Friday, August 17, 2012

Single-Bevel DuoSharp Sharpening

The setup: DMT DuoSharp Black/Blue (extra-coarse/coarse), leather strop, and DuoSharp Red/Green (fine/extra-fine) in a holder, with 30 degree sight block, Flexcut Gold compound, and a spray bottle. The upper edge of the holder has crescent-shaped finger notches rasped out to make it easy to lift the DuoSharps and flip them over. But here I just used the Blue and Green sides.

After I posted a link to my Convex Double Bevel Sharpening post on the UKWorkshop Hand Tools forum, Mark Rhodes, one of the readers there, said I was spending too long on my sharpening, and his chisels were sharper than what I showed in my video. He said he sharpens with a few swipes on a DMT fine stone.

It's clear from Mark's blog that he's capable and knows what he's talking about. He's a trained and experienced professional, having started with a 4-year apprenticeship with one company, then later a 4-year stint with John Barnard, where he says he relearned all the techniques he had thought were set in stone. He works with a mix of power and hand tools.

There are a number of variables in sharpening, and I'm always curious to know what different people find effective, especially when they have the experience to back it up. So I asked him a few questions about his technique, since I have a pair of DMT DuoSharp stones. Thanks, Mark!

My goal is not so much to find the "best" method as it is to learn proficiency with a variety of methods. As with other skills, I like the versatility of knowing that no matter what setup is available, I can get good results with it reasonably quickly and efficiently.

Am I doing better than the other guy? I don't know. I don't see it as a competition, I see it as developing a range of choices. Then I can pass that information on to others who have similar setups, and we can all learn together.

Mark uses a single flat bevel, or two points of contact if he's just hollow-ground the bevel. He said sometimes he'll follow the DMT fine with a piece of slate, and usually strops. He uses a side-to-side motion up and down the length of the stone, like Mike Dunbar's Sensible Sharpening method.

So I gave it a try, with an emphasis on minimizing the length of time I spent. However, note the update at the end of this post; I didn't get it quite right. Nevertheless, I got some interesting results. You can see the corrected method at Hollow-Ground DuoSharp Sharpening.

First I reshaped the bevel of my chisel on a DMT extra-extra-coarse Dia-Sharp plate to get it to the 30 degree single-bevel. Then I went through a few cycles of sharpening on the DuoSharp coarse (blue) and extra-fine (green), followed by stropping with Flexcut Gold on a homemade leather strop to practice the method.

I may not be doing it exactly the way Mark does, but I'm using the same general parameters. While he doesn't use a coarse stone, I went ahead and used mine since I have it, but if you include his slate, we're both using two grits before stropping. I also found in subsequent practice I could get away with fewer strokes on the strop than I show in the video.

Side-to-side on the DMT coarse, with the sight block on the end to show me the angle to maintain.

I experimented with a couple different grips on the chisel, with my forefinger wrapped around as shown above,  and extended down its length. I got similar results both ways, but different blade shapes would clearly benefit from a variation in grip (think of the differences between a short spokeshave iron, a thick mortising chisel, and a wide plane iron).

Unlike the convex bevel methods, here I'm trying to maintain a very consistent flat bevel, so I want a grip that keeps it steady up and down as well as side to side. I still ended up with a very slight convexity, but should be able to improve that with practice.

I also tried sharpening a little longer, but clearly there's a point of diminishing returns. Stop when it's done, any more is just wasting time and metal for no further benefit. You just end up building up a bigger flap of wire edge that takes longer to remove.

You can tell from the scratch pattern whether you're contacting the full bevel surface and how much progress you're making. While learning, it's best to check it every few strokes to make sure you're not getting off track. Adjust as necessary.

The results were excellent. With just a very quick sharpening, I was able to slice copier paper by resting the chisel on the edge of the paper, and it took clean shavings off a scrap of pine end grain, leaving a glassy smooth surface (incidentally, I learned about this pine end grain test from Garrett Hack in the same article that describes Dunbar's sharpening method).

The paper is a threshold test: either it will or it won't. The pine is a comparative test: it allows me to compare the shavings and surface resulting from multiple blades to see if one is better than the other. These allow me to evaluate the relative effectiveness of various methods as well as tell if I've done a good job from one time to the next.

As with other methods, if I didn't wait too long to resharpen, I was also able to do a very quick maintenance sharpening, just the Green and a few quick strokes on the strop. That got it back to paper-slicing sharp.

The chisel slices smoothly through copier paper.

Taking a pine end grain shaving.

So this is indeed a quick and efficient method, another good one to keep in the toolbox.

Update: Before you get too excited, it turns out that that I misinterpreted an important bit of information: Mark always bases this on a hollow-ground bevel. So he's not working a full flat face, he's working the two small flats at the ends of the grinding arc. That avoids the convexity (which he says would have earned him a telling-off from the foreman) and allows him to resharpen with a minimum of effort. I'll be covering this in an upcoming post.

Another thing to note is to avoid confusing this with single-bevel sharpening of Japanese tools, which are composed of a thin layer of hard steel for the cutting edge laminated to a thicker layer of soft steel. Jim Kingshott, in his book Sharpening, The Complete Guide, said that this soft steel abrades more easily, so the whole tool can be sharpened easily with a single bevel. Conversely, he pointed out that the reason for the western double-bevel is that removing metal from thick irons composed entirely of harder steels is time-consuming, so it is initially reduced with the primary grinding, leaving a smaller amount of metal to be honed at the secondary bevel. He also mentioned that he had met one or two craftsmen who sharpen all their western-style tools with a single bevel.

See the corrected Hollow-Ground DuoSharp Sharpening method.

1 comment:

  1. Great post on sharpening. I have been using Paul Seller's method for a couple of months now and I'm happy with that. But I still like to read/see what others are doing.