07 May 2008

The Knife Hospital


This is a Japanese hangi-to, the carving knife that's used most often in traditional moku hanga. The hangi-to, pronounced hahn _ gee (hard g) _ toe, is used to closely outline the design on a block. Then larger areas are cleared away from these outlines using gouges and chisels. The hangi-to is essential for carving the beautiful thin black lines that are always found in ukiyo-e prints. Although I rarely use such linework in my own prints, I do use the hangi-to quite often, so I was pretty upset when I dropped mine on a hard floor and the point broke!

Interestingly, even though I'm using these woodworking tools on a daily basis, I'm not really a tool kind of person. I've tried to learn the subtleties of sharpening, but it all makes my eyes glaze over a little bit -- maybe it's a girl thing. I have managed to learn to use a leather honing block and some small water stones to keep the edges sharp on my little tool set, but the broken point was too much for me and my attempts to reshape the knife just made matters worse. I briefly thought about getting a motorized sharpener, but found it to be cost prohibitive.

When in doubt, consult the Yellow Pages. I found a local precision tool sharpening company with the welcome words "No Job Too Small" on its advertisement. For a mere $5.00 they were happy to reshape my knife for me. I'm hoping to pick it up tomorrow, as good as new.


Eli Griggs said...

I hope your blade will be all you want it to be when it is returned to you but I want to encourage you to do this work yourself in the future. It is really very easy and will allow you to work uninterrupted when the Muse is upon you.

A set of diamond 1 x 3 inch 'stones' in coarse, fine and extra fine are ideal for reshaping these knives' geometry in a mater of a five minutes or so. Reserve the coarse stone for major repairs. After the repair/reshaping, hit them with the water stones to remove the deep scratches from the diamond surfaces and to polish to a smooth surface. Hone as usual and you're good to go.

A very good reason to learn to do this yourself is that toh tips break frequently and it's just not practical to break your pace of carving to send out for repairs. Pauses for periodical touch-up sharpening are easy to work into the woodcutting routine and will give you a small rest as you carve as well as the best edge for the job.

It is likely that the knife sharping shop will use machines to shape/sharpen your tool and very likely will overheat the relatively small blade so it will not hold a good edge. The best nature of the bi-layer blade will not be easy to recover if it is overheated and you will have to remove significant amounts of material to get to metal that has not had its' temper stolen by careless handling and too much heat.

If they use a vertical wheel grinder it will also cause a concave 'face' in the blade which might not properly support the cutting edge which is a more brittle than the soft steel/iron backing that absorbs a lot of shock that bi-layer tools experience while being used. This concave face might also effect the handling of the knife while carving.

A grinder will often remove much more metal than needed for a good edge. These are very small tools and they need a deft touch. It may be too much of a stretch for the run-of-the-mill tool sharpening tech to go from sharpening mower blades or kitchen knives to the very specialized nature of the Japanese toh.

A shop may also neglect to properly flatten and polish the back of the single face blade for a truly sharp woodcut tool.

If they use a mechanical cloth or felt buffing wheel, then the odds are better than good that the fine edge of the blade will be blunted and kept from achieving a truly sharp cutting edge. Wood hones are best for this as the material will not roll over the edge and dull the blade. Firm leather, backed by wood is a good second choice. Even paper rubbed with honing compound and backed by a piece of plate glass or wood is preferable to a buffing wheel, cloth, felt or leather.

I hope you will let us know how well the shop handles your knife and how it performs on your next block.


Annie B said...

Eli, thank you for this great information. I have some small water stones in coarse, medium and fine as well as a leather honing block and honing compound, and I do an OK job of keeping my tools sharp, but I was flummoxed by the sheer amount of metal that I would need to remove in order to reshape the toh after losing the point. My stones seemed too small for the job, but the biggest issue for me was holding the toh consistently at the correct angle for each face. I just couldn't seem to do it.

I sent the knife out to the shop with the full knowledge that I might not be satisfied with the result, but I was already unsatisfied with my own efforts, so we'll see what happens. I'll report back.

Lana Lambert said...

Hi Annie,

I keep a stock of course emory cloth for just such emergancies. If you can hold your knife at an angle pretty consistantly, you can grind off enough area to get rid of the break and quickly enough to make it efficient. Because emory cloth is backed on fabric and not paper it works great with water stones. It won't fall apart when it gets wet but it will smell like a wet dog. :P Good luck!

Anonymous said...

Hi Annie,

I use a motorised device with the wheel turning through a water bath and it works like a dream. I understand your frustration with hand sharpening. But Eli is right you do need to do this work yourself. Can I suggest you try a honing guide. It will help you hold the tool at a set angle. Google will take you to Rockler tools and a nice image of a guide they sell for $14. The guide is a little wobbly but once you work out a way to clamp your tool in place it does


Eli Griggs said...

Annie, keeping the knife in the correct position can be difficult but there is a simple method you can use that works well for me.

First of all, it's very important that your water-stones are flat, without dished-out areas or grooved tracks.

It will be almost impossible for you to get a good face/edge on your knife if is is has to ride in and out of an unleveled surface. Small stones are just fine for these small tools and it is more important to have well maintained stones than how large they are or are not.

