After a series of positive comments and very good suggestions by shabaz beacon_dave and mcb1 it's time to go ahead another step. I mentioned the users whose comments helped me to address what in my opinion seems the best solution to proceed. What I consider important in this process is the fact the constructive critics focus the attention on possible alternative solutions as well as are helpful in identifying possible mistakes in the project. Despite I have an almost clear idea of what I aim to reach as the final result, every step reveals how changes and corrections are needed. What I am sure at this point is that the final result will be adapted in a different way than what I have supposed when I started the design.
Moving the Torso: the Stepper Motor
After thinking and simulating the possible solutions the most reliable seems a teeth belt rotating a pulley (only of some degrees) driven by a stepper motor to grant the needed precision but also to have the required torque. By the way, I hope that with a 140 mm diameter pulley the Nema17 stepper motor can move the upper part of the mannequin without problems.
The sketch in the image above is the first design on how to position the stepper motor to control the rotation. The main advantage of this solution is that the space between the dorso top-side and the top plane of the legs is the most reduced possibly. The thickest part is the motor that results submerged inside of the right leg. A small further change has been applied designing the 3D printable object with Fusion360.
The rendering below shows the support from the bottom side. I have added a small contour where the stepper motor can fit; this means that the only visible part of the motor is the shaft with the teeth pulley and 3 mm thick of the upper part of the motor support.
After 3D printing, I found the farthest point where the support can fit leaving as much space as possible to the center area as the rotating platform should be as large as possible. Then, I have cut the hole and prepared the support to be kept in place.
The motor has been screwed to the support with foud Allen screws, accordingly with the measures shown in the Nema17 data sheet. The image below shows the motor in position ready to be connected to be connected to the rotating platform.
Don't miss the making of the rotating platform in the next episode.