I’ve fielded a few questions lately about saw sharpening (obviously from woodworkers who haven’t yet read my book on sharpening for woodworkers) and thought this would be a good time to give away (most of) a sample chapter. I hope you enjoy it and that it helps you to keep your saws in optimum condition. — Ron
From Chapter 9 of The Perfect Edge, the Ultimate Guide to Sharpening for Woodworkers:
A saw is a long row of small chisels – sort of. It helps if I keep that image in mind when
I think about sharpening one. But the row-of-chisels analogy is only so good; my imagining must also include the set of the teeth. As the row of chisels cuts through the wood, the rest of the saw, the flat blade that the teeth are part of, follows along through the
cut. The path through the wood, the gap or slit, created by the teeth is the kerf, and if the kerf is the same width as the saw blade, the saw will rub, drag and bind during cutting, making the cut difficult if not impossible to finish. To solve this problem, the teeth of a saw are set by bending them slightly outward, one tooth bends one way, the next tooth the other way and so on along the edge of the saw, so that they cut a wider kerf and allow the rest of the blade to travel unimpeded. Bending the teeth is possible because saws are usually hardened to a mid range hardness — hard enough to hold an edge for sawing but still soft enough to set and to sharpen with a file.
Unfortunately, my “row of chisels” image does not take into account the fact that most saw teeth are sharpened to a negative rake angle and actually cut more like scrapers than chisels. My powers to visualize reach only so far.A saw tooth’s singular purpose is to remove wood. While a chisel or a plane iron may be called on to do the same thing, we often demand that they leave behind a fine, smooth surface at the same time. To ensure that fine surface, we hone and polish the edges on plane irons, chisels, carving tools, anything that we use to create a smooth surface. Though all the woodcutting edge geometry applies, saw teeth are sharpened to achieve a different goal: they must simply remove material efficiently. A smooth, finely honed, polished cutting edge is not called for; therefore we sharpen saws with a file — an efficient tool to do an efficient job of making an efficient tooth.
Some contemporary saws have induction hardened teeth that can’t be sharpened or re-set; they’re too hard for filing or bending. You can tell induction hardened teeth by the rainbow discoloration at the toothline. These saws will tend to stay sharp for a long time but when one is finally dull, you will have to replace it. Being able to file a saw yourself isn’t only about keeping it sharp. You can also modify the shape of the teeth for different cutting situations. A saw’s edge geometry can be adjusted for optimum performance in, say, green softwoods, which require more set compared to dry hardwoods.
There are two fundamentally different sawing tasks: ripping and crosscutting. Ripping is cutting more or less in the same direction as the grain runs in the wood (with-the-grain) while cross-cutting, as the name implies, is cutting across the grain (cross-grain, see Chapter Four: How Wood is Cut). A rip saw is usually used to narrow a board while a cross-cut saw usually shortens one. Each cutting action calls for a different cutting edge geometry. A cross-cut saw is used for almost any angle of cross-cutting, whereas rip-sawing is most commonly used to cut close to parallel with the grain.
Ripping is done with teeth that resemble square-edged chisels held at a near-vertical scraping angle. Each tooth makes a small, curly shaving much like a small version of the shaving made during with-grain planing. Rip saw teeth are usually filed with a rake angle between an aggressive 0° and a more laid-back 15°. The larger rake angle makes it easier to start a cut; the more vertical teeth will cut faster. Pete Taran (www.vintagesaws.com) suggests a 4° rake as a good compromise for rip saws.
Cross-cutting wood fibers requires some special cutting action. The end-grain fibers that line the kerf must be severed then rolled up and out of the path of the blade. So, to facilitate cross-cut sawing we add an angle to the tooth of the saw in order for it to achieve the fiber-severing goal, allowing a clean cut that leaves a relatively smooth surface along the sides of the kerf. This angle on cross-cut teeth is called fleam, which is also the old-fashioned term for the lancet used to open a vein for bloodletting. Fleam is an angle on the face of the tooth (and generally on the back as well) much like a skew chisel, compared to a rip tooth’s similarity to a square-edged chisel. The sharp point of the skew cuts deeper than the rest of the tooth in order to sever the wood fibers along the sides of the cut, while the rest of the tooth cleans out the kerf.
Rip-saw teeth are filed straight across, the file held at 90° to the saw blade. Cross-cut teeth are filed at an angle from 10° to 45° with 15° being popular for a general-purpose cross-cut saw. Cross-cut teeth have a less aggressive, negative rake angle of 12° for a fast, aggressive cut, to 30°, which is more typical and gives a smoother though slower cut. Another aspect of saw tooth geometry is slope, which refers to the angle of the file from horizontal.
Though it may seem a daunting task (all those teeth!) you can sharpen your saw yourself. Most power-saw blades are either carbide, which is sent to a specialized shop for sharpening, or disposable and simply replaced with new blades when dull. Keep in mind, though, that a good hand saw is another of the many hand tools that will last for generations if cared for and kept sharp. There’s nothing difficult about filing a saw; like everything else it’s a skill to learn and with a small amount of practice, you can make your
saw work better than it did new — a finely tuned tool that you’ll find myriad uses for.
These days, much of your sawing tasks are likely done with one of the power saws that we all seem to have, but there will always be a place for hand saws and hand sawing in any fine-woodworker’s tool kit, and skill set. As your saw sharpening skills improve, you’ll find yourself sharpening your saw more often because you are using it more often, and vice versa. A hand saw is frequently your best tool for many sawing tasks. When you keep your hand saw in good fettle, you’re likely to find yourself turning your back on the noise, dust and danger of power saws in favor of your uncle’s classic, well-tuned Disston (et al), or
one of the beautiful new-generation of saws being produced today.
This concludes part 1 of this excerpt from The Perfect Edge, the Ultimate Guide to Sharpening for Woodworkers by Ron Hock. Stay tuned for part 2.