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.
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.
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.
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.
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?