Chisel backs

[Corneel]

Some philosofing and a bit of experience with vintage chisels, made me start a thread on chisel backs. I am especially interested in how these theories can be used in practice to reduce the amount of work to bring an old one back in shape and use it with pleasure.



The ideal shape of a chisel has a flat back. Ideal, but hard to get in practice. But in use when paring wood this should work very nice. Light pressure on the tip and the chisel starts to cut. Also when registring the chisel from a reference surface this should be ideal, but I don't think that is a very common occurence in woodworking. A question with this shape is, how to keep it flat over time?




The concave back. This is often regarded to be ideal too. At least it is easy to sharpen. And over time it will tend to become flatter, which is more ideal probably. But in paring it might be a bit grabby? Tend to dive into the wood too easy?




The convex back. Often regarded with dismay, but very common in the field. If the belly isn't too bad, how awfull can this be? It makes for a chisel which is easy to stear in the wood. Also sharpening this isn't difficult as long as you keep fingerpressure on the edge. Over time it will tend to become rounder, not flatter.




The true backbevel. This is probably something to avoid. The chisel doesn't want to cut when paring. Sharpening is also difficult.



And then something about wear and sharpening. An edge wears round, thus creating a wearbevel both on the front and the back.

Usual practice is grinding back the bevel until you get a wire edge. That means grinding back to the red line which removes both wear bevels. Then just a light polishing wipe on the back to remove the wire edge. At the other hand, what if you want to avoid this scenario and give a quick hone often, what do you do exactly to avoid a backbevel?

Personally I'm not afraid for a heated argument but please try to remain civil to each other. My ideas in this are far from set in stone. I hope to learn something from the collective wisdom.

 

[woodbrains]

Hello,

A lot of this makes sense and the bits about back bevel and wear bevel you describe, are pretty much exactly as I was saying in the other thread, but as I also pointed out, someone's mission was to always contradict me, no matter what. It just becomes exasperating for me, so good luck here.

I would say that the concave back illustration is a longitudinal section and therefore the chisel is hollow in both dimensions, rather like a Japanese chisel, rather than just a banana shape? Am I right. If so, the chisel would not be grabby, as the sides would contact the work and it would act no differently to a chisel with a flat back. A banana shape would be terrible.

Paring to a reference surface is done quite a lot in dovetailing, and other paring tasks for fine fitting joint making, so I would always avoid the convex back. I don't think it is worth getting a lovely cast steel chisel, especially if it is for little money, and not bringing it up to the best working standard it could be. It takes workto get them flat, but not a prohibitive amount. The drawing is an exaggeration, I've not come across a chisel out by more than ( estimating ) about 10 thou. Most are much less and flatten on coarse wet and dry in minutes.

Mike.

 

[Corneel]

Indeed, pictures are exagerated.

I have a small set of Japanese chisels and they are by far the best chisels I have. Totally flat on the back (with the hollow of course), easy to sharpen but a very long lasting edge. They are quite ideal, but rather short, which sometimes is not helpfull. And expensive of course.

I also have two very long Sorby paring chisels which are definitely banana shape. You can bend them though, so it isn't too hard to press them flat on a surface. I am not sure yet what to do with them.

And then I have a fairly large accumulation of Dutch, Swedish and English chisels, most are more or less convex. I use them often, also for paring duties and don't find the convexity to be very problematic.

I'n mot quite sure how you use a reference surface when dovetailing? As far as I can see it is mostly a freehand excercise. Also for example when cleaning up a halflap joint after the initial chopping. All the surface is rough, so there is no reference. I then work from both sides inwards, without reference either, probably leaving a bulge in the middle, sloping down to the sides. No reference to speak of. I'm just lying the back flat on the wood and work on the bulge until I shoot out on the other side. A convexity doesn't hinder this aproach. The only reference surface I can think of is with a guide, for example when paring the bed of a plane using a beveled block clamped to the top as a guide.

 

[Corneel]

Oh, and I don't know why we can't get coarse sandpaper overhere that makes leveling chisel backs a matter of minutes. :evil:

In the past I've spend hours on a single chisel. Especially when going from the sandpaper to the stones, I always seem to have created a convexity.

