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The down shear bit leaves a very crisp edge but is less effective with chip evacuation, which can lead to excessive friction and tool wear if not run correctly. Down shear bits are a good choice for cutting shallow grooves or dados with visible edges. One other note – down shear bits are generally run slower than up cut bits due to the chip evacuation speed. This is another reason for cutting shallow grooves in multiple passes vs going deep on one pass. The slower feed speed allows the bit more time to evacuate chips which will help avoid excessive tool stress or friction. The down shear carbide spiral bit is far more popular because many router tables have less than efficient, vacuum hold-down which makes the up-shear bit want to lift the panel with it’s upward cutting motion.  This results in unacceptable vibration and movement of the panel.