Let me ask a burr question

Re: My take

Dave Mount, hey, we broke 0 F today!

>Caveat -- the following is based on common sense and experience/observation, not materials science expertise.

I think you're basically right, but not capturing all that happens. I think the burr grinding into the edge (and yes, I definitely get one too) is formed from the metal getting bent over by the impact of the abrasive particles coming into the fine edge. Think of what would happen if you ran 36 grit sandpaper into the edge of brass shim material -- it would bend rather than break. This is consistent with the observation that the coarser the abrasive, the coarser the burr. The finest grit grinding I've ever done is on the Tormek, and it definitely leaves a burr grinding either direction. But of course that's not the finished edge.

When you are working off the burr on a stone (you mentioned grinding, but now I've moved to a stone), you are also thinning the bit of metal that is holding the burr on. When it does break, it is not a perfect arris but a failed edge as you say. but the width/thickness of the failure is reduced by working the burr off (and/or forming new, smaller burrs) on finer abrasives. At some point, either the burr quits forming or is too small to be sensed -- I don't feel any burr coming off an 8000 grit water stone, and I still follow that with LV green stuff on MDF.

I need to go look at Leonard Lee's book again, but it also strikes me that at some point, you have to have a blunt edge rather than a perfect arris, because a perfect arris would be infinitely thin and therefore simply fold over when it met resistance (as in wood). At some level, a degree of bluntness would be required in order to have enough metal there to support a cut.

Makes sense it my little world, may be total BS in the real world. . .

Best,

Dave

Arts and Mysteries

Warren in Lancaster, PA

>Bob, I think you are entering the arts and mysteries realm here. It seems that how workers treat the burr has more to do with experience and observation than with scientific analysis. As you have suggested, what actually goes on at this level is rather complicated. You paint a pretty gruesome picture of the edge. I'm sure that at the molecular level things are not all that uniform and refined, but somehow we manage to sharpen tools and make them work. The answers may be different for different steels, different stones, or different sharpening techniques.

I think it is helpful to try to visualize what is going on as you work. Observations over many sharpenings also give one a feel for what is going on. When an experienced worker uses a certain number of strokes on the back of his chisel, his intuition (judgement based on experience) tells him when to stop.

WOW

bill tindal, E.TN

>that made my head hurt. If we knew where to look the answers to all these sharpening questions are in the technical literature somewhere. Imagine the studies that Gillette must have made. Can we find someone that works there?

Re: My take

woodburnbob

>Thanks for responding, Dave. It's always reassuring to know others are experiencing similar delusions.

I agree, into-the-edge burrs are likely the product of bending, for example, particularly when I'm pushing a blade down on that Tormek under the wish that it wouldn't take so long.

I think you are spot on with everything you wrote.

I particularly enjoyed: "...because a perfect arris would be infinitely thin and therefore simply fold over when it met resistance (as in wood)." It makes me want to take a leap and say something a little paradoxical like: an edge can be too sharp.

Anyway, thanks for your words.

Re: Arts and Mysteries

woodburnbob

>Just so, Warren. No argument here. Still, I think you'd agree that steel is pretty empirical earthly stuff and that it obeys the laws of physics and such. I'm not so sure we need to recruit "arts and mysteries" to guide us here.

It's my understanding from looking at old metalurgical textbooks that we sharpen by fracturing the stuff along fault lines in the otherwise highly organized lattice structure of the hardened steel. We're not sharpening anything sharper than the structure allows. My thoughts are in the direction of what might we be doing in the vicinity of those little fractured off clumps we break off with our expensive abrasives...along the lines of: Do we do any damage to what remains by all this rubbing and bending.

Allow me to quote you, When an experienced worker uses a certain number of strokes on the back of his chisel, his intuition (judgement based on experience) tells him when to stop.

And, that's just what I think about these long, warm, winter, Oregon nights lately, when it's so hot I can't sleep: Why would it matter when we stop our strokes? Is it that more strokes are a benign waste of time and energy, or does it mean that an edge can be "over-sharpened"? Why is stopping on time a virtue?

Re: too sharp

Dave Mount, hey, we broke 0 F today!

>While a reductio ad absurdum argument might be made that a (theoretical) infinitely sharp edge might not stand up to cutting, I'm not sure that is ever actually achieved -- I certainly wouldn't claim that I've ever made an edge "too sharp". Infinite is just that, and is never actually achieved.

