Single-Head VS Multi-Head CNC Router
Single head CNC router has one spindle to work, which need to cut one by one. Multi head CNC router with multi spindles can do multitasks at the same time.
You will understand how does tool sensor of CNC router work for for automatically measuring the distance between the router bit and the material in this video.

Tool sensor (Automatic tool calibration pad) allows easy and accurate "Z" zero position for single and multiple tool changes through the program. This can automatically measure the distance between the router bit and the material.
Single head CNC router has one spindle to work, which need to cut one by one. Multi head CNC router with multi spindles can do multitasks at the same time.
This video shows ST6060 small hobby CNC mill cutting aluminum sheet. On the basis of CNC router, CNC milling machine has increased spindle power, servo motor.
Granite CNC cutting machine STS1325 can be used to carve and cut on marble, granite, sandstone, tombstone, milestone, ceramic tile, and hard materials.