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+--- Thread: Bevel angle vs sharpness (/showthread.php?tid=86)
Bevel angle vs sharpness - grepper - 05-22-2017
Simple question: Can a blade sharpened with a 20° bevel angle be as sharp as the same blade sharpened to a 15° bevel angle?
Seems like a silly question, but what is your experience?
RE: Bevel angle vs sharpness - Jan - 05-23-2017
It is a good question, which is only apparently simple.
Following the BESS concept, sharpness is measured by a force necessary to cut a test media, however stress at the blade’s edge is of crucial importance.
At an initial stage the edge elastically deflects the surface of the test media. Then, at a critical contact stress, the edge penetrates the surface of the test media. The thinner the edge, the greater the stress on the test media surface. From this point of view the determining parameter is the width of the edge (diameter of the apex).
In my opinion, the bevel angle of the blade affects the sharpness indirectly, because with a 15° bevel angle it is more probable that honing will produce a narrower edge than for 20° bevel angle.
RE: Bevel angle vs sharpness - wadebevan - 05-23-2017
I have pondered the same question.
For the most part, I sharpen at about 18* as a compromise between the higher and lower angles.
The Japanese knives I have (Shun) arrive with 16*, but, over time they end up at around 18*.
From a BESS point of view, I agree with Jan.
From a daily usage point of view, my experience says the higher angles do last a bit longer.
Just my .02.
TW
RE: Bevel angle vs sharpness - SteveG - 08-04-2017
(05-22-2017, 07:06 PM)grepper Wrote: Simple question: Can a blade sharpened with a 20° bevel angle be as sharp as the same blade sharpened to a 15° bevel angle?
Seems like a silly question, but what is your experience?
I'm new to this forum (just bought a PT50), and not a great sharpener, but have an opinion that I'd like to share (mostly so you can give me feedback if it's wrong). It seems to me that all surfaces will deform to some extent when an edge applies pressure, and prior to the surface being separated (i.e., the start of the actual cut), some amount of the blade will penetrate into that deformation. The steeper angled edge will apply more pressure straight down, since it's not deforming (or wedging) as much material sideways. So, all other things being equal, the smaller the edge angle, the sharper the blade (assuming it has the strength to actually make the cut). It's probably not as simple as that (I'm simple-minded), and I'm assuming a push cut, not slicing, so would be interested in what others have to say.
RE: Bevel angle vs sharpness - grepper - 08-04-2017
Welcome to the forum Mr. Steve.
I’m sure you will enjoy your PT50! It’s amazing how informative the thing is. I sharpened for years without one, and now I would feel lost without it.
Everything you say makes sense to me. Subjectively people do indeed say a thinner bevel cuts better or at least requires less effort to make the cut. From my experience nothing beats a thin blade with an acute bevel for ease of cutting, and it makes sense for the reasons you speak of.
I’m wondering something slightly different and that is the sharpness of the edge without consideration of the ease of cutting something. I’m pretty sure that the answer is going to be above my pay grade, and has something to do with the properties of steel or something along those lines.
My suspicion is that for any particular type of steel there is an optimal bevel angle at which the edge can be the sharpest, i.e. thinnest.
Just curious- what kind of stuff do you sharpen?
RE: Bevel angle vs sharpness - SteveG - 08-04-2017
(08-04-2017, 07:14 PM)grepper Wrote: Welcome to the forum Mr. Steve.
I’m sure you will enjoy your PT50! It’s amazing how informative the thing is. I sharpened for years without one, and now I would feel lost without it.
Everything you say makes sense to me. Subjectively people do indeed say a thinner bevel cuts better or at least requires less effort to make the cut. From my experience nothing beats a thin blade with an acute bevel for ease of cutting, and it makes sense for the reasons you speak of.
I’m wondering something slightly different and that is the sharpness of the edge without consideration of the ease of cutting something. I’m pretty sure that the answer is going to be above my pay grade, and has something to do with the properties of steel or something along those lines.
My suspicion is that for any particular type of steel there is an optimal bevel angle at which the edge can be the sharpest, i.e. thinnest.
Just curious- what kind of stuff do you sharpen?
Thanks for the warm welcome!
I've only had the PT50 for one day, but I anticipated how useful it would be, and I'm overjoyed with what I'm seeing so far - having numerical feedback instead of subjective is incredibly helpful.
I'm a woodworker, so I mostly sharpen plane blades, chisels, lathe tools, and that sort of thing. I don't have any problems there, either with jigs or freehand, as far as my own needs are concerned. But I have several carving tools which I normally just strop, but are getting to the point where they need more than that, and I question my freehand skills. I also have a few knives, which I also sharpen sometimes with jigs, sometimes freehand. The freehand isn't too consistent, but it doesn't bother me as far as the usefulness of the knives, but I'm determined to improve my sharpening skills, hence the PT50.
I believe you're right about the steel making a difference, as far as the minimum width/radius you can theoretically achieve on an edge. I think the grain structure makes a difference (on my woodworking tools, I can get O1 sharper than A2, using the identical jigs/methods - that's been a subjective judgement until now, I'll need to prove that with measurements soon). I have also read that some carbides, notably vanadium carbide, have grains over a micron in diameter, so they're fairly easy to tear out when honing or polishing with grits larger than that, and impossible to reduce with finer grains unless you're using something harder than the VC, such as CBN or diamond. But I'm still not clear how the geometry fits in. There's a lot to learn, and every time I start to think I "get it,' I find something that tells me I clearly don't.