 

[Jacob]

Theorising is interesting (I suppose :roll: )but it can get in the way of the work especially if you are driven in the direction of flattening and polishing everything!
But in reality any sharp edge will work but the different configurations (bellied, back bevelled, flat, concave etc) might entail small variation in technique which would be more or less unconscious. You might also be altering your sharpening technique, without giving it much thought.
There is only issue if a particular shape is definitely causing a problem in which case you either pick up another chisel or reshape the one you are using.
In reality a chisel would have to be very concave to make "paring to a reference surface" a problem. You can see the logic of it ( that's the trouble with armchair theorising!) but who has encountered this in a real working situation? I certainly haven't and a lot of my chisels are vaguely concave on the face - including nice old ones. IMO flattening them spoils them and makes sharpening more difficult.
In any case it is logically difficult to "pare to a reference surface" as you are altering the surface as you go and directing the tool as you want. Chicken and egg - you can't have a reference surface unless you pare to it, starting from a position without a reference surface. Erewego, armchair theorising has no limit!

 

[woodbrains]

Hello,

OK you rumbled me, I just theorise from the comfort of my armchair. Here are some of my theoretical dovetails in some of my theoretical furniture.

I really cannot understand the want of some here to try and circumvent doing what is necessary to get a tool sharp and well fettled. I cannot think of anything in life that benefits by missing out steps and pretending the result is just the same. Why waste time thinking up new and novel ways of not doing what is necessary, my tools work; if they were not sharp they wouldn't, nor flat backed, nor have fine bevels etc. And if my tools work, why wouldn't I tell others what I have found to work best.

Mike.

 

[Corneel]

Just been looking at my Sorby parers. The wide one is in fact not bad at all. It has a hollow in the back with the front and the sides in one plane for several inches up the face. The narrow one isn't so good, it is definitely more banana shaped.

Mike, I still wonder exactly where the reference is when making dovetails?

And why don't I want to make the chisels totally flat? Because of the work involved.

Nice work BTW.

 

[Jacob]

Hello,

OK you rumbled me, I just theorise from the comfort of my armchair. Here are some of my theoretical dovetails in some of my theoretical furniture.

Very neat. Mind you I'm not too keen on that irregular spacing - I know it's fashionable (everybody's doing it :roll: ) but it just looks a bit untidy to me. Also I prefer the single kerf triangle (sometimes called London Pattern for no apparent reason) rather then the fat ended trapezium. And it's easier. I guess you are obeying the arbitrary rule of a gradient of 1/8
Here's some of mine

I really cannot understand the want of some here to try and circumvent doing what is necessary to get a tool sharp and well fettled.

Because it doesn't seem to be necessary to th extent which is fashionable nowadays.

I cannot think of anything in life that benefits by missing out steps and pretending the result is just the same. Why waste time thinking up new and novel ways of not doing what is necessary, my tools work; if they were not sharp they wouldn't, nor flat backed, nor have fine bevels etc. And if my tools work, why wouldn't I tell others what I have found to work best.

Mike.

I'm sure they work but I think you probably spend too much time doing unnecessary things in the way of flattening, polishing etc. not to mention the struggle with honing jigs and buying unnecessarily expensive tools. :lol:

 

[Corneel]

Show offs!

 

[woodbrains]

Hello,

I did actually write a longish reply to Corneel's questions, but it seemed to disappear when I presented submit, might do again later, if I have time.

When I do dovetails, I choose assymetrical ones ( not irregular spacing!) for constructional soundness, not fashion. If a board is going to cup, it can perhaps let go along the edges. Spacing the dovetails so there are more towards the edges helps with resisting any cupping of the board. Of course it might not, but I'm not going to be here in 100 years to witness it. I try to build longevity into my work, central heating puts a lot more stress on furniture than would be seen even 30 years ago and available wood is not as stable as Cuban mahogany, either.

London pattern dovetails are nice, but not for carcasses.

There are about 13 individual tails per edge on the carcass in one of the photos. (More than 50 coming together all at once.) I do not want a tool that will not behave and do something I cannot predict or have to make allowances for as one over cut will ruin the whole set. Once the chisel backs are flat, they are done forever and I can always rely on them.

Corneel's, in half blnds, the pins are partially cut with a saw and this provides a reference surface for the chisel to continue the pin socket into the corners. There are others, if you think about it, even holding the chisel vertical to make a chopping cut at the baseline will not be possible if the chisel is convex or has a back bevel. Once the first chop here establishes a flat and vertical face, it serves as a reference for continuing the cuts. It is much harder to cut a flat surface with a non flat chisel.

Mike.

 

[woodbrains]

Oh and do they look like one in eight to you?

Mike.

