Chisel Backs, Stop Lapping

It’s an age old arguement. How flat do chisel backs really need to be. Some flatten the entire back right up to the bolster, others just flatten the 1/2″ or so near the tip. In either case, the arguement has always been that the back of a chisel needs to be absolutely flat in order to do its job properly. Today, much of this work is done by the manufacturer, especially on higher end tools like the chisels from Lie Nielsen. However, there are reports of some folks spending 2, 3, 4, even 5 hours or more lapping the back of a single lesser quality chisel, with little progress being made on achieving that all important flat back.

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How the flat back craze started I’m not sure. I will admit that I subscribed to the flat back camp at one time. That is, until I started buying old tools. After spending a rediculous amount of time trying to flatten a few of the antique chisels and plane irons I had acquired, I began to think that there had to be a better way. There was no way that our ancestors would have spent so much time on such a task that had no financial value to them at all. They needed to get their tools sharp and get back to work. Let’s face it, flattening chisel backs does not put food on the table.

So rather than trying to find a faster way to flatten the backs of these old tools, I began to think more about the tools, and challenge conventional wisdom about tool backs being absolutely flat. This was about the time that David Charlesworth wrote his first article about a technique now commonly referred to as “The Ruler Trick”. In essence, you place a very thin ruler on the edge of your honing stone when chasing the burr off the back of the blade, in order to focus all of the effort right at the cutting edge. This of course creates a small back bevel on the tool, but it eliminates the need to lap the back flat. The one caveat was that it was not to be done with chisels, under any circumstance, only plane irons.

So I tried the ruler trick with a few plane irons, and I liked it. It was great for old irons that would have otherwise needed a lot of work to lap flat. However, chisels were still a challenge. So I still challenged the flat back wisdom. I took a look at the old chisels I’d acquired. Inspecting the backs of them, one thing became immediately evident. Not a single one of them had a flat back. None. Really. I mean it. Not one!

I did a little research and found out that properly made chisels should actually be concave along the length of the back. Think Japanese chisel. Traditionally, toolmakers would harden and temper the chisel before the bevel was ground. This way, after the heat treatment, they could identify which side of the tool had warped concave, and grind the bevel on the opposite side. Doing so would put only the very edge of the chisel in contact with the stones when chasing the burr. This made perfect sense to me, as it focussed the work only where it was needed, just like a Japanese chisel. Unfortunately, not all of my chisels and plane irons seemed to be made that way. Some had convex backs, the hardest kind to flatten.

About this time, Adam Cherubini wrote an article for the Arts & Mysteries column of Popular Woodworking (November 2006). The article was about period sharpening. In it, he experimented with an old grindstone and old (not flat) honing stones. One of the outcomes of his experiment was exactly what I had seen on many of my old plane irons and chisels. The backs had been dubbed over and the cutting edges were almost double bevel in design, like a knife or a carving chisel. The dubbing was the result of using dished stones. This made sense to me. Period tradesmen wouldn’t have any effecient way to remove dishing in a stone once it was there. They would just continue to use the stone.

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What Adam found was that such an edge really didn’t affect the use of the tool. But it certainly was by no means flat. This prompted me to do a little experiment of my own. So I took a few old chisels I had (not wanting to back bevel my Ashley Iles chisels) and sharpened them using the ruler trick. Guess what. They got real sharp and I didn’t have to lap them flat. Guess what else. I didn’t notice any difference in use either.

Today, I don’t use the ruler trick, but I don’t lap my chisels and plane irons either. For the tools that I have with concave backs, the concavity does the work for me, focusing the effort right at the edge. For the tools that I have with convex backs, I will lift the tool slightly to make sure the very edge is where the work is done. In essence, I will create a shallow angled back bevel on the tool, even on chisels (gasp!). In use, I have found no disadvantage in using chisels with shallow back bevels vs. chisels with dead flat backs. Carvers have been using double beveled chisels for centuries and they can still make good flat work. It’s no different for bench chisels.

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The main arguement for using a chisel with a flat back has to do with using the back of the tool as a flat reference for chopping & paring tasks. However, the geometry of this just doesn’t work. The bevel of a chisel is a wedge. The action of a wedge is such that the direction of travel as the wedge is driven is along a path that bisects the angle of the wedge. So a chisel with a flat back will not move in a line that is parallel to the back of the chisel. Instead, if you lay that flat back on a flat surface and push forward, the chisel will want to dive into the wood, as pictured at left.

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This same tendency for the chisel to dive into the grain when paring with the chisel flat along the grain tends to cause undercutting when chopping down through end grain, as in chopping the waste between dovetails. Similar to the picture above left, the chisel wants to travel in a direction that bisects the bevel angle. So if the waste section you are removing is too thick, providing enough resistance to the downward force of chopping, the tendency is for the chisel to move backward if it is held vertical during chopping. If you chop with the chisel right in the scribe line with too much material remaining on the waste side, you can crush the baseline and cause small gaps in the assembled joint. This is the reason most dovetail instructions have you stay away from the baseline until there is barely a whisper of material left to chop or pare away before you go for the final cut right in the scribe line. The alternative, if you really want to chop straight down, is to angle the chisel back over the baseline by an angle equal to half the bevel angle of the chisel. However, this too would damage the baseline as the wedge shape of the bevel drives the fibers apart.

One benefit of the small back bevel that I have noticed is a greater ability to “steer” the chisel out of a cut. Similar to using a lathe, where you always want to ride the bevel during the cut, having this small back bevel on the chisel allows you to steer the chisel out of a cut that wants to dive. The very shallow back bevel gives you just enough clearance that you have to lift up ever so slightly on the handle of the chisel during a paring cut. Conversley, dropping the handle back down causes the back bevel to lift the cutting edge up and out of the cut. I find that I can control my paring cuts better by steering the chisel this way rather than just trying to hold the back flat against the stock.

So I’m sure this post will ruffle some feathers and damage my credibility with flat back worshipers who still insist that a chisel has to have an absolutely flat back. That’s OK. I’ll spend my time working on wood while they’re busy lapping away. If you’re a little more open minded though, and like to experiment and think outside the box, take an old chisel and give it a try for yourself. It may take you a few minutes to find the cutting edge if you are used to holding the tool absolutely flat while paring, but with just a little experimenting, you’ll be steering your chisels like an Indy car driver through a street course. Not to mention spending a lot less time at the stones.

Any one want to buy a 12″ x 18″ granite surface plate?

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12 thoughts on “Chisel Backs, Stop Lapping

    • The back of a bench firmer simply needs to be flat enough that when rubbing the back during honing (chisel registered ‘flat’ to the stone) the burr is chased back to the front (bevel side). That’s all, however that tends to be pretty darned flat!

      If it’s not, then successive honings will be at increasingly steeper angles to the back (in order to chase the burr back to the bevel side) which results in what is ultimately probably an undesirable shape, or too severe a one at least – a knife-shaped edge.

      A flat socket in endgrain (dovetails, bridle joints, etc.) is achieved through a specific methodology and order of paring (not willy-nilly with a knive shaped edge which might be more forgiving absent a specific order of work – though this isn’t the way of the craftsman). The best write-up I’ve ever seen on this is an Ian Kirby FW article (black and white years) entitled “Chisels, and how to pare.” Kirby covers it all in this article, as per usual, from position of feet, torso, elbows, all the way to the tip of the chisel.

  1. Thanks for this Bob – I’ve started trying to be lazier with my sharpening since my natural tendency goes closer towards the “can’t I get the edge smaller than 1 atom?” side of the sharpening discussion. I’ve done this a few times but always feel guilty about it, but now I’m just going to add it to the list of things I do on purpose. šŸ™‚

    I’m quite pleased that even without using a sharpening guide all the time, slightly imperfectly flat backs, and bevels that aren’t perfect mirrors I can still cut and shape wood just as well as before. It’s very nice to know where the quality bottleneck is (lack of skill and experience).

  2. Bob,
    It is interesting that we were just having this same conversation here last night. The same discussion can be heard for flattening the backs of drawknives. While I have been flattening my chisel backs I don’t flatten my drawknife backs. I brand you a churlish troublemaker.
    Mike

  3. what a great explanation bob. you truly have a gift for identifying and illustrating all the stupid stuff we do with out making us feel like boobs for having done it ;).

    the more i learn about woodworking, the more i realize that the ‘arts & mysteries’ (over used, i know..) are really about saving us work. your video on squaring lumber was an excellent example of that and here’s another.

    perfect/flat/dimensions/etc. are dangerous concepts to casually throw around near an engineer as he enters this hobby and like shaun, you’ve done quite a bit to jolt me out of needless, time robbing obsessions.

    so, thank you bob. again.

  4. I don’t bother being too fancy on the backs of most of my chisels – for the majority of work, I find it doesn’t need to be perfectly flat, and like you say, being not quite flat almost helps it steer a little better (although if I really need to steer, I should be using the chisel bevel down)

    That said – I don’t have a router plane, and a lot of times find myself using a chisel to clean up tenon faces or the bearing surfaces of lap joints or cross joints (think the bottom, single stretcher of the PW sawbench) – often using a guide block or as Chris Schwarz does in his sawbench video, using a hand screw clamp as a guide surface. I often do the same with mitres, using a guide block like Roy did on the corner cabinet episodes.

