Inspecting cutting edges under a microscope

T

taiwanluthiers

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You wouldn't believe it but I was walking around and someone decided to throw out a perfectly good microscope. All it needed was to have its eyepiece and objectives cleaned and it worked fine.

But since it's a microscope for biological use, it's not really suited for looking at surfaces, so what I did was hold a flashlight onto the cutting tool while looking in the eyepiece. Unfortunately no photos because the flashlight happens to be my phone... I used to have a trick of taking photos from microscopes by sticking a cell phone camera into the eyepiece when I was in college.

Anyways it was eye opening. I could see carbide inserts are NOT completely sharp, even "sharp" ones have rounded cutting edge. Only aluminum inserts have a somewhat of a sharp cutting edge. Furthermore it's very easy to spot edge wear.... you couldn't see it with the naked eyes. Great tool to have if I am shopping at the used insert stores (can quickly reject the badly worn ones).

It makes me wonder about getting good surface finish with say HSS blanks because those are completely sharp when you grind them, but from looking at carbide inserts they are not actually completely sharp. But then there's no way for me to machine in that roundness either.
 
Carbide inserts are what they are. Look at the third letter in the (ANSI) designation. If it's "M" - yeah, it's probably not going to be too sharp. If it's "K" it's probably as sharp as the HSS you're looking at.

GsT
 
I have found that whenever the surface finish degrades, it's 99% certain that there's a chip or crack in the insert. Looking at the inserts under high magnification shows that even new inserts aren't that nice, but that poorly performing inserts are obviously damaged. Rarely, one can change the angle of the cutter to use a slightly different position of the nose. However, it's far better to replace the cutting tip, by either rotating to a new tip, or total replacement of the insert. When you buy cheap inserts, like I usually do, you have to deal with this.
 
Depending on the type of material, the sharpness is "not too big" of a concern. What was found under the scope was that they're irregular edge, and that's bad, probably due to very low cost inserts.

But, because they can't control the straightness of the edge, they certainly can't control the thickness of the edge either, which is a also bad. The only good think about them is that they're cheap. A diamond wheel in a grinder can help with this.
 
The microscope I got wasn't particularly cheap looking... it had 4 objectives, a 4x, 10x, 40x, and 100x (oil). Eyepiece is 10x. I only find the low power objectives useful when inspecting cutting edges. The rest is probably better for looking at bacteria and stuff... It appears to be made in Japan, made by a company called Erma or something. The stage has a XY traversal knob.

It really does open my eyes on the state of carbide inserts. It looks nice and sharp to the naked eyes, and under a microscope it is full of chips and cracks. And sometimes grinding or sanding them (with diamond of course) only makes it worse...
 
And when viewed microscopically, your polished ways look like the Alps being inverted and dragged over the Pyrenees. Or so said my favorite physics professor. I've said it before, there are no straight lines or hard edges in reality. Everything is means and gradients.
 
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