Sunday, February 19, 2012

Are you flat on your back?

I think of bench chisels as the fundamental woodworking tool, so I have been fascinated by the debate  over flattening their backs.  The consensus of expert opinion is that you should expend a great deal of time and effort flattening the backs of your chisels a considerable distance back from the edge.  Frankly, doing so is extremely tedious and it is even possible to destroy a narrow chisel if you don't pay attention.  Trust me, I know.  Alternatively, you can buy premium chisels with backs that are already flat, like I did after this embarrassing incident.

Then, lightning struck.  Bob Rozaieski and Adam Cherubini suggested that chisels didn't usually have flat backs in the eighteenth century and argued against arduous efforts to flatten the backs of chisels.  I am not an expert, but I do know that the back doesn't have to be flat to create a perfectly sharp cutting edge.  The accepted wisdom is particularly curious because of the widespread adoption of the "ruler trick" for plane blades:  you use a thin metal ruler on your honing surface to create a shallow, narrow bevel on the back of your plane blade to save time and effort.  We are admonished never to do this to a chisel.

The argument I've read for not using the ruler trick on chisels is that it would impede paring because you want the chisel back to ride on a reference surface and trim in the same plane.  Let's think about that.  The ruler I use for the ruler trick is .02" thick and I generally rub the cutting edge on the stone about 2" from it, so, if I remember my trig right, the back bevel angle is about .57 degrees.  If you assume I hone a back bevel of 1/16", the cutting edge is raised .0006" above the reference surface when I pare.  Yes, it's an issue, but a mighty small one and it can be counteracted by raising the handle slightly.  Maybe there's an argument for tedious flattening of backs, but I haven't read a convincing one.

In all other chisel operations I can think of, the back bevel is actually a slight advantage.  We all know that, when we chop, the chisel will move away from the bevel because it makes the chisel want to dive and there is no reference surface to prevent it.  The back bevel tends to counteract this slightly.

Draw your own conclusion.  Mine is to not worry about being flat on my (chisel) back.  My premium chisels came flat, so I leave them that way.  I use the ruler trick on my second set.

For me, this debate raises a more basic question.  Maybe some of the other consensus rules we accept should be reexamined.  At the least, it is useful to ask ourselves why from time to time.


  1. Ask your self this question, have you ever not been able to do a task with a chisel because the back was not flat? In my case the answer is no, so I don't worry about it.

  2. A chisel will certainly take a sharp edge and cut wood just as well whether the non-bevel side is flat or not. But, if you don't mind, I'll take a contrarion view here for the purposes of discussion. Why do you suppose those premium chisel makers go to the trouble of making the backs flat? It isn't just aesthetics. Maybe it's because our current paradigm is for flat backs, and that is what customers expect. But if I received a Blue Spruce chisel with a bellied back and I called Dave Jeske to complain, I doubt he would say "Eh, put a back bevel on it and you'll be fine". I do use the back of a chisel as a reference point, and a bellied back would frustrate me in those instances.

    I flatten my backs not because back bevels don't work - they most certainly do - but once I flatten the back of a chisel or plane iron, that's one less thing I need to worry about when I sharpen. As long as the back is flat or has a hollow, it takes just a few passes on the back at the end of sharpening to remove the burr and refresh the polish. The more factors I can take out of the equation when it comes to sharpening, the better. For me, it's a case of spending more time upfront to save time down the road.

    I really like how Adam and Bob continually challenge our current thinking, though. I learn more from their perspectives on woodworking than a year's worth of magazine articles from some periodicals (like, ahem, Fine Woodworking).