Printed circuit board milling is the process of removing areas of copper from a sheet of printed circuit board material to recreate the pads, traces and structures according to patterns from a digital circuit board plan known as a layout file. Similar to the more common and well known chemical PCB etch process, the PCB milling process is subtractive: material is removed to create the electrical isolation and ground planes required. However, unlike the chemical etch process, PCB Milling is typically a chemical free process and as such it can be completed in a typical office or lab environment without worry. High quality circuit boards can be produced using either process. In the chemical etch process, the quality of a circuit board depends on the accuracy and/or quality of the photo masking and the state of the etching chemicals. In the case of PCB milling, the quality of a circuit board is chiefly determined by the system's true, or weighted, milling accuracy and control as well as the condition (sharpness, temper) of the milling bits and their respective feed/rotational speeds.

Hardware

In general, a PCB milling system is comprised of a single machine that can perform all of the required actions to create a prototype board, with the exception of vias and through hole plating. Most of these machines require only a standard 120v AC outlet and a shop-type vacuum cleaner for operation (vacuum and setup are covered later in this document).

Most machines can perform such routine operations as milling, drilling, and routing; some machines can also perform other operations such as solder paste application, board digitizing, and measurement, but these are not required for the actual production of a board. Milling is the act of cutting an isolation path through the copper cladding to create the required electrical isolation around board features. Milling is also used to remove large areas of copper; this is often referred to as rubout. Holes are drilled in the board for through-hole components and vias. Routing is the milling of mechanical features like board outlines or large mounting holes. Some PCB Milling machines allow for limited 3D routing and contouring.

Mechanical System

The mechanics behind a PCB milling machine are fairly straightforward and have their roots in CNC milling technology. A PCB milling system is similar to a miniature and highly accurate NC Milling table. For machine control, positioning information and machine control commands are sent from the controlling software via a serial port or parallel port connection to the milling machine's on-board controller. The controller is then responsible for driving and monitoring the various positioning components which move the milling head and gantry and control the spindle speed. Typically this drive system is comprised of non-monitored stepper motors for the X/Y axis , an on-off non-monitored solenoid or pneumatic piston for the Z-axis , and a DC motor control circuit for spindle speed, none of which provide positional feedback. More advanced systems provide a monitored stepper motor Z-axis drive for greater control during milling and drilling as well as more advanced RF spindle motor control circuits that provide better control over a wider range of speeds.

For the X and Y axis drive systems most PCB milling machines use stepper motors that drive a precision lead screw. The lead screw is in turn linked to the gantry or milling head by a special precision machined connection assembly. To maintain correct alignment during milling, the gantry or milling head's direction of travel is guided along using linear or dovetailed bearing (s). Most X/Y drive systems provide user control, via software, of the milling speed, which determines how fast the stepper motors drive their respective axes.

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 which is attached to a pressure foot assembly that limits the milling head's downward travel. The rate of descent as well as the amount of 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 - this system functions in the same manner as the solenoid type, pushing against a spring stop/pressure foot assembly. Air for the cylinder is provided by an external compressor with the air flow being controlled by a manually operated regulator and 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 provide no positional feedback while in motion, and are therefore useful for only simple 'up/down' milling tasks. The final type of Z-axis control uses a stepper motor with dynamic positioning feedback. This system allows the milling head to be moved in small accurate steps up or down through its whole range of vertical motion. 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.

External Resources

• http://www.pcbprototyping.com/udocs.htm