This model is probably difficult to assemble, so take your time. The better you understand automatic transmissions, the easier the assembly process will be.

I was always interested in how automatic and manual transmissions worked, this is an exercise in understanding and design I did in my freetime. I appreciate any and all constructive criticism to help improve this model.

1. print every piece, the number of pieces necessary is mentioned in the file name, if you need more than one.
2. place “planet carrier back” on a flat surface, then put three planetary gears on the provided studs/ axles. The fit should be loose to ensure easy rotation of the gears.
3. place 1st sun gear in the middle to mesh it with the planetary gears, fit should be tight, but the gears should all rotate freely.
4. slide “ring gear (print 3 times)” over all three planetary gears, again tight fit but easy rotation once correctly mounted.
5. place  “planet carrier front (first set)” on the three studs of the back planetary carrier to complete the first planetary gear set. Be careful to not squeeze the gears.
6. insert “intermediate shaft” into the first sun gear, and slide "spacer between planetary gearsets onto that shaft. Fit sould be loose
7. push “front planetary carrier second set” all the way onto the previously mounted spacer and insert the pins from that piece into the corresponding holes in the first ring gear.
8. push the second sun gear all the way up to the fron planetary carrier of the second set along the intermediate shaft.
9. seperately assemble “planet carrier back (print 3 times)”, 3 more planetary gears and the second ring gear and set them down next to the previously assembled parts.
10. lower the previously assembled parts through the hole in the “planet carrier back (print 3 times)”. This part is tricky, make sure the planetary gears and the sun gear mesh properly. The three pins, which also serve as axles for the planetary gears, should into “front planetary carrier second set”. Now the second of three planetary gearsets is assembled, almost done!
11.  put the pins  on “inside connector” inside the three corresponging holes on “planet carrier back (print 3 times)”, be careful not to brake them!
12. push the pins on “front planetary carrier third set” all the way into ring gear of the second set, again beware of damage to the printed part.
13. take the last three planetary gear, “sun gear (third set)” and “planet carrier back (print 3 times)” assemble them in the same manner as done before. Make sure all gears can freely rotate. The part “inside connector” should fit throught the larger hole on the third sun gear.
14. fit “connector sun gear to C2” into the smaller holes on the back of the third sun gear
15. fit “C2 clutch disc” on the very end of the “inside connector”
16. fit “intermediate shaft connector” to the very end of the intermediate shaft
17. fit “C2 clutch counterpart” onto the three fingers sticking out the side of “connector sun gear to C2”
18. last piece: fit the “driveshaft” into the shape of the back of the c2 clutch counterpart.

Added some more picturs to better show which pieces go where and how they interact. A video to further explain how an automatic transmission works and how to select gears can be found here: