Propeller Launcher gun

I wanted a propeller launcher that was simple to operate. The motive force is a rubber band that winds around a spindle and stores the propeller launch energy.  But rubber bands break so it needed to be external and easy to replace.

You’ll need a couple M3x10 cap screws and nuts to hold the gun halves together.  On EBay or other source purchase a supply of 3mm cap screws that are 10mm long (i.e. M3x10) and 3mm nuts unless you already have a stock. You may also need a couple M3x20 cap screws for seating the 3mm nuts into the nut traps. Get a hex driver or metric Allen wrench if you don’t have anything that fits the cap screws.

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 25% hexagonal fill.  Print the gun halves with support and about 5mm of brim for adhesion.  The gun insides and the propellers just need support turned on.  Two spindles need to be printed at the same time since they are tall and small.  You need the hot end to deposit hot plastic on one and then the other to allow cooling.

Assembly

Drive out the “support fill” in the screw holes from the inside of the gun using the hex driver. Then check to make sure that there’s no stray filament threads at the bottom of the pockets.  They can provide enough spacing so that the screw threads won’t start.

Assembly will need two M3x10mm screws plus nuts (I used cap screws so I could use a hex driver). The nut traps are tight (final assembly is a pain if they are loose). So, start the nuts onto the end of the screws then (drive, hammer, pound) i.e. force the nuts into the nut traps. OR, put an M3x20mm screw through the hole from the inside, start the nut, and use the hex driver to pull “cam” the nut into the bottom of the nut trap. OR, thread a M3x30 screw through from the inside, start a nut, heat the nut with a flame and pull the nut into the nut trap using the head of the M3x30 screw.  I’ve used all approaches but prefer the brute force method (the second method works well but just takes a few minutes longer).

Trim off the brim from the gun halves and any sharp edges that can cut a rubber band.  Clean out the support material on the square ratchet and the spindle shaft.  Note: the trigger shoe tab fits loosely into the square ratchet hole but sometimes support residue partially blocks the opening.  Slide the spindle shaft into the square ratchet and the trigger shoe tab into the hole in the square ratchet piece.  Then snap everything into place in a gun half.  I found that assembling the left gun half worked best since it was easier to get the “elephant’s foot” on the trigger spring over the brace bar.   Press the gun halves together and put in the screws.

Connect a “fat” rubber band to the spindle post using a slip knot.  A round toothpick works well in threading the rubber band through the recess. Stretch the rubber band down the right side of the gun and slid it through the post slot below the trigger guard then loop the end up over the post.

Operation

Place a propeller on the square pin at the muzzle (ratchet toward the gun).  Wind the propeller clockwise (looking down the gun muzzle).  Note: Instruction Repeat: i.e. twist the propeller counter-clockwise when holding the gun by the grip and looking down the barrel.  The trigger should chatter as the ratchet slips.  When the propeller is tight enough aim slightly high and shoot. Over tightening will break the rubber band so keep that toothpick and spare rubber bands handy.

I tried #64 rubber bands (3.5”x ¼”) i.e. (89mm x 6.4mm) but found some ¼” wide rubber bands in a colored assortment that seemed to be stronger and worked better.  Experimentation is the key to greater range.

Final Comment

This has been a fast one week design/development cycle.  I’ve not had time to optimize the propellers.  So I’ve included STL files for an array of propeller designs.  Try them out and send a comment on the best performance you obtain.