I intend to expand on this instructable when I have a serious go at implementing it. But in the interim I thought I would share some ideas I had.
The first problem I considered was how to implement the tracks. These require quite tight tolerance engineering so that the device is rock solid when the router or whatever is in operation.
But if you are like me, with a shed full of wood working tools but only the bare basic metal working tools, this seems rather daunting to try and implement from scratch with pats from Bunnings Warehouse.
And then someone nearby chucked a set of draws out on their nature strip and I did some late night scavenging.
The draws were using the type of runners in the photo and I thought these would be perfect to form the tracks of a CNC device. They are rock solid and all it would require is some sheets of MDF of the right width - easy to achieve if you have a good set of wood planes. All the hard engineering is done for you, potentially at no cost if you are a scavenger like me.
Now you could mount an engine with cutter on it on the Z-axis and route the traces directly. Although that would require you to solve the problems of copper clad boards not always being flat. I guess one solution you could try is to stick the board firmly to the base with some spots of silicone sealant - once the board is finished you could just slice through the silicone sealant with a thin blade.
Or alternatively you could have the device etch an acid resist instead. That would simply require a sharp point mounted on a suitable spring system on the Z axis. There would be no issues with vibration control etc as with routing the copper directly.
And one ideal resist would be RustGuard. It has very low viscosity and you can apply a very thin coat to the copper. In fact I have used it to coat my Tesla Coil secondaries.
Once dry it won't rub off but can be scratched off fairly easily.
And I know for a fact it will dissolve in a mixture of 30 percent acetone and 70% ethanol/metho. The precise proportions are not critical but metho is cheaper than acetone.