Editing Printed circuit board milling (section)

===Z-axis control===
Z-axis drive and control are handled in several ways. The first and most common is a simple [[solenoid]] that pushes against a spring. When the solenoid is energized it pushes the milling head down against a spring stop that limits the downward travel. The rate of descent as well as the amount of [[force (physics)|force]] exerted on the spring stop must be manually set by mechanically adjusting the position of the solenoid's plunger.

The second type of Z-axis control is through the use of a [[pneumatic cylinder]] and a software-driven [[gate valve]]. Due to the small cylinder size and the amount of [[air pressure]] used to drive it there is little range of control between the up and down stops. Both the solenoid and pneumatic system cannot position the head anywhere other than the endpoints, and are therefore useful for only simple 'up/down' milling tasks. The final type of Z-axis control uses a stepper motor that allows the milling head to be moved in small accurate steps up or down. Further, the speed of these steps can be adjusted to allow tool bits to be eased into the board material rather than hammered into it. The depth (number of steps required) as well as the downward/upward speed is under user control via the controlling software.

One of the major challenges with milling PCBs is handling variations in flatness. Since conventional etching techniques rely on [[optical mask]]s that sit right on the copper layer they can conform to any slight bends in the material so all features are replicated faithfully.

When milling PCBs however, any minute height variations encountered when milling will cause conical bits to either sink deeper (creating a wider cut) or rise off the surface, leaving an uncut section. Before cutting some systems perform height mapping probes across the board to measure height variations and adjust the Z values in the [[G-code]] beforehand.{{Cn|date=March 2025}}