Top of the CNC mill electrical work items list was making sure it had a functioning Emergency Stop button. Once that was completed, the next work item was the Z-axis homing switch. For a vertical mill, we want the Z-axis homing operation to take it to its highest position, furthest away from workpiece. This is reverse of many 3D printers, which home against the print bed. This is because 3D printers have the luxury of starting from an empty print bed as a characteristic of additive manufacturing. A CNC vertical mill, representing subtractive manufacturing, could not make such an assumption since the workspace would have work fixtures and material stock.
Our initial Z-axis homing switch was appropriate for our initial orientation of the mechanism, allowing it to home to the top of its travel. But once it was flipped around, the homing switch is now sensing the bottom of its travel instead. We need to find another mounting position before we could have a good Z axis.
This switch had the luxury of sitting next to the motor and conveniently sensing the approach of the carriage. Its height was sized to match the length of the motor ballscrew coupler, engaging the switch just before the carriage would run into the coupler.
This linear actuator have no convenient location to sense the other end of the range of motion. Since there was no coupler on the other side, a similar mechanism would subtract from valuable range of motion. We didn’t have a good place until we installed the spindle motor mount plate, whose top edge gave us an feature we could use to trigger the homing switch.
From that point, it was a matter of running through a few 3D prints to find the correct dimensions to trigger a homing switch while maximizing useful travel distance. Now our Z-axis homes against the top of its range of travel again, let’s give it an useful surface to work on.