I have a similar machine (but which moves the bed rather than the gantry for Y axis - photo is some random one off the web). I've used it (and still use it) for single-sided prototypes and think 8mil should be doable but you'd need a lot of practice to achieve it reliably. I second the other members who suggest JLCPCB for serious or non-prototype jobs.
However, there is a certain amount of satisfaction in DIY PCBs that come out close to expectations, and just as some people use wood chisels and lathes to turn doorknobs instead of just running down the shop and buying a proper one for peanuts, if you want to make your own PCBs then don't listen to the killjoys who will never know the fun of doing so - you don't need a reason or excuse to do it, and they should be ashamed at always trying to shame others into not doing it.
One aspect often missed is that the mill can be used for other stuff too. I just recently cut some LED filter panels from 2mm red perspex (let's see JLCPCB supply that!) and, luckily, I didn't waste loadsadosh sending the drawings off somewhere to be done because I got a hole positioned slightly wrong. No problem - design fixed and another panel in my hands within 15 mins. If you don't have one of these mills around you'll never find a job for it to do, but if it's there you can do many useful things with it.
Re PCB specifically, there are lots of blogs explaining how to cope with warped boards and stuff, specialised height adjusters, etc. This, which I've also mentioned elsewhere on EEVBlog, works for me (and I've tried a lot of things) but YMMV:
1. Get a half-inch (12mm) perspex slab and bolt it to the bed with recessed bolts. Then fly cut it so it is perfectly flat and at right angles to the Z axis. This is both your flat base and sacrificial base board (which you don't need now). Although you don't want to, it's OK to drill too far through the board and into this, OK to mill an outline a bit deep, etc. Photo shows a section of mine - it's perfectly OK and doesn't need surfacing yet.
2. Use 2" (50mm) carpet tape to stick the PCBs down. Carpet tape is very thin, double-sided and, more importantly, consistent thickness. That will keep them flat and in place without the need for clamps or any sensors or anything. Unless your PCB copper thickness is really terribly inconsistent, you'll get a consistent depth of cut.
3. Use 60 or 90 degree cutters from LPKF. Seems counter-intuitive since the slightest change of depth is amplified in width, but it works fine if you've got the PCB stuck down flat. LPKF are expensive but you can sometimes find them on Ebay, and treated right they last a long time (see 4). The photo shows LPKF on the left, typical 'CNC PCB' cutter on the right. Forget the right one or anything like it.
4. Smear some oil over the PCB (I use 3in1 but only because that's what's to hand - anything of similar consistency would be OK). You don't want to be cutting dry copper or FR4 because your bit will last approx no time at all. With oil you'll be wondering why they haven't worn out yet. A bonus is that the oil will trap the debris into nice piles alongside the tracks, so it's easy to clean up and doesn't go down your nose (or require extractor fans). Doesn't need to be deep, but keep it wet (might need to refresh as the job goes along). You shouldn't need to worry about oil being sprayed everywhere because the centrifugal force at the cutting tip will be quite low (see 5).
5. Use a high speed motor. You need to cut the copper within a certain range of speeds (surface feet per minute), which is the rate at which the cutting edge bites into the copper. That speed is a function of the tool diameter and rotational speed, so with a really tiny tip it amounts to not very much at all for normal drill speed and 20krpm isn't very high for this. The speed at which the cutter is being pushed along the track determines how wide the strip of copper being cut is. You don't want it too wide, so slow rotational speed means slow axis movements too.
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