Thread: Lapping backside of chisel

Andreas

It sounds like ...

1. The steel is harder than it should be (since you are expected to remove a micron each time you sharpen by lapping the back - otherwise the edge would reach the hollow).

2. You are rocking the blade, or it was curved to begin with and continuing this.

The chisel sounds defective. Japanese chisels are generally very easy to lap and set up.

Regards from Perth

Derek

But if a blade is curved to begin with aren’t there an effective way to get it straight? Like removing more in the middle of the chisel with a file or something to make it concave. Probably won’t work with this since the file probably would be of softer steel....

Hi Andreas,

I'm confused, Japanese chisels backs are hollow, how did you make it convex?

Can you post a picture.

Regards
Ray

The chisel is convex along the hollow of the chisel, the same way as the handle goes.

If we say that the line that goes along the handle is the x- axis and the line that is the cutting edge is the y axis. Then the x axis will be convex and the y axis straight along the plane that forms the backside of the chisel. If still confused then the x axis is the longest part of the chisel and the y axis is 90 degrees of the x axis. Will post pic tomorrow, middle of the night in Norway and now I’m off to bed.



Derek is probably right with option 2, it was curved to begin with but I think I made it worse with rocking. Know of a jig/trick that can make up for my lack of skill?

Hi Andreas,

Quick suggestion.

If you've still got a reasonable hollow in the back of the blade, then try this...

Take your chisel, make sure it's centred on the hollow on your sharpening medium (whatever it is, doesn't matter) and rub the back in the x axis (as you describe) with pressure more at the tip than anywhere else. You should be able to establish a flat area that will swell out from the hollow and should be easily visible because of the longitudinal scratches. Once that area becomes large enough that the chisel can be held down on a flat surface without rocking on that area, carefully use it to establish a flat back.

Go carefully and slowly and check it often. If it tends to run more in one direction (sides, tip, handle, etc.) then focus pressure away from that area and keep working/checking until the flat is nicely balanced.

Yeah, I have chisels that didn't have a flat back, but they do now.

The really interesting point with the hollow in the back of a Japanese chisel is that it actually allows the back to manufactured a right mess, but the hollow allows you to create and then expand a genuinely flat area easily.

It's got to be a fairly poor chisel to not allow you to get it flat again. Poor chisels do exist, but many folks out there seem to be blind to that possibility, considering all Japanese chisels (and tools) to be great, good or garbage with no thought that they're all made in different places by different people with different priorities.

Good luck with it,

Stu.

Andreas

Depending on how much of a crown you have on the chisel, you may have a very difficult task ahead of you. If you have already taken off a large amount of material, the area on the back that is hollowed out during manufacture will have decreased and the flat areas will have increased. Meaning that you will now be trying to remove more material during the honing process, which is therefore going to take a lot longer. I suggest to first use a bench grinder ( very carefully !!) to increase the hollow out section. Then use the edge of a 800 grit waterstone ( making sure it is flat ) so you can work on the crown of the convex area, working across the chisel, and keeping pressure on the crowned area. If you are not confident in using a bench grinder you could skip that, but it will take a lot longer to get back to a flat surface, and you will have a lot more work when honing the back in the future. Another option may be to use a narrow diamond plate to work on the crown of the convex area if you can find one that is suitable.

A file will definitely not work as the chisel will be much harder.

The process will not be quick, but you will learn from this

you could also try posting in the japanese tool section of the forum to see if someone has a better idea.

I hope all goes well for you

Regards

Basil is going in the same direction as myself.

Think of the back of a curved chisel blade as you would jointing a curved edge of a board - If you continue to plane the full length, all you will do is continue the curve. What you need to do is first plane out the centre, create a hollow, and then plane across the full length. It is easier to create a flat side from a hollow.

1. Determine where the centre of the curve lies.

2. I would mark the blade back about 1" either side and then colour this in with a texta or paint on layout fluid. This is the section that you will grind away, preferably on a wet grinder such as a Tormek. If not, then a high speed grinder and do it gently, and wet/cool frequently.

3. Once you have a slight (emphasize slight) hollow, then you may begin lapping the back of the blade directly on this hollow, widening it.

4. Use diamond paste. 40 microns at least. Remember, the cutting steel of Japanese blades is harder than Western steels. It is also thin, so be careful not to grind too deep.

5. I question whether if is worth the effort. I suspect a cheap chisel. Can it be returned?

Regards from Perth

Derek

Hi thanks for all the grate advice!

