How to tell if a cutter is sharp?

Revisiting a topic that came up in a different thread. I wanted to know how to tell a dull cutter from a sharp one. I tried a few ways to feel the difference, but I don’t have much confidence in any of them.

@RichCournoyer mentioned that he uses a jeweler’s loupe to check his. I happen to have a Supereyes B005 1 ~ 200X Handheld USB Digital Microscope, so I figured I’d look at them through that.

Here’s a cutter that was diagnosed in this forum as likely being dull:

And here’s one that I just received, brand new, from C3D:

I think I can see a difference, but I’m not totally sure. What should I be looking for?


The “Dull” one looks pretty fresh to me, however if your taking very shallow cuts, you’ll have to focus your inspection to the tip of the tool.

Try putting them side by side?

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you uploaded the same video for each one i.e your videos shows the same cutter, not one dull and one new.

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I don’t know how it happened, but you were right, I posted the same video twice. I blame my computer, and my malfunctioning brain.

Fixed now. Thanks for the catch!

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The fact that you can “see” the edge is the indication of dullness. When a cutter is brand new the cutting edge offers no reflection, and you won’t really see it at all. The more you can see a reflection or a bright line on the edge, the more dull your cutter is.

In this case, the easiest way to compare is to just look at the tip-end vs. further up the flutes. The cutting edge “disappears” about 50% up the flutes, indicating that it’s sharper for the upper half of the cutter than the lower-half.

Duller cutters have to work more to remove material, but they’re still fine for roughing, especially in softer materials that don’t tear out too badly, such as polyurethane foams/lighter-weight tooling board, and waxes.

You can also frequently get a tool re-sharpened for less than buying a new one, if you have them processed in batches. You’ll still have lost any special coatings, but a sharp uncoated carbide tool still works just fine in much of what we’re cutting around here :wink:


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