When Rounding Over Occurs?
- This topic has 11 replies, 5 voices, and was last updated 12/20/2018 at 3:31 pm by
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Stropping is the only stage of the sharpening process where what I would consider “rounding” to be a potential issue. If you don’t have the angle sufficiently matched as you progress through the grits you can miss the apex and end up with an edge that doesn’t preform as well as it should. This is where the use of magnification can be very helpful because it can help you see if you’re reaching the apex with every grit. Forming a burr on both sides of the blade is another way of verifying that you have reached the apex with your stones.
If you can post pictures of what you’re seeing on your USB scope that would be helpful. Also, can I inquire as to what your measure of sharp is? Is there a particular cutting task that you’re trying to achieve with your freshly sharpened edges that you haven’t been able to do (cut news print, push cut news print, shave hair, whittle hair…)?
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Thanks for replying. Although I’m still in the learning stages, I am having a hard time with this. When I’m done sharpening, I usually grab a piece of phone book paper and test the ease of going thru it. Other than just feeling the edge, I have no other way of testing, nor have I ever with plane irons and chisels. If the blade enters the paper edge easily, in a slow, slicing motion and continues to blade tip, then I’m usually satisfied. The quieter it is, the happier I am. If the paper edge is pulled forward any, before the knife edge enters the paper, then I figure I didn’t get the edge sharp enough. Hope that’s clear explanation.
During the sharpening process, I do dangle a piece of that paper and run it over the edge, just to see my progress. If I’m cutting good at 1k diamonds, then go back a bit after stropping, then I figure I’ve stropped at too high an angle. I’ve tried backing off 2*, but it seems I’m only seeing polish at the bottom of the bevel, so I decrease angle, 1/2* at a time, until I see some shine on the entire bevel. That usually happens when I’ve adjusted back to my original angle. All that said, I was very pleased with my first knife. It is a higher quality paring knife. The whole process went just as expected.
I’ve tried backing off 2*, but it seems I’m only seeing polish at the bottom of the bevel, so I decrease angle, 1/2* at a time, until I see some shine on the entire bevel. That usually happens when I’ve adjusted back to my original angle.
Don’t go by the polished appearance. Try the edge for sharpness after 1.5º to 2º reduced angle stropping. It is sharpness we’re seeking first. Get your technique down and accomplish sharp edges. You may not produce a polished edge working only up to 1000 grit and with 5µ and 3.5µ strops, especially this early in your sharpening experience. It takes time, practice, and consistency throughout the entire sharpening process, (start to end) and maybe even finer grit stones and finer grit strops with good technique and attention to detail to see a truly polished edge.
Marc
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Derek: Qualitative evaluation of a sharp edge is difficult, simply because the different uses you might use it for are so diverse. For a given edge (let’s assume a 40 degree inclusive, honed to 1 micron, with no apparent rollover) you will see the effects of the bevels – how wide they are from shoulder to shoulder, and how polished they are. This is clearly demonstrated when you take an already sharp edge and work the bevels toward a convex, highly polished shape. Unless you know the difference, you’ll swear the convex edge is sharper, even though the edge (apex) hasn’t changed.
Perceived sharpness is influenced by the included angle, the degree of refinement of the edge, the width and shape of the bevels and the degree of polish. Add to this the toughness and friction component (lubricity) of the material being cut. Many materials are quite resistant to the penetration of a polished edge/bevel. There, a toothier edge may easily outperform the more refined and polished edge.
Your use of slicing telephone book paper to detect defects along the entirety of the length of the edge is probably the best method, other than purchasing one of the scientific devices, which ignore all but the very edge. I don’t see it used much lately, but a few years ago, the big thing was to push-cut the corners off of a phone book. Rockstead has/had a video showing them push-cutting through a thick hemp rope dzens and dozens of times. They use ZDP-189 with convex bevels. If you have a letch to buy a $4000 knife, I highly recommend them.
The “real” standard is whether your knife (more accurately, your “razor”) is “hair-popping” sharp. There are five levels:
Standardization of the test:
- Moisten the hair.This avoid all possible variations in outcome, due to fluctuations in humidity of storage conditions. A good way to do this, is to wet the thumb and index finger, pinch the hair and drag it through. Allow the hair a few seconds to settle.
- When performing the test, hold the hair at the root side and slightly angle the edge of the razor away from you. This maximizes the possibility for the edge to catch between the cuticle shingles.
- Scale of possible results:
HHT-0 – shave
The hair can be shaved immediately at the holding point. This is strictly spoken not a true HHT, but it does tell us that the edge is capable of shaving. [all other attempts must be made at least half an inch from the holding point]
HHT-1 – violin
The hair doesn’t cut, but it “plays violin” with the edge. This is due to the shingles catching the edge, but it’s not sharp enough to penetrate. On a full hollow razor, a faint ringing sound can be heard. On all razors it can be felt with the fingertips that hold the hair.
HHT-2 – split
When it is dragged across the edge, the edge catches the hair and splits it lengthwise.
HHT-3 – catch&pop
When it is dragged across the edge a bit, the edge catches the hair and pops it. The severed part will jump away.
HHT-4 – pop
The hair is popped immediately when it touches the edge. It still jumps away.
HHT-5 – silent slicer
The hair falls silently as soon as it touches the edge
This is all well and good, but is heavily dependent of the quality of the hair. I have never been able to pop my own hair, which is exceedingly fine. My wife’s hair is somewhat less fine and I have many times been able to pop hers. Shaving arm hair is pretty easy, as you can influence the pressure and angle of attack, or even wet the area with saliva. I’ve achieved HHT-2 a number of times and have heard from others that they’ve reached HHT-4, but I think HHT-5 requires included angles only seen with straight razors. (The one I have is configured to 14 degrees inclusive) I have several packs of disposable surgical scalpels and am not particularly impressed by their sharpness, They have a 30 degree included angle and are polished on one side only. Consider that they have to cut slippery tissue, so a toothy edge would seem to be a requirement. Some disposable razors I’ve dissected have 20 degree or less inclusive angles.
Some years back, there was a video on the ‘net of someone slicing hairs with a diamond edge. I think this is it:
Wrong video. See next post.
My advise is to not become obsessed with reaching some level of sharpness Nirvana. Know that your knives are sharper than any your friends have ever used and take what joy you can from that.
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Very cool video, but what kind of hair is that? It’s exceedingly thick. Each one of those slices are about the diameter of one of my hairs.
-Clay
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It’s hard to tell what it would score on the HHT because he’s only slicing along the fiber instead of trying to bisect it. The fiber is also very rigid. Note how it hardly deflects while it’s being cut.
-Clay
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It does look more like a bristle than a hair. Even so, it’s amazing how many slices it cuts without going all the way through.
When I’ve done the HHT2 test, I get a curl that stays attached to the main shaft. Hair isn’t a uniform rod of solid material, as the video might suggest – it looks more like the trunk of a palm tree. Once a sharp edge bites into the point where a “scale” offers an edge, it tends to cut all the way through or to flip the hair away from the edge, similar to what’s called “barber poling” in tree felling,
I’d like to see that edge under Clay’s 2000X ‘scope.
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