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Thread: Grinding/honing/lapping/superfinishing..

  1. #1
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    Grinding/honing/lapping/superfinishing..

    Reading a bit tonight threw up a perspective which may help explain some of the positive results people report from stropping - even though the grit/particle sizes may not at first look be that much different.

    Grinding and to a fair degree honing seem essentially to be abrasive cutting processes where each grit acts like a cutting tool and removes material from the in our case blade by gouging out a groove. The result is a fairly heavily scored surface - a network of criss crossing scratches running in whatever direction the blade was moving on the stone.

    Superfinishing is interesting. It's widely used on engineered parts where low friction and wear in use are very important - stuff like precision gears, car steering parts etc. It's realised in multiple forms depending on the shape of the parts to be processed (try Googling 'superfinishing' and also YouTube - it brings up all sorts of diverse links), and to a degree involves using an abrasive, but with a very fine grit indeed - or in fact materials/media which are not necessarily very abrasive at all. e.g. steel beads and all sorts of things. Movements tend to be short and multi directional, and to result in a highly polished or if less fine slightly hazy surface.

    The key point seems to be that superfinishing like this takes the scratched/gouged surface produced by cutting grits in grinding and honing, and by rubbing or burnishing the surface rather than cutting knocks the peaks off the ridges - and possibly also moves a micron thin surface layer about to smooth things out even further. It can reduce the CLA/RA reading by a factor of up to 10 it seems. Guessing, but there could very well be work hardening going on too.

    It's also offered for use on parts like the disc type knives used to slit sheet and film materials in many continuously strip fed industrial processes - and (in one example anyway) is claimed to greatly reduce web friction, and to extend life between re-sharpenings by a factor of four or five. One factor mentioned in this is straightening and/or removal of any wire edge.

    Lapping may be somewhere on the overlap - perhaps depending to quite a degree on the nature of the abrasive used. e.g. very hard gits like diamonds may cut/scratch more than some of the more traditional compounds that break down quite a lot. Reading about stropping and burnishing compounds there seem to be quite a few about that break down like this.

    Maybe this relative softness/friability is an important part of how they work. (the emphasis becomes burnishing/rubbing rather than any cutting action/removal of metal) Maybe it's one of the reasons why stropping with the right compound delivers better results than the basic grit size information might suggest. Might explain too some of why a leather or dry MDF lap (with no abrasive) may do a good job, and why a fine waterstone using e.g. a nagura to produce a slurry that beaks down may give a very hard wearing and free cutting edge compared to another of nominally the same size but much harder grit.

    Here's a thought. It seems that lots of low amplitude multi directional movement helps with superfinishing. (it would presumably tend to increase the amount of burnishing/rubbing going on) Wonder if anybody has ever tried using a vibrating platform on which to mount very fine grade waterstones or other very fine sharpening media while using it?

    ian
    Last edited by ian maybury; 08-06-2014 at 6:12 AM.

  2. #2
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    Good question.
    Although my edges are so sharp now that I can shave curls off single hairs and if I don't sneak up on the arm hair they all jump off my arm before the blade gets to 'em.

    What's the next step in sharpness testing ?
    Sharpening is Facetating.
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    Having the hair just jump off while the blade is still laying on the bench. Now that's sharp!
    You never get the answer if you don't ask the question.

    Joe

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    Likely you are doing it (super finishing) already Winton, depending on how you are finishing off your blades - the finest of waterstones my be getting into this sub micron territory. Especially if their grit breaks down with use. I seem to be finding (?) that very light pressure at the end and short circular movements help to produce a better edge. I can't manage the remote hair removal (maybe some of the ladies would be interested?) , but casually testing one of the first finished chisels on my arm a few weeks ago it came as something of a shock to be effortlessly left with a sizeable bald patch. (i'm new to more careful use of waterstones)

    The insight that the info on super finishing triggered was that while my thinking was in terms of abrasive cutting processes, it may be that right at the end that some sort of 'frictional wiping' is actually more desirable - and what is mostly going on in the final sharpening steps. i.e. stropping/burnishing/buffing. Next up is to test the MDF laps and felt buffing wheels.