Best,

Dave

Re: Arts and Mysteries

Moses Yoder in White Pigeon, MI

>There is a limit to how sharp an edge can be. An experienced workman has a pretty good idea what he must do in order to achieve maximum sharpness, and sharpens the tools in such a way that the edge best suits his purpose. That is to say, a combination of honing at the apropriate angle and honing to the highest grit stone that is available, and not wasting time honing to long. When I hone a piece on my 8000 grit stone I can pretty much tell by look at the reflection in the edge when the maximum sharpness is reached. The burr is definitely metal worn to such a thin edge that it breaks away from the tool. As has been mentioned, a coarser stone will cause a coarser burr; I think this is because the grooves cut into the tool with the coarser stone cause metal to break away around the grooves; a finer stone makes finer grooves, thus less matal failure. The goal in steel of course is to create a tool steel that can be honed to a very thin edge without chipping; the thinner that the metal can be made before failure, the sharper the edge. I think we have a long way to go in development in this area; I think we need to focus on ways to build and align microscopic atomic structures in steel. That should give you something to think about; I myself have been concentrating on my fly tying skills during these warm winter months in Michigan.

Re: too sharp

Richard L. Darin

>The sharper the tool the less force is required to cut. A Sharp tool stays sharp longer.

Re: Arts and Mysteries

Warren in Lancaster, PA

>Bob, you use the words "highly organized lattice structure of hardened steel". When we harden steel (quench) we form a disorganized collection of crystals. I cannot remember the sizes of the crystals relative to the abrasives, but I think the fact that the hardening process is successful argues for a small size relative to the fineness of the edge. In other words, if the crystals were large compared to the fineness of the edge, the individual crstals would act like unhardened steel.

I was taught how to handle the burr in 1962, when I was 12, and have been thinking about it ever since. When I took a metallurgy course in college in 1969, I had already sharpened a knife about twenty thousand times, so the material was meaningful and helpful. Yet I think that most of the decisions I make regarding sharpening are based on experience or intuition rather than engineering knowledge. I am familiar with my stones and my steel.

My grandfather taught me to shave with a straight razor, also in 1969. Despite having prepared an edge daily for sixty years, when asked how many times to strop he said "an indeterminate number" and how often to hone "when it seems to need it". Today people marvel at an edge tool that you can shave with, but if you look at 16th century paintings, even the peasants seem to be cleanshaven. The average guy was maintaining a fine edge with just a hone and a strop.

My take on burrs

Brent

>I have though a lot about what happens during grinding and honing and have a couple of ideas.

First, burrs form whenever you work the edge itself, regardless of how fast you grind or how hot the metal at the edge gets. I get burrs when I grind the edge using coarse bench stones - there is little heat involved. I get burrs when I hone with 5 micron SiC sheet abrasives. The fact that both hand and powered grinding produce burrs does not make them the same - powered grinding produces much more heat and airborne filings and grit.

The burr is not a smooth. The tool face is ribbed, with gouges formed by abrasive particles - the burr is similarly ribbed. The thickness of the ribs is related to the depth of the gouges, which is in turn related to the grit size, uniformity, and density on the abrasive surface.

With precise microbevel honing at increased angles, the burr left by one grit is removed by the next grit, with a new finer burr being created. That is, the old burr is honed off. At largish grit sizes, the grinding action affects the structure of the metal under the surface. At very small grit sizes, the effect is reduced. See the section on scratching on my bevels page.

This paper suggests that damage below the surface occurs only for larger grit sizes. For smaller grits, a plastic movement of metal around on the surface is taking place rather than fracturing.

Based on this theory, I try to not actually grind the edge itself - whether using powered or hand grinding. I stop a little short of the edge when grinding - using large grits to reform the primary bevel. Only when honing - using smaller grits to form the edge - do I apply abrasive at the edge.

Rather than intuition, I rely on geometry to tell me when I am done. By using jigs, I can be assured that a microbevel of a certain width corresponds to the removal of a depth of metal at the edge that ensures the fractures caused by the previous abrasive have been removed. The page on bevels discusses this in detail.

