Main Gear Box (MGB) for Helicopter driven by 2-Engines has complicated gear train in inside.
To understand this mechanism, I designed the internal visible model.
There are many different designs for each actual helicopter, therefore this model is just a sample.

The features of MGB are;
1. 2-Engine Inputs
2. 1-Tail Take-Off Output (moved to the opposite side to clarify)
3. 3-Stage Reduction (including 1- Planetary Gear) and 2-Stage Reduction + 1-Stage Increase
Note: (1) Gear types used in this model are changed due to ease designing and printing.
Bevel Gear → Spur Gear + Face Gear, Helical Gear → Spur Gear
(2) Clutch and Accessary drives are omitted at this time.

Download files includes;
-files: STL Files
-images: Parts Breakdown and Sub-assy photos for assembly Aid.
-Excel File (ZIP): Parts List with related hardware

Purchase Parts Information;
-Bearings; No metal Ball/Roller Bearing is used. Then some grease for plastic is recommended for lubrication.

-ABS Tube
See Parts List

-Screws and etc.
Micro screws M1.4x3.5L - 5 ea - Direct screw-in to 1 mm diameter hole (No nut),
Screws M2 x 6L - 20 ea, See Parts List
Screw M2 x 25L and Nut+ Some tube for Turning Handle (Optional)

Printing parameters recommended (0.4mm Nozzle is used)
- Gears: 0.2mm Layer Height, 100% infill
- Bodies: 0.2mm Layer Height, 40% above infill (for threads)
Raft and Support: Depending on your experience

Assembly Points
- Standard Skill (such as Drilling, Tapping, Filing, Tightening, Painting and Gluing etc.) is needed and perseverance too.
- For gears: Deburring (removing the sharp edges etc.) is very important for smooth rotation.
- If you do not give up to finish the gear teeth, then you can get the smooth rotational MGB model.
- Good luck!!

Total Net Print Time: Approx. 28HR
- (Estimated as case of PLA, 0.4mm Nozzle, 0.2mm Layer Height, 40% infill and No raft and support)
Note: When at actual print, each parameter should be adjusted by your experience.

【Optional Parts】
TTO drive is moved to same side as Engine input. See photos.
- Body-Mid201.stl → Body-Mid901.stl

I do hope your success!!