Help me understand "toothy" vs "polished"

[REK Knives]

Great discussion here... enjoying it on my part =)

I'm not sure that this is still applicable to the original conversation anymore, the exact definition of "toothy" has always been a bit slippery to pin down, kind of like describing what is "spicy". Past a certain point is obvious, but prior to that not everyone will agree.

Exactly. It is somewhat relative because what may be toothy to one may be refined to another.

Toothy: Think of a coarser blade that isn't as refined/polished and it has what seems like micro serrations due to the groves that are still in there from the more coarse media that was used to sharpen it. These act like little saw blades and helps it in slicing applications.

Polished: More refined and those micro serrations are far less noticeable so it doesn't grip and want to dig in and act like a saw blade but prefers to be pushed through an object with ease like a very smooth wedge and there are very little serrations to hinder this in this use. So it works better with push cuts.

They can both be sharp, extremely sharp no less. It's a question of what application you are using the knife for which determines which finish would work better but by the nature of it neither is better than the other. Though a lot of us find a happy middle ground we like, with me I personally love the finish off of my norton economy stone the india version on the fine side and think that probably the best all around finish I can get from everything I own and tried thus far.

Essentially, sharpness is about pressure - force/area. We can increase pressure by increasing force or decreasing contact area (sharpening). There are two ways we can decrease the contact area; making the edge narrower (uniformly along the apex) or by producing high points (irregularities). We can also imagine that those high points will wear first, protecting the keen edges in the low points. As an example, a sewing needle easily penetrates skin, but a blade sharpened to the same geometry (in 2D) is too dull.

Great descriptions/definitions, and I agree. What has to be differentiated is the difference between a push cut and a slice/draw cut. The former being for highly polished (i.e. straight razor shaving) and the latter being more for cutting rope (i.e. slicing through it w/ EDC knife). Also, Todd, your statement about the high points wearing first also seems to coincide with some work Cliff Stamp has done revealing that coarser edges have longer edge retention (I am not saying this is always absolutely true, just a general observation).

And while I can't find the reference right now, it also seems that a highly polished edge formed cleanly will actually cut through any given medium w/ less force than it's equivalent toothy edge when measured on a scale.

 

[JasonJ]

I think it would be helpful if someone posted comparison pictures of a toothy vs polished edge as well.

 

[ToddS]

I think it would be helpful if someone posted comparison pictures of a toothy vs polished edge as well.

That is the problem, few of the toothy-feeling edges have anything resembling microscopic teeth

And while I can't find the reference right now, it also seems that a highly polished edge formed cleanly will actually cut through any given medium w/ less force than it's equivalent toothy edge when measured on a scale.

But this is not what you reported for your BESS scale measurements!?

Not exactly. If the waterstone has a somewhat soft binder, or loose abrasive on the surface, it can grind a very clean edge with inconsequentially small or no burr formation - provided the amount of pressure and approach angle are well controlled. Keeping in mind, abrasive mobility or lack thereof is one of the major determinants in the amount of burr formation.

It is my belief that this mechanism produces fine irregularity along the edge but with a relatively narrow cross section. On a similar sized abrasive with a leading pass, the edge will be more uniform. On an edge with the same amount of irregularity done with a leading pass on a fixed abrasive, the cross section will be a bit wider across the edge.

Also keeping in mind the above is speculation based mostly on performance and somewhat on what information I can glean from an optical microscope...

...

Would you expect a Shapton Pro 320 to behave this way? That one seems like a "muddy" stone to me.

 

[HeavyHanded]

Would you expect a Shapton Pro 320 to behave this way? That one seems like a "muddy" stone to me.

I'm not so sure about that one. When they get down in the low end, the grit is so large it becomes problematic on its own. The best I have managed was backhoning off of a 240 grit jointer stone - the edge was so toothy it couldn't chop through a piece of heavy paper on a laminated desktop with moderate pressure. When I picked the edge up, the paper was stuck to it, individual high points could be seen poking through it! I never tested that one as I suspect it really would bust down fairly rapidly, but man would it draw cut. If going straight off the stone my experience tells me best results start at 2k or so.