    It seems from the above that if there is a gain with an appropriate regime of this sort it might not be additional sharpness so much as improved edge robustness and reduced friction as a result of smoother blade surfaces....
    Last edited by ian maybury; 08-06-2014 at 6:50 AM.

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    Quote Originally Posted by Winton Applegate View Post
    What's the next step in sharpness testing ?
    See :14 of the demonstration video below -


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    Quote Originally Posted by ian maybury View Post
    It seems from the above that if there is a gain with an appropriate regime of this sort it might not be additional sharpness so much as improved edge robustness and reduced friction as a result of smoother blade surfaces....
    I would agree, it feels smoother.
    I notice this most in paring, not so much in planing.

  7. #7
    Quote Originally Posted by ian maybury View Post
    Reading a bit tonight threw up a perspective which may help explain some of the positive results people report from stropping - even though the grit/particle sizes may not at first look be that much different.
    There are a couple of things in the extreme end of sharpening that don't make quite so much difference with tool use, but do with razors. The 1 micron stones around (naniwa 12k superstone, shapton pro 15/glass 16k, sigma power 13k) damage thinner edges when they're used too much. I don't know how much they do to something sharpened at 25 degrees, but the damage they do is clear on a razor edge. Strangely enough, if they are used for 20 light strokes, no problem. If they are used for 100, the edge of the razor will break out and become fairly dull.

    Stropping with graded particles doesn't seem to do this. I don't know what causes the stones to do this damaging, maybe particles aggregate, maybe they fracture the thinner steel to some extent, who knows? I'm not sure if the 1/2 micron stones do it (shapton 30k or gokumyo 20k), but I don't think they do it as much. the Gok is reported to not do it at all. This difference doesn't really have any use in woodworking that I know, because if the damage occurs at steeper bevels, it doesn't seem to be much, and it's at a sharpness level that's not that important.

    As far as superfinishing, if it's not occurring as part of a fairly quick regimen, it won't have much use in wood. The wood is going to wear an iron too quickly, no matter the finish, to be able to tell anything other than initial effort in thin shavings with paring or very thin with planing (I personally don't feel any difference in planing resistance beyond the 1 micron stones, and i don't see it, either. Paring, maybe something, I don't know, but the SP 13k paring edge (that being a 0.73 micron stone) doesn't exhibit any flaws on wood, and neither, I guess do any of the 1 micron stones).

    In terms of stone particles breaking down, I think the term is used a lot to describe what is actually a combination of a loading stone and/or particles in a natural stone that are affixed in place and that become dull on the surface. I don't think much actual particle breakdown is occurring, because natural stone edges seem to be best on "clear water", and not on slurry.

    http://www.tzknives.com/razorbevels.html

    And a picture of the edge damage (this I've experienced on choseras, shapton 15k and the sigma power 13k..I haven't sharpened a razor on the Gok yet). When you're new to honing razors and you figure you just need to keep working a little longer, it never occurs as to why the edge doesn't seem to be getting sharper, especially when the polish is brighter than a natural stone that's yielding a greater sharpness - though all of the very fine natural stones impart a bright polish when used properly for maximum sharpness - it' just isn't quite as bright as a powder abrasive polish when you look really closely).

    http://jendeindustries.wordpress.com...g-controversy/

    Anyway, with all of this, I'd still conclude that there's no practical reason to use more than two stones (after grinding), and if the final stone leaves a bit of wire edge residue, a clean bare leather strop. I never would've thought, when I was a 1 micron stone addict, that using one would be perfectly fine, too, but under the right circumstances and with some experience, it works very well (and fast).

