Saturday, April 14, 2012

GNHW Sharpening Demo

For the April meeting of the Guild Of New Hampshire Woodworkers Hand Tool Group, held at the Homestead Woodworking School in Newmarket, NH, Roger Myers, Dave Anderson, and I gave a demonstration of different sharpening methods. The goal was to demystify the process while showing some of the choices available.

This was the number one item when members were polled on meeting topics last year. You can read Roger's take on the day here.

Sharpening is one of those subjects that tends to get people worked up. One of the attendees compared it to the Lilliputian factions in Gulliver's Travels, the Big-Endians who break their hard-boiled eggs from the big end, and the Little-Endians who break theirs from the little end. They fight repeated wars over the difference.

Roger showed hollow grinding with a Tormek wet grinder and waterstones, I showed double- and convex-bevel sharpening with oilstones (I also demonstrated sharpening a card scraper), and Dave finished the day by showing micro-bevel sharpening with a Veritas Mk II power sharpening system.

As different as these methods are, it's more important to focus on the similarities:
  1. Foremost, they all achieve a sharp edge that allows you to do your work. 
  2. They all involve working front and back sides to form two planes that meet at the sharp edge.
  3. They all involve working through progressively finer abrasives.
  4. They all involve controlling the blade being sharpened to get a desired bevel angle of 30 degrees at the edge.
  5. They all finished up with a stropping method.
  6. They all took only a few minutes. We spent far more time talking about them than actually sharpening.
One other point on which we all agreed: you should do it frequently. Frequent quick sharpenings keep your tools tuned. A couple of minutes is all it takes to maintain an edge once you've achieved the initial shape.

My attitude is that any method someone has been using successfully for years to get their work done is a valid one, regardless of what others might think of it. Beyond that it's just a matter of personal preference. You can spend all your time arguing over cost, speed, efficiency, ease of use, accuracy of angles, etc. or you can get back to woodworking.

Roger started out demonstrating hollow grinding. He used a Tormek wet-grinder to establish an initial hollow in a chisel bevel, then used waterstones of 1000, 4000, and 8000 grit. He finished the edge by stropping on a leather strop charged with green chromium oxide compound.

The actual hollow-grinding is an infrequent operation, and the most time-consuming step, taking 5 or 10 minutes. The Tormek uses a jig to hold the blade at a specific angle to the slowly-rotating abrasive wheel. The wheel turns in a bath of water which clears the swarf and keeps it cool, to avoid overheating the metal. Overheating ruins the temper, a common mistake people make when using a dry power grinder.


Roger sets the angle on the holding jig using a guide.


With the wheel turning away from him, he applies thumb pressure to the chisel end as he moves it side to side on the guide. That motion ensures even wear on the wheel.

On the waterstones, Roger established the bevel angle by resting the end of the chisel on the stone, so that the front and back points of the hollow contacted it. He freehand sharpened by drawing the chisel down the length of the stone, then flipped the chisel to run the back up and down the edge of the stone. It only took a few passes on each stone. Then he finished by drawing it down the strop.

Roger said he repeats the stropping frequently to refresh the edge as he works. Eventually, he has to go back to the stones. After several resharpenings on the stones, he's worn the hollow almost flat, and goes back to the Tormek.


Roger pulls the end of the chisel down the waterstone, maintaining its angle by keeping the ends of the hollow-ground arc in contact with the stone.

Next I showed sharpening on oilstones, Norton India and Arkansas stones, using my portable sharpening station and drugstore mineral oil. I use oilstones for a combination of practical and frivolous reasons:
  1. They're reasonably cheap and fast.
  2. They're portable.
  3. They last a long time.
  4. They're the more traditional method, and I like keeping the tradition alive.
I prefer not to use a jig, because I want to develop the hand control, and I find jigs too fidgety, too time-consuming setting an angle consistently from one time to the next. They are useful in training your body to the angle of the blade.

