Variant construction set

So, this is Variant construction set, which was originally designed about 40 year ago by one Czech company and it is still produced till nowadays:
https://www.kaden.cz/variant-stavebnice (please support the producer by buying the original set if you can).

It's a LONG time ago since I was playing with it, but I still clearly remember the hours and hours of joy and entertainment it brought, as well as the skills I acquired by building different constructions.
Recently I got into possession of one of these original sets and gave it to my son (3yo). He also likes it a lot. And then I realized that with 3D printing I can create more and new parts, limited just by my imagination. So I recreated most of the original set in SCAD and added a lot of new parts.
From the original set I only did not try to model the chain gears since the chain consists of quite small parts and would be difficult to print accurately enough. On the other hand, I added many different gears to the original ones (12T, 24T).

For quick overview see: Part list with print instructions.pdf
(Also because Thingiverse was unable to generate thumbnails for all the parts)

For ideas from the original set see: Variant - original manual.pdf

Regarding printing material, the original parts (especially the beams and flat parts) are quite flexible but strong, I would say they are from nylon.
I recommend to print most of the parts from PETG (including screws), but PLA would also do. The only problems with PETG I have is warping (even with heated bed and enclosure), therefore I made a version of the most parts with localized brim in corners to improve adhesion. These parts are with "_F" in the name. But you can also use full brim in Cura.
Only components I print from PLA were gears because I did not want to bother with brim on those. Also boxes did not need PETG flexibility and were easier to print with PLA.

Regarding layer height, most of the components is really insensitive and I recommend 0.28mm. Only anything with thread I do with 0.2mm.

Regarding infill, 20-30% for most of the components, 60-100% for screws and nuts.

Regarding threads accuracy, I designed and tested everything to be mix-and-match compatible with the original parts. For that I chose 7.9 mm for M8 screws and 8.8 mm for M8 nuts. I also prepared 8.6 mm M8 nuts in case you find it better. Anyway, I recommend to print the screws and nuts one-by-one to reduce stringing. And I also use M8 tap to clean the nut threads after printing. It works great for me. If you find thread too tight/loose, you can also change a bit material "Flow" factor in Cura (too tight -> reduce by 5%, and vice versa).
For the big (M14) threads for axle clamps, I use a tiny bit of grease to reduce the friction. Also with great success. BTW these clamps are a bit fragile for handling, but due to the backwards compatibility, I did not change the design. In the worst case I print a new part :-)

Regarding holes accuracy, the original holes are 8.5 mm, while the screws and axles are 8 mm. I designed the holes 8.6 mm because I ended up with about 8.2-8.4mm holes after printing and clean-up. Only the center hole in the gears and everything with threaded clamp is designed 8.4 mm to have more tight fit.

Gears
Not all gears are compatible, the rule is that the sum of teeth of both gears must be multiple of 18.
e.g. 12T + 24T (1:2 ratio) = 36T = 2*18.
The multiple will also tell you how far apart the gears are placed. In the example 2 holes apart.
I prepared these gears and combinations:
6T + 12T (1:2)
12T + 24T (1:2) - the original gears
9T + 27T (1:3)
6T + 30T (1:6)
6T + 48T (1:8)
Worm gear + 24T (1:24)
Worm gear + 6T (1:6)
You can also make many more combinations using the attached SCAD file.

Regarding other, non-printable parts:
8 mm wooded rods (original lengths are 12 cm, but this is up to you). I buy beech wood rods 0.6 euro per meter from my local hardware store. Only remark: check the diameter with precise caliper when you buying it. I found that they can way piece by piece from 7.7 to 8.3 mm. For me 7.8-8.0 mm fits the best the holes of the threaded clamps.
I would toss in a few M8 washers for when you want to reduce gears side friction.
Also a few 8 mm ID o-rings are handy for adding friction to when needed.

Last thing: I plan to extend the set of components in the future. In that way I'm also open to your suggestions. And I'm also looking forward to you creations :-)