Final answer:
In machining, a facing toolpath normally cuts down to the top surface of the workpiece, with the depth of cut set relative to this surface as predefined in the toolpath parameters.
Step-by-step explanation:
When discussing a facing toolpath in machining, unless otherwise specified, the tool will typically cut down to the top surface of the material being worked on. The facing operation is intended to create a smooth, flat surface by removing material from the very top layer of the workpiece. The depth of cut is usually predefined in the toolpath parameters, and this depth is relative to the original top surface before any machining starts. It's important to set the correct reference surface in the machine's controls to ensure the correct depth is achieved during the facing operation.