Slingshot - lock and load trigger

The objective of this build was to explore and understand trigger mechanisms. In particular, how to achieve a light but reliable lock mechanism and one that re-cocks both the trigger and the bolt in one motion, so that a true ‘lock & load’ process is followed. Secondly, to make the lock pull out of the way quickly to minimise deflection of the projectile. To generate sufficient load to test it, the resulting slingshot had to be larger than the print bed foot print on a MK4S. Hence the slingshot body had to be broken down into multiple parts that could all be accommodated on a Mk4S print bed. Assembly is required.

There are two versions. The ‘short’ version adds the minimum to the length of the slingshot to create a sufficiently strong mount for the elastic and sling. The ‘long’ version has a mount that is as tall as it can be within the print dimensions of the prusa mk4. For those with larger hands a ‘long’ version of the stock butt has also been produced. Both models are intended for two handed use. The siting lugs also act as a cocking lever and additionally as deflection to help protect eyes in the event of the elastic giving way.

For indoor use, the smaller slingshot and PETG 10mm balls should suffice. For external experimentation the longer slingshot used with 8mm clay balls can be used. The length / number of latex rubber strips you use is at your own discretion.

Safety. This model has been made to investigate and explore trigger mechanisms. By definition; this model will be subjected to strong forces. Wear safety glasses when experimenting with it. My infill ratios may not be sufficient for the rubber band power you place on the device and only you know how well tuned and reliable your printer is when printing parts for stress use. Ensure the discharge area is properly prepared and no other person is at risk when testing, I used a pillow to discharge the slingshot into. That said. I created the ‘long’ mount because it was clear that the power being put through the short mount wasn’t troubling the release mechanism at all.

Performance: With the amount of latex-elastic I am using (max so far 4 x pieces 110mm long and 15mm wide by 1mm thick), the release mechanism still appears strong and smooth, even with the increased forces. Within the limits of the mk4 print area and without adding a stock extension (which I won’t do). It can’t get any ‘longer’ and so I do not know what the limits of performance are for either the slingshot or the release mechanism.

You can simply link the trigger and the lock with one spring (rather than 2 independent springs). It works but makes the trigger noticeably heavier, so defeats the object.

Print Notes: The parts are printed in various orientations (to maximise inter layer strength) and with fill density modifiers. The Mk4S gCodes include the following. I used gyroid fill in all cases. The through screws are useful for location, but I also superglued all joints

Trigger. Prints on its top face (the trigger pointing up). 30% infill and build plate supports active

Bolt. Prints on its side. 30% infill and with enforced supports for the trigger flange and the spring flange, plus the voids between the bolt buttresses and the sight buttresses. In addition, a modifier is added that increases the infill to the front half of the bolt to 60% (the end with the bolt buttresses).

Left & Right sides. Prints on side with outer face down. 30% infill and with enforced supports for the various bolt head recesses. In addition, a modifier is added that increases the infill to the upper front arm to 40%.

Front Mount. Prints upright on the end of the fixing post (that connects to the side panels). To ensure bed adhesion I added a 30mm brim. 40% infill. Enforced supports around the fixing post and for each of the rubber band lugs. Enforced supports for the central top arc section of the central cut-out.  In addition, a modifier is added that increases the infill to the upper parts of the mount including all of the rubber band mount extensions to 60%.

Bottom of handgrip. Printed on its end with the fixing lugs upper most. 20% infill since no real stresses on this part. No support required.

Ammunition ‘hex’ balls. No support but a 5mm brim to ensure bed adhesion. 100% infill (GCode prints x5 at a time).

Assembly Hardware (both versions – the short version has been updated to use 2x 20mm – one from each side, for the fixing point of the catapult front mount. It is a more reliable fixing)

2.9mm x 30mm (2.5mm hex roundhead) bolts x3. 2.9mm x 20mm (2.5mm hex roundhead) bolts x6. 2.9mm x 10mm (2.0mm hex countersunk) bolt x1. Source RS Components.

Springs. 5mm dia 10mm extension spring (for bolt fly-away). 5.5mm dia 18mm compression spring (for trigger return). Mixed spring pack, source Amazon.

Bottom of handgrip is a non-structural, non-load-bearing push fit (see, previous - superglued)