Ha-Ganna : Sharpening 2

In the previous post on ha-ganna sharpening I started to talk about having difficulties getting a good cut with my 60 degree blade. My other blades all cut fairly well. Yet despite trying a few different sharpening techniques, and doing my best to get a really sharp edge, the 60 degree blade continued to cut poorly. So I started to think a lot about how the blade was cutting through the wood. When you use ha-ganna you are always cutting across the grain. With a cross grain cut the more you can slice through the wood with a shear cut the cleaner and easier the cut will be. Particularly with certain soft woods a shear cut can really help to get a clean slice. Imagine using a chisel to cut across endgrain. If you hold the chisel so that you are cutting perpendicular to the grain the cut takes more force and it usually less clean than if you shear across the grain with the blade at a slight angle. This is especially the case as the blade starts to get dull.

As I was thinking about all this I decided to model my blades in sketchup. I also started to search around the internet for any information I could possibly find on ha-ganna. One thing I noticed was that blades from another maker looked like they were forged at tighter angles than my own. The angle of my 60 degree blade is actually really close to 60 degrees. Several of the blades that I found online for cutting 60 degree kumiko, looked like they were forged more like 45 degrees. So I started wondering how the forged angle of the blade would affect the cutting edge geometry. If the blade is formed at a tighter angle like 45 would that still give a workable edge geometry for cutting 60 degree kumiko? And how would it compare to my current blade?

I ended up reaching out to a friend who had ordered some ha-ganna from Japan a while back. He ordered his set from a different supplier so I was curious if his blades differed from mine. He sent me some photos of his blades and sure enough the blade he has for cutting 60 degree kumiko is forged at a much tighter angle, around 44 degrees. Thanks for the help Shaun Davis!

In contrast, here is the forged angle of my 60 degree blade

Here I measured the angle of the sharpening stone I’m using for this blade, and you can see it’s right around 61 degrees.

Both Shaun’s and my blades’ were sold as 60 degree ha-ganna. Yet clearly the angle they were forged at is drastically different.

From here I decided to model the blades in Sketchup to get an idea for how the forged angle would affect the edge geometry of the blade. Below is an image from sketchup showing a model of my current 60 degree ha-ganna blade next to one with a forged angle of 45 degrees like Shaun’s. I set both at the same angle as they would be if they were in dai (plane block), and both have bevels that produce a 60 degree cut in a imaginary piece of wood.

Here is the blade forged at 60 degrees, producing a cut at 60 degrees.

And here is a blade like Shaun’s, forged at 45 degrees but producing a cut at 60 degrees.

The edge geometry of each blade as it would slice through an imaginary piece of wood, is completely different. The blade on the left (forged at 60 degrees) has a strong vertical attack. While the blade on the right (forged at 45 degrees) has much more of a shearing cut action.

It seems to me that the shearing cut would produce a much better cut. Particularly since it’s going to slice the fibers at the surface of the wood first, helping prevent tearing of fibers. It’s my guess that this is the main culprit behind my own difficulties getting a clean cut with my 60 degree ha-ganna blade. I’ve seen numerous videos of people cutting really soft wood like sugi (Japanese cedar) using ha-ganna with beautiful, clean results. Yet no matter how much I tried I couldn’t achieve anything like that using my 60 degree blade.

My general takeaway from all this is that it’s best for a blade to be forged at a tighter angle than the desired cutting angle. I could certainly be wrong, but clearly the difference between my blades and Shaun’s are quite large. Also the effect on cutting ability and edge geometry is clearly affected by what kind of angle the blade is forged at.

To test this theory a bit more I modelled my 60 degree blade with an edge geometry for cutting 45 degree kumiko. (45 degree kumiko ha-ganna leave a 90 degree cut in the wood). Below on the right is the 60 degree blade producing a 60 degree cut. On the left is the same 60 degree blade producing a 90 degree cut. The edge geometry is completely different.

Here’s the 6o degree blade, producing a 60 degree cut.

And here is the 60 degree blade producing a 45 degree cut.

Finally I couldn’t help but put this to the test a bit. So I took my 60 degree blade and reshaped the blade to produce a 90 degree cut (as described above). And it worked great. It cut really smoothly, leaving no tearout or jagged grain in even really soft sugi.

Below is the modified 60 degree blade, along with the blade that I bought for cutting 45 degree kumiko pieces. Both make an actual cut of 90 degrees through the wood, but clearly the edge geometry is vastly different. (The blade sold as “for 45 degree kumiko” is forged at a 90 degree angle).

So I’m really curious how the blacksmith who made my blades decided on the angles to forge each blade. My pessimistic guess is that the blacksmith probably didn’t really know any better. Maybe the Blacksmith forged the blades at the same angle as the desired cut without thinking about the effects. I could also be completely missing something. I am far from an expert on ha-ganna and my experience is still limited, so take all this with a grain of salt.

In any case, hopefully this doesn’t come as disappointing news to those who bought blades from the same shop as me. I think these blades will probably work quite well with more tolerant woods, like basswood. But for really soft woods the edge geometry starts to become a problem. I’d be interested to hear from anyone else who has started to play around with ha-ganna, and curious what your experience has been like.

~Jon