Anatomy of a Saw Blade

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The saw blade – irrespective of how good the saw is, how flat the table, how rigid the fence, if your saw blade is substandard, everything else is cheapened. The blade tends to be somewhat overlooked (ok a bit of a generalisation), as it just seems to keep going and going and going. What it is however, is a series of tiny chisels. A hand chisel gets looked after, cleaned, sharpened and stored correctly, and the saw blade should receive the same attention.

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These days, the blade is often laser cut from high carbon alloyed steel, with the cutting edge being a Tungsten Carbide Tip (often a Tungsten-Cobalt alloy) The tip is very hard, but brittle, and is brazed onto the body of the blade. It is used because it will retain its edge for about 10 times longer than steel. The width of cut (the kerf) is primarily the width of the Carbide Tip, although there are other factors that come into play. In general, the kerf is 3mm, but there are also thin-kerf blades which are good for minimising wastage, and to get better performance out of lower-powered saws.

During the cut, the tips are cutting or peeling wood away, and to allow it to get away from the cutting edge, a gullet is provided below the TCT. The size of the gullet dictates what the blade is for. Crosscutting requires a small gullet, and consequently a much higher tooth count is possible (the number of teeth on the blade). The more teeth, generally the smoother the cut. However, when ripping (down the length of the board), this is an easier operation, and so you don’t need as many teeth for a good finish. Lucky, because the operation also generates significant amounts of shaving, and you need a much larger gullet. (Obviously once the tooth is clear of the wood, the gullet is free to empty) If the gullet clogs during the cut, cutting performance is seriously restricted, it becomes much harder on the saw, and the wood is much more likely to overheat and burn.

The operation generates quite a bit of heat, and to stop the large disk of the blade warping, expansions slots are provided. They are also called noise reduction slots, but in the end, noise would be caused because of the speed of the edge of the blade (up to 200km/hr), and any distortion of the disk will significantly increase that noise. Distortion of the disk is far more important for other reasons – rougher cut, wider kerf, and really, I don’t want a distorted disk spinning that fast. I don’t see noise as being as big a deal (especially in a circular saw – the noise of the saw is significantly more than the blade.) This may not be so true on the quieter saw tables, but I haven’t experienced them enough to say where the primary noise is being generated.

At the end of the expansion slot is a cut circle. This is sometimes filled with solder (to reduce noise), but the function of the hole is not affected. It is there to stop the expansion slot growing (and if it grew, the blade could literally fall into 2 pieces, or fly in 2 pieces!) The end of a crack is a major stress raiser, and an expansion slot is just a deliberate crack – it needs the same treatment to prevent disaster. This same technique is used to stop cracks in glass growing – drill a hole at the end of the crack, and the crack is arrested.

Some blades (particularly ripping blades) have a feature on the back of the tooth to decrease the likelihood of a kickback. To be honest, I don’t know how effective they would be – after all the majority of kickbacks happen at the rear of the blade, catching and throwing the workpiece, rather than the saw trying to eat more than it can cut.

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There are some different ways that the teeth can be set, and I’ve only got a couple of examples here. The first is an alternating top bevel. This cuts (or slices) left, then right, making for a smooth cut, which is like running a sharp chisel down either side of the cut.

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The second is called a triple-chip blade (I think incorrectly – because the raker tooth is full width, and doesn’t have the corners removed). I think it is actually known as a Planer-Combination, with a left cutting tooth, a right cutting tooth (as is the case with the alternating top bevel blade), but then has a raker tooth, which removes the v-shaped piece left in the middle from the alternating teeth. This results in a much smoother bottomed cut.

3 Responses

  1. I’m curious about the face of the teeth. Are the face of the teeth ground at 90 to the body of the blade or does that angle vary?

    • Definitely vary for different effects. They can have a positive, neutral or negative rake.

      Generally, positive rake (where the top of the tooth is sloped forward) is used for timber, while negative rake is common on metal cutting blades

      • I’m sorry, I didn’t mean the rake. What I mean is the angle side to side.

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