How Much Vacuum Can the Pump Tolerate?
Sometimes people think they need more vacuum, when in fact, they need more flow (a larger pump). If you have small pieces, you generally do want more vacuum, since atmospheric pressure is what holds the part down. If the part is very small, then atmospheric pressure may not be enough to hold the part even at full vacuum, since it is limited to a holding force of only about 15 PSI. For example, if your board has a surface area of only one inch, then you can have no more than 15 pounds of force holding it down. If the cutting head is applying, say, 10 pounds of side pressure because the board may be thick, the part may move (since sliding force is less than vertical force).
It is the in-leakage of air to the table that is the source of most CNC vacuum hold-down problems, so doing as you have by blocking unused portions of the table is good. You said you were running at -12 PSIG, which is 24 in. HgV. That's a decent vacuum level for most jobs. If your pump is running near its maximum limit, you can add another pump in parallel, or increase the size of your pump.
One way some people who must make small parts deal with the problem is skinning, or cutting almost all the way through the part on the first pass, then removing the skin. This works because you have eliminated most of the side forces from the cutting head while the entire board is under vacuum. The small parts can often still be held in place well enough to remove the skin, since the side force is negligible. If not, then you either have to eliminate more leaks, or get a larger pump.
From contributor G:
The previous poster covered the subject well. I would add that you need to make sure both sides of your spoil board have been surfaced to allow for better flow.
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