cut the ends of the filament piece flush with the base walls using a side cutter.

4. Screw the force dial into the underside of the base.

5. Before inserting the release piece, make sure the edge that'll hold down the arm is sharp enough. Try pulling on the underside of your thumbnail with it to test. If the edge was printed too round, try carefully sharpening it with a knife.

6. Push the release piece into the corresponding hole behind the arm. Make sure to push it all the way in so that the bolt holes align, then secure the release in place with the M3 screw.

7. Try pushing the catapult arm down; the release should be able to hold it down reliably. 
   If doesn't, try dialing the shot power back a little. 
   If that doesn't help, take the release back out and sharpen the edge a little before inserting it again.

8. With the catapult arm pushed down, let a marble roll into it - it should automatically release. The marble needs some momentum to push the release aside, of course! 
   If the catapult doesn't release, then the filament used for the release likely isn't flexible enough. Try re-printing with one of the materials recommended above until it reliably releases.

All done! Have fun and enjoy your little one's “AGAAAAIN!!”

*Credits to the makers and their models shown in the videos:

• Bridge and 5x High Stone by Seppel

• Vertical Drop & Catcher by Highcooley

• Tunnel and Switch by J.S.

• Crossings and Junctions by urfin