From: "Finn Lassen finn.lassen@verizon.net" <flyrotary@lancaironline.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Bell mouth 3D printed
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Date: Thu, 19 Sep 2019 09:15:35 -0400
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My first experience with 3D printing.

I had bought a new Ender-3 and a 1Kg spool of ABS for less than $200.

Article and pictures here:

https://drive.google.com/open?id=1LbQcJIGnjsfNWfL5qiwz5ev7ze9IWAzm

I copied the "best" picture from the RET_Bellmouth_Sept.pdf article, 
pasted it into Paint and saved as Bellmouth.bmp.

Imported that into Fusion360 as canvas. Calibrated it so throat was 
19.972mm (1.18")

Made a spline matching the left curve up to the top.

Added a tangent arc (radius).

Added a 2mm rectangle that extended the throat about 1" with an 1.25" 
ID. (The bell mouth will slide 1" down on the 1.25" OD pipe.)

Added a "looks good" spline from part of the arc down to the throat. (I 
believe that in our case, where most of the air comes from behind the 
bell mouth, air should have a smooth path along the outside of the pipe 
and radius of bellmouth.)

Revolved the whole thing to get the solid bell mouth.

[The result is actually a bit smaller overall diameter than the 
recommended 2.13 * 30 + 2 * (0.08 * 2.13 * 30) = 74mm]

Exported to a .stl file.

Downloaded and installed Ultimaker Cura 4.2.1.

Opened the .stl file with Cura and "sliced" it. 20% infill. This changes 
the solid inside to a criss-cross pattern. Looking at it being printed 
it appeared?? to produce about 3/32" thick walls by default.

Saved result to .gcode file.

Printed with the Ender-3. Took 3 hours and 15 mins.

Nozzle temp default for ABS is 240*C. Spool says 230-260*C. Seeing how 
some of the strands didn't stick and pulled across the mouth, I think 
I'll use a higher nozzle temp on the next print.

I printed it with mouth down. Unfortunately that resulted in a radius 
quite different from the design model. I'll try to smooth the lip on my 
first attempt with epoxy or JB Weld and sandpaper. If I print it throat 
down, it'll probably need support along the outside that'll have to be 
cut after printing.

Will ABS be strong enough? I think so. The airflow should keep temps 
well below 85*C.

Finn