Finally found a really easy guide to making worm gears! So now here is my take on the flex drive extruders.
There are 2 different worm gear sets. The one labeled 1-40ratio is a 1:40 gear reduction to get the same resolution & power as the flex3drive. The non-labeled one is a 1:13 gear reduction to be used with a titan or bondtech extruder which has a 1:3 reduction already. So it should have a 1:39 gear reduction total and perform much like the other worm drive with a not geared extruder.
There are still parts that need to get modeled to make this work but I have found a ptfe shaft that is 4mm o.d. & 2.5mm i.d. on amazon. Its the perfect size to fit a 3/32" coated coated stainless steel wire rope/cable. which i think will work perfectly.
Edit: Totally forgot to mention this uses threaded inserts for M2 & M3 screws. 4x M2X6mm inserts, 4x M3X6mm inserts, & 2x M2X4mm inserts.
You would also need to buy two 5x8x2.5mm bearings. I think I used these in earlier projects: https://www.amazon.com/5x8x2-5mm-Metal-Shielded-Bearings-MR85ZZ/dp/B019I3C4MQ/ref=sr_1_20?ie=UTF8&qid=1542311084&sr=8-20&keywords=5mm+bearing
If it doesn't work I will post but the goal is to find something that fits in a 4mm ptfe tube.
The instructions for making your own worm gear are here: https://imgur.com/a/UzLSB?fbclid=IwAR1BSC21u6nS84WB9XDvFAJJTpcNzj6kwcFWTRNCK5i58TgcOC_oXi2ApAc
Update: Moving 1 step closer to the real deal! So I found out the that Rhino does not produce proper geometries when using the sweep commands. The worm gear set was not correct. I found a way around that by lofting which took way too long but worked. The tolerances are tight enough, less than .001mm I think.
I've also changed how the worm drive couples with a flex shaft. You should now be able to easily attach designs to fit any flex shaft. The coupler and bowden tube clamp are for a 2.2mm flex shaft used for rc boats & 4mm ptfe. I found a place where i could get 1m flex shaft for $30 after shipping and I ordered some for testing. You can also buy it in bulk on alibaba if you want to make a lot of them & get much cheaper per unit.
My current plan moving forward is to print the parts that i can print & have the worm & gear printed at high resolution in resin. The resin parts will then be cast & reproduced in a better resin for mechanical parts.
I think this is the best method for producing them accurately, in larger numbers, & to make replacements.
You can always try 3d printing them. You'll probably want pva or hips supports and a .2mm nozzle though. Let me know if you manage it.