In woodworking, the bevel angle is more a function of use than of the steel (then again, there are only a few steel types most woodworks deal with). For hard use (chopping), big angles. For paring, small angles. The second consideration is the wood (soft or hard). Pretty simple until you start carving, or something like that.
RE: Bevel angle vs sharpness - Ken S - 08-05-2017
Caveat: This is largely based on opinion rather than documented evidence. That said, With router bits, carbide bits stay sharp longer by a very wide margin than high speed steel cutters. However, new hss bits could be sharpened more keenly than carbide. The choice was higher initial sharpness which was shorter lived, or slightly less sharpness which holds up for a long time. Longevity generally carries the day; high quality hss router bits are rare today.
Lie-Nielsen used to make bench chisels in both O1 and A2 steel. Once again, the longevity of the A2 steel edge seems to have carried the day over the shorter lived sharper edge of the O1. O1 bench chisels are no longer listed in their catalog. A2 chisels are generally sharpened with a 30º bevel, whereas O1 are generally ground to 25°. In the past, much of the quality of a chisel depended upon the skill of the heat treatment. I do not know the answer, however, I wonder if A2 is less dependent upon skilled heat treatment, and, therefore, less costly to manufacture.
Not all woodwork requires the same level of sharpness. A finish cut on a delicate carving is much more demanding than chopping out a mortise or dovetail.
Thanks for bringing up an interesting topic. I look forward to more dialogue.
Ken
RE: Bevel angle vs sharpness - SteveG - 08-05-2017
That's a good point about carbide edges. I have some new/unused carbide edges in my shop, I'll test them when I get a chance. It might even be worth sharpening one of them to see how far it can be taken.
I didn't know that A2 was normally sharpened at a different angle from O1, that's an interesting piece of information. Most of my chisels are A2, but I have a few that aren't, though I doubt I've left any of them at the factory angle to check for that. I've heard a lot about Lie-Nielsen's heat treatment process, and it seems quite elaborate, The cryogenic process they use has very controlled and specific rates of temperature changes, and periods to remain at specific temperatures. I'm sure it's all automated, so not hugely expensive, but I'm guessing more expensive than O1. I do like their steel, though; I can always seem to get a better edge on it than on my Veritas tools.
I agree with your statement about the level of sharpness. I think that's why I've become obsessed with sharpening - I'm always disappointed in the sharpness of my carving tools, and really want to be able to bring them to the level I think they should be at. Now that I can measure them, I'm hoping my practice is more productive.
Which leads me to a question (maybe more appropriate in a different thread): I measured one of my new gouges today using a disposable test clip, and the result was quite low. The guidance on the disposables was to expect low readings, and multiply by 1.2 to get an actual value. Does that sound about right from experience. I ask because I tested a straight edge on both the spooled block and with the disposable, and the ratio there was about 1.6, but I only measured once each, so not a great sample.
RE: Bevel angle vs sharpness - Ken S - 08-06-2017
Steve,
Don't apologize for obsessing about sharpening; we are all afflicted with it to some degree (mostly a lot of degrees°°°°°). I think that is a good thing, within reason.
If you have not already found them, the videos on the L-N website and youtube channel are very informative.
I think a high level of heat treating, and even cyrogenic treatment has become a customer expectation with premium tools. Why else would many of us willingly pay $55 for a L-N chisel instead of $8 for an Irwin (formerly Marples) Blue Chip chisel?
We do have different levels of expectation with sharpness. If you do not have Leonard Lee's Sharpening book, your library is incomplete (and you are missing a good read. The accompanying DVD has much good information and a good dose of LL's good dry humor. Purchase and enjoy both.) LL mentions the collection of odd chisels reserved for rough jobs, like repairing the garden gate. These wobegone tools get only quick, cursory sharpening. Pampered tools for fine cabinet work would expect much more.
One would expect a higher level of sharpness from the sharpening booth guy at the farmers market than from an on the fly quick touch up. On the other hand, one might expect to invest more time and/or money in sharpening a five hundred dollar Japanese kitchen knife.
You mention a most interesting topic with factoring and sampling with the new disposable test media. Would you post it as a separate topic in the BESS section. I think we might generate more replies there, and it is a timely, worthy topic.
Keep posting.
Ken
RE: Bevel angle vs sharpness - EOU - 08-07-2017
Hello Steve and welcome to the Exchange! Thought we'd weigh in concerning results using test clips vs. standard test fixtures. As you've already noted, there is a 20% differential in scores obtained using the two systems (add 20% to test clip results). This differential was calculated using knife and razor edges. If the test clip is first de-tensioned by wiggling one leg back and forth then the differential between the two becomes negligible. If you are witnessing greater than 20% differentials when measuring various wood working tools we would advance this theory; It is very possible that the shape and edge geometry of the tool accentuate the delta between the two results. It is often difficult to take extremely accurate and repeatable measurements using the standard test fixture due to it's physical constraints relative to the shape of the tool being measured. This is one of the primary reasons that disposable test clips exist. If this turns out to be the case we would think that 120% of the clip measurement would be the best representation of the edge sharpness level. You bring up an interesting point here Steve and one we would like investigate further.
Once again, welcome to the Exchange and thank you for posting!