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Why is my end mill and coin overheating?

I use my Boxzy to cut and build magic effects for magicians. It does a wonderful job with cutting the wood. My biggest problem is I use coins that are brass with about a 30 micron nickel plate. The process I try to run is very simple and straight forward. I need to cut a 1” hole in the coin, using a 1/4” bit. It’s a simple 1” in diameter path that I configure in Easel to follow along the path. Cutting per pass is 0.050 mm, the Makita is running at setting 4.

After about the 3rd pass the end mill and coin get so hot they are glowing orange, which in turns burns the jig and I have to start all over again from the jig to the pockets. My feed rate is 127mm and plunge is 76.2

What am I doing wrong? Thank you in advance for any input.

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Brass should be a very easy cut. Needless to say, those are concerning results. One problem is likely the small size of the coin, it leaves a very small heat transfer surface. Consider mounting the coin in an aluminum fixture to allow heat dissipation through the fixture. I have a few questions, presented below:

  1. What kind of endmill are you using? (How many flutes, what's the helix angle, up or down spiral? What Is the endmill material?). Ideally you'll want a carbide, 2 flute (always), high helix, upspiral endmill. Is your endmill still sharp and undamaged? You may have damaged it with a poor chipload and high heat. A smaller endmill will give a lower surface speed at a given spindle speed. You want lower endmill surface speed at such conservative feedrates, to reduce heat, so it's worth considering moving to a smaller endmill, like 1/8”.
  2. You stated 127mm feedrates, Im assuming that's 127mm/min. Is that correct? That's much too slow, the excessive time over the material while spinning and re-cutting chips will cause additional heat build up. The chips being displaced are the primary source of cooling while milling metal (they take the heat with them). You need more chips over a given period of time. With a 1/4” endmill, I wouldn't go lower than 800mm/min, as high as 1500mm/min. Fast feedeare is necessary with such a small depth per pass and a big endmill. With a 1/4 inch endmill, I cut brass blocks at 1200-1500 mm/min and .5mm depth per pass (10x more than 0.05mm), or 800-1200mm/sec at 1-1.5mm depth per pass, typically. Easel is too conservative for metal in many cases. A low pass depth is ok, but crank the feedrate up. The bigger the endmill the faster the feedrate needs to be.
  3. What speed have you set your spindle to? You want lower speeds when at lower feedrates. A faster than necessary spindle speed is a very bad thing in regards to heat. With a 1/4” endmill, I run the spindle at 3.5 to 4.5 for Brass with the feedrates I mentioned earlier. I would go even slower on the spindle, but power reduces as speed on the spindle reduces, which can lead to chatter. Another way that a smaller endmill is helpful; faster spindles speed still get you lower surface speed (because the circumference is shorter per rotation). But a 1/4” endmill should still work fine.
  4. Are you using lubricant? WD40 works well, not because it's a great lubricant, but because it evaporates slowly and lingers on the metal longer than many others. It reduces cutting friction and allows heat to evaporate away. I add a little (several drops) every two or so plunges, the small coin may benefit from even more.

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Thank you for your guidance in this matter. I tried your settings and it I finally was able to accomplish a successful cut.


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Glenn Auer will be eternally grateful.
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