The cutoff tool is used to part off small stock less than an inch in diameter. The riser blocks are available so that stock up to 6" in diameter can be turned. So if you can't part off large material, why make a cutoff tool riser block available?
The answer is than many people prefer to leave their riser blocks in place at all times, even when turning small work, and they need a way to part off the work. The cutoff tool holder riser block goes under the P/N 3002 cutoff tool holder to allow it to be used with the riser blocks in place.
To increase rigidity and reduce "chatter", the riser block is designed to be used on the "back" side of the work with the blade held upside down. This means the direction of rotation tends to lift the tool rather than push down on it, making for a smoother cut. It also has an extra wide base with two mounting screws to further increase strength.
Download PDF 1296 Instructions