Coarse apexed vs. Fine apexed edge

So I came across this blog done by Todd Simpson which has EXCELLENT photography of knife edges. I asked Todd and he did not mind if I post some photographs here, so here they are!


 

Following the bevel set on the Chosera 1k, the razor was honed on Shapton glass stones; 2k, 4k, 8k and 16k. At each step in the progression, SEM imaging was performed.

The side-view images, at 5000x magnification are shown below. As expected, scratch depth and width decrease with each step. Although the Shapton 2k is rated 7.35μm grit and the 16k is rated 0.92 μm, the observed scratch depth is much shallower, less than one tenth the grit size.

CHO_1K_P
Chosera 1k
Shap2k_P
Shapton Glass 2k
SH4k_5kx
Shapton Glass 4k
Shap8k_P
Shapton Glass 8k
Shap16k_P
Shapton Glass 16k

The above side-view images provide little insight into the edge geometry or keenness. Edge view images, taken at an inclined angle to provide perspective, allow direct assessment of the apex width. The edge width (or width of the apex) decreases with increasing hone grit rating.

edge-on view images after Chosera 1k
edge-on view image after Chosera 1k, showing an apex width in the range of 0.7 microns.
Shap2k_E
Edge-on view images after Shapton 2k, showing an apex width in the range of 0.4 microns.
Shap4k_E
Edge-on view images after Shapton 4k, showing an apex width in the range of 0.3 microns.
Shap8k_E
Edge-on view images after Shapton 8k, showing a apex width in the range of 0.15 microns.
Shap16k_E
Edge-on view images after Shapton16k, showing an apex width in the range of 0.1 microns.

Cross-section images for the progression are shown below.

CHO_1K_X_measured
Cross-sectioned edge-on view images after the Chosera 1k. Sharpness: 1.4 microns width at 3 microns from the edge.
Shap2k_x
Cross-sectioned edge-on view images after the Shapton 2k. Sharpness: 1.2 microns width at 3 microns from the edge.
Shap4k_X
Cross-sectioned edge-on view images after the Shapton 4k. Sharpness: 1.05 microns width at 3 microns from the edge.

No significant change in the bevel angle or width at 3 microns occurs after the 4k level. No measurable difference is observed between the 4k and 16k level, other than a reduction in the apex width, observed in the edge-on views (above).

Shap16k_X
Cross-sectioned edge-on view images after the Shapton 16k. Sharpness 1.05 microns width at 3 microns from the edge.

The convexity near the apex is removed by the 4k level, and the measured geometry is unchanged with higher grits beyond the 4k. This result indicates that the bevel set is completed at the 4k level, not the 1k level as commonly believed. Beyond the 4k level, the bevel polish is refined and the edge width is reduced from about 0.3 microns to about 0.1 microns.

Sharpness is achieved during the bevel setting stage up to 4k grit. Beyond the 4k grit, Keenness improves, but Sharpness is not improved further.

 


 

What this teaches me is explained very well by knife guru, Cliff Stamp (see Cliff’s forum here):
” A low grit finish will generate a much thicker apex than a high grit edge which is one reason why a low grit apex will be stronger (resist lateral deformation) more than a high grit edge at the same micro-bevel angle. However the main issue is the type of cutting which is why I started with “If they are slicing soft materials (cardboard, ropes, foods, etc.) …” . In this case the apex will typically not be under high loads to deform it, it will dull mainly from slow abrasion. To combat that, use a low grit edge to make an irregular surface which will take a lot of abrasion before it is worn smooth and stops cutting and use a low angle apex to both enhance the roughness and maximize the amount of wear required. Because the loads are small the angle you can use can be very low and there is no real risk of deformation.

However if they are push cutting hard materials this type of finish won’t work because the apex will come under heavy loads to try to deform it and if you use a low angle and finish the apex with a low grit finish it is very likely to collapse almost immediately. For example that finish in the above which easily slices though ~2000 piece of 1/2″ hemp will fail dramatically in less than 10 cuts if you try to push cut because the forces will be so high that the apex will just deform and can crack right off. In order to make the apex perform well under cutting that type of material you have to raise the polish and then increase the angle to prevent it from deforming. Now of course continuing to increase the angle past the point it stops deforming doesn’t help. It is just that this point will be a lot higher in push cutting a hard material vs slicing a soft one.”

In other words, a coarse edge will have higher edge retention at very low angles when cutting soft material. I believe it will even have higher edge retention at the same angles as a polished edge, although a polished edge may excel in some push cutting tasks.

Polished Vs. Toothy Edge?

Below is an article by a sharpening expert, Joe Talmadge. In my own findings, a highly polished edge is not only not needed very much, in most cases it actually hinders the cutting ability of the blade. High polish is for push cutting (i.e. “pushing” through a cut vs. “slicing” through a cut) such as straight razor shaving or wood carving. It has very limited uses. In most other situations, you will want and use a toothier edge.

” Many treatises on sharpening tend to focus on getting a polished, razor-like edge. This is partially the fault of the tests we use to see how good our sharpening skills are. Shaving hair off your arm, or cutting a thin slice out of a hanging piece of newpaper, both favor a razor polished edge. An edge ground with a coarser grit won’t feel as sharp, but will outperform the razor polished edge on slicing type cuts, sometimes significantly. If most of your work involves slicing cuts (cutting rope, etc.) you should strongly consider backing off to the coarser stones, or even a file. This may be one of the most important decisions you make — probably more important than finding the perfect sharpening system!

Recently, Mike Swaim (a contributor to rec.knives) has been running and documenting a number of knife tests. Mike’s tests indicate that for certain uses, a coarse-ground blade will significantly outperform a razor polished blade. In fact, a razor polished blade which does extremely poor in Mike’s tests will sometimes perform with the very best knives when re-sharpened using a coarser grind. Mike’s coarse grind was done on a file, so it is very coarse, but he’s since begun favoring very coarse stones over files.

The tests seem to indicate that you should think carefully about your grit strategy. If you know you have one particular usage that you do often, it’s worth a few minutes of your time to test out whether or not a dull-feeling 300-grit sharpened knife will outperform your razor-edged 1200-grit sharpened knife. The 300-grit knife may not shave hair well, but if you need it to cut rope, it may be just the ticket!

If you ever hear the suggestion that your knife may be “too sharp”, moving to a coarser grit is what is being suggested. A “too sharp” — or more accurately, “too finely polished” — edge may shave hair well, but not do your particular job well. Even with a coarse grit, your knife needs to be sharp, in the sense that the edge bevels need to meet consistently.”

By Joe Talmadge