The advantage of the portable sharpening station is that it's fast to setup and use. Secure it to the work surface with clamps at the beginning of the day, no matter where I am, then when I need to sharpen, lift the lid and go right to work. No fussing around pulling out stones, setting them up, putting them away.


Illustrating single-bevel, double-bevel, triple-bevel (behind my hand), hollow-ground, and convex-bevel. Photo by Roger Myers.

I showed both double-bevel and convex-bevel methods. Historically, the double-bevel resulted from doing rough shaping on a large hand- or foot-powered grindstone to remove the bulk of the metal, followed by careful honing at a higher angle on bench stones to form the cutting edge.

Whether with modern powered grinders or by hand on stones, the primary bevel does not require any real precision. It's the secondary bevel that requires care. The heavy metal removal on the primary bevel reduces the amount of work required on the secondary bevel.

As with Roger's method, the secondary bevel can be resharpened multiple times until it's time to re-establish the primary bevel. Even then, you only need to regrind the primary bevel back enough to leave a starting secondary bevel. Thus over many repeated sharpenings, the two bevels chase each other down the length of the tool.

A triple-bevel puts a third shallow bevel behind the primary bevel. The purpose of this is to reduce the amount of metal you have to grind to re-establish the primary bevel. That means three small flats, none of which ever requires a lot of work. However, not many people use it.

The convex bevel combines the two angles of the double-bevel (or the three angles of a triple-bevel) into a single continuous curve. People object that this is rounding over the edge, but it's not. The edge itself is still at the same cutting angle as any other method. The rounded part is behind the edge.

What I like about the convex bevel method is that it's easier for a beginner to learn. The most difficult part of freehand sharpening is maintaining a constant angle. The convex bevel turns that difficulty into an advantage. Then as you develop control, you can decide if you want to keep doing it that way, or change to one of the flat-bevel methods.

One thing I always recommend is to buy some really cheap chisels and replacement plane blades at the home center and use them for sharpening practice. It doesn't matter how well they hold an edge, just use them to develop the hand skill on the stones. Spend a few hours repeatedly sharpening them and dragging the edges on sandpaper to dull them. Just like sawing dovetails, with a little practice you can get it down. Then you won't be so nervous about sharpening your finer tools.


Honing a secondary bevel on my chisel.

For the double-bevel method, I showed how to hone the secondary bevel at approximately 30 degrees freehand. Plus or minus a degree or two doesn't make a whole lot of difference. Since this was just a demonstration using a tool that was already shaped, the primary bevel was already established at 25 degrees. That can be very time-consuming. I use a DMT extra-extra coarse diamond bench stone for that; it's faster, and I avoid wearing the India stone.

I started out on the coarse stone for about 30 seconds, moving up and down its length, until I could just feel a tiny burr on the back of the edge. Then I advanced through the medium, fine, and hard Arkansas stones about 30 seconds each. I flipped the chisel over and honed just the end of the back, sideways up and down the length of the Arkansas stone. Sometimes this is enough to see the wire edge come off in the oil.

Then I finished up on the strop, pulling the bevel toward me about 10 times, then flipping it over and pulling the back the same number. This should remove the last bit of wire edge. If not, flip it back over and repeat. This edge can be maintained by repeated stropping, but when I eventually have to go back to the stones, I only go back as far as the medium or fine, depending on how long I waited to resharpen. Regardless, restoring the edge never takes more than a minute or two. Lately, I've been using Flexcut Gold compound instead of green compound. It seems to leave a better edge, but it's hard to tell.

One of the problems learning to sharpen is finding a reasonable way to measure sharpness. There's no gauge or measuring device that will tell you objectively. Taking end grain shavings on pine is a useful subjective measure. Because it's soft, pine end grain will crumble and catch if the tool isn't sharp. But when the tool is sharp, you get clean shavings like a pencil sharpener.

I demonstrated the sharpness of the edge on the end of a piece of pine. Time to put my money where my mouth is: I stood the edge on a sanding block and pulled it along to dull it, accompanied by groans from the audience. I tried it on the pine, and it barely cut, digging in.

