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Aug 04, 2020, 08:43 AM
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Build Log

Bell 206 JR on OMP M2 continued


Slicer
For anyone who is not familiar with the 3d printing process, the slicer is the software that converts the CAD output (typically an .obj or .stl file) into a printer input file (.gcode). The slicer controls lots of printer parameters, such as nozzle and bed temperatures, fan speeds, layer heights, extrusion rates and head speeds - all of which can vary during the print. I've gone with Prusa's slicer (a version of the open source Slic3r) because they have profiles already set up for various filaments and nozzle types, which means many less things to worry about for initial prints. Everything is tweakable later, if you know what to tweak and what the consequence will be (I don't, so I'm going with the stock settings for now).

Filaments and Nozzles
I'm starting with PLA filament, as it is widely recommended to do so. It is regarded as the easiest material to print with and has fairly good mechanical properties. The downside is that it can degrade outdoors, it is somewhat brittle, and it has a low glass transition temperature. The plan is to get some experience with this and then move on to ColorFabb's XT (tougher than PLA) and XT-CF20 (stiffer than PLA). The later has 20% carbon fibre in the mix and requires a harder nozzle due to its abrasive nature, so I have a tungsten carbide nozzle waiting in the wings. That nozzle is 0.6mm compared to the standard 0.4mm brass nozzle that comes with the printer. The larger nozzle should also result in faster print times and stronger z-axis strength, at the expense of slightly lower resolution. Lots to experiment with.

First Prints
It was a strange feeling to have invested so much time and effort in the design, working solely in the virtual/digital world with nothing physical to show for it yet. There is also a point in the design process where you need to know what kind of tolerances to leave for parts to fit together, or how thin a bit can be made through 3d printing. I was just hitting this point when I got the printer up and running.

The first part I selected was the horizontal stab, as it is a simple rectangle with an airfoil cross section. I decided to print it sitting vertically on the bed (so the z axis along the span) to get a nice clean cross section. This worked well enough and led me to increase the thickness of the trailing edge to get the full cord width, as 0.8mm or so was about minimum thickness produced. That equated to about 2 perimeters during the print, which is kind of obvious if I had thought about it.

Next up was the vertical stab. This is slightly more complex as both ends taper out from the middle. This was also printed on end, and unfortunately fell over near the end of the print. Lesson 2 - you need to put a brim on tall tower like objects to securely anchor them to the bed. How the horizontal stab managed to stay upright was a bit of beginner's luck. This was re-sliced with a 4mm brim and at the same time, I set up the tail boom to print along side it. The tail boom is a much more complex part. Much of it is a thin shell that is similar to what is planned for the fuselage, there is a cut out where the horizontal stab slides in, and the end where the tail motor mounts has lots of cut outs including an odd shape that the vertical stab locates onto.

These prints came out well with just a bit of fine 'hair' stringing as the printer moved between the two parts. The horizontal stab slotted exactly into the tail boom (where I included no tolerance) and the vertical stab located onto the boom with some wiggle room (where I had build in some tolerance, which I can now remove). Now I need to test fit the tail motor and try tapping some threads to see how well a screw will hold the vertical stab on. And move onto test printing the fuselage . . .
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Aug 19, 2020, 09:05 PM
My name is Naga
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Looks like you are making good progress.

In these photos the parts appear to be very finely printed, I cannot see any layering.

Was the tail boom printed on end too?
Aug 31, 2020, 09:40 AM
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Yes, the fuse and tail boom were printed on end.


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