Summary

I have changed the "bucket" to be wider for bigger "projectiles" and added a thumb wheel of easy range adjustment.

But the bigger redesign is the fulcrum. I split it into two parts to be printed easier.
Now the hole for the axle goes now through all the way, so a longer threaded is needed

https://youtu.be/aHacG3KJ2HM

EDIT 4/30/2019

As mentioned below I managed to break/wear out the spring rather quickly.
Hence I added a beefed up version. This might not be the engineering approach of Leonardo as he had presumably no access to rubber bands. I use my catapult as an office toy and am therefore not too concerned with historical accuracy.

P.S. I will need to redesign the spring and strengthen it. It suffered material fatigue due to tightening the rubber bands to much, because I wanted to increase range and power. Stay tuned!

P.P.S: Here is the original:
https://www.thingiverse.com/thing:3436588

Print Settings

Printer Brand: 

Prusa

Printer:

i3 MK3

Rafts: 

No

Supports:

No

Resolution: 

.1

Custom Section

Materials Needed

M4 x 25mm Cap Screws - 6rnM4 Nut - 1rnM4 x 15mm Cap Screws - 2rnM4 x 20mm Cap Screws - 3rnM4 x 35mm Cap Screws - 1rnrnM6 x 35mm Cap Screws - 6rnM6 Threaded rod 150mm longrnM6 Hex Nut - 9rn(M6 Nyloc Nut - 2, replace 4 regular nuts)rnM6 Washers - 7rnrnAn assortment of elastic bands (smaller ones will give greater power for the arm