STEMFIE Desktop Catapult

The STEMFIE Desktop Catapult is perfect for shooting ping-pong balls or crumpled paper. The ball-holding beam moves using the energy stored by two rotor springs.

To arm this catapult, you rotate the beam until it locks into place held by the trigger. Then, you can shoot ping-pong balls into cups or hit and flip the STEMFIE shooting target, which is included in this project.

• Video showing step-by-step assembly

Start by 3D-printing the three build plates containing the parts for this catapult. Each build plate corresponds to a different colour, as seen in this instructable illustration.

You can download the attached STL files. When slicing, you can disable support structures. All STEMFIE files are designed to 3D-print without the need for support structures.

Once you 3D-printed the build plates, you will realise that there are several different fasteners in length and type. The two different types are: threaded and cam-locking.

The most common STEMFIE fastener is the shoulder screw, which has a smooth shaft ending in a 5 mm long threaded tip. There are also full-length threaded STEMFIE screws using the same type of threaded nut.

The cam-locking pin looks similar to the shoulder screw – having the same rounded head and smooth shaft – but instead of a threaded tip, there is a connection shape for attaching the cam-locking nut. This nut needs to be orientated and aligned to the end of the pin to be inserted. Before you rotate the nut 90 degrees, please make sure that the tip of the pin and the outer part of the nut is flush.

To attach two or more stacked STEMFIE parts, the length of the shoulder screw needs to be such that the tip protrudes 5 mm from the last part on the stack (including a washer), leaving enough length for the nut.

The STEMFIE standard is defined by a block unit (BU), which equals the thickness of one beam and the centre-to-centre distance between two holes. A STEMFIE block unit measures 12.5 mm if 3D-printed at 100% scale.

Smaller increments of STEMFIE part sizes are defined in BU unit fractions, such as 0.75, 0.5, 0.25 and 0.125. Typical fractional sizes are braces and washers, measuring 0.25 BU.

The calculation of the proper length of fastener for a given stack of parts can be as follows.

Attaching one beam to another beam to a brace and a washer equals the following: 

(Beam 1BU) + (Beam 1BU) + (Brace 0.25BU) + (Washer 0.25BU) = 2.5BU

The correct length of shoulder screw for this stack is 2.5BU, which corresponds to the part: Shoulder Screw RHD RH BU02.50 - SPN-SSC-0050.

The trigger is the only part of this catapult held in place by a pin and cam-locking nut. Note that this nut is slightly different from the threaded nuts. 

You only need to insert the pin into the nut and rotate 90 degrees clockwise to lock it in place.

All other remaining fasteners in this STEMFIE project are threaded screws and corresponding nuts.

Fasteners:

• Pin CL RHD BU01.50 - SPN-PIN-0046 (1x)

Place the previously assembled beam and trigger onto the U-shaped STEMFIE beam. Use the four equally long shoulder screws and corresponding spacer to secure the beam.

Note that this step – the base frame – uses the largest of the two U-shaped beams in this project.

Fasteners:

• Shoulder Screw RHD RH BU04.25 - SPN-SSC-0057 (4x)

Attach the STEMFIE sign to the front of the base frame.

Fasteners:

• Shoulder Screw RHD RH BU02.25 - SPN-SSC-0049 (2x)

Attach the two rotor springs to the lower part of the arm. Make sure the loose end of the spring – the round attachment hole – is closest to the tip of the arm where the payload-holder is mounted.

The arm will rotate around a shaft in the hole close to the rotor springs. If the shaft does not rotate freely, you can use a round needle file or a 7 mm drill to clear the hole for the shaft.

Fasteners:

• Shoulder Screw RHD RH BU01.50 - SPN-SSC-0046 (1x)
• Shoulder Screw RHD RH BU01.75 - SPN-SSC-0047 (1x)

Use the smaller of the two U-shaped beams in this step. Attach the upper box-shaped beam to the two open ends. At the closed side of the U-shaped beam, attach the barrel nut. Ensure this is the barrel nut with a threaded hole in the centre.

Fasteners:

• Shoulder Screw RHD RH BU01.50 - SPN-SSC-0046 (4x)

Insert the arm into the frame and make sure springs and the payload-holding loop align correctly.

Insert the arm and its frame into the base frame. Refer to the illustration to align the correct holes and then insert one of the two identical shafts to hold the frame.

Insert the second shaft into the frame and rotate the closest spring to match its hole to the inserted shaft. Continue moving the shaft inwards and insert it into the second spring's hole before exiting through the opposite part of the frame. The spring is loaded, forcing the arm and its frame to rotate forward.

This long threaded screw adjusts the inclination of the arm-holding frame and, thus, the initial angle of the ballistic trajectory.

Start by inserting the screw into the knob, then proceed with a round washer, the remaining barrel nut and finish the stack with two long threaded nuts.

Adjust the position of the two nuts until the barrel nut is gently trapped, and then counter-rotate them to lock the position.

Thread the long screw into the arm-holding frame's barrel nut until it protrudes roughly 10 mm. Proceed to place the long screw's barrel nut into the front position of the STEMFIE sign. Lock the barrel nut in place using two (1.5 BU) screws.

Fasteners:

• Shoulder Screw RHD RH BU01.50 - SPN-SSC-0046 (2x)

Attach the target to its base using the remaining long screw and corresponding washers. Then, attach the nut avoiding over-tightening and make sure the target can flip freely. If necessary, you can use sanding paper or a flat-file to create the required gap in this stack.

Now that you finished assembling your STEMFIE Desktop Catapult, it is time to have fun playing with it. Once you have it all working, I recommend 3D-printing another one so you can play with your friends and try to hit the target as well as throw ping-pong balls into cups filled with some water. If you don't have a ping-pong ball, you can easily crumple a sheet of paper or tin foil.

You can remove the shaft holding the spring in tension when not playing with your STEMFIE Catapult; it will let the spring's plastic relax and retain its power for the next time you want to play. In addition, if any part of the catapult breaks – such as the rotor springs – you can quickly make a replacement with your 3D printer. 

I hope you like the STEMFIE Catapult, and I wish you lots of fun playing with it! If you have some time and filament to spare, please make an additional catapult and gift it to a child in need of some playtime; there are far too many children in places where a stranger's gift would fill their day with joy! :-D

Cheers,

/Paulo Kiefe

STEMFIE.org