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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 found in the PC board's layout file data. Like the more common and well known chemical PCB etching 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 used in a typical office or lab environment without worry. High quality boards can be produced using either process. In the chemical etch process the quality of a board is governed by the accuracy/ quality of the photo masking and the state of the etching chemicals, both of which are the board designers control. In the case of PCB milling the quality of a 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 all of which are directly under user control.
Generally speaking a PCB milling system is a single machine solution on which all actions required for the creation of a prototype board - with the exception of vias and through hole plating - can be handled. For most machines only a standard 120 VAC outlet as well as a shop type vacuum are required for operation, vacuum and setup are covered later in this document.
The common operations supported on most machines are milling, drilling and routing, in some cases other operations such as solder paste application, board digitizing, and measurement are provided, 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. Drilling is, as the name applies, the creation of holes in the board for through hole components and vias, while routing is the milling of mechanical features like board outlines or large mounting holes. The intent in routing is to separate the circuit board by fully piercing the board material. Some PCB Milling machines do allow for limited 3D routing and contouring.
The mechanics behind a PC board milling machine is fairly straight forward and has its roots in CNC milling technology. At a basic level a PCB Milling system is 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 or parallel connection to the milling machine's on-board controller. The controller is then responsible for driving and monitoring the various positioning components that move the milling head and gantry and control the spindle speed. Typically this drive system is comprised of non-monitored (does not provide positional feed back) 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. More advanced systems provides 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 axis.
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 decent 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 feed back 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 feed back. This system allows the milling head to be moved in accurate steps up/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. Control of the depth (number of steps required) as well as the downward/ upward speed is under user control via the controlling software.