 

[G S Haydon]

First off, compliments to those showing the quality work. Both examples are very well executed indeed =D>

Also, I do not want to change how anyone prepares their tools or claim their methods are no good or what I think is best.

Ok, so until I started looking at forums (feb this year) all I had ever done was sharpening. This concept of flatness was all new, I felt quite ignorant. I bought a cheap set of chisels and took some time to flatten the backs a little. I did not fully flatten them because I was too lazy. Reflecting on this approach I wish I had not bothered for what is a general purpose set of chisels. Every single one of those cheapies had a slightly hollow back. Perhaps a 12mm flat behind the blade would be more than enough for the type of work I do. The really nice thing about the hollow back is that it focuses the honing when removing the burr, and it would seem speeds up removing the wear bevel.
I very much like this article on the subject. http://www.toolsforworkingwood.com/store/blog/243

I have never back beveled a chisel.

I don't think I have never used a convex bellied chisel. I think you could use one, cant say really if I have never tried.

Each to their own

 

[Corneel]

I see what you mean Mike. But that's only a very short bit, 3/8" I suppose in a carcasse, I suppose? Even with a convexity all the way to the edge you can pair into the corner and steer the chisel wherever you want it. My bellied chisels actually have a bit of flat on the front, about 3/8 I guess, so that would take care of this issue. Anyway, I am in no hurry to start flattening the backs of these chisels, until I find a situation where I really need a flat back.

Backbevels on a chisel seem to be unwise, so I understand why you won't want to strop this side.

Nice article G.S. Joel sounds pretty rational in that article.

 

[Jacob]

...... It is much harder to cut a flat surface with a non flat chisel.

Mike.

In the ordinary way of things they are never so non flat that you couldn't cut a DT as you describe and in any case a concave curve would conveniently undercut and make a better dovetail!
Come to think a lot of shoulders need undercutting - a purpose made under cutting chisel could be good - I'm surprised nobody has thought of this, I'd buy a complete set (with PMT69 steel and bubinga handles) immediately!
Mind you it could be done with a convex faced chisel (within reason) with no difficulty at all.

 

[David C]

One of the worst aspects of the bellied chisel, is that the wire edge will not be honed away on a fine stone, without some very dodgy handle lifting.

Unless of course the belly exactly matches the hollow of the stone! I suspect this is the cause of convexity on many old tools.

David

 

[Corneel]

No problem reaching the wire edge. The secret is putting finger pressure on top of the chisel near the cutting end. So you don't need to do any dodgy handle lifting. From experience I know it is no problem to get the edge sharp.

We are talking very light convexities here, a piece of paper just slips under it. The picture is exagerated of course. It will get worse over time, but I have no idea what timeframe we should think about.

 

[Jacob]

One of the worst aspects of the bellied chisel, is that the wire edge will not be honed away on a fine stone, without some very dodgy handle lifting.

Well yes but nothing dodgy about it - needs must when devil drives!

 

[woodbrains]

No problem reaching the wire edge. The secret is putting finger pressure on top of the chisel near the cutting end. So you don't need to do any dodgy handle lifting. From experience I know it is no problem to get the edge sharp.

We are talking very light convexities here, a piece of paper just slips under it. The picture is exagerated of course. It will get worse over time, but I have no idea what timeframe we should think about.

Hello,

This is where I get confused. If the convex only is slight, how much effort could it be to correct to flat? But if the slight convexity eventually becomes more exaggerated over continued honings, as I agree, it must, surely it is essential to correct the problem early on.

Mike

 

[Corneel]

I see your point, but I hate the work. Sitting here on the sofa isn't usefull either, but at least it is pleasant...

I did a bit of calculating. 0.2mm convexity. The chisel is 1.8cm wide and the area about 10cm long. Lets say the shape is triangualr instead of convex. That makes 0.5 x (0.02 x 1.8 x 10) = 0.18 cm2 of hardened toolsteel to be removed with sandpaper. That's a lot!

And as long as it doesn't present real problems and as long as I have other chisels when it really must be flat, I remain lazy.

 

[Jacob]

I suppose what I'm suggesting is that you shouldn't look to closely at your chisel. Just sharpen it and get on with whatever you are doing.
If you find there is a problem in spite of all you best efforts then have a closer look.

I had a 7 plane which just wouldn't do a good straight edge however I tried. It turned out to be slightly concave along the length so I put it on ebay and bought another one. Job done! There's no point in looking for problems from the armchair - they'll show soon enough.