Any edge defects related to the burr produced by the 0.5 micron abrasive have little effect on wear. I suspet that are in fact smaller than the grinding done on the edge during use - grinding by the wood itself, on the upper and lower surfaces of the blade.

Good theory , but data lacking, and specualtion

bill tindal, E.TN

>Someday I hope we have data to answer the question of whether sharper edges last significantly longer. It seems just as reasonable that an edge produce by a 1 micron abrasive would wear to the dullness of an edge produced by a 5 micron abrasive in 10 feet of planing, or maybe not. If we were to plot dullness vs feet of lumber planed for the best edge possible to produce, what would the curve look like? Would it rise sharply and then level out, or is dulling a more linear function? Or, is the initial slope more flat and then it rises as the edge is used? I expect it will turn out the first description will be observed. I'm sure Gillette knows.

I know this, by the time I decide to sharpen in practical planing situations (not talking contests or diddling here) it is vastly duller than a freshly prepared 1 or 5 micron edge. So, I expect most of my planing is done well up the dullness curve when I am fitting something by planing bits off.

Re: WOW

Joe Rogers, Northern Virginia

>Wow is right. But in truth I believe the edges we try to generate are "sharper" than production razor blades. The minute serrations from a razor serve to grab and hold whiskers allowing them to be sliced off. In a harder medium like wood those same serrations serve to fold and fail. At least that's my hypothesis :-) jJR

Re: Good theory , but data lacking, and specualtio

ThomD

>Bill I think we do have quite a bit of data on that. I think planing contests prove it. The theory there is that it's worth sharpening to hyper levels in order to win these contest. It takes a sharper edge to engage a finer shaving, and when the edge is gone it's stops holding, so the improvement must hold for multi passes or it would not deliver on complete shavings and would be a looser. I'm sure we could design an experiment that is so expensive that nobody would do it. The contest guys are at least minimally using tools like microscopes to do some quality control on the edges so I think they are probably doing early 20th century stuff if not dead current science.

I also think most workers are looking for the point where diminishing returns of sharpening comes into it, which is essentially the same issue. 99% of my work does not require a peak edge unless it's an increase in durability also. In the end the data really needs to be from some kind of practical perspective. It would be one thing to find an answer in a lab that arise from processes a human can't replicate, it would be valid as science, but the human factor has to be included if the conclusions are to apply to real work.

I remember, back when 2 blades was a lot and Gillette had brought out a few version of multi blade and gadget improvement, still only 2 blades. Popular Mechanics said the multi blades were largely gimmicks. That what Gillette was doing was improving the edge on the razor (I took that to mean the improvement was in terms of the amount they could improve it for a given cost), but that the best way they had found to sell that improvement was through big campaigns on technology like 2 blades. They could get a much better response if they gadgetted it up rather than just saying "now with sharper blade". Does seem to imply they were moving the whole performance curve along, or the improvement would not have been durable, and customers would have immediately seen that the moment the edge wore a little the new technology stopped working so they wouldn't have bought into it. Hardly conclusive, but interesting.

what is a planing contest?

bill tindal, E.TN

>I have never seen a planing contest so I could not follow your argument. If the contest consists of planing 6' of soft lumber then I don't see how the results of the contest provide any meaningful data on a practical dullness curve generated by walnut that would span dozens if not hundreds of feet. The relevant data is out at 100', more or less. After 100', to pick an arbitrary number, is the 1 micron edge 1% sharper than the 5 micron edge or is it 20% sharper? Or to put it another way, if I can plane 100' before sharpening with a 1 micron edge how many feet can I plane with a 5 micron edge?

I think the whiskers example is also not relevant. Gillette is after the best 1st shave, maybe like the planing contest. The data we need are whether the sharper blade delivers 10 more shaves. I shave like I plane. When the blade pulls the whiskers out I change it.

If those twin/triple/etc. edge razors are so good

Bill Houghton, Sebastopol, CA

>why do I get a better shave from my double-edge safety razor?

Bill, living in fear that those double-edge blades will go the way of push drill bits, 828 film, and other things you can't get new anymore

Re: data

paul womack

>Someday I hope we have data to answer the question of whether sharper edges last significantly longer.

