Saws, Teeth & Tooth Spacing

The following is excerpted from “Set & File: A Practical Guide to Saw Sharpening,” by Matt Cianci.

In this book, Cianci (aka The Saw Wright) teaches you the fundamentals of maintaining backsaws and handsaws: how to file and joint your saws with the correct rake, fleam and pitch to keep them cutting sharp. You’ll also learn how to deal with saw teeth that are in good shape but dull, plus how to successfully doctor teeth that have been abused.

Plus, Matt shows you how to identify and restore vintage saws (i.e. is that yard sale box lot a good buy?) – and basic sawsmithing – how to hammer out a bent blade, and how to re-set the spine on a backsaw.

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Do you remember sitting in junior high geometry class and trying your best to stay awake? Me too.

While we don’t have to talk about the Pythagorean theorem in this chapter, it is important to understand the basic shapes and angles related to saw teeth. We’ll also cover basic terminology. Grab your highlighter.

Hand Saw Anatomy
Western-style hand saws come in many forms, but the two most common to woodworking are the handsaw and the backsaw. A hand saw (two separate words) refers to any saw worked by hand power. This includes large two-person timber saws for felling and bucking trees to pit saws for sawing boards from a log, and even tiny keyhole saws and dovetail saws for fine joinery with all types in between. Hand saws can have a thin, narrow blade tensioned in a wood or metal frame, or the blade can be unrestrained and of sufficient gauge to remain stiff and resist buckling in use. A handsaw (one word) is a particular form of hand saw that includes a wooden handle with closed grip on one end of an unrestrained blade, which is used for making straight, dimensioning cuts in boards of all kinds. Handsaws are identified in size by the length of the toothed edge of their blade, which can range from 12″ to 30″. The most common size is 26″, which is the prototypical carpenter’s saw.

A backsaw is a hand saw with a thinner blade and an applied metal rib along its top edge to provide stiffness in use. It has an open- or closed-grip handle ,and is used for making precise joinery or fine dimensioning cuts in wooden components of all kinds. Backsaws are also identified by the length of the toothed edge of their blade and typically range in size from 6″ to 20″ , with 12″ being the most common. A special variety of backsaws, called miter box saws, are designed to be used in manufactured miter boxes, and can be up to 30″ in length.

There is common language used to identify the parts of both handsaws and backsaws that you should understand before you learn about saw teeth. The thin steel body of the saw that contains the teeth is referred to as the blade. The toothed edge of the blade is called the toothline, and the opposite edge is called the back. The other major component of a handsaw or backsaw is the handle, which is self explanatory and usually made of wood. The two ends of the toothed blade are distinct. The end with the handle is called the heel; the opposite end is called the toe. Unique to backsaws is the metal reinforcing rib, called the back, along the top edge of the blade. These can be made of iron, steel or brass.

Parts of Teeth
The teeth of Western saws are shaped like triangles, and each part of these teeth have a specific name and function. Understanding these elements is the first step in learning to sharpen your saws. The point is the acute apex of the tooth where the wood fibers are first cut. Depending on how you file a particular saw, these points can have different geometric distinctions and cut wood in different ways. The face of the tooth is the side of the triangle that first contacts the work when the saw is in use. Because Western-style saws cut on the push stroke, these faces are always on the toe side of the toothline. Logically, then, the opposing side of the tooth is called the back. The site where the back and face meet is called the gullet.

In use, all four parts of the tooth work together to cut a kerf into a piece of wood. The kerf is the empty space created by the saw as it cuts. Here’s how it happens:

1. The acute point of each tooth is pushed down into the wood as the saw is thrust forward. As the point penetrates the surface of the wood it cuts and lifts up a tiny chip of wood.
2. This chip then slides up the face of the tooth as the saw moves forward.
3. As the chip reaches the gullet it breaks apart and collects with other chips in the gullet.
4. As the tooth exits the work on the backside of the kerf, the chips of wood fall out of the gullets. The saw is drawn back and the process repeats.

