[quote quote=30064]Thanks, Todd and Josh! That’s insightful.
A practical question: how can I, who only has a caliper and no electron microscope, measure the thickness directly behind the edge. Maybe my caliber is precise enough to actually measure this thickness, but how do I position it in such a way that it measures the thickness at, say, 10 microns behind the edge. Kind of the most precise I’ve been able to get is about 5 mm behind the edge, with an emhasis on “about”.
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If there is no convexity, then the thickness can be determined from the bevel angle and simple trigonometry. In the case of a microbevel, you would need to measure/estimate the width of the microbevel to calculate the thickness behind the microbevel. Light stropping will only convex the bevel very near the apex and will not affect the thickness appreciably 1mm or more back from the edge - again you could calculate it from the angle.
You don’t need to measure edge width - the ability of the shave or cut paper can tell you all you need to know about that.
[quote quote=30063]Your point is valid, in that softer metals can take a finer bevel due to glass’ brittleness, but you also said yourself – that edge will fold, and if it folds within the first stroke of the blade (such as with hypodermic needles) then all subsequent incisions will be tearing cells instead of cleaving them. There’s a reason glass and diamond blades are used in ultramicrotomy. But as we said, the risk of chips from the inherently sharper, but more brittle glass type blades getting sheared into the wound is why they’re still not fda approved. And like you said, given applications are everything. I am purely arguing surgical instruments and cell histology, where what we would call a toothy edge is not what you want when you’re bisecting tissue. At a 100 micron level, everything is toothy, but it’s not what we’re talking about because at a microscopic level, everything is irregular.
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I believe that in days long past, the lab technician responsible for microtomy could hone his carbon steel blade to a level surpassing that of a modern diamond microtome blade.
I am not picking on them, I am pointing out that they make a blatantly false claim that metal knives have an edge radius of 600nm. Gillete razor patents explicitly state that they have an edge radius less than 1/10 of that dimension - and I have measured such blades to confirm this to be true. Even cheap, disposable scalpels are nearly as keen as Gillette Fusion blades.
Thanks, Todd and Josh! That’s insightful. A practical question: how can I, who only has a caliper and no electron microscope, measure the thickness directly behind the edge. Maybe my caliber is precise enough to actually measure this thickness, but how do I position it in such a way that it measures the thickness at, say, 10 microns behind the edge. Kind of the most precise I’ve been able to get is about 5 mm behind the edge, with an emhasis on “about”.
If there is no convexity, then the thickness can be determined from the bevel angle and simple trigonometry. In the case of a microbevel, you would need to measure/estimate the width of the microbevel to calculate the thickness behind the microbevel. Light stropping will only convex the bevel very near the apex and will not affect the thickness appreciably 1mm or more back from the edge – again you could calculate it from the angle. You don’t need to measure edge width – the ability of the shave or cut paper can tell you all you need to know about that. [/quote]
Todd, is that true? I’m indeed talking about the case without convexity and without a microbevel. But don’t you need to know both the angle of the edge and the length of the edge? I can easily find out the angle of the edge (particularly if I sharpened the knife myself), but how do I find out the length of one of the triangles of the edge? For example, if the edge is 15 degrees per side and the edge runs for 1 mm on every side of the blade, the thickness directly after the edge will be different than when the edge runs for 2 mms on every side.
[quote quote=30071]..
Todd, is that true? I’m indeed talking about the case without convexity and without a microbevel. But don’t you need to know both the angle of the edge and the length of the edge? I can easily find out the angle of the edge (particularly if I sharpened the knife myself), but how do I find out the length of one of the triangles of the edge? For example, if the edge is 15 degrees per side and the edge runs for 1 mm on every side of the blade, the thickness directly after the edge will be different than when the edge runs for 2 mms on every side.
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You would need to measure the length of the edge with some modest magnification - a USB microscope with scale function, or laying your calipers across the bevel.
Thanks again, Todd. But that was the original problem : my caliper and/or my hands are simply too fiddly to measure such small distances. But your USB-microscope idea seems indeed doable. I’m gonna try it!
: my caliper and/or my hands are simply too fiddly to measure such small distances. But your USB-microscope idea seems indeed doable. I’m gonna try it!