I will try to grind out the hollow as I already have removed a lot of the back. The chisel was expensive but I got it about 4 years ago so I can’t send it back. There’s only one shop in Norway that has Japanese tools, and the price don't match quality. It cost 120 dol when new. Daylight robbery! But I was not aware of the prices outside of Norway; I will never buy more tools from that shop! They do not know who made the chisel, or what the steel/hardness is .... I was completely new to Japanese tools at that time so i did not know better (I'm still quite a noob when it comes to hand tools, so not much has changed)

Hope I manage to get it flat now!


Thanks all !!!

Hi Andreas,

If it was me, I wouldn't be touching the hollow at all.

On the Norwegian store, there might be a new one starting up very soon...

They should have some rather decent Japanese tools and their pricing will hopefully be reasonable, although I can't do anything about that part of things, only to make sure they have good quality tools.

Stu.

Originally Posted by Schtoo

Hi Andreas,

If it was me, I wouldn't be touching the hollow at all.

Stu.

Stu

I have previously "gently" ground the hollow to reduce the size of the lapped areas on blades so as to reduce the amount of metal that has to be taken off to get a flat back. This does require care, as emphasised in my previous post, as it is easy to damage the flat areas of the blade. Is the latter the reason for you not touching the hollow ? or is there some other reason ?

Regards

I've tried regrinding a hollow a few times, with not important chisels, and it's always ended up being a bit of a mess. I'll have to find out how it's actually done and get shown how to do it, if I can. I don't mean a video or book, but more a case of asking one of the folks I know who make chisels how they do it.

Besides, I'd prefer to use a hammer and tap out the tip enough to get my flat there, and call it good. I'm definitely not one of the 'flat back club', so long as the tool will do what it should I'm not worried about it being something less than ideal.

What I'd mentioned above is how I get one particular brand of chisels notorious for having convex backs and make them flat. The hollow will allow you to start a flat easily, and then grow it out to the edge, but the hollow needs to be decent and present. Altering it, you run the risk of not being able to use it to get things flat again.

But I fear that Andreas here has managed to get something decidedly less than wonderful in a chisel, and we can give a dozen suggestions and none of them might work for him. One of us might make it right, but I don't know how good he is at these things. Not a reflection on Andreas at all, but I get asked all kinds of questions, and need to make sure that the skills of who's on the other end are sufficient to get the job done without fouling it up.

That's all.

Stu.

Originally Posted by Schtoo

Besides, I'd prefer to use a hammer and tap out the tip enough to get my flat there, and call it good. I'm definitely not one of the 'flat back club', so long as the tool will do what it should I'm not worried about it being something less than ideal.

Stu.

Hi Stu, unrelated to Andreas's problem but a question for you or Des: Is Ura dashi carried out on chisels as well as plane blades? I only see examples doing it on plane blades, I don't know why. And I see lots of (mostly western) people lapping the backs of chisels to keep the hollow at bay.

Originally Posted by mic-d

Is Ura dashi carried out on chisels as well as plane blades?

Wow, this is a bit like asking what's the best way to sharpen. Endless debate with no real resolution. There are two schools of thought on this. Obviously ura-dashi is out on the multi-hollow chisels, but is possible on the wider, single-hollow chisels.

Personally, I'm in the "not at all necessary camp". If you have a look at the profile of the most commonly used chisel, the oire-nomi, you will see that the deepest part of the hollow (called the ura-suki) is closest to the kubi, or the "neck" of the chisel, rather than in the central area. This characteristic allows the chisel to retain the hollow until the very end. If the deepest part of the hollow was in the centre, as the blade became shorter and shorter through years of use and sharpening, the hollow would eventually disappear while there was still plenty of life left in the chisel.

Therefore, to me, the chisel blade is designed so that ura-dashi isn't required.

The ito-ura (the flat) is different on a chisel and on a plane blade, and unlike that on a plane blade (unless a consistent technique foul-up takes place), the flat on a chisel will follow you back as you sharpen. There shouldn't be any real need for heavy lapping of the back of the chisel after the initial tuning process - just the normal back maintenance in the course of sharpening.

Des

As I understand, it is not the hollow that is the problem here. It is also not the curve in the back. This can be removed. The problem is the hard steel that is laminated to the soft iron backing. It has a finite depth. One may end up removing the hard steel while flattening the back of the chisel and not have anything to cut with. I have a paring chisel like this at home, bought on eBay, where the Seller flattened it past the hollow, and now it cannot hold an edge.

Regards from Perth

Derek