For conceptual purposes, it has long been suggested that the cutting action of saw teeth is similar to the cutting action of many other edge tools, like chisels and knives. And while this is true for an individual saw tooth, it does not give the full picture. Planes, chisels, knives, axes and all other edge tools possess a single cutting edge, whereas a typical handsaw can have more than 200. For a saw to cut well, each of these teeth must not only be sharp, but also must be uniform and harmonized with all neighboring teeth. Uniformity in partnership with sharpness is the goal for saw teeth. This is why saw sharpening can be so challenging for even the most experienced woodworkers. The efficient and accurate function of the saw is not solely dependent on the quality and geometry of a single edge, but on the quality and uniformity of many edges in relation to each other.

Now that we have defined the parts of a saw’s teeth and how they cut in general, let’s look more closely at how changing the size, shape and geometry of the teeth can make them better suited to different kinds of work.

Tooth Spacing
Tooth spacing determines the size of a saw’s teeth and is measured by the number of teeth a saw has within one linear inch. There are actually two different but often confused units of measurement common to hand saws for tooth spacing, and they are points per inch (ppi) and teeth per inch (tpi). They are, in fact, not the same unit. On handsaws, tooth spacing was traditionally stamped onto the heel of the saw on the medallion side of the blade right below the handle. Backsaws had no such marking.

To identify the ppi tooth spacing of any saw, take a rule and place it on the toothline of the saw. Line up the 1″ mark on the rule with one of the tooth points at the heel of the saw. Begin to count the number of tooth points starting with the point aligned with the 1″ mark and progressing up to and including the point under the 2″ mark. This count is the ppi tooth spacing of the saw.

To measure the tpi tooth spacing of a saw use the same rule, but this time line up the 1″ mark with the gullet of a tooth at the heel. Now count the number of full saw teeth (gullet, back, point and face) from one gullet to the next until you reach the gullet under the 2″ mark. This count is the tpi spacing of the saw. If you completed this task correctly, you should arrive at a tpi spacing that is one less than your ppi measurement. This means that a points-per-inch measurement of a saw will always be one greater than the teeth-per-inch unit. Represented in an equation: ppi = tpi +1. So, a 6 ppi saw is not the same as a 6 tpi saw. A 6 ppi saw is equal to a 5 tpi saw, and a 6 tpi saw is in fact equal to a 7 ppi saw. Got it? Good. Of note, I only use ppi when describing tooth spacing.

If you followed the steps above to identify a particular saw’s tooth spacing, and you are finding that the points (if you are using ppi) or the gullets (if you are using tpi) are not lining up with the exact inch marks on your rule there could be a couple of reasons why. Historically, full-size handsaws were commonly available in half-point increments in coarser sizes. So that means that 6-1/2, 5-1/2, 4-1/2 and 3-1/2 ppi saws are commonly found. In fact, 5-1/2-point ripsaws are about as common as any other ripsaw spacing in the wild. A discrepancy in measurement could also mean that your saw has morphed over years of careless filing into an odd spacing. This is especially common on backsaws with very small teeth that are easy to misfile. For handsaws, look for the heel stamp (noted above) to confirm the original spacing. Also, always measure teeth at the heel of a handsaw because ripsaws were commonly toothed one point finer at their toe to allow for easier starting of the cut. This means that a 6-point ripsaw has 6 ppi spacing at its heel and 7 ppi at its toe.

So why are these distinctions about tooth spacing so important? Because more than any other element, tooth spacing affects how a saw will cut, and unlike the other geometric features of saw teeth we will discuss later in this chapter, tooth spacing is not typically changed once it is established. Tooth spacing determines how large or small the teeth are on a saw, which in turn determines how large or small a bite of wood each tooth will take. This is due to the distance from one tooth to the next and the resultant gullet volume the teeth create. In use, a saw tooth will continue cutting chips of wood as long as its point is able to reach the bottom of the kerf. Once the gullet in front of a point fills with chips, it is overcome and can no longer effectively bite into the wood. So a saw with a greater amount of space between each tooth point, and hence a greater chip capacity in its gullet, can go on cutting and creating chips longer than a relatively smaller tooth. But this speed comes at a cost. Larger teeth may cut faster, but they leave a rougher surface on the wood and are not as inclined to the precise work of joinery. Conversely, fine-toothed saws for joinery sacrifice speed, but they allow a smooth, even stroke, whose tiny teeth gently nibble away at the wood to cut a finer line.

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