I'm looking for some flat head 5/16" nylon bolts to make counter sunk mounted sacrificial board that I then machine flat.
#Open deskcnc upgrade
My next upgrade is to not use just scraps from the same board underneath, but machine a few slightly varying heights of plastic to tune the board flatness. While CopperCAM allow up to 4 layers, I've had no problem indexing dual layer boards. By turning the rod through to T-nut to the base first, the position is fixed. If I position my holes to line up with my T-slots, then I can pull the board down to a flat reference with small washer and hex nuts into this threaded rod. I then turned down the 5/16" rod from the T-nut to a smaller diameter on the lathe and threaded (to 8-32 I think). I use a T-slot table and have some threaded rod that goes into them. If these are positioned so that board flit remains aligned, then you are set. CopperCAM allows you to add drill locations for layer alignment. I make multiple pads over the existing slots and just ignore the slotted NCDrill file for CNC milling. This is a little annoying, as I can't use the exact same file for real fab. I've started using multiple holes to emulate slots where they are needed. For the most part, I've been happy with it for non-slotted designs. I am using CopperCAM to convert Gerbers and drills to G-code. 2 - Drilling, Routing of larger holes, and edge cut out will all erode the surface such that hold down force is no longer present. Now as far as vacuum hold down, I see two problems: 1 - Some PCB material is bowed enough that vacuum might not pull it flush. Then I raise it until the bar just passes under the cutter. Lower the cutting tip with a very slow Z jog. For setting Z height, I use a 0.250 brass rod. It isn't causing enough pain right now to cause me to drop a couple hundred on changing the setup. However, it means replacing the stock controller board that came with the Probotix kit. If I had the input capability for a touch probe for PCB CNC, I would do it immediately. I've really been happy with the quality of bits and service from Think and Tinker precise bits. Luckily most of the quick prototypes are small enough in area that I mount as flat as I can and set height in the center. I'm using a Probotix Fireball X90 setup for all kinds of general CNC prototyping with machining plastics and also PCB prototyping. Unless you are in the situation where you constantly need to be turning around designs on 2-3 day lead times I don't really see the benefit of this method any more. You also get the added benefit of soldermask and silkscreen. When you consider the cost of your own time, the machine costs, material and of course bits it probably works out cheaper to get them done externally. Perhaps things have improved in the 5-6 years since I gave up on milling PCBs but personally I think you are much better off just paying for one of the fast turnaround fabrication services. Bear in mind these were EU and USA built specifically for PCB work. Broken mill bits are another pain, you sit listening to the machine drone (plus vacuum and air compressor) for hours while you do other things only to realise the mill bit broke 10% of the way in. Even the slightest warp in your FR4 can destroy your results and leave you with multiple efforts. However they often needed a lot of attention to get the right results, particularly on larger boards.
+1 to what Mike said, previously I've used LPKF and T-Tech milling machines and sometimes they work beautifully so you are can go from concept to PCB in a single day. I'm not a good enough circuit designer either so I have to build my circuits to make sure they work otherwise I wouldn't bother. However, when I mill a one-off I can do it many times faster with carpet tape. Both methods will pay off if you're milling a batch since changing stock is very fast.
I've never done this myself but I heard that drilling/milling into the vacuum holes ( or grooves ) is not a good idea - weaker vacuum, abrasive swarf going into the pump, etc., so you spend time designing your table for the board you're making. Everything can be machined except the horizontal hole which you would drill, tap and screw in the vacuum port. Another way is to clamp a 1" thick piece of HDPE and make the vacuum table out of it. The fastest way to make sure is to mill the top surface flat however, since vacuum table is expensive and your machine may not necessarily be good enough to mill aluminum you spend time with indicator in the spindle measuring and shimming. You mount a vacuum table (or any other jig for this matter) on the machine and start wondering if its surface is orthogonal to the spindle.