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02 Stuff to 3D print
In the repo, there is a folder called "CAD files". This contains the design files for the motor mounts, T-mounts for spars, etc, in the .scad format of the free software OpenSCAD (which, by the way, is awesome for this kind of prototyping design, provided you're OK with coding rather than a GUI tool).
These need to be 3D printed! Within the folder, there is a subfolder called "STL files for printing." The STL files, in contrast to the .scad design files, are suitable for directly feeding into a 3D printer (almost all of which accept .stl files as input).
By the way, if you click on any of the .stl files in the GitHub folder, a 3d viewing window pops up allowing you to see, rotate, and zoom the part. I'm not sure if this works in all browsers, but it works on all of my computers in Firefox and Chrome. Do take a look!
The main question here is what spars you are using, 15mm carbon fiber (or 15mm aluminum) or 17-20mm bamboo (a marvelous and inexpensive option, highly recommended).
Please don't use anything flimsier; if you use, for example 10mm carbon fiber you'll get unacceptable levels of vibration, and 15mm wooden doweling will do the same (unless you use really hard wood, in which case you'll be adding a lot of weight). Less than about 16mm bamboo is also likely to vibrate a lot; we recommend aiming for about 19mm average-diameter bamboo spars (excluding the fatter nodes; we're talking about the main shaft diameter).
If you are using 15mm carbon fiber, you will need to print 4 of the "Combo_motor_mount_for_15mm_spar_D2830_motor", and 4 of the "Tmount 15mm rod 10mm thickness." The T-mounts allow you to strap the spars onto the fuselage, and the motor mounts do just what it says on the box (mount the motors).
An alternative to the combo motor mounts is to print 4 extra T-mounts, and 4 of the "Motor_mount_X"; this obliges you to use small bolts to attach the x-mounts onto the t-mounts but means all of your mounts are interchangeable (both mounting the spars and motors is done with the T-mounts).
Print 4 of the "Motor mount for 20mm bamboo spar and D2830 motor." Those are necessary no matter what, but for the rest you have some choices!
You need something to put on the opposite side of each motor mount to clamp it to the spar. You can print 4 more "Motor mount for 20mm bamboo spar and D2830 motor" and use the extra motor mounts as clamps; put two motor mounts facing each other on either side of the bamboo, put bolts in them to clamp them together over the bamboo, and Bob's your uncle. This way, if a motor mount breaks, you can just reverse them and continue. Alternately, you can save a bit of printing time by printing either 4 of the "Motor mount bottom clamp for 20mm bamboo spar" or 4 of the "Motor mount bottom clamp half-width for 20mm bamboo spar" (this saves a bit of printing and weight, and the full-width bottom clamp is probably stronger than needed).
Now you need to mount the spars onto the fuselage. You'll need 4 of either the "Spar T-mount for 20mm bamboo" or 4 of the "Spar T-mount half-width for 20mm bamboo". We usually use the half-width, as the full width is not necessary unless you're using particularly soft foam for your fuselage. To clamp these onto the spars, you'll need 4 more "Motor mount bottom clamp for 20mm bamboo spar" (or the half-width equivalent if you've used half-width T-mounts).
So for a quad with bamboo spars, you'll be printing 16 pieces altogether: 4 motor mounts, 4 clamps for the motor mounts, 4 T-mounts, and 4 clamps for the T-mounts. Whether you use full or half-width for the clamps and T-mounts is up to you, but we recommend the half-width unless you have a specific reason to want more strength.
The two materials usually used in 3D printing are ABS (Acrylonitrile butadiene styrene) or PLA (Polylactic Acid). ABS has a bit more flexibility and give, while PLA is stiffer and more brittle. For this reason, ABS is a much better choice for these parts, which are intended to flex a little bit to grip onto the spars. PLA does work, but is more likely to crack if over-tightened, and we don't recommend it unless you have no way to print with ABS.
The print settings we use are 30% infill, with 3D honeycomb infill pattern (this will make sense to a 3D printing person). If your parts seem flimsy due to low-quality plastic, by all means increase the infill density, but keep in mind that too high a density will result in brittleness; the parts are designed to be a bit bulky to contain some air and facilitate a bit of flex. Don't go to 100% infill (maybe 75% max). Not all printers support 3D honeycomb infill; other types are likely to work nearly as well.
Whatever you do, print a few test pieces first, and try them out! Then print your main run.
Also, make a few spares. Nothing more frustrating than being in the field and not able to fly due to a broken or missing part that would have taken 7 minutes to print.