Tag: myths busted

Dual Grit Sharpening

A few months ago, a video was posted by Pete Colligan on his Cedric & Ada YouTube channel demonstrating the enhancement in edge retention of a knife sharpened by a novel technique, where one side was sharpened with a coarse diamond plate and the other side with a fine grit aluminum oxide hone.

Gabe Kirkwood, the man who provided the knife to Pete, later posted a video demonstrating his sharpening technique.

At the time, I watched Gabe’s video and it was apparent to me that his technique would produce a large burr, but it wasn’t at all clear why this burred edge would perform so well. After all, burrs are known to be bad, the veritable Bogeyman that keeps young sharpeners up at night.

Some time later, Gabe contacted me to see if I would be willing to analyze a knife sharpened with his dual-grit technique. Gabe had the idea that sharpening makes scratches or grooves and those grooves create serrations in the edge, and he produced this very nice drawing (below) conceptualizing what he imagined his technique produced.

Although I’ve demonstrated repeatedly that this isn’t typically how sharpening occurs; that scratches generally don’t form serrations in the apex, the so-called toothy-knife myth is still widely believed. For this reason, I was happy to have Gabe send me a couple of knives, sharpened with his dual-grit technique.

Gabe sent me three Victorinox paring knives; one where the last stroke was on the coarse diamond plate, one where the last stroke was on the fine hone and one finished blade that had been stropped on a pine board loaded with 0.5 micron diamond.

Following my initial SEM analysis I performed my usual cutting tests and evaluations and found the knife to perform very well. It would easily slice paper, and nearly push-cut with just a small amount of draw or tilting the knife (which is the same as draw cutting). It wouldn’t meet my definition of shaving sharp, but it could easily scrape hair off my arm. I then did some moderate cutting with the knife, 20 end grain and 30 cross-grain slicing cuts into the edge of a maple block and then I cut about 3m of corrugated cardboard. Following this cutting exercise the knife still performed well, easily slicing paper. The blade was analyzed in the SEM again at this point.

The first image shows the coarse side of the blade prior to stropping. The edge is non-linear with height variations on the order of 10 microns and for the most part the height variation does not correlate to scratches.

Side view of coarse side of the dual-grit sharpened knife where the last stroke was on the coarse side.

The next image shows the fine side. Again, we can observe the non-linear edge, but we can more easily see that some of that non-linearity is due to the presence of burrs.

Side view of fine side of the dual-grit sharpened knife where the last stroke was on the fine side.

From the edge view direction, we can observe the burr clearly, noting that some areas appear to be partially broken.

Inclined edge-view SEM image of the dual-grit sharpened knife prior to stropping.

Now for the good part. The cross section view shows that there are actually two burrs, one very large triangular burr, and then a more typical thin, broken burr on top of that.

Cross-section view of a location along the edge of the dual-grit sharpened blade (prior to stropping) shows that there is a large burr (metal outside the triangle of the sharpening angle) and also some smaller, tenuously attached burrs at the apex.

Stropping the blade on a pine board loaded with 0.5 micron diamond did clean up some of the loose burrs.

Side view of the fine side following stropping. The concave area near the apex is apparent from the fact that the strop has only contacted the last few microns of the apex but not the next 10 or 20 microns in from the edge. The edge is still non-linear, but the more tenuous burrs have been removed.
Side view of a cross-section through the apex of the dual-grit sharpened and stropped blade shows that the triangular burr is still present after stropping, but the apex has been cleaned up and made more keen by the strop.

At this point I did a moderate amount of cutting with the knife, enough to remove the initial sharpness and allow me to observe the early stages of the dulling process.

Edge view of the stropped knife after moderate use cutting maple and cardboard. Blunting is apparent along most of the apex.

To understand why this blade performed particularly well cutting sisal rope, it is important to understand that sisal is formed from bundles of fibre which are on the order of 10 microns diameter individually. It should therefore be expected that the last 10 microns of the apex will contribute the most the cutting of sisal rope.

SEM image of a few strands of sisal rope, showing a typical fibres on the order of 10 microns thick.

In the cross-section view, we observe that the smaller edge burr is still partially present but blunted, while the large triangular burr is still intact. There is little change in the 10 microns from the edge that we expect to contribute to sisal rope cutting.

Edge-view SEM image showing the cross-section of the dual-grit sharpened knife after moderate use. Although the apex is blunted, the sharpness as measured by the width 10 microns from the apex is only 4 microns.

Although blunted, the knife is still has a remarkably sharp geometry with a width of 4 microns at a distance of 10 microns from the apex. At the neck of the burr, 8 microns from the apex the width is only 2.75 microns, which corresponds to an included angle of just under 20 degrees, less than that of a new razor blade or disposable bladed box cutter.

Following some moderate use, the irregular burr heights are mostly levelled.

To summarize, the dual grit sharpening produced a burr much like sharpening just one side with the coarse diamond plate would, while the fine (6k King) stone pushed it back towards the center-line without removing it. This produced a low-angle triangular apex which is by definition a burr because it exists outside the triangular sharpening area. However, since this acute triangular “burr” is so thick and its steel undamaged, it does not fold or tear off with use.

There it is. A good burr. Sleep well.