What a tiny defect in an endmill can do

How To
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Some of you have been on the receiving end of discussions that contain me saying “Look at the endmill under magnification” - here’s why.

This HSS endmill was being used to cut aluminum. There is a tiny chip out of the corner of the cutting surface (yes, it’s a chip, not just dust, gunk or something stuck to it)
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Here’s a wide-ish view of the cut it left. It’s deceptive… If you run your finger over it, it feels like the router is out of tram.
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If we look closer, it’s not! That ridge is about .001" high, give or take, and it’s not an undercut on one side of the cut path.
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That tiny chip, which is essentially impossible to see, is leaving a little ridge of mashed up chips along the outside of the cut. (was a facing operation with a 80% stepover on a .25" end mill).

So, if you’re getting cuts that aren’t of the quality you want, the first, easy thing, to do is look at the endmill under a loupe at 5-10x magnification. You cannot inspect an endmill by plain-old-eyeball and expect to always see the problem. Chips in the cutting edges are very hard to see.

6 Likes
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Short of a microscope, any recommendation on inexpensive magnification providing good resolution?

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A loupe is really cheap. The one I use was about $10 from Amazon, no light, has two lenses, one for 5x, one for 2x (together, 10x). The pictures above were a cheapo USB “microscope” - but the loupe works great.

Here’s one with a light, 40x which would be great too:

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Thanks, I thought those pictures did not come from a loupe.

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Yep, I’ve been finding this more and more important to pay attention to examining my endmills frequently. Thanks for some great microscope images and analysis.

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