Ok, my printer is still broken, despite attempting a short-term epoxy fix. My replacement parts are still are not here but due in the next couple of days. I had planned to walk through a first printed piece, from positioning in the slicer to holding it in my hand.
Instead, here are few things I have learned about 3D printing, and specifically, 3D printing aircraft.
Once your model is ready to print, the output from your CAD software is usually an STL file (an abbeviation of stereolithography). This file is a representation of the shape exported from CAD, made up of lots of little triangles and containing no other information about the model, i.e. material, colour etc.
If your STL is a solid body, say a cube, the file contains no information on how to print this. This is where a slicer program comes in. The slicer, as the name suggests, slices the model up in to layers and generates a path for the print extruder to travel. At the moment the model is a solid body but the slicer can decide how to handle this. we'll come to that.
There are a number of slicers out there, some free and some paid for. I have found myself using Cura (free) as it does everything I need it too, but I can't tell you why you shouldn't use a different program.
Lets load our cube into Cura. The first thing that you'll notice is you can adjust the positioning of your model, for our cube all the sides are the same so it doesn't matter. For an aircraft, choosing which part of the model touches the build plate is critical, as demonstrated here.
Using just the basic options we can look at a sliced version of the cube. With 100% infill the slicer generates a path that fills the cube, it will take ages to print, use a lot of material and be pretty heavy (from a flying point of view).
Here is 30% infill, its still pretty busy in there, but the print time has been reduced from 13 hours to 4 hours for a 50cm cube. The weight is also reduced from 160g to 60g for a negligible reduction in strength. I have used a cubic infill pattern here as it provides strength in all directions, I'll leave you to play with the different patterns. They each have their strength and weaknesses.
Notice also that the walls of the cube have 3 lines of filament. This is the wall line count.
Lets see if we can get the weight down even more. Using 10% infill we halve the weight, down to 33g. What about the number of walls? Changing the wall thickness to 0.4mm ( the extruder diameter) and the wall line count to 1, brings the weight down to 24g.
You can see how critical the slicer settings are to the weight and structural strength of the model.
Moving on from a simple cube to a section of the aircraft. Taking the aft firewall of our Cessna we can look at how these settings combine. During the design of the model I chose a thickness for the fuselage edge/wall (I don't want to call it wall thickness as I don't want to confuse that with slicer wall thickness). I made sure that this was more than 2 X 0.4mm knowing that giving the slicer the instructions to print the model with a single wall 0.4mm thick would produce a print that has two walls separated by a connecting infill (this is because it treats our STL as a solid body). This has a number of advantages. First, it is lightweight and strong. Second, having two walls allows a bit of redundancy incase of any small print defects, which in a single wall design could be catastrophic.
This firewall section of the model is a unique case. Generally I want to keep the infill percentage low, just enough to glue the two wall together, however here you can see two potential problems with this.
You can see the four small cylindrical posts sticking up, they are for the screws from the motor mount. Having a low infill level leaves this with out much reinforcement, the bottom layer is only 0.8mm thick. It's possible on the from firewall where the motor is pulling away from the firewall that the screws would simply pull out of the model. Also the top layer of the firewall needs to be bridged, the higher the infill percentage the easier it bridging is.
We are left with a compromise, increase the infill percentage to support the screw 'holes' and bridging at the expense of weight. This is another reason to think carefully when sectioning the model. In order to help the compromise, we can change another setting for this model which is the bottom thickness, raising this to 2mm gives the screws more to hold on too without having to needlessly increase the infill across the whole component.
A few other settings to think about.
Single wall printing. Layer adhesion is really important since we only have one wall and a low infill percentage. As such I tend to print with higher temperatures and lower fan speeds. 3D printers (and filaments) differ so it is important to realise that each individual is going to need to tweak their own settings
Print bed adhesion. It goes without saying that the part needs to stay on the print bed. Having a correctly levelled bed is the number one thing to check but when printing aircraft it inevitably has tall thin parts such as ailerons and elevators. For these parts I will usually use a brim adhesion, and make the adhesion bigger than standard. This is so that the part has more support when it is under stress from extruder drag towards the top of the model.
I hope this article has been helpful, fingers crossed we can start printing the Skymaster tomorrow!
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