Milling Printed Circuit Boards by David Slatter G0CJG
Over the years I and other club members have used many different techniques for producing circuits from copper clad boards. Many of these techniques involve nasty chemicals which aren’t great in the home environment and most are not that reliable or precise enough for surface mount ICs. Mechanical milling of boards using engraving cutters is routinely used in industry for prototyping single and double sided boards but until recently has been too expensive to consider for amateur use. I’ll describe my experience producing PCBs using a Chinese Engraving machine I acquired on eBay.
The machine I bought is a TS 3040C 4 Axis engraving machine. (see picture). The chuck and thus the tool tip is moved by stepper motors in a volume of 300(X)mm* 400(Y)mm*80(Z)mm. The spindle motor rpm tops out around 20,000 rpm and a flexible drive connects it to the chuck on the gantry of the engraver. The stepper motors are driven by the Control Box which in turn is driven by a standard parallel interface from a PC.
I design boards using Eagle PCB software. I run an EaglePCB plug-in called PCB-GCODE that produces the G-CODE used by numerically controlled machine tools. Another software package called Mach3 turns your PC into a machine tool controller and turns g-code commands into pulse streams on the parallel port. PCB-GCODE is free and free versions of Mach3 and EaglePCB and are available but are limited in terms of number of g-code statements and size of board that can be designed.
The engraving tool I use has a tip width of about .25 mm and a tip angle of about 60 degrees. So for every .025mm (.025mm=0.001”) of depth into the material you get an increase of width of about 0.03 mm. So it’s important to control the depth of the cut if you want to control the width. The copper thickness on a 1oz board is .035mm (0.0014”). This in turn means that you need to get the PCB material as flat as possible. I used double sided adhesive tape to stick the material onto a sacrificial board of MDF (medium density fibreboard). I experimented with a few tapes to find one that was strong enough. (3M double coated #9578 1” tape worked for me)
This picture shows the PCB fixing arrangements. Note that you can cut out curved boards with an end mill. Also note the cut down black plastic plumbing T-piece attached to the chuck. This moves a few mm above the cutting surface when in operation and connects to a dust extraction vacuum system. It is essential for health and safety to use a dust extraction/containment system.
The above picture shows SMD mounting pads milled on the system. The pads are on a 25 thou pitch. The pads are approximately 9 thou wide and the gaps 16 thou wide.