To restore the edge, I repeated the entire sharpening process, but using the convex bevel method. The only real difference is the motion. Starting at the near end of the stone at my desired angle of 30 degrees, I push forward, dipping the chisel down as I go. Pulling back, I raise it back up, being careful not to raise it any higher than 30 degrees. About 30 seconds of work to raise a burr, then on to the other stones, finishing with the back on the Arkansas.

Stropping goes the same way: starting at the far end of the strop, draw the bevel back as I raise the end of the chisel. Then strop the back.

The moment of truth: I was able to take clean end grain shavings again. The edge was just as sharp as with the double-bevel method.

For the card scraper, I used the method I learned in this video by Scottish furnituremaker Dougal Charteris. Most of it is pretty standard: put the scraper edge up in a vise, file the edge flat and square to the faces, then polish the edge on sharpening stones, and polish the two faces along the edge. This process forms two sharp arrises in a couple minutes of work.

The next part is where people tend to have problems, using a burnisher to turn the "hook", the cutting edge. Dougal angles the burnisher (he calls it a strop) down a bit and takes one swipe. That's it. Most people do what I used to do: run the burnisher down the edge a couple times to slightly mushroom the metal on both sides, then take multiple passes angled down on each side to turn the edge. It's all those multiple passes that are the problem, plus its easy to use too much downward force. You end up rolling the edge past it useful point.

By limiting it to one swipe, or maybe two if the scraper doesn't seem to be getting a bite on the wood, you turn a tiny but sharp edge. This will take very fine shavings, not dust, once you bow the scraper with your thumbs and angle it on the wood just right. This all takes a bit of practice and finesse, but once you get it, you realize what an amazing tool the scraper is, especially considering it's nothing but a rectangle of sheet steel. I use a Hock burnisher, which is a good hard tool steel.

I also showed how to take the edge off the scraper by laying it flat and running the burnisher up and down flat on the face. Then I put the edge back on. This is another thing to practice, putting the edge on and taking it off, learning to control it. Just like the chisel edge, eventually you need to stone it again, then go all the way back to the file.


Another view of the sharpening setup as I file the edge of a card scraper in the vise behind it.

Dave Anderson gave the final demonstration. He makes and sells marking knives, each with two beveled edges, so he does a lot of bulk sharpening. He uses a Veritas Mk II power sharpening system. Where the Tormek is an abrasive wheel turning vertically in a water bath, this is a horizontal spinning platter with dry sandpaper disks adhered to it. Similar to the Tormek, it has a guide rod and jig for holding the tool at the proper angle.

Dave went through 3 grits of paper, applied to 3 platters which he exchanged as he progressed. A platter can have paper on each side, so once one wears down, he can flip it to the other side. Because the backing of the finest grit is thinner than the other grits, it ends up increasing the grinding angle by a hair, resulting in a micro bevel at the edge, a very small double-bevel.


Dave sharpening a chisel on his Veritas Mk II. With the wheel spinning away from him, he applies pressure to the end as he moves the chisel side to side in its holder.

For stropping, Dave used 1 micron diamond paste on hard maple plywood. The diamond particles get embedded in the wood and hold in place as the tool passes over them. He said the wood turns black from the metal swarf almost immediately. A tube of paste lasts a long time.


Stropping on a plywood block with diamond paste.

While they vary in the details, all these methods achieved sharp tools quickly. Any one of them will allow you to get back to work. There are yet other methods that will do the same. Sharpening can be turned into a hobby in its own right, an obsession, but it's really just another practical skill.

2 comments:

  1. I think you opened a can worms Steve. The point to take away is find a way that works for you and stick with it. Get your tools sharp, return to woodworking, and then argue about which is the best way. By the way, I have a tormek for restoring bevels etc, and I now use diamond stones and strops for my sharpening. And I don't use micro bevels on anything.
    ralph

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  2. Yes, the next question is whether you'd like those worms fried, baked, or grilled. They're all tasty no matter how you do it!

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