I am having an intermittent issue with my Fineline machine. When I go to run a program it will start with the z all the way up to the safe Z height, and then drop to the piece. Randomly sometimes it will make a barking noise and just plunge all the way into the piece like it gained a bunch of steps and dropped through the leadnut. I have since learned to hold my hand on the Estop button and listen for that when starting a new program, but would like to fix it permanently. I tried lowering the safe Z height and lowering the velocity and acceleration of the axis, thinking it would be a decent bandaid, but it still happens starting 2.5" lower. It only seems to be an issue in the top half of the Z travel. Before I start tearing it apart to figure it out, could it be something else? Are the ACME leadnuts a wear item that needs regular replacement?