That would be because you set the Z offset to +15 in the Fusion360 post options. The default is -10, which prevents it hitting the limit switch. If -1 is enough to avoid hitting the switch then use that. Always in MM. make sure your machine size is set correctly in GRBL settings. perfectly normal, industry standard stuff. since you will normally set the part 0,0,0 at bottom left of the part, you then have positive coordinates on the part, the negative machine coordinates just don't matter. Hover your mouse over each of the options in the post dialog in turn to see the help for each item. Read the help. Knowledge is power (-:
