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This is my latest design, and I'm really stoked with how tough it is, and how easy it is to do common repairs.
I like taking risks with my flying (to my quad, not people) and as such value ease of repair over big-brand, expensive setups. So, I came up with this design, and it's already been subject to a heap of abuse and been back in the air within minutes of a crash. All you have to do is take some cable ties when you head out! :-)

The following will show the steps I went through with this particular build, but this Instructable is mostly intended to illustrate the general design, and it's up to you to adapt it to whatever are the critical components it needs to conform to, eg. motor size, prop size, etc. The essence is the durability and easy serviceability.

Step 1: Frame Geometry and Design

The photo below shows the basic structure of this frame design--the sandwich of two layers of MDF wood, with the carbon fibre square tube arms. You can't see them, but I also put five nylon spacers (10mm in length, to match to square tubing) in between, to add some extra strength to the frame, and also help prevent twisting of the frame.

The nylon spacers also serve as 'stops' for the arms, when pushing them back in after a repair, so that they sit at the correct depth into the frame.

But, the real ease-of-repair comes from using the cable ties to secure the arms in place. NO GLUE. For the front arms especially, they typically will be impacted towards the rear of the quad, and in these cases when the arm is free to slide inside the frame sandwich, all that typically happens is one of the cable ties just breaks.

As long as the arms don't twist then the quad will fly perfectly fine. Also, in my build, the arms can even shift forwards/back slightly, but even that doesn't affect the flight characteristics. Just as long as the arms don't twist!

For this build I used 10mm square tubing. Here's a link to product page: 
http://www.hobbyking.com/hobbyking/store/__9012__carbon_fiber_square_tube_750x10mm.html

Nylon spacers: 
http://www.hobbyking.com/hobbyking/store/__29540__5_6mm_x_10mm_M3_Nylon_Tapped_Spacer_10pc_.html

Step 2: Planning the Frame

Again, the specifics of what will work with your particular RC bits and pieces will vary a lot, and you can come up with your own specific geometries, for whatever fits your gear.

The drawing below is what I used for the Micro FPV Quad (also here in Instructables), if you want to use that as a reference guide. This build used the exact same proportions, but scaled up by 50% (compared to the micro). I think the diagonal of motor to motor is 440mm in this build.

Step 3: The Upper and Lower Plates

I decided to make the upper and lower MDF wooden plates different, so that the upper one has extra surfaces at the front and back, for things like the camera, and transmitter components. Below are some photos of the MDF sheet, with the pieces drawn out, prior to cutting, and then after drilling some holes through the result.

The holes only save a very negligible amount of weight, but I find the holes incredibly useful later, for doing the wiring! You have much more sneaky routes to get things hooked up :-)

Also, the drawing on the MDF has three small circles where I planned to put three nylon spacers. These are the ones that act as stoppers for the arms. I added a further two, in the middle of the frame later on.

Step 4: The Motor Mounts

I used a similar design for the motor mounts as with my Micro FPV. I made custom carbon fibre plates to match the motors I am using. Again, you can customize this to whatever size motors you might be using. Below is a photo of the paper template I drew up first, and the four mounts that were cut out. 

This is a very fiddly step, and it pays to do it carefully! But, essentially, you're just making something that you can put two bolts through to secure the motor, and two places at which cable ties can go around it to secure it to the square tubing arms.

Also below is a photo showing the end result of one of the mounts, but this is jumping ahead slightly.

Step 5: Getting the Mounts Level

Usually, the square tubing arms should result in four LEVEL motor mounts, one they're placed onto the arms, but not necessarily. I found that using a bit of Epoxy on the ends of the arms, then placed upside-down onto the four mount plates (as pictured) results in a more precise result. The epoxy will squish slightly if there are any irregularities in the arms, ie. and slight twisting. Once dry, then the mounts can be cable tied to the arms.

NOTE: be sure to place the mount plates fairly accurately into position before allowing the epoxy to dry!

Then end result should be like in the next photo. (although, I haven't yet attached the cable ties in that photo)
I was also checking the weight before the electrics were installed, and in this case came in at about 170 grams.

Step 6: Wiring It Up

There are many ways to skin a cat, and the same is with wiring. I found that my ESCs fit nicely in between the frame sheets, so they could be tucked away.

Also, a week or so later, I actually snipped the main cable ties holding the arms to the frame, and unscrewed the nylon spacer screws, and took apart the frame, and laid out a lot of the wiring, and then screwed the frame back together.

but, for the first test, I just did an ad-hoc wiring like in photo below.

Step 7: The Final Build

Below is the final layout I came up with. Even though this is a bigger frame, compared to my Micro, it can still get pretty crowded when all the gear gets put in.  Again, this will vary a lot depending on whatever FPV gear you have and want to install.

The final touch was to carve out a custom foam canopy, which is held in place at four points on the frame with strips of velcro (ie. hook-n-loop tape). This gives much need protection to all the electrics, but with the velcro makes for easy serviceability of all the components.

Step 8: The 'Crash' Files

I've had a number of pretty big crashes with this quad, and the worst has been a bent motor mount plate. (I just whipped up another one from my drawn template). Most crashes have been one or two cable ties, and back in the air!

The second photo was two cable ties. It looks pretty bad, but was quite a simple on-field repair!

Anyway, I hope you get some good take-aways from this IN-deSTRUCTABLE :-)
<p>This one looks like it would work remarkably well. </p>
Yes, it does actually :-) Also, recently I've hooked up a 4S lipo (instead of 3S) and it just screams along now!

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