Some serious progress....
After dinner with the family I went back to my office (radio room, work office, home office, man cave, etc) and fired up Eagle again. I used the same board example and output two versions of the board to support a 0.2mm 30 degree vbit.
The reason that I created two files is that I wanted to compare the output with different settings from the Eagle - PCBGCode ULP (v188.8.131.52). There are a couple of ways to setup the isolation cut messurements within the software. (In the end both worked the same at the mill when I ran the boards.)
My mill is nothing overly magical. The table moves on the X/Y axis with two stepper motors and the tool moves in the Z axis with a stepper. In my case I using a Dremel model 395 (ebay) because it fits the mount for my mill. I bought the mill in the summer of 2012 at a hamfest in St Paul, MN. (Tailgate Swap Fest) The table has holes drilled every 0.5" and tapped for 4-40 screws. There some basic clamps that are held down with 4-40 screws. The grid can also be used to zero and align the board for two sided work. (I tested that a couple of weeks ago and it does work pretty easily if you think through your steps.)
Part of the magic if you want to work with small parts, tight density, or surface mount parts (SMD/SMT) is that you need to know the width of your bit @ the cutting depth and have some idea of the run out (wobble) on your system.
The test board that I laid out in Eagle and have been using for a couple of weeks has traces from 0.001" to 0.025" laid out 0.050" on center. I am able to hold 0.001 / 0.002" traces if I can figure out tooling width. (I would never actually work with traces that small in the real world.) The short story is the you change the tool (bit) width in the software and keep re-running the board until you stop over cutting (erasing) your trace. Your are basically backing into your measurement with real world tests. Once you understand the bit size it is pretty easy to setup in the software.
Last weekend while I was out of town I was doing some online reading and found a spreadsheet that someone had started to figure out the approximate tool size. The table (above) shows what common bit sizes should cut at a depth of 0.004" which is what my system is setup for.
I am having really good results with a 0.2mm 30 degree vbit. In fact I might be cutting a little too deep. It appears that it is cutting an isolation trace of ~0.008" to 0.010". It is small enough that I can't accurately measure it with the tools that I have.
I think that I am close to being able to work with medium sized surface mount components. I think that I need to be able to cut an isolation trace of about 0.008" and I think that I am in the ballpark. (I have some more bits on order too that might be a little narrower.)
Overall I am extremely happy. I am starting to get good response & correlation between Eagle, PCBGCode ULP, LinuxCNC, and my milled circuit boards. --I like using the mini CNC mill so much that I would like to get something bigger for working with metal, wood, and plastic projects.
The time this afternoon was very rewarding and well spent. I am ready to tackle some homebrew projects!
73 de NG0R