Here's some images I've collected with a description and magnification. Is optical, but backlit seems to give a better feel for the depth at apex. Focal depth at 1000x is only about .2u. Is pretty clear that the irregularities function when cutting. They induce drag on a pressure cut, aid separation of materials on a draw. As the edges become more uniform, the amount of friction carried into the pressure cut decreases, as does the amount of tearing/separation of materials when drawn. Not quite serrations, but when you think about the low points still being sub or low single digit micron across, and then speculate on the formation width along the slopes - they will act somewhat like a serration. One of the interesting impressions one gets working on an optical microscope - if I can get a substantial stretch of the edge perfectly parallel with the stage and lined up laterally, I can slowly slide the stage past the viewing area and you can see all the teeth turn into a blur of what would be material separation.


Edge off 800 King stone stropped on plain paper 400x, This one would sryshave facial stubble but a few minutes later itched like mad!:

Norton Crystalon fine side and burnished with a smooth steel - treetopping:

Off my Washboard with 320 wet/dry - ripple is caused by movement of the microscope, is not actual 400x draw cutting across newsprint, some armhair:

Off WB with 600 wet/dry - ripple also not actual 400x. Shaves armhair and crosscuts newspaper with a push - noisy as it goes.

WB with compound 400x whittling, crosscuts newsprint quietly.

Stock compound on carbon steel at 1000x

Green CrO on one sheet of paper over WB 400x. Treetopping

Same as above - Green CrO on one sheet of paper over WB 1000x

Green CrO on two sheets of paper over WB 400x. Whittling

Same as above - Green CrO on two sheets of paper over WB 1000x

 

[BluntCut MetalWorks]

Martin, your sharpening skills definitely a step or 2 above mine, nice clean & flat edges. Apexes from pics have high undulation, I am guessing - you didn't flatten these edges prior to sharpening, right? I wonder how much teeth from an overwhelm/aggressive coarse grit would remain after smoothed out by subsequent fine grits progression. Of course, stroke pattern would affect how much irregularities remain/generate on the apex, e.g. sliding x-hatch would produce much smaller irregularities.

 

[HeavyHanded]

Martin, your sharpening skills definitely a step or 2 above mine, nice clean & flat edges. Apexes from pics have high undulation, I am guessing - you didn't flatten these edges prior to sharpening, right? I wonder how much teeth from an overwhelm/aggressive coarse grit would remain after smoothed out by subsequent fine grits progression. Of course, stroke pattern would affect how much irregularities remain/generate on the apex, e.g. sliding x-hatch would produce much smaller irregularities.

For sure a sliding X will reduce many of these. How much irregularity might remain, likely effected by a number of factors. I suspect the more firmly fixed the abrasive is at a given value, the more side to side deviation. Sharpening to a burr would leave more of these behind (maybe) compared to working off of a 'softer' surface. Or it might be the exact reverse...or it might all come down to the technique. All of my images show a fair amount of deviation till I get into the 6-8k and higher range, and that might be due mostly to unidirectional(?) grinding. Have never taken images of my edges after doing a sliding X.

 

[REK Knives]

great images martin =)

 

[Dry-cell]

Here's a older video made by jdavis882 called "Microbevels, Polished, & Toothy Edges: A Close Up View".
[video=youtube;iQJOAe1Xzis]https://www.youtube.com/watch?v=iQJOAe1Xzis[/video]

 

[FlameCycle]

Here's a older video made by jdavis882 called "Microbevels, Polished, & Toothy Edges: A Close Up View".
[video=youtube;iQJOAe1Xzis]https://www.youtube.com/watch?v=iQJOAe1Xzis[/video]

That is a great video. It would be interesting to see a similar one with even higher magnification. I'm especially curious to see the example of a polished edge he shows closer up to investigate why it still appeared to have a micro bevel (jdavis attributed it to stropping, I wonder if it is essentially a tiny lined up burr, and just how uniform the peaks and valleys are on that edge comparitively).

 

[Dry-cell]

That is a great video. It would be interesting to see a similar one with even higher magnification. I'm especially curious to see the example of a polished edge he shows closer up to investigate why it still appeared to have a micro bevel (jdavis attributed it to stropping, I wonder if it is essentially a tiny lined up burr, and just how uniform the peaks and valleys are on that edge comparitively).

He hasn't been posting on that channel anymore, he moved over to another one since he started making knives. This is his other YouTube channel, DavisBladeWorks.