Implementing a physical stop-block would be quite a project. First there's mechanically mounting it to the z-axis member without interfering with the rotation of the chuck. But even once you've done that, you still need some way of lowering the stop block to contact without digging into the copper surface, a sort of "soft" push down if you will. My LPKF uses an electromagnet to pull the tool block down.

I too use the feeler gauge method to set my positions (not just z-axis) on my CNC machine (the mill/lathe in my machine shop, not my circuit board plotter). It works well for tolerances of ±0.001". But with a 90 degree tool, 0.001" of depth corresponds to 0.002" of channel width. If you're off by just a few thousandths of an inch in depth, you can easily obliterate a small trace. If your machine can maintain 0.001" over the entire surface of the board then you have a pretty good machine.

If I had to use z-axis steppers without feedback, I would probably switch from v-groove bits to flat end-mill bits. That way the channel width would be independent of the tool depth, and my traces would be dramatically more uniform in width. That will be a real issue if you ever have to do controlled impedances.

You might be interested to know that I am now testing a new lamination press. The one I've been using was based on a benchtop hydraulic press, but it turns out that I need much greater control of the applied pressure, both in magnitude and uniformity. The hydraulic cylinder had to be manually adjusted constantly throughout the entire lamination process (which means the pressure varies significantly), and the load distribution members I built didn't make the pressure uniform enough across the whole panel. So I built a frame and four pneumatic cylinders from scratch. I've connected the cylinders to a compressor that gives me up to 335 PSI of regulated pressure output, which is more than I need to laminate my panel. I just finished testing the new press for pressure fluctuations and it looks good so far. Tomorrow I'll be testing it for real with a new panel. Wish me luck.

Joe