This is a stop block that fits a Festool track. It is designed to set starting and stopping points for a track saw. An M6x12 bolt is used to tighten the block with a corresponding M6 nut trapped in a plastic adapter that goes into the aluminum extrusion. A knob (made with the brilliant nut knob generator: http://www.thingiverse.com/thing:713300) grabs the hex head of the M6 bolt, making it easy to secure and release the block. A washer can be added to serve as a spring, between the knob and the block, but this is not mandatory. The block has a round notch that catches the corner of the base of the saw so there are two models: a start/front block and a stop/rear block. Typically print one of each block and two knobs plus two nuts. Printed in PLA at a Z resolution of 0.22mm and with a 25% infill. Nuclear green is recommended for the knob. Don't even think of using another color ;)