Round vs. Square Arms
Above is a picture of my cheap quadcopter frame before adding all the electronics, motors and wiring. It has less than $10 worth of parts in it and flies well. The most important thing I have learned as a novice builder/pilot about frames is they need to be square and level. As you can see, my frame uses square wood arms. I think it is easier to make sure everything is square and level by using square arms. The motors and legs are square and level with the frame and ground by design when using square arms.
However, most quadcopters I see use round arms where nothing is square and level by design. With round arms the motors and legs can be rotated around the arms. So, they have to be in just the right position to be square with the ground. This involves eyeballing and measuring to try and get it right. On the other hand, it does allow for micro adjustments, whereas square arms are pretty much set in position.
Anyway, I opted for square arms and tried to build my frame using steps to make sure everything is square from the start. The complete bill of materials are in my previous post and here are the steps I followed to build it…
Step 1 – Marking the Center of a Ciricle
The first step was to mark the 5″ acrylic circles I used for the center plates. I needed to find the exact center of the circles to secure the arms in a square position for drilling. First, I placed the corner a square at the exact edge of my circle then marked two points where the sides of the square crossed the edge of my circle. I connected these two points. Next, I did the same thing again after moving the corner to another location on the circles edge. The two lines intersected at the exact center of the circle.
Step 2 – Marking the Alignment of the Arms
Either of the lines I drew above could be used to line up one of the the wood sticks I used for arms. I needed another line exactly perpindicular to line up the other arm. To draw the second arm alignment line, I just used the same square and lined it up at a 90 degree angle to the first line.
Step 3 – Drilling Center Holes.
Next, I drilled holes, with a 2mm drill bit, in the center of my acyrlic plates and arms. Since I have two center plates, I put them on top of each other before drilling them. I also marked the sides of both plates at the edge so they could be easily relaligned when seperated.
I decided to make my arms 15″ long from the center for a total of 30″ across. This allows an extra length of wood to extend beyond the motor and protect the propellers in a hard landing. So, I marked two wooden rods at 30″ long and 15″ in the center. I then cut them at 30″ and drilled a 2mm hole in the center at 15″.
Step 4 – Drilling Holes to Attach the Arms
The center holes I drilled were used to attach each arm to the center plates (with a 20mm long x 2mm screw and nut) for drilling some holes. These holes are for securing the arms to the center plates. I screwed each arm between the two center plates, aligned them with one of the lines I drew in step 2, and drilled two 2mm holes on each side of the center. I drilled one hole 1 inch from the center and one hole 2 inches from the center on each side. By drilling the holes with everthing attached at the center and aligned, I insured that I would get a square fit. After drilling one arm piece, I removed it, screwed the other arm piece in the opposite direction and drilled the holes for the second arm piece.
Step 5 – Cutting and Attaching the Arms
Since both arms could not be screwed between the center pieces at the same time, I measured and cut a section out of one of the arm pieces to allow the other arm piece to cross through it. Finally, I took off the paper covering the center plates and screwed it all together.
Step 6 – Measuring and Cutting the Legs
Another nice thing about using square arms is that it makes it very easy to attach any rigid material to them at a 90 degree angle for landing gear. Like the motors, if the landing gear is carefully cut, it will keep the quadcopter square and level. I made my legs out of 4 equal sized sections made from a 5″ piece of round acrylic. I used the same method I used in steps 1 and 2 to divide my circle into 4 equal parts. Next I cut the acrylic along the lines with a miter saw to get my 4 pieces.
Step 7 – Marking Holes on the Legs
To attach the legs to the fram with zip ties, I had to drill some holes into the legs. I marked the spots to drill by laying a scrap piece of the wood used to make the arms across the top edge of on of the legs, aligning the bottom of the wood with the two corners at the top of the piece. I drew a lines on each side of the wood. This is where the zip ties will be centered after drilling the holes. I also drew lines down, one width of wood from the center, to intersect with the lines across. The intersection of these lines is where I drilled my holes.
Step 8 – Drilling Holes in the Legs
To makes sure all of my legs had holes at the same place, I stacked them all together and drilled through all of them at once using the top piece that I marked as a guide. Also, since the pieces were not exactly the same size, I lined them all up with the point at the bottom. This way each leg would be attached at exactly the same height from the ground. So, I drilled all of the holes, removed the paper from the acyrylic and attached the legs to the frame. Finished!
Almost Finished, Anyway
This is obviously a very basic frame and I left out everything about attaching the battery, the cover and all the other stuff.
The battery I use fits well in an X quadcopter configuration between the screws I drilled at one inch from the center. I attach it with zip ties through holes I drilled in the bottom center plate. It could also be attached with velcro, but I find the large zip ties I use hold it securely without replacing them each time I change the battery. I just squeeze it in there.
To use a velcro strap to secure the battery, a larger slit would need to be cut through the bottom center plate and the wood arms would need to be cut back in the middle to allow it to slip through.
For a lid, I swiped a 5 inch Rubbermaid bowl from the kitchen. I just use some scotch tape to secure it on four sides.