The set screws on the Z axis Lead screw pulley came loose and allowed the router to drive down into the work piece and spoil board.
It was running a 40 min toolpath and had gone approx. 30 minutes. I had left it unattended in order to work on something else while it was running. When I came back sparks were flying and my garage was filled with smoke. Had I been much longer it very likely would have caught on fire. My router runs but it may be damage from the heat concentrated on the shaft; it is discolored. It was new for use with the Longmill.
I intend to find a better method of securing the pulley and use locktite on the set screws (one fell out and I haven’t found it yet). I have already had other issues with screws loosening on the Longmill.
So, a loose set screw not only cost one project, but most likely a router and the bit that was in use.
Since the weight and stress of the router is supported by just 2 little set screws, perhaps it would be better for the lead screw to have a groove machined for a retaining ring to rest on the bearing.
Bottom line is: Make a routine of checking the torque on screws and bolts, especially on the XZ gantry.