A question about sharpening a bottom-cleaning chisel

Jesse
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A question about sharpening a bottom-cleaning chisel

Sat May 25, 2013 3:55 pm

Hi all:

I joined last week but haven't started yet, so sorry that my first post is a question, and a bit out of left field at that.

I bought a sokosarai-nomi from Hida Tools recently and I can't find any resources on how to sharpen it. I used a fine diamond plate to get the semblance of an edge, and then a waterstone, both flat on a table, but it is hard not to get a number of facets doing it freehand. I am concerned that I may have borked the geometry of the blade. Before I turn this mortise cleaner into an expensive earwax cleaner, I decided to stop.

Should I make a jig of some kind? Use small stones? If anyone can point me to a reference, I would be greatly obliged.

Thanks in advance,

Jesse
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Chris Hall
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Re: A question about sharpening a bottom-cleaning chisel

Sat May 25, 2013 11:45 pm

They are tough little buggers to sharpen, exceeded only in difficulty, I think, by the knives on kama-kebiki (the marking gauges with the blades bent 90˚ at the ends).

The main problem is the two surfaces you want to work, the bottom and the bevel are small and geometrically awkward, especially in terms of holding the lower end of the tool steady and in the same position for each stroke.

Once you have lost flatness on the bottom of the tool, it is very difficult to re-establish it as the tool becomes even more prone to rocking. You need to get back to a clean slate, getting the bottom of the tool dead flat and polished.

I would suggest you make up a block of wood - a jig - to fix the tool rigidly so that you can move the stone over the bottom of the blade to get the bottom flat again. The wooden jig only needs a slot cut into one side so that the shank of the tool can be fitted, and a recess on the side where you will be moving the sharpening stone for the cutting head to sit flush to the surface. A wooden wedge or screw clamp of some sort is needed to lock the blade firmly in position, and to allow for adjustment.

Once you have the tool positioned as well as you can in the jig, take a couple of swipes with the finish stone first and observe the pattern on the blade's bottom. If the pattern of scratch marks is centered on the surface, then continue with the flattening, starting with a coarse stone and working up. If the scratch marks are concentrated on one side of the bottom or another, adjust the wedge and re-position the blade, then take a test swipe with a finish stone and repeat the process if necessary until you have the scratch marks centered. The jig must not allow the blade to squirm around while you are working it.

Once you have the bottom flat and taken up to your finish stone, then you can work the bevel. For that, I place the stone on a 4x6-ish block of wood so that the chisel can hang down to one side and the bevel can be worked. After each grit progression, dress off the bottom by hand on the finish stone to remove the burr. Repeat until done, and try out the tool to see how it cuts.

Once you have the bottom nice and flat and polished, subsequent manual work of the tool over the finishing stone on the back, since it removes so little material, will not tend to deform the tools bottom any further. If it does become non-flat over time, drag the jig off the shelf again and re-set. Make sense?
Jesse
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Re: A question about sharpening a bottom-cleaning chisel

Sun May 26, 2013 12:11 pm

Hi Chris. Thanks for taking time out from your weekend to answer my question.

Before reading your reply, I was thinking of truing the sokosarae as I would true the blade of a router plane: by running a jig sitting on two parallel tracks of MDF (or plywood blocks) over the sharpening surface. But I don't know if the shaft of the sokosarae is stiff enough to not flex or torque as it goes across the surface of the plate or stone. The shaft on a router plane is much shorter and straight. Because I had already removed so much metal just to get an edge on the foot of the chisel, and there is precious little to begin with, I didn't try it. Your method eliminates the problem of flex. When I get the stones and get it sharpened, I will report back with the results.

Best

Jesse
Jesse
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Follow up: a question about sharpening a bottom-cleaning chi

Tue May 28, 2013 10:04 am

Hi Chris,

I spent some time Sunday browsing for small stones online, but I could see that it was going to take some time find stones that would fit the bill. So Monday, in a fit of impatience, I decided to use what I had at hand, and build the jig you suggested but use it upside down. A bit rough, but it worked. The foot was rounded at the center, and I did get it flat fairly quickly, but I think my earlier free-hand attempt changed the angle of the foot and bevel. So the cutting edge is now too acute--it cuts, but the edge is too fragile. I will have to adjust the bevel. Sorry for the poor photo quality: I used my old-school cell phone to take the pictures. Whether this is a positive example or a negative example...well...
Photo-0002.jpg
Photo-0002.jpg (238.01 KiB) Viewed 4533 times

Not pictured is a small strip of wood that I attached to the small rail with double stick tape to keep the chisel upright.
Photo-0003.jpg
Photo-0003.jpg (183.69 KiB) Viewed 4533 times
Photo-0006.jpg
Photo-0006.jpg (205.8 KiB) Viewed 4533 times
The sharpening surface is a DMT fine diamond plate. I moved the jig and the two squares of plexiglass across 1/2" MDF held down with double stick tape.
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Chris Hall
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Re: A question about sharpening a bottom-cleaning chisel

Tue May 28, 2013 6:31 pm

Good job! The only thing I might change on that jig would be to attach a 'fin' and second clamp, to the bottom, so that a wedge could be slid in between it and the edge of the chisel to fine tune the side-to-side tilt of the tool. I would worry that a single clamp could let the tool squirm a bit while sharpening, but maybe I'm just paranoid :geek: . Looks like you're getting the job done though!

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