    Otherwise, any "superfinishing" is going to be blasted off almost immediately in use, vs. in machine use where you can suspend two surfaces apart with a layer of oil - we don't have that luxury. The standard of excellence for super fine edge is getting a specific angle (try 25) and doing a hanging hair test, where you hold a hair about a half inch from where it sits on the edge you've honed, and see if it will violin (vibrate), catch and split, catch and pop, or just sever immediately (the closer to the latter, the sharper an edge is) without appearing to catch. It's not unusual to be able to get that level of sharpness, but I think most of us don't clean up an edge well enough to do it. The higher the bevel angle, the harder the test is to pass. Trying it a couple of different places on the bevel will ensure you didn't just find a clean spot among an otherwise damaged edge. It's not woodworking, it's satisfaction of curiosity, but it's interesting. I haven't tried to hang a hair off of a washita, but with proper stropping, I could probably do it - it wouldn't be nearly as easy as it has been with other stones.
    Last edited by David Weaver; 08-06-2014 at 9:13 AM.

  8. #8
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    Beilby flow

    Quote Originally Posted by ian maybury View Post
    Reading a bit tonight threw up a perspective which may help explain some of the positive results people report from stropping - even though the grit/particle sizes may not at first look be that much different.
    . . . . .

    Maybe this relative softness/friability is an important part of how they work. (the emphasis becomes burnishing/rubbing rather than any cutting action/removal of metal) Maybe it's one of the reasons why stropping with the right compound delivers better results than the basic grit size information might suggest.
    You might find Sir George Thomas Beilby's 400-page: "Aggregation and Flow of Solids - being the records of an experimental study of the micro-structure and physical properties of solids in various states of aggregation, 1900-1921", interesting. It's free.
    Last edited by David Barnett; 08-06-2014 at 9:24 AM.
    διαίρει καὶ βασίλευε

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    This definitely isn't one for the no theory guys. Thanks for the links, it took a little while to process. It sounds like we're at one level moving beyond what's directly relevant to current methods of woodworking tool sharpening. It has to have implications though.

    A quick trawl through the discussion you linked on edge breakdown on razors as a result of excessive working with very fine stones (David W - it seems quite likely that some of this effect could arise at 12,000 grit too), and some of the stuff that comes up if 'Beilby layer' (David B) is Googled begs all sorts of interesting questions.

    This is just speculation - but the most fundmanental seems to be the possibility that damage to the surface layer of the steel as a result initially of cuts and fractures made by coarse grits in the early stages of honing, and later on the plastic working of the surface (folding over of peaks into valleys, highly localised melting, and trapping of debris and contaminants) by fine buffing/stropping/polishing may create a fatigued, cracked and otherwise weak and damaged layer of up to 0.003 to 0.005 in thickness. This if true is more than enough to be significant in respect of the robustness of woodworking tool edges - although whether it shows up in practically observable terms might be another matter.

    The discussion on edge chipping as a result of the use of very fine stones on razors theorises mostly in terms of cutting/abrasive processes, but it seems quite likely (given the small grit sizes) that it may also be the result of overworking creating a damaged layer as above. Could be too that the grain size in the metal plays a part.

    The question is what it might imply (presuming it has consequences) in terms of tool sharpening technique. It's a kind of a catch 22 problem. Overly aggressive coarse polishing and insufficient fine polishing risk leaving deep scratches behind (?), but too much polishing risks creating a damaged surface layer as above. It sounds like once the surface are formed during initial tool set up that there might be a 'porridge stone' (just right - not so coarse as to scratch deeply, but just coarse enough to quickly remove the wear bevel and any damaged surface layer) that could then be followed by a very quick final polish of not too many strokes with something very fine. Wonder if a hard strop might not contribute to this - have the advantage of doing minimal damage as it works???

    Maybe you are on to something David W with your using two stones and a strop - what two grits would you use for re-sharpening/touching up a previously good edge?

    Could be too that it takes several re-sharpenings (using a pair as above) to get past the deeper damage done during the initial grinding/flattening of the bevel and the back - the edge will improve.

    There's mutterings in various places (although not in the context of sharpening) about electropolishing being one way of getting past the issue of the damaged surface layer…..

    ian
    Last edited by ian maybury; 08-08-2014 at 7:59 AM.

