Is there such a thing as too much suction? It seems that small pieces on the back of the table (by the origin) always seem to move a little bit (or right off the table). I have 12 suction points and covered 5 of them that we don't utilize (6 x 12 table and we primarily cut 4 x 8). The first sheet of melamine I put on I had -12 psi (-.08~ bar) of pull on all 4 gauges. We have two 20 hp vacuum pumps and the last thing I want to do is blow one up.
From contributor C:
The answer to your question is, it depends. There are different types of vacuum pumps, and each has a range of operating vacuum levels. If you have a liquid-ring pump, too much vacuum can cause the pump to cavitate; if you have an oil-flooded pump, high vacuum is good; if it is a dry vane pump, they usually have a maximum operating level, above which the pump can overheat. Most pumps have built-in protection from operating outside of their limits, unless that protection has been altered.
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.