Each tooth on a saw is essentially a single point cutting tool. There are just a lot of them in a row. So the basics of cutting tools apply directly to saw teeth. Each tooth must have a rake angle, back clearance, and side clearance just as a lathe tool would have.
The front face of the tooth forms the rake angle. It is measured from a perpendicular to the line of action as it cuts. It can vary depending on the material the saw is intended to cut. Wood saws would have a larger rake and metal saws a smaller one but this is not hard and fast and you may find examples contrary to this. Most saw teeth have a combination of back and side rake: more on this below.
Any cutting edge of point must have clearance in order to actually cut. If there is no face clearance, the edge can not penetrate the material being cut: the side lands on standard twist drills are an example of this. These lands on the sides of the drill flutes give it a zero sideways clearance so that they will not cut sideways (at least not easily).
Saw teeth have back clearance formed by the rear face of the tooth. A problem arises here since usually the same cutter is used to cut the front of one tooth and the back of the previous one. The simplest cutter used to sharpen saws is a file, ususlly a triangular one with 60 degrees between adjacent faces. So, the back clearance on many saw teeth is relatively large. Using a triangular file, the clearance angle automatically becomes 90 degrees - (the rake angle + 60 degrees) or 30 degrees - the rake angle. Hence the most common saw tooth form that you will see. I guess this turns out to be a good thing as most saws are sharpened in this manner. However, a second sharpening operation on the tip of the tooth can reduce the clearance angle to any smaller amount desired.
Side clearance is also needed or the saw will stick in the kerf. This can be generated in different ways. One is to offset the teeth alternatively; left, right, left, right, etc. This is done by just bending them and special tools are made for this purpose. They can also be offset in other patterns such as the wavy set where they are offset in a sine wave like pattern and ten or more teeth form one cycle of this pattern. Another way of providing side clearance is a hollow ground or tapered saw blade. This is where the cutting edge of blade is thicker than the rear or inner part for circular saws. So the cutting edge of the teeth is wider than the blade behind it and it effictively has side clearance.
I spoke of a combination of back and side rake above. Saw teeth that have an alternate offset will often have alternating side rake also. This allows the teeth to cut sideways a bit better, which is good for allowing you to follow a curved line when cutting. Or to make adjustments to follow a straight line better. So when sharpening staggered teeth, it is desirable to change the angle of the tool for every other tooth. Or more likely, do every second tooth and then change the angle and then do the other half. This can also be done on wavy set teeth, but several teeth would be ground at each angle.
The above is just some basic ideas. There are many, many tooth forms on saws designed for many different cutting situations. One of the most important things to do when sharpening saws is to observe the original form and duplicate it as appropriate. Also, any good shop book should have information on saw tooth form and sharpening.
On a grinder you would probably want a cup style wheel with a 60 degree edge. But much can be done with a triangular file. Same number of strokes and same pressure on each tooth. With a little practice, angles can be set by eyeballing it. Observe the results under magnification (5X or 10X) and adjust technique accordingly.
Last edited by Paul Alciatore; 03-08-2009 at 06:41 PM.
Make it fit.
You can't win and there is a penalty for trying!