I haven't milled my waste board but I have attached various smaller sub-boards / jigs to the machine waste board depending on the job. The smaller sub-boards /jigs can be milled flat if necessary, however the overall rigidity of the machine isn't of course anywhere as good as a professional machine. Coincidentally, today I measured the z runout on my c-beam XL wasteboard with a Mitutoyo Dial Indicator, and was surprised to find how good it is; less than 0.1mm in 300mm movement. For wood routing this is good enough, but I know that under cutting conditions, the machine will deflect more, but I've done a lot of machining now on the c-beam and I find it's perfectly good enough for wood. Don't know what accuracy you require for PCB milling but I'm guessing that the cutting loads won't be too much? You could stiffen your machine with another Y axis like on the XL if there is too much deflection for your application.

 

I don't know of any other option than physically leveling the bed in order to get the accurate flatness that you need in order to get good PCB routing results. Even with your Dremel as a spindle it shouldn't be too much of a problem. I don't know what size PCBs you are going to do, typically, but I used a 5/16" bit that came with a Dremel bit set that I had bought (or did it come with the Dremel like tool I got? don't remember) and made a pocket that is the size of the PCBs that I use. I think it's 4"x6". Didn't take too long to do it. I did it with an overlap of 70%.

 

When I first built my CBeam Machine I used a 0.25" router bit chucked into the router on the CBM and ran that over the spoil board at a constant Z. Took a while, but it worked.

 

The main issue with PCB milling is the fact that the PCB itself is never flat. It doesn't matter how flat your spoiler board is, the PCB determines success or failure. If it's a small part, you're generally fine, but anything near the 4" either dimension tends to require deeper than necessary milling to achieve proper trace separation over the entire are of the board. You can support the entire perimeter of the PCB and still have this problem. Other factors include the whether the board is double or single sided as well as the thickness. PCB creation with a cnc machine is better done with an ink plotter.

I've personally made a ton of parts in many sizes, but they are ultimately only good for proof of concepts or "raw" undemanding parts. I use 20 degree v bits. It's cheaper to outsource if you are making a quantity of anything. My experience is on three different machines: 4'x8' Thermwood ($100k+), a 2'x'2 CNC Shark (garbage machine btw), and a 13"x13" Sphinx.

 

I've tried a 15° bit on my boards and it broke the tip almost instantly. I since use 30° and have no breakage anymore.

 

Have a look at this video here:
This bloke has written an open source program that scans the surface of the PCB to be milled and then makes adjustments to the g-code for any height variations. This is about his third or fourth iteration of the program and he explains it extremely well. I haven't used it as yet (although prototyping PCBs was one of the reason I got a C-Beam) but if his results are anything to go by, it looks like a good simple solution

 

Glad to hear that it might be useful and thanks for the feedback ....