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In our discussion of sawtooth geometry last month, hook angles of circular saws were explained. Incorporated with the hook angle, the clearance angle and sharpness angle provide the profile view of the sawtooth in circular, band and other saws. The combination of the clearance, sharpness and hook angles must equal 90 degrees. The clearance angle allows the saw to pass through the lumber so as to not to hit the heel of the tooth. Usually ground from 8 to 12 degrees, this clearance angle must allow for spring-back of the wood after the cut is made. Research material suggests a larger clearance angle will positively improve the cut. This is true to a point; however, as a clearance angle is increased, the result is a smaller or sharper sharpness angle.
If we follow this thought process and continue to increase the clearance angle, a clearance angle that is too large will reduce the sharpness angle and weaken the support material and the hardened tooth, causing premature breakage of the tooth. This discussion results in a compromise situation where the clearance angle is ground to maximize tooth strength, while minimizing drag from compressed materials. The resulting angles are published and should be considered when it is known how hard (potential of spring-back) the material has.
Top bevel angles
The other angles involved in sawtooth geometry are not profile angles, but are viewed from the front of the tooth. The geometry of top bevel angles was developed as a result of the type of material to be cut and the resultant shape of the saw cut. For example, if the saw cut needed to be a square bottom, all of the teeth may be ground square.
Alternate top bevel
Other configurations of the saw teeth may also result in a similar bottom with a slight profile. The illustrations of different types of top bevel angles include an alternate top bevel (ATB), where the top bevel of each subsequent tooth is opposite. The benefit of an ATB ground tooth is the superior finish. They do require a slower feed than the straight flat grind, as in Figure 1.
Triple-chip tooth
Another common top bevel tooth grind is called the triple-chip tooth or TCT. Utilizing an ATB set of teeth matched with one straight ground tooth, the resultant shape is almost square. The benefit of this grind is a very good quality of cut and long lasting when cutting panel composite products. Variations of a TCT grind are described in Figures 3 and 4. The resultant shapes are similar. The benefit of the hollow ground grind in Figure 5 is the cutting action is more efficient than the other two ATB ground tooth designs.
Dado saw sets
Dado saw sets are made with two saw blades and several hogger blades that are placed together in a configuration to make a resultant groove (dado) or a tenon. The two saws are placed on the outside of the set and provide stability and sharp corners. Each of these blades is sharpened opposite to each other, and they are called single side top beveled ground teeth. Since the tendency of the saw is to pull to the side, the saw body is thicker and stronger, resulting in a wider kerf and increased strength. The hogger blades may have only two teeth and are ground flat to give a flat bottom to the cut.
The final word: Circular sawing of wood requires attention to detail. The proper saw will be safer, give a better cut or finish, and do the work faster. Sharpen your saws regularly and inspect them before and after every use. Handle with gloves and always "Lock out and Tag out" your machine while changing blades. It is best to change to the proper blade.
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author: Steve Ehle