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Milling Capacitor Plates

capacitor plates

Back to my loop antenna project. Nearly there with milling capacitor plates.

Boy, I was starting to think I had bitten off more than I could chew. As a new CNC user, my plan to cut some capacitor plates from aluminum sheet was perhaps too ambitious. On the other hand, what a wonderful opportunity to learn about using my Mostly Printed CNC.

As you can see in this picture, the aluminum sheet was still not completely flat on the machine. As a result, half of the edges were not milled deep enough. This makes it hard to remove the plates. But, I am getting very close. A careful look at the picture shows a significant improvement in the quality of the cuts, compared to my first try.

The aluminum sheet is only 0.025” or less than 1 millimeter thick. So cuts need to be exact.

Learning from Milling Capacitor Plates

In future articles, I will describe some of the things I have tried and learned on more detail. For now, here is a summary.

  • Feeds and speeds. The holy grail of CNC milling appears to be something called “feeds and speeds”. Feeds refers to the feed rate, or how fast you feed your tool across the material being cut. Speeds refers to the rotational speed of the cutting tool. Both of these are set to achieve a proper chip load, which refers to the amount material that is cut during each revolution of the cutting tool on the spindle. After some experimenting and learning the formulas (or, in other words, both practice and theory) I settled on a feed rate of 10 inches per minute (IPM) and a rotational tool speed of 20,000 RPM. This produced some great cuts.
  • Depth of Cut. Typically, you can do a depth of cut up to one times the diameter of your tool, which in my case was a 1/8” end mill. But, to get a good cut, I reduced the depth of each pass to 0.002” or about 1/20 of a millimeter. Thus, each profile required around 12 passes using conventional milling.
  • Work Holding. After cutting the aluminum sheet into four inch squares, I used two methods to hold it to the spoiler. First were some 3D printed plastic fixtures which were screwed into the table. This held down the edges of the material. Second was the use of double-sided tape spread across the material to prevent it from lifting during the cut.
  • Leveling. When you are making cuts in layers of 0.002” per pass, the material must be level. I used my Canadian Tire feeler gauge to check for a level tool path, and made leveling adjustments using shims. Not quite there yet, but close.

I will see if I can rescue these capacitor plates with a utility knife and tin snips to complete the removal of the material.

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