On sharpening and surface finish

[Boink]

Just read John Juranitch's book and checked out the video. He espouses providing a relief that effectively grinds away any surface finish a knife maker may have applied to the blade (satin finish, DLC coating, etc. etc.).

Is this what one can expect when learning to sharpen knives? Meaning that a user blade is definitiely a user because the blade surface as provided by the knife maker is ground away by the process of properly sharpening the knife?

 

[Cliff Stamp]

Boink :

... the blade surface as provided by the knife maker is ground away by the process of properly sharpening the knife?

This isn't necessary on all knives. David Boye and Phil Wilson are two stock removal makers who grind very thin edges on thier knives, there is no need for a relief grind.

Most of the Japanese blades, even the cheap production ones ~15$, also don't need any extensive edge work because the primary grind extends way down to the edge and it is just 0.005" - 0.010" thick.

The relief grind is only necessary if the edge is very obtuse, and thick. This is however a problem on most production knives and many customs. It isn't inherently a design flaw, but indicates preformance geared towards durability more so than cutting efficiency.

Most custom makers, and even some small shop production outfits like Busse Combat, will modify the initial edge bevel to the customers specifications. Thus when you buy a knife be specific about what you want, and you won't need to apply a relief grind.

That being said, normal use will scratch the hell out of a blade anyway unless you always cut soft and clean materials.

-Cliff

 

[Boink]

Thanks, Cliff. Very succinct response.

It then follows that one should know what they are cutting, know the blade shape for that type of cutting and get the blade with the proper relief from the beginning, if possible. And, yes, what you are cutting will have great effect on what happens to the finish anyway. Thanks again.

 

[Jeff Clark]

Back when I was in college I used to go door-to-door sharpening knives, scissors, and gardening tools. When you sharpen for customers you take great pains not to cause cosmetic damage to blades while you work on them. I would generally reduce the edge bevel down under 15-degrees, but I would be careful to leave a clean and uniform bevel. This didn't work any better, but it looks a lot better. When working on blades with a black coating I would be even more precise. On some military knives I would do a lot of work with a file so that I got a really precise transition between bare steel and coating.

This background is one of the reasons that I am not a bigger fan of slack belt tapered edges. They just don't look right. I normally make as clean and flat a bevel as I can at 15-degrees or so. Then I use a slightly slack fine grit belt to slightly smooth the shoulder where the edge bevel meets the blade bevel and slightly increase the final edge angle. Then I finish off with fine hones at around 20 degrees. You can be neat and have a relatively high performance edge. If it is just for my personal use I sometimes throw that all out the window and hollow-grind the blade. Sometimes a guy just needs a throwing knife that he can shave with.

 

[Boink]

What does a slack belt tapered edge look like? Is it way different than a hollow grind?

 

[Jeff Clark]

What I meant by a slack belt tapered edge is what is more commonly called a convex bevel (I just couldn't recall the term while I was typing my previous comment). You take a blade and lay a considerable portion of the primary bevel against an unsuported section of a sanding belt. The belt forms an arc that grinds a convex curved bevel onto the blade. Sometimes the blade's primary bevel is placed so flat against the belt that the entire bevel is marred and significant material is removed from the spine side as well as the edge side of the bevel. The idea is to curve and extend the blades bevel to blend right up to the edge. This provides an edge that is acute yet strong.

If a blade is not originally manufactured with a convex bevel you are real likely to marr the appearence of the bevel if you try and make it convex after the fact. When I do this to a blade I usually only convex the last 3/16 to 1/4 inch of the bevel. It takes some work with multiple belt grits to sort of match the finish on the blade.

Once have a convex bevel that looks ok (maybe just purchased from the store) you sharpen by stropping or using sort of a slack belt with polishing compound. You need to maintain the edge well since you have to remove a lot of material if the blade gets very dull. This contrasts distinctly from a hollow ground bevel. A hollow grind is essentially a concave bevel (the opposite of a convex bevel). The concave bevel thins the blade in the region behind the edge. While not as strong as a concave bevel it is simpler to sharpen a hollow ground blade. You simply hone with almost anything at almost any angle and you will get a thin edge with high slicing ability. You could use a slack belt at around 15 degrees and get a reasonably strong edge with minimal effort. You could also use simple v-mounted ceramic rods to sharpen just the edge. When you sharpen a hollow ground edge it is easy to not marr the appearance of the blade. You just need to not lay the blade flat on your hone.

 

[Boink]

Jeff - it's very easy to visualize what you are saying. Thanks.

I have a Mayo TNT that I think has a convex bevel on the edge. It's very sharp, it's polished-looking and has that rounded appearance in section view. There's also a slight hollowness to the blade. But I figured that this would be hell to sharpen, so I have never cut with it with the exception of cellphane once in a while. I can see what you mean by removing lots of metal in the event this goes dull. I read once about a poor man's convex bevel by doing two or more straight bevels in series, with each one more acute than the previous.

The way I am understanding it, a hollow ground blade already has good relief built into it by it's nature, so that you can avoid having to grind in a relief. Although I can picture that if it's your favoriate knife, it will eventually get used down to where the hollow gets thicker, necessitating creation of a relief.