After you begin to work with your CNC machine, you will quickly find that zeroing your z axis becomes a pain because you have to do it every time you turn your machine on and every time you change tools (unless you are flush with cash and can afford an automatic tool changer). I am not at the moment, so flush with cash that I can afford to spend $5-$10k on an automatic tool changer. That would, in fact, more than double what I have invested in the CNC router at the moment and I do not have a large enough demand for salable products yet to justify making such an investment. Therefore, I am going to be changing bits by hand for quite some time. As such, I put a little thought into how I would improve my work flow and reduce my setup time. As I am sure that you have found, one relatively cheap and very easy solution is to use a printed circuit board (PCB) blank as a touch plate.
If you have seen my Pogo Pin homing switches, you would know that the CNC Router Parts router is ideally set up for direct contact limit switches because every part of the metal structure is solidly grounded. Therefore, all one has to do is run a single wire to an isolated contact to act as a touch plate. This is an extremely accurate way to home or zero an axis. I tested my pogo pin limit switches, and found that I could not really measure the error even with a dial gauge. I repeatedly tested the unit and found that it would home to the very same step on my motors every time which is about 0.0005″.
I had seen other people use a plug and clip to set up their z axis touch plate. However, I wanted to have it handy wherever the router head was and wanted to be able to quickly grab it, set it, and put it back, so I decided to make a mount that would attach to my router spindle mount so it would be handy wherever I was setting up the machine. What I came up with was a fairly simple design and uses the flexibility of the wood to act as a spring to hold the PCB blank in place.
Check it out: