Researching the Burr

Re: Researching the Burr

david.marcus.brown


>I'm just guessing here, but isn't the burr just the bending upwards up unsupported metal?

Re: Researching the Burr

Thom D


>I think that is basically it. One grinds away at the edge, and there comes a point, as the edge thins, where the leadingmost part of the edge is unsupported sufficiently to be pushed down on the stone enough to be cut. The more one grinds past that point the more material one removes and the burr grows because it is further undermined. Sorta like crayoning without peeling back the paper tube.

The burr on a scraper is cold formed and that is a different process.

Re: Researching the Burr

David Barnett - SW Florida


>"I've sure read about the burr being due to molten steel being smeared across the ground surface, rolling off the edge and then solidifying into a burr when a fellow sticks a tool up against a grinding stone turning at 3750 RPM. That's kind of easy to imagine. But I can produce a burr with my water cooled 100 RPM grinder and can do it with any of my whetstones."

I really wish I had the time to delve into this subject so near and dear to my heart but will have to offer only a couple resources. What you are describing is at 100 RPMs is a series of process rather than a single phenomenon which was once great fodder for debate, namely Beilby Flow Theory of Polishing, put forth by Sir George Thomas Beilby in three landmark material science papers during the first two decades of the last century*. Although temporarily refuted in 1981, it's been reprised and modified into a modern theory that includes frictional surface disturbance and hydroxyl formation (aqueous chemical destruction) and other elements to better explain what happens when metals and hard minerals undergo iterative destruction and refinement of outer molecular layers. Once believed to be due to frictional heat alone, tests showed water cooling and lubrication to be so effective that temperature rises were held to around six degrees Celsius at the abrasive contact juncture -- negligible, in other words. Obviously, it's an explorationally rich research domain for material scientists, solid state physicists, and tribologists with wide-spread applicability in optics, gem faceting & polishing, industrial metallurgy, and more.

So, in a nutshell, burr formation-removal is a lot easier to do than to understand. I'd say more, esp. about using waxes and greases as mediums for diamond grit lapping, Twyman effect, Poisson's Ratio, fracture process zones in ductile to brittle transition and other such matters, but I'm still moving (mostly unpacking for the next few months) and I'd like to see it done in my lifetime.

Sisyphus

*The Minute Structure of Minerals, Surface Flow in Crystalline Solids Under Mechanical Disturbance, and Aggregation and Flow of Solids.

Great. Now consider this...

woodburnbob


>Thom, would you agree that the burr, according to that scenario, has a finite thickness? The burr can be no thicker and no thinner? Especially, all things remaining equal, you can't make the burr thinner. Do you agree?

When you break off the burr (either by "refining" it or simply using the edge), you might go back and continue honing in the hopes that you might further narrow the width of the stub at the edge. But that is misguided. You are already at the failure point to begin with. So, this new shorter burr will just be too short to easily break off, until you grind away some more, simply to get this second burr long enough to break off. Aren't you back to where you started? Back to the point of a futile effort to get an edge narrower (the arris sharper) than the burr.

Would you further agree that burr thickness is a direct function of stiffness, or modulus of elasticity, or whatever term might best define that point of failure of reversible deformation in that particular piece of steel's stress-strain curve

...AND...

the force (vertical vector representing), downward pressure, on the surface of blade edge that is being ground (honed, whatever)?

Isn't that the limit of sharpness, for that steel-force combination, no matter what abrasive, technique, etc. used?

Okay, I'll indulge at least this one edit:

David Barnett - SW Florida


>"describing is at 100 RPMs is a series of process"

should read:

"describing at 100 RPMs is a series of processes"

Sorry

You da man!

woodburnbob


>Sisyphus, wasn't he the patron saint of the Luddites?

I predict you'll be back to this thread before too long. In the meantime I'll get started looking up those references. Thanks.

Re: Great. Now consider this...

Thom D


>I won't take on all those questions until we consider:

a) the effect of taking the burr off from the other side, which is done at 90 degrees to the face, with all the bevel on top to support the edge.

b) The effect of moving to a lower grit, whatever that may be.

c) the effect of oversharpening. one is trying to get a burr initially since it indicate a near zero edge thickness, but in subesequent grits one is not interested in oversharping that much since they cut so slow.

There may be a burr developed at finer grits but it is a finer burr.

Not sure what is really happening, but the burr is certaqinly not the sharpest edge.

Re: You da man!

Thom D


>If one is talking about heat distribution of a kind of metal smear wouldn't that accumulate mostly at the rear end which in wheel grinding is the edge other than the cutting one?

You can create a burr filing metal also. Sharpening broadheads being one example and the burr can be visibly wide and like foil. I don't believe there is anywhere near enough heat for that to happen, while my finger is supporting the edge.

It takes surprisingly little pressure...

David Barnett - SW Florida


>to separate and ignite steel particles, which is what happens when a steel of sufficient carbon is struck against a sufficiently hard mineral such as chert (flint, jasper, agate, and, of course, novaculite). It isn't the burning carbon alone that ignites your tinder but the steel itself. The carbon makes the steel brittle enough to fracture into small enough fragments for the molecular tearing at crystal junctures to generate the required 1370C ignition temperature. You can see the entire edge emit light when honing, even when immersed in water or other coolants-lubricants. This is why tool steel takes a fine edge as opposed to other ferrous metal combinations. The carbides allow for smaller grain fragmentation.

Even a large burr has to start somewhere.

(You were right, Bob. I have no self-control, heh)

Thanks Dave.

V Parisian


>This is a great board. Gems like your response are the reason we hang out here.

Victor in Rainnnnnnnnnnny Houston - glub

Just email me...

Scott Burr in Ben Lomond, CA


>"A burr is formed when sharpening" Not always ;^)

But, I am edgey...

Not 'just' the bending

David Barnett - SW Florida


>"I'm just guessing here, but isn't the burr just the bending upwards up unsupported metal?"

Mechanically, yes. But that fine burr on tool steel raised by honing has undergone changes that render it not only thinner but inferior in other ways, too; softer in some sections, harder and more brittle in others, which is why its removal is desirable. The finest of secondary bevels removes the burr nicely and consolidates the edge by removing the remnants of the compromised metal that often remains when the burr is simply torn away or broken off, usually polishing and work-hardening the edge, however slightly, for added durability. (This is an oversimplification but it'll have to do)

I used to think I was pretty smart

Stephen Kirk in Quakertown PA


>But I'm not so certain now.

How many Kids ya got??? :>)

Derek

data on the topic

bill tindall


>The CPM 3V that we have been using is exceptionally tough. Perhaps related, it form tenacious burrs. It is easy to delude oneself that the edge is not sharp because the burr has not left with typical fine grinding. One has to either straighten it(knife) or go to smaller grits to be rid of it. (Makes awesome scrapers for turning, but the folks over there didn't want to hear about it so I quit bothering them). With a carbon steel as we go to finer grit the burr eventually just parts from the edge in little flakes that are redily seen on the abrasive.

Beilby, Twyman, Poisson, Etc. *LINK*

woodburnbob


>Until Prof. Barnett returns to continue the seminar, anyone taking this class, or auditting, might want to take a look at this link. I got half way through and realized I'm now late for work.

If you substitute "tool steel" for "glass", I think you'll see the hand tool and sharpening relevance...not that I know the substitution is a legitimate one.


Journal article

Zoooooommmmm...

Scott Burr in Ben Lomond, CA


>Anybody else hear a big wooshing sound go right over their head?

Re: Beilby, Twyman, Poisson, Etc.

David Barnett - SW Florida


>Aha! That's the very article I had intended to *LINK* to my post (I'd promised a couple resources) and then forgot in my mad dash (which looks for like slow-mo in reality) to find and unpack my cookware so I can eat something other than sandwiches. The differences between ceramic and metal are what I'd like to say more about -- most of my interest is cold-working glass and faceting gems, of course, but the lines blur at shaping and sharpening very hard gravers, for which purpose I use the same equipment that facets sapphire. Now I'm looking for my notebooks but this may be a lost cause for awhile, at least. Fascinating subject. At any rate, sorry for my slapdash sloppiness and lack of attribution.

People should take note of Bill's post...

David Barnett - SW Florida


>Those guys at Crucible are amazing. You can literally hammer this stuff through tough modern gun alloys without losing your point far longer than lesser steels, which is pretty much all of them, and in more forgiving metals, it's not even an issue. A quick polishing every couple hours and you're good to go.

In fairness...

David Barnett - SW Florida


>...aside from getting the optics and geometry right for good dispersion and light return, faceting is pretty much polishing, so we who do this sort of thing, whether for money, for competition (yes, imagine that), from irrepressible OCDs or just for fun -- it's a great hobby, btw -- are never far from it. Fortunately, though, like sharpening, you don't have to know it all to do it. When it comes to slippery slopes, my interest in faceting came from my tendency to buy a handtool just to sharpen it. So now my tool sharpening Jones is well-fed and when I finally grind and polish something, it's worth at least 10 times what I paid for it.

Sweet...

Scott Burr in Ben Lomond, CA


>I'll send you my No. 45 blades... ;^)

And I almost forgot...

David Barnett - SW Florida


>One of the biggest differences between glass/gemstone and steel, and a simplifying praxis, is the use of diamond for sharpening vs. other polishing oxides. As Bill Tindall pointed out in recent thorough and well-written posts, diamond is the fastest and most economical honing medium for hardened tool steels.

Unfortunately, many woodworkers have been dissappointed in what they perceive as rapidly deteriorating performance of plated diamond bench hones. I won't go into the reasons for this here but the initial abrasiveness of the hones were never intended to stay that way due to the friability of the medium*. Sintered diamond hones offer fresh cutting surfaces but are more expensive initially and must be maintained. So loose diamond's the way to go.

Plasticity and elasticity are differing factors for glass or steel, so pay attention to Poisson's ratio and Young's modulus. And for honing with traditional stones try tribology journals for new wrinkles on lubricants. All sorts of ways to get into trouble if you look for them and plenty of unanswered questions to explore. How might one consistantly strop, polish, burnish a plane edge to increase performance without deforming (dubbing) it? There's a question. And what speeds, hand or machine, for what oxide compounds on what substrates? And what about burrless sharpening? And how 'bout them powder metallugies? And should I just order in a pizza strike now that I've again neglected my unpacking?

*And yes, steel is 'gummy' and will pull the diamond crystals out of some substrates like caramels dislodge ill-fitting tooth fillings.

You'll think "sweet" until...

David Barnett - SW Florida


>...you realize "removing the burr" means that in your case I might just keep them.

Oh No! Never thought of that.

Scott Burr in Ben Lomond, CA