Think & Tinker, Ltd.
CNC router bits and carbide cutting tools with tutorials and technical guides

Sidewall Precision

Precision wooden gears

One aspect of metal cutting that has been missing from woodworking is the precision and accuracy of the shape and size of a finished part. At the heart of the problem lie the distorting effects of the grain of the wood and the inconstant density encountered as the cutter moves through the material. Wood, even hardwood, is a compliant media that tends to press away from a cutting edge and then swell back into place after the edge has passed. As a result, pockets tend to be smaller than expected and sidewalls end up in the wrong place. Efforts to overcome this tendency by increasing the attack angle and sharpness of the cutting edge, often result in a cutter that pulls into the wood, creating pockets that are bigger than desired. Making matters still worse, a poor match between the shape of the cutting edge and hardness of the wood can result in the grain being ripped apart instead cleanly cut.

The SMF family of cutters are designed to cancel the tendency to push away from the wood by using a novel geometry that exerts a very slight pull into the wood, even when climb milling.  The result is pockets that are exactly the size that they should be and edges so precise that clocks with wooden gears can be made to keep time with a precision of +/- 30 seconds a day. All of the SMF tools are intended for full plunge operation eliminating the sidewall stepping commonly encountered with multi-pass cutting. Notice the smoothness and polish of the gear tooth faces in the magnified image. Pretty neat, huh? ( 0.0313" dia., 0.125" depth of cut, 32 in./min. feed @ 40,000 RPM).