Forest Products Research Lab

And I could have sworn either Steve Elliot or Brent Beach had done something in this line. A brief look failed to find it though.

http://bladetest.infillplane.com/

http://www3.telus.net/BrentBeach/

BugBear

Re: Let me ask a burr question

j smith

>It would seem to me that one can take sharpening to a point of being "too sharp". I would not want to put my carving chisel edge on an axe . it wouldnt last long because it was "too sharp".

seems to me that sharpness is also defined by function of the tool.

The Wire

Warren in Lancaster, PA

>Brent, you write "the burr left by one grit is removed by the next grit with a new finer burr being created. That is, the old burr is honed off." I do not see what mechanism you understand for doing this.

I have long noticed that the burr is less prominent on waterstones than on oilstones. With oilstones, if you raise a substantial burr on a coarser stone, it remains substatial to the finest stone. That is to say that it is obvious that the original burr remains until it is removed. Sometimes the burr comes off on the Arkansas stone or the strop as a single fine wire with no visible thickness, seen only in very good light. Somtimes you can see small sections of wire, sometimes not at all.

I have not seen this happen with waterstones, and I have wondered about it for many years. Perhaps with all the loose grit floating around the delicate burr gets beat up and gradually wears away during the honing. It is someting of a mystery to me. If you have a model for how the first burr is removed I would be pleased to read it.

My take on razor blades

Brent

>My site contains photomicrographs of a number of different razor blades.

First though, initial sharpness is a geometrical property of an edge. It is not magic, it is not rocket science, it is simple geometry. An edge with a smaller included angle is sharper than an edge with a larger included angle. (Sharpness in this case refers to the ability to cut the way a razor cuts when shaving, which is similar to the way a plane iron cuts when planing: the force required to push the edge through the object.)

Based on these images and measurements of the thickness of the blades, certain conclusions are clear.

The final micro-bevel on all blades I have examined has been honed with an abrasive at least as good as 0.5 micron. They do not deliberately leave any scratches on the edges.

The older single blade razors (BIC disposables) which support the blade rigidly, use thinner and hence sharper blades, than the new floating blade razors. The fixed blade is supported just like a plane blade: pressed between two rigid surfaces. The shaved surface conforms to the shape of the blade.

With the new floating blade razors, each floating blade moves back and forth in a slot, pushed forward by a small plastic flange that acts as a spring. This means that the group of blades can conform slightly to the shape of the surface being shaved, provided that the pressure does not exceed the strength of the plastic springs. (I believe this effect to be very small, perhaps negligible.)

When the individual blades float, they have to be much more rigid. The manufacturer achieves this extra rigidity in two ways. First, by using a slightly thicker piece of steel. Second, they weld this piece of steel to a rigid right angled piece of metal. The new technology is in the welding process, not the actual cutting edge. (Perhaps the cutting steel is thicker because they were getting warping from the weld with thinner steel.)

So, the multi-edge razors with floating blades are initially duller than the older single-edge BIC disposables. Same fine honed surface at the edge but thicker metal.

Durability is another question. While affected by geometry, it is more a materials problem. Do they use better steel (more durable steel) in the multi-edge blades? I don't know.

Finally, in my opinion the real science in razors is not in the making, it is in the selling.

Re: The Wire

Brent

>Warren

When I go from 15 micron to 5 micron I also use a small wooden slip under the jig. This means that the honing begins at the edge. In fact, the first thing that happens is the burr left by the 15 micron abrasive is honed off. Check out a typical sharpening session.

As to your observation that oil stones produce bigger burrs than water stones, that makes sense because (almost all) water stones are synthetic and more uniform in structure than natural oil stones. The size of the burr is dependent on the size of the larger abrasive bits.

I expect that the size of the burr left by a modern synthetic oil stone would be the same as that left by a modern synthetic water stone with the same grit size.

Re: The Wire

Warren in Lancaster, PA

>Brent, you write "the size of the burr is dependent on the size of the larger abrasive bits". Yet one could have a burr that is a 1/8 inch wide strip on any oil stone if he had but time and patience (and did not mind losing tool length). The burr continues to lengthen as the tool is worn away. This phenomenon is quite noticable when putting more camber on a plane iron. Those areas that have been worked harder have more prominent burr and the burr continues to lengthen as more work is done. One can get just as prominent a burr with a fine India stone as with a coarse Carborundum stone, albeit with considerably more labor.

The burr does not disappear even temporarily if the tool is honed at a slightly higher angle on a fine oilstone.