Hi,
How do you plan to flip the board and achieve better than 0.1mm alignment between top and bottom? I use alignment pins, but others use a pocket milled into the spoil board,Observation #1: When FlatCAM tells you to "include alignment pins" in the layout,
yet others use an optical microscope to align to one or more fiducal marks....I don't really care what method you choose, but you must have a strategy to align the board when you flip it
or you will have a failure. The fact that I use alignment pins which are in turn suggested by the board geometry as represented by the CAM artwork is just what I find easy and convenient, no more
no less.
The specialised tape I bought for the thick copper boards I was doing looked much like that, in fact might have been as much as 2mm thick. You might call it an adhesive foamObservation #2: I never actually "made a technical selection" on the tape,
almost. It worked well, in fact so well that I struggled to remove the PCB from the spoil board. Unless you are using coolant then I think it's overkill.
Just ordinary doublesided tape that you get from any stationery store will be fine. The only proviso is that the material of the spoil board have a smooth, firm surface for which the tape can adhere.
The brand I'm using at the moment is 'Pomona', but is in fact just a cheap generic doublsided tape, the sort of stuff kids use at school day in, day out.
In the early days, about eight years ago, after having difficulties with clamps pulling surface distortions into the blank, I tried a few vacuum experiments. I use vacuum forming for model aeroplaneObservation #3: The "vacuum holddown" has a certain appeal,
parts so I had a pump and a few useful vacuum parts to choose from. You need to be able to seal the PCB blank to the spoil board, and the smaller the board the more critical that sealing becomes,
as the area will determine how much 'clamping' force the vacuum will exert. If you are using just one or a few size PCB blanks then having a vacuum jig with an O ring around the periphery would
be doable, but if you are making random sized board it would become extremely tiresome very quickly.
There was a guy on the Autoleveller forum whom used vacuum very extensively, and he made many boards a day. He had a set of procedures that worked very well and I was very inclined
to copy him. Then I tried doublesided tape, and that worked so well that I never bothered following up on vacuum hold down any further.
The last part.... '(all have internal levelling)'.......is the single most critical part to successfully using mechanical isolation routing for making circuit boards. You have to achieve a vertical height controlObservation #4: I have enough time while waiting for the new tips (Wednesday), I printed out the instructions, I can ATTEMPT to get bCNC or Chilipeppr or Candle running (all have
internal levelling).
within say 20um to 50um over the entire PCB 100% of the time 100% reliably otherwise you will always be disappointed and have unusable PCBs. Whether you choose to use a software solution like
Autoleveller as I have, or another software, or some mechanical version of 'milling the spoil board flat',or combination, does not really matter....what does matter is that you achieve 20-50um vertical precision over 100%
of the PCB 100% of the time.
I have attached a pic of my spoil board. It's in the vice in my mill. Its really very convenient, I can remove it from the vice to fit a new blank, or flip a blank or whatever, and then put it back in the vice.
The spoil board jig has a screw head such that I can put it back into the vice in precisely the same location as when I removed it earlier. No need to re-zero or any of that rubbish, as I say, very convenient.
You may notice that this spoil board has been used several times before as evidenced by the grooves/divots/holes left over from previous PCBs where the tool/drill has passed right through the PCB
and slightly into the surface of the spoil board. I will use this spoil board for about 20 or 30 PCBs before the surface becomes so scarified that the doublesided tape loses adhesion. I will then flip it over
and use the other side. Once that side is 'used up' I discard it, after making about 50-60 PCBs, say a month or so depending on customer demand. The piece of acrylic, 200mmx200mmx 8mm costs
about $7.00NZD, say $5USD.
Just as a matter of interest I would use one or maybe two engraving tools over this time, unless I break them, and that does happen from time to time when I don't concentrate.
Craig