As I reported earlier, I had achieved great success in cutting PCBs at a 0.05mm depth, without auto-leveling, but instead only by pre-leveling my sacrifice board. I use MDF, and it seems to warp (at least more than 0.05mm, across a 4"x6" board) over time. Pre-leveling resulted in most, but not all, of my PCB’s cutting very nicely. But, the occasional exception frustrated me. In search of an answer, suspecting that some of my cheap Chinese boards are not as flat as they look, and inspired by some other recent posts, I purchased a vacuum table (about $130, plus $60 for a pump, on Amazon). Three PCB boards later, I believe I have achieved pretty near perfection.
I recall an only post in which the poster seemed to say he used MFD as a vacuum board – he pumped air out of it and it ostensibly held the circuit board firmly in place. That did not work for me, although MFD does pass alot more air than I would have guessed. I ended up drilling a bunch of holes in my MFD sacrifice board, to correspond to the holes in the vacuum table, and that (with painters tape around the perimeter of the PCB) pulls enough of a vacuum to hold the board very flat.
And I think this explains why autolevel didn’t work very well for me. The autolevel probe never actually exerts any downward pressure on a PCB, since it stops as soon as there is electrical contact. So, it produces a faithful map of the pcb surface, but does not account for the partial willingness of the PCB to flatten under the pressure of actual cutting. (And it seems that relatively great force is necessary to penetrate the PCB copper layer, but once penetrated, it pops right back up.)
That sounds exciting. I always wanted to try out a vacuum table, but never was ready to pull the trigger. I recall looking around and not seeing for anything at that price.
I am still experimenting, but it seems to work well to drill only on the second side and only to a depth of 1.65mm, which is just slightly deeper than the board (at least my board) itself. Since the top and bottom traces are all cut by that time, a little loss of vacuum (through the vias as they are cut) doesn’t really matter. My sacrifice board gets slightly pocked up (mostly from cutting the perimeter), but since I re-level anyway (cutting off 0.05mm), it really doesn’t matter. My backup plan is to use a second sacrifice board, but if Plan A continues to work as well as it is seeming to, that won’t be necessary.
And, speaking of vias, I discovered a trick recently that HUGELY reduces the time it takes me to solder up a board. In the past, I would cut little pieces of wire and solder them through the vias, but that is sloppy and time consuming. I found really small rivets (1mm wide) in amazon. I can plug those in and solder them up really quickly.
My experience with vacuum and MDF is you need an industrial-level pump to pull enough air through to hold down a piece.
On our 5x10 CNC we seal the edges of the MDF using paint/varnish before fixing it as the spoil board, then level it using a 2" flat cutter.
It works great, but much better on large pieces, such as full sheets of ply/mdf, than on anything small.
When using smaller bits of material, we use thin painted ply sheets to block off the unused areas of the CNC bed, otherwise it just bleeds around the edge.
For my home unit I’ve tried a number of different methods, including drilled MDF, channels with hold-downs and found that for small, flat items, 3M double-sided tape or carpet tape is perfect.
My MDF board is only about 5" x 7", and holes (10mm apart, as I recall) are in a grid of about 4" x 6". I believe that is small enough that the vacuum created by a $60 consumer vane pump is more than adequate to pull a piece of PCB firmly to its surface. I also use painters tape to cover the edges of the pcb and adjacent MDF holes, but primarily to block the holes rather than to hold the PCB in place. I had intended to seal the MDF, but do not consider it necessary for the small size I am using.
I also tried 3M tape, but found that the PCB surface was not sufficiently flat (and it was a PIA to put on and take off).