I built this tee at 7 7/8" outer diameter so that flexible duct can fit over the ends. The parameters in the included f3d file can be modified in order to create other sizes.

Assembly

This fitting is designed to be screwed together using 4 drywall or construction type screws. I used #8 x 3/4" torx screws and they provided more than enough bite to hold the pieces together. The built in screw holes are long enough to cover 1 ¼" screws but are open ended so that longer screws could be used if needed.

Printing Notes

Due to the large size of this model I printed it using a 1.0mm nozzle with 0.5mm layer height, 1 perim and 7% infill with 5mm max attachment length. The large nozzle produces very substantial single layer walls and the sparse infill provides a structural framework without adding too much weight. Total weight of both parts was around 700g, total print time around 10 hours.

With larger nozzles, it may be a good idea to use random seam alignment to avoid having a deep seam that compromises the structure of the part.

I printed my first 2 layers of each piece in TPU, adding a pause at layer 3 to switch to PLA. This provides a flexible gasket that helps seal and hold the pieces in place against each other.

Safety Notes

As with any custom fabricated object, use materials appropriate for the application. If any kind of heat or fire resistance is needed, use a metal part. ABS filament with fire-retardant additives may also be a viable option.