Hmmm... Should it really be called PCB? It's not really printed, right? But it is a circuit board. This would be more appropriately called MCB, or Milled Circuit Board. It's traditionally called PCB Isolation routing when these come out of a CNC Machine through a subtractive process (Machining/Milling). Circuit isolation routing is just the removal of copper from a copper clad board using a machining process rather than a chemical process. This is considered the cleaner way and more environmentally friendly way to create circuit boards. The main challenge in creating circuits with a machining process is how tight the clearances from each trace can be made. Although Duncan's example does not show this aspect, he did a great job in making some really small traces with the tolerances he did not expect from the blackToe CNC machine.

Such a tiny item milled by a machine with a routing area of 2'x4'. Duncan wanted to show me this example to illustrate the fine work that is possible from this machine. Here is the information provided by Duncan: I don't know how much really fine work your machines get to do, so I thought I'd send you this pic of a PCB I made on your 2'x4' machine. The traces are 0.25mm wide. I didn't think we would be able to get this kind of detail out of this machine, I'm impressed.

There are two main types of bits that can be used to produce isolated traces on a circuit board using a CNC machine. There are the conical bits, like the one I sell, and there are the flat bottom (square end) type of end mills, which is used for clearing out more material during the machining process. There are pros and cons for each type. The conical bit is great for creating ultra thin clearance between traces, but if the level is not accurately positioned (z-axis), the isolation routing can become too wide (the end mill is set too low), or may not remove enough material to completely isolate the traces (the end mill set too high). The square end (which will also have the appearance of a conical bit, but contains a flat at the end and is cylindrical to a certain height above this flat end), will mill at the diameter of the specification of the end mill's flat end. This can provide a circuit with large clearance planes, but the space between traces is dictated by the diameter of the flat end of the end mill. Setting the end mill too low may not be detrimental to the process.

I asked Duncan what bit he used and where he finds them. From Duncan: We get our bits from Performance Micro Tool, mostly because they’re really cheap and we broke a lot of bits trying to find the right feed speed and depth increment for aluminum with our smaller machine, a Colinbus Profiler. 0.65mm milling bit, if you're interested. Looks kinda funny attached to the big router.

So how does Duncan create these impressive PCBs? From Duncan: The gcode is created by the pcb-gcode ULP for Eagle CAD. Works really well with Mach3, especially accommodating the manual tool changes.

This is a tool that I use too. It is worth the effort to learn how to use the Eagle ULP if you do this type of thing. There is a very active group around this tool.