My objective is to build a butterfly capacitor for a magnetic loop antenna.
Capacitors that can handle high voltage are expensive. Building your own is an option if you have a CNC machine. The plates for the butterfly capacitor can be made from thin aluminum sheet. But before I try cutting aluminum, I need to design the capacitor plates.
This gave me a reason to get my first experience with HSM Express, a computer aided manufacturing (CAM) add-in for various CAD programs.
There are two types of plates needed for a variable capacitor: rotors and stators. As shown above, the rotor plate in a butterfly capacitor is shaped, not surprisingly, like a butterfly. The rotor will rotate between two pairs of stators (or stationary plates.) These can be held in place using threaded rods, with spacing between plates set by inserting washers, nuts or other kinds of conductive spacers. An example of the finished product is shown on the top right of the picture.
Each section will provide a certain amount of capacitance – highest when the rotator plate is fully enmeshed between the pairs of stators. To build up to the amount of capacitance you need, just add more sections. The amount of capacitance of each section depends on the area of the plates. The area of the butterfly rotor plate shown above on the left is about six square inches. A larger air gap between the sets of plates determines the amount of voltage the capacitor can handle without arcing. Unfortunately, increasing the air gap also decreases capacitance.
So, there are tradeoffs to be made when I do a proper design. For now, my objective was to use CAD for drawing the shapes, and then use HSM Express to create the G-Code for manufacturing the plates.
Milling a Butterfly Capacitor – First Try
My first try, of course, was to cut some Styrofoam. First, I did a simple sketch of what each plate might look like using the CAD software. The sizes used were just guesses, but will be close enough to the final requirements for this proof of concept. To give you a sense of size, the holes in the plates are 1/4”. Second, the sketch was extruded to a solid depth of 0.025 inches, which will probably be the size of the aluminum sheet I will eventually purchase from Home Depot. From what I have read, sheets of 25 or 40 thousands of an inch thickness are typically used.
Second, I used HSM Express to create the tool paths of each part (stator and rotor) and create the G-Code. When doing so, I used a post processor script to ensure that the G-Code was formatted for my CNC.
Third, I fired up my MPCNC and milled both parts in Styrofoam, using my 1/8” end mill. The work turned out pretty well. The pockets and holes were cut using four passes, each one removing a small sliver of material to get the final 0.025 inch depth.
Now that I have learned the process, I can move on to a proper design. Fingers crossed!