  10. #10
    I use one stone and a strop, though if I were to use two, they'd go like this:
    * washita (conditioned to cut) and hard arkansas of some type (translucent, hard black, etc....not just a generic hard that is not trans or black) and then strop
    * washita and then jasper slab, condition jasper slab as needed to get it to cut
    * washita and then charnley forest (charnleys are a terrible option here, but I think you should be able to find one at boot sales there at some point - they are just a slightly softer novaculite finisher)
    * fine india and then hard arkansas and strop
    * For modern waterstones, I'd use any medium stone (1000 grit type stone) and any fine stone (8k grit or higher) and then no strop. Waterstones don't allow an organized wire edge to develop like natural stones do, they cut narrow grooves and the wire edge is more like a bunch of little bits of stuff that you can just palm strop off if you need to strop. My favorite setup ever is the shapton 1k pro and the shapton 15k pro.

    My favorite setup now is a single washita stone and a bare leather strop. If I have to pare something and really think the washita doesn't quite cut it, a cheap jasper slab both chases the burr and polishes the edge to a higher degree than a hard ark. Plus it's a novelty - it cuts a little different, more like a modern waterstone.

    The reason for two stones is basically because the first stone is going to raise a wire edge. The next stone's job is to remove it and even out just the very cutting edge. No performance is gained by doing anything else because the point of contact will be worn back from work and will never be the more coarsely honed part of the bevel - it'll always be the worn area.

    I know there are a lot of washitas in the UK, and a lot of india stones. Washitas aren't such a great choice if you use modern irons of A2 and such, and especially if they are thick or especially hard. The india stones will cut those, but any stone that doesn't refresh itself will get a little tired cutting those and need some surface conditioning to wake it back up.

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    Thanks for that David, I'll have a play with the Shapton options for the moment as the other stones would tale some research and then have to be sourced.

    The other big variable in all of this is pressure, speed and pattern of movement. Looking at videos and write ups the razor guys use minimal pressure and very few strokes to finish off. I seem already to be finding that light pressure, few strokes and short/circular movements are a good idea in the final stages of the finer stones....

  12. #12
    Yeah, you don't have to worry about what the razor guys do with fine pressure, etc. They need to do that for a few reasons:
    1) having the abrasive cut as shallow as possible
    2) raising as small of a burr as possible (and contrary to some talk, a tiny burr is still raised, but you want it as small as possible so that after it comes off, the edge is as narrow as possible)
    3) not flexing the razor while sharpening, which leads to less bevel support and some serious problems

    For tools, that stuff is all moot. The level of sharpness such stuff imparts is blasted away in a stroke or two, and that stuff takes serious time to do nicely. If it makes you feel good, you can use light pressure for your last few strokes on a polishing stone and alternate the bevel and back back and forth just a bit. Someone using a washita would do just that to cheat and try to get a little extra from the stone.

    Vintage razor texts show pretty much one basic stroke for razors, which is a heel first type stroke that is designed to keep the geometry of the edge in good shape, and to allow a razor to be worked on a stone narrower than it is long. A lot of the discussion of various methods on the razor boards is a bit overblown and that classic stroke is the most successful at making a fine edge.

    If I can get you to catch the straight razor disease, those things are important things to do.

    The zowada link I showed (was it in this thread?) with the various levels of polish shows just how well natural stones do on clear water when the cutting action is mitigated, but such things can literally take a couple of minutes after an otherwise finished edge and the levels of pressure are difficult to obtain on tools.

    Fast and reasonably fine is great policy for tools. And if finer is desired, go finer in abrasive rather than in pressure. It's cheap to do that these days.

    But keep your eyes out for coticules, charnley forests and eschers at car boot sales. they are not great for tools, but they are fabulous for your wallet if you find them cheap.

  13. #13
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    Think I have enough addictions without getting into razors David - but sharpness is a fascinating subject. It sounds though like I'd do better to focus on the woodworking for a while and see how what I've got already performs. As before it's almost magical the way a very sharp edge materialises with the use of a relatively straight forward process with waterstones...

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