CAD Design for Arm

Now that the main body is done, the next step is the arms. Now I know exactly how they have to fit into the frame, because I designed the fitting earlier, and the female part of this is already built into the frame centre.

The first problem I noticed is that the radius of the propellers I’ll be using is about 130mm – the same as the length I had set between the body and the motor centre. The frame is angled so that motors this distance from the frame will be equidistant, but I obviously need some clearance between the propellers and the frame, and so I have regretfully extended the arm length to 160mm to allow for this.
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Base Parametric Design for the Frame Centre

Ok I’ve started modeling the CAD file for the main centre of the frame, which will contain the electronics and hold the arms in place.

The first step is to create a base sketch that maintains the shape that I want. One thing I do want to ensure is that the motors are all equidistant; that they form a square. This, while not strictly necessary, means that the quad will react in the exact same way in both directions.

Because I’d rather have a longer and thinner frame than a big fat one, I’ve decided to have a rectangular space for the components in the middle and have a separate space on each end for the attachment of the arms:

A quick sketch of what I mean
A quick sketch of what I mean

In the CAD software (Autodesk Fusion 360, which is essentially the same as Inventor but cloud-based), I have opted to make extensive use of ‘parameters’, akin to variables/constants in code: I can set dimensions to ‘centre_width’ or ‘fitting_depth’ etc.
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Test Print of Attachment

I’ve done a test print of just the attachment between the arm and the frame, to check that everything lines up and fits properly; it works beautifully.

Looks a bit ugly but that's because I haven't cleaned it up - it's actually a beautiful print
Looks a bit ugly but that’s because I haven’t cleaned it up (the lines are from where the support material was) – it’s actually a beautiful print.

There are a couple of small problems that this has revealed with the design. First, the screw won’t physically fit underneath the attachment for the bottom mounting, let alone a screwdriver: something that I overlooked. The same will be true for the left and right sides when it is part of the bigger design for the frame.
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Fitting from Arm to Frame

Before I can get together a final design for the frame of the quadcopter (now that I know what the inside is going to look like) I need to decide on and model a connection from the four arms to the central body. They can’t all be printed at once because my printer isn’t big enough, and anyway it’s always better to print smaller parts in case anything goes wrong or you have to change something.

At my disposal, without having to go and buy anything, are lots of countersunk M3x10, M3x8 and M3x6 screws, and so I’m going to use these along with some nuts to attach the arms to the frame.
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