    In those situations, where I’m trying to pare down a slight hump, I often find myself frustrated if I don’t use one of my chisels with a flatter back – if the back is convex at all, it rides up and over the hump, rather than slicing through it. If I try and english the chisel into cutting the hump, if I’m not careful, the direction of force, like you diagram in the photo, ends up with the chisel wanting to dive into the work. A flatter chisel doesn’t dive into the work if I keep downward pressure on the chisel behind the bevel, as the body of the chisel behind the bevel can’t move into the work, so this becomes no problem. I guess I just need to experiment with it more; but do you have any hints for making this work with the less-flat chisels? Because I can’t get it to happen.

    As far as lapping goes; whenever lapping begins to turn into an hassle, I pull a trick from Garrett Hack’s handplane book – I take a small grinding wheel on drill, or a dremel tool (electrons, I know, I’m sorry! Although I bet you could make it work with a with any wheel grinder with care) and put a subtle hollow grind on the tool. Particularly if I have a pronounced “belly” on the tool – even just grinding enough of a hollow to give myself two bearing surfaces makes lapping the make much more effective, as the tool will no longer rock on the stones.

    As an addendum, the theory I had heard about hollow honing stones, was that it was much more common for folks to hone all variety of things in their households – knives, scissors, farm implements, and razors, of course; so there were a whole lot more old oil stones being used for normal household tasks than by joiners and cabinetmakers. A lot of those items are a lot more forgiving of using a dished stone than chisels and planes would be. Some razor folks even say they prefer a dish.

    Whether this has any bearing on if woodworkers would flatten their stones, or move their less-than flat stones to household tasks is pure conjecture, though, I suppose.

    I feel like most of the “old” sharpening info I’ve seen do mention to use the whole stone, however. If you’re careful to use the whole thing with oilstones, (as a free-hand sharpener ought to) I know you can keep them flat for a *very* long time.

    • That is a really cool idea with the Dremel tool or similar to put the slightest of hollow on the back. I never even considered that. I think I might give it a try on a chisel that I am having a bear of a time with.

  5. You’re right. The flat back idea is an IDEAL and it is not an ESSENTIAL. One can get by perfectly well with a slight back bevel from the ‘ruler trick’ and not really notice it.

    One point is that, IMHO, it is a good idea to try to use the very thinnest shim to create your back bevel using the ‘ruler trick’. My experience is that a very (and I mean VERY) slight raising is all you need to get the benefit – more than that is creating the issues you mention. I now just put a piece of masking tape on the blade where the ‘far’ edge will rub on the stone and that’s enough.

    And I do that the first time only. Each subsequent sharpening, I just rub the back of the blade on the stone. I find that I get the back flatter over time (in effect getting rid of the back bevel) and I still wipe off the burr that honing the main bevel draws.

    The consequence is, in my experience, that I can get a blade (chisel or plane or whatever) to a good usable standard and keep it there AND over time get the back flattened out so that, in time, I don’t need to use the ‘ruler trick’ again, unless I run into another pit in the back of the blade a bit further up the blade!

    IMHO, sharpening is about getting back to work ASAP with a tool that will do the job easily and safely. You don’t get marked on sharpening! If it works well and safely, it is sharp enough.

  6. I would agree that obsessively trying to produce an “absolutely” flat back on old chisels is an exercise in . . . obsessiveness. That said, for paring I prefer a more flat back over a less flat back, and I would not deliberately grind a back bevel on a paring chisel. I don’t want to guess at what angle above flat my chisel edge will produce a surface that extends the surface behind the cut–which is called a register surface for a reason. No dogmatism, no obsession, just a preference for a flat(er) back for paring.

    PS–I see that Adam Cherubini has reprised his own declarations on this topic, in the latest issue (Oct. ’11) of Popular Woodworking.

    • I saw that article too. I thought it was a pretty funny coincidence. But it more or less confirms what I’ve found during my experimentation. And I guess I should clarify that the back bevel is an absolute last resort. If I can get the chisel sharp quickly by registering the chisel flat to the stone, I’ll certainly do that. But if the edge is already dubbed over slightly, I won’t waste time flattening the dubbing out. I’ll just back bevel ever so slightly. It is a very mild angle that is more or less undetectable until you try to register the chisel flat on a flat stone.

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