A cambered iron is a blade that has a curved cutting edge rather than a straight edge. I camber most of my plane irons, but for different reasons.
For heavy material removal a cambered edge works better than a straight edge. When planing across the grain (which I often do when flattening a rough board) the curved blade scoops out a large shaving with ease and minimal tearing. Straight blades can get wedged under the fibers and have to rip through them where the blade ends, so it takes more effort, leaves a rougher finish, and can clog the plane’s mouth easier.
There are two major reasons I like to camber a jointer’s blade. First and foremost, I can correct edges that are out-of-square with ease. (Click here for more on this technique) Second, it minimizes or eliminates plane tracks. This saves time when I switch to my smoother. I don’t want to have to remove a bunch of .005″-.008″ deep tracks left by a jointer when I’m using a smoother taking shavings around .001″-.002″ thick.
Only one reason and it’s probably the most obvious, no plane tracks on my finished surface. Duh.
How much camber do you need?
There are two factors to consider when determining the amount of camber (radius of the curved cutting edge). First, how thick of a shaving do you want to take? A jack plane takes heavy shavings and needs a large amount of curvature. A smoother, however, takes a thin shaving (typically 0.002″ or less) so the blade projecting below the plane’s sole should be sticking out around 0.002″ in the middle of the cut, but fade to nothing just before the blade ends across it’s width. If you place too much curvature on your smoother and take a 0.002″ you will only get a narrow shaving because most of the iron is curved up inside the body of the plane. Hopefully this paints the picture that if you’re going to camber, you need to be precise about how much curve you induce in your blades.
So, if you want a blade that goes from 0.000″ at the ends to 0.002″ in the center, is that the arc you want? Nope. There is still the second factor to consider; your blade angle. The lower angle your blade is pitched at, the greater amount of curvature you need for the same functional camber. Don’t get confused here, this is a fairly simple (though often overlooked) concept. Think of holding a disk, like a frisbee. If you look straight at it, you see the full curvature, like looking straight at your plane blade. As you begin to lay the frisbee flat, you will see the curve reduce more and more until you’re looking at the side of the frisbee at which point there is no curve at all. You just see a straight line. The same is true with the plane’s iron. Standard angle blades have moderate curvature to achieve a desirable functional camber, but low-angle (bevel up) planes need more curvature for the same amount of functional camber. The diagram below might help to clarify this point.
What shouldn’t you camber?
This is pretty obvious, but it’s worth stating. Some planes don’t benefit from cambered irons. Primarily, skewed blades (fillisters) or straight blades mounted at a skew (shooting planes) should never be cambered. The same goes for rabbet planes (shoulder planes, rabbet block planes, etc). As a matter of personal preference, I don’t camber my block planes either. You absolutely can, but for the type of work I do and simplicity of sharpening I’m happy to leave them straight.
How to camber
To save time, I’m going to assume you have a basic knowledge of hand tool sharpening. If you’re using a new blade or modifying an old, straight blade, you might choose to start on a grinder (at least for large cambers where you may have a decent amount of steel to remove). This is fine, but the usual grinder warnings apply (use quality wheels, don’t grind the bevel right to the tip or you will burn the metal, safety concerns, etc). With the shape roughed out on the grinder or if you’re just trying to add a slight camber to a smoothing iron, mount the blade in a honing guide and go to your sharpening stones. I use the Veritas MK.II guide with a cambered roller wheel.
This works very well for me and is fairly self-explanatory. Place more pressure on the outer ends of the blade and/or take more strokes while applying pressure on the ends and the cambered roller allows the guide to pivot left or right. This removes more material at the ends than than at the middle of the iron and thus an arc is formed. The more pressure and more strokes you take on the ends, the more camber you will create. Check your progress as you work and before long you should have a uniform curve. I usually eye-ball the amount of curve rather than trying to measure exactly how much arc I’ve created. When I use the blade in the tool, I make note of how I did. If camber is too severe I take extra strokes down the center of the blade the next time I sharpen. Not enough camber obviously means I take more off the ends the next time I sharpen.
If you don’t have the MK.II you probably have the old Eclipse style honing guide. Fortunately, the wheel on this guide is narrow which means you can tip the blade by applying pressure to the ends of the blade and create an arc. If you have the MK.II but do no have the cambered wheel, get one. The wide, straight wheel on the MK.II will prevent cambering. Creating a camber on the Eclipse guide is similar to using the MK.II though slightly less automated. Mentally (or literally) number your blade’s edge 1-5, dividing the blade into 5 equal zones. Zones 1 and 5 are at the ends, 3 is in the middle, 2 and 4 are halfway between the ends and the middle. Using your course stone, place high pressure over zone 1 and take several strokes (we’ll start with 10 for now, but this will vary person to person). Repeat the same 10 strokes with the same pressure over zone 5. Reduce the pressure slightly and take 6 strokes over zone 2 and repeat over zone 4. Finally, with less pressure still take 3 strokes over zone 3. The idea is obviously to symmetrically remove more material from the ends than in the middle. Zones 2 and 4 are just to create a more even arc. Repeat this process as many times as needed to achieve the desired radius. You may need to modify the number of strokes in each zone to increase or decrease the radius. Once the shape is defined, hone your micro-bevel in a similar fashion on your fine stone and lap the back in your usual manner (I use the ruler trick).
-Take an equal number of strokes from each side to keep the camber centered. If you’re slightly off, use the lateral adjustment lever on the plane to center the cut.
-Bevel up planes can capitalize on curved blades just like standard irons can, just be aware that bevel up usual means you’re using a low angle tool and that means you need a greater observed camber (c) to achieve the desired functional camber (f).
-If you’ve never cambered an iron, start with a jack plane. Even if you do a poor job the tool will work fine. Then move to a jointer and finally a smoother. Smoothers have the least material to remove, but they require the most precision. You want a shaving the tapers out to nothing on the ends, but is only 0.001″-0.002″ thick in the middle. If you’re not precise, you will either have plane tracks on your finished surface or your plane will cut a very narrow shaving… neither is desirable so practice on the more forgiving tools first.
(I wonder how many people actually read all that?)