When you set-up your water-stone, have it running left to right in front of you. You should be in a comfortable position, standing or sitting, that does not require you to bend low if standing or reach much above mid-torso if sitting. Either of these extended postures will throw you off balance and you'll not be able to sharpen correctly.

Assuming you're right handed, take the knife lightly in the right hand and place the entire face of it, on the right edge of the water-stone.

It's important that the cutting edge run parallel to the ends of the stone so you keep the same geometry.

If the edge is broken or you want to change the line of the blade to a more acute or more shallow angle, simply adjust the knife so the angle you want it to have is in parallel to the end(s).

Now place your left hand index finger on top of the back of the blade so it pushes down directly over the part you're sharpening. You should feel the full face of the blade contact the stone as it 'rocks' into position. You will apply direct pressure to this part of the blade the entire time it is on the stone and even if you lift the blade, you can return to this position every time. Let the face of the blade guide you to the correct position each time you put it to a stone.

You will find that it is easy to detect when the blade face is lifted too far forward or rocked back so the entire face is not in contact with the stone. It is a must that the face is always in contact with the stone while sharpening.

The right hand is only used for pushing and drawing back the knife, the left keeps the face/edge in position. Always keep the edge parallel to the end of the stone

It is helpful if you also place the left hand middle finger on top of the index finger nail (sort of like crossing your fingers) to assist with applying pressure and reducing fatigue.

All you do now is push the along the length of the stone, right to left, keeping the face in full contact with the stone.

Until you build a reasonable degree of confidence that you're in fact getting the edge you want, I suggest you lift the blade at the end of each pass, AFTER you've stopped pushing and return to the starting point, reposition the knife again for the next pass.

This is more a more deliberate pace but you won't be as likely to rock the blade out of position as you would if you pull the blade back along the stone.

When you are a bit more skilled you can push and pull the blade back and forth in the more common manor.

A few end points.

Keep the blade face flat, without a secondary bevel to the edge.

Lap the back of the blade and keep that flat as well.

When sharpening, you must raise and completely remove a small wire edge from the cutting edge of the blade. Your knife won't be truly sharp until you do so. This is mandatory to most all wood tool sharpening. The wire edge can be very small so long as it runs the complete length of the edge. A loop or other magnifying glass is useful to examine the edge.

Remember, if you look at the edge straight on and can see light reflected back, your knife needs work.

Don't ever skip grits to get done faster, it's a false economy of labor and you'll work harder in the long run.

Finish up with a good chromium oxide honing compound and always PULL, never push, the blade across the strop with pressure applied to the blade as if sharpening.

I should also mention that I left out a medium diamond stone from my list. Diamond stones cut much faster, deeper and with less pressure than water-stones and are ideal for repairing and reshaping tools. They should be part of your kit, IMO, in addition to water-stones. They should be used wet with water.

I personally like the ones that are backed by 1/4 inch steel. Avoid the type that are in handles that won't lay flat on a table.

I'm sorry for the extra long posting but I hope this helps.

Annie B said...

Tom, I think some kind of honing guide would be a good solution for me. Also, I looked at the Rockler web site and noticed a couple of motorized wheels that are cheaper than the ones I looked at previously and can be used with water. Maybe one of those...

I think I can make the big flat bevel if I do it carefully as Eli suggests. But, correct me if I'm wrong, I think I see a clear secondary bevel on my toh, and that's what I can't figure out how to recreate. That's where I imagine a guide would help me.

Eli Griggs said...

Annie, just to touch base, is it possible you are looking at the thin line created by the thinner lamination of hard steel backing that forms the cutting edge?

If your blade indeed has a secondary bevel, at the edge, it would be best if you worked the face into a single face.

A secondary bevel is not needed or desirable in a dedicated woodcutting knife. They are common to other types of knives in general but here you only want the two planes of the knife to come together to form a single perfect edge.

If however you see a deviation from a single big bevel towards the back of the cutting face, where it emerges from the shaft, don't wory about it, so long as it doesn't impact your making the desired cutting bevel.

It is absolutely necessary that your tools back remain flat, without any hint of a bevel.

About cutting guides; you may find them too fiddly to bother with and if you only have small stones, you will need to upgrade to new ones that are large enough for the guide and blade to run on.


Annie B said...

Thanks again, Lana Eli & Tom for taking the time to respond. I've uploaded a photo of the toh in a new post if you want to take a look and I've also queried the Baren Forum to see if the shape of my toh is unusual.

Anonymous said...

Hi Annie,

Just to make it clear the honing guide will not work with water stones because they are little awkward bits of rock that only allow for small hand movements. A honing guide is best used with carborundum (wet and dry)paper which comes in large sheets that can be adhered to a flat backing like a piece of timber. The wheel can run over the sheet and you can build up a smooth momentum. As with the stones you will need a range of grits. If you are careful these sheets will give you a lot of service, if you are unlucky you will cut into them, but they will never develop the uneven surface that make the stones hard to use.

Annie B said...

Eli, you were right. I was interpreting the difference in metal color as a bevel. My sharpener guy understood what HE was looking at, thank goodness, and he didn't make a secondary bevel. I'll have to wait and see if his process damaged the metal or not, but for now it's behaving well.
Tom, thanks for the clarification about using a honing guide.