Hollow ground circular saw blades are a sub-category of sheet goods blades recommended for making smooth cuts on coated boards as well as for extra fine crosscutting. These are specifically designed for use with coated boards, such as double-sided melamine and laminates, Medium Density Overlay (MDO), High Density Overlay (HDO) and Phenolic plywood. These engineered wood products have a thin, resin soaked overlay, made of a fibrous material (similar to thick paper), which is soaked in resin. This makes for an extremely hard and smooth exterior surface, which is excellent for workbenches, signs and concrete forms. The surface veneers on these sheet goods are both hard and brittle. As such, they are prone to chipping, more than splintering. Cutting them cleanly requires an extremely sharp blade. The resin impregnated exterior veneer is very abrasive to blades, dulling them rapidly. For these two reasons, only carbide tipped blades are used (often, coated carbide tipped blades).
Hollow Ground Teeth
When manufacturers refer to a blade being hollow ground, they are referring to the face of the teeth. If you were to stand the blade in such a way as to make a tooth horizontal to the floor, the face of the tooth would be slightly “hollow” like a bowl. This provides a sharper cutting surface to the material being cut, rather than the typical 90 degree sharpened saw tooth. The resulting tooth geometry provides for an extremely accurate and clean cut. However, the tradeoff is that these blades become dull much easier than other types of circular saw blades, which is the major drawback of these blades. They are also harder to resharpen, as specialty equipment and grinder wheels are necessary to accomplish the hollow grind.
Melted Glue
Another serious problem when cutting these sheet goods is heat. When a blade heats up, some percentage of the heat transfers to the material being cut. This will cause softening of the resins and glues used to hold these panels together. Melted glue or resin that connects composite boards, along with sawdust, can cake onto the blade, further increasing friction and heat due to high cutting speeds. This issue is related to hollow ground saw blades as well as for any other saw blade type specialized for veneers. Very hot saw blade, melted glue and sawdust may represent a serious threat to woodworker. Choosing an appropriate blade can prevent these undesired effects. To battle this problem, these blades are normally coated with carbon, and might even be Teflon coated. Both the blade plate and the teeth themselves need this coating, in order to reduce the risk of this sticking. Coating not only protects from sticking, but guarantees resistance to rust and friction. Teeth wearing are the main disadvantage of hollow ground blades, this can be compensated by the quality coating too.
Characteristics
Hollow ground blades can be found with a number of different tooth counts, ranging from 40 up to 150 teeth. While the vast majority of woodworkers use the higher tooth count for a smoother cut, especially when crosscutting. Teeth mostly have negative hook angle in order to prevent over-feeding while cutting. This is crucial for double sided composite boards and veneered materials. Hollow ground blade is usually characterized by a thicker kerf and thinner blade plate. The width of a cut is always equal to kerf thickness, or precisely, width between left and right sides of tooth. A special version, usually highlighted when purchasing, hollow ground planer blade has saw-blade chisels assigned to cut moldings. After using a planer blade there is no need for planning the wood or sanding.
Use
Some woodworkers like to use hollow ground blades as general purpose blades. In addition to being extremely good for cutting the sheet good panels mentioned above, they are excellent for crosscutting softwoods, such as mahogany, which splinter and split easily. Furthermore, cutting fir, pine and cedar also leaves splinter-free cut. The sharper angle of the tooth geometry helps ensure that these softwoods cut cleanly, without chipping.