Back beveling bench plane irons
Recently, a lot of folks have asked me what my opinion is about back beveling bench plane irons. I’ve tried to be courtesy as I explain to them that this is like asking a theologian about the number of angels you can comfortably seat on the head of a pin.
Back beveling seems harmless enough. But, in fact, there are a number of things to consider before you do anything other than develop a bright, mirror-like, flat (yes, I said FLAT) surface on the back of your bench plane irons.
Just as an aside; many of those asking for my opinion are folks who are not happy with the performance of their bench planes. My first response to this is to ask them if their irons are sharp, really sharp. It still amazes me how many people, experienced woodworkers, have never learned to sharpen their tools. Sharpening is THE gateway skill. So, for the sake of argument, I’m assuming that your irons are SHARP!
WHAT IS A BENCH PLANE?
There are two types of planes. The BLOCK plane iron is seated with the bevel pointing up, toward the user. The BENCH plane iron is seated with the bevel pointing down, away from the user. In most cases, bench planes will be fitted with a chip breaker. The bedding and cutting angles on a block plane are not the same. The bedding and cutting angles on a bench plane (unless a back bevel is employed) are the same.
PLAYING THE ANGLES
If you’re going to be serious about hand planing, you have to be aware that there are a number of angles which determine cutting efficiency. The plane body itself provides the BED ANGLE. When we talk about the iron we have to consider the INITIAL (ground) ANGLE, THE SECONDARY ANGLE and the BACK BEVEL, if one is to be employed. The sum of all of this information is the CUTTING ANGLE.
What is performance? How are you going to use the plane? Will you be using the same plane while working various species of wood? Will you be using the plane for initial truing? Will you be thicknessing with this plane? Will you be using the plane for smoothing the finish surface on softwoods? Do you want the ability to vary shaving thickness? Is your goal to remove the thinest possible shaving? Will the plane be used to smooth highly figured stock, such as curly maple?
Well, if your answer was that you want to be able to smooth highly figured stock, then continue reading. If you were answering yes to the other questions, keep the back of your iron flat and polished and have a nice day.
UNDERSTANDING THE CONVENTIONAL WISDOM
Conventional wisdom is based on the collective experience of many people who have separately accomplished the same or similar tasks. In the case of planing wood, this conventional wisdom has been developed over, at least, fifteen hundred years. And the conventional wisdom is this; when using a bench plane, a forty-five degree cutting angle provides the best overall performance over a broad range of wood species and job requirements?
WHY DOES BACK BEVELING CHANGE THE PERFORMANCE CHARACTERISTICS OF THE PLANE?
In a word, back beveling changes the CUTTING angle (some folks like to refer to this as the attack angle). It has long been known that optimal cutting angles vary, depending on the wood species being planed. For example; soft, straight grained species like redwood or cedar are best planed using a low cutting angle, something between 35 and 40 degrees. Highly figured hardwoods like curly hard maple need to be planed using a cutting angle that could be as high as 90 degrees, but 60 to 65 degrees seems to provide the best performance.
KEEP BACK BEVEL LENGTH TO AN ABSOLUTE MINIMUM
After you’ve determined your new cutting angle (cutting angle minus bed angle equals back bevel), you’re ready to start honing your back bevel. Remember, the back bevel really doesn’t need to be any longer than the thickness of the maximum shaving you intend to remove. 1/64 of an inch is sufficient. Making the bevel length any longer than this creates two potential problems. First, if you should want to eliminate the back bevel at some point, you’ll find that you need to waste a lot of stock in order to re-establish a true single bevel. Second, back bevel length actually determines the position, and subsequent support (strength), of the cutting edge (arris).
One of the rules of the woodworker’s universe is this – increasing cutting angle means an increase in required effort. In other words, you’ve got to work harder to remove a shaving of the same thickness as you increase the cutting angle. So the logical conclusion is that you should expect to keep the shaving thickness to an absolute minimum.
I, like most other hand planing fanciers, recognize the therapeutic value of removing “whisper” thin shavings and leaving a “glass-like” surface. But, if I have to take a 1/4″ of twist out of a board, I’d like to remove the thickest shaving that I can, without having a heart attack, of course. So I won’t be putting a back bevel on anything but my smoothing plane.
As cutting angles and effort increase, so does potential for vibration or “chatter“. You may find that the chip-breaker needs to be bent back in order to provide more “purchase” on the iron. Yes, one of the things that the chip-breaker does is to dampen vibration, especially in thin irons. It’s interesting to note that many very expensive smoothing planes have no chip breakers. Their performance depends on the use of very thick irons and very small mouth openings.
If you find that back beveling helps you in working with certain species and figures, by all means, do it. But…chose the plane (or planes) that you’ll use and dedicate it (or them) to that purpose. An alternative solution, is to have additional plane irons that are back beveled to provide a range of cutting angles. It’s not a bad idea to use a “magic marker” to indicate the cutting angle on the iron.
A FINAL THOUGHT ABOUT CHIP BREAKERS
Gramps would tell me to set that chip breaker as close to the cutting edge as possible. And, generally, like all things that he taught me, that is true. However…when removing very thin shavings, the contact point between the chip breaker and the iron, if not perfectly seated, can easily become clogged. This is especially true when a back bevel is employed. There are two simple solutions to prevent this: First, make sure that the chip breaker has been undercut and honed dead flat across. This will insure constant contact between the chip breaker and iron. The lead outside surface of the chip breaker should be smooth and free of any nicks. Second, set the chip breaker well away from the trailing edge of the back bevel (1/64 – 1 /32″). Remember that the back bevel itself will actually begin to “roll” the chip before it ever comes in contact with the chip breaker.
Well, enough talk. Happy planing!