Re: Smallest table worth using vacuum hold down?
If you are thinking of a porous spoilboard table to hold down larger sheets with a high power regen blower, I suspect you need several square feet to hold that down properly or maybe less when using only 1/8" bits as Ger21 mentioned. I would guess you may be able to flatten 1/4" sheets but probably not 3/4" . If you have a full sheet size machine that is of course all no problem.
For anything smaller a high vacuum/low volume pump with pods if the ticket. I use them all the time and clamp or use tabs only for odd shaped and extremely small parts. A double piston pump can get 25" or better of vacuum with a well sealing pod/gaskets and will hold (from my experience) with at least 5-10 square inch surface when cutting with an 1/8" bit (80-120 ipm). I need maybe 50-100 square inch surface to cut with 1/4" bits (200-300ipm). If you have fences and don't rely on the gasket friction you may even be able to cut smaller parts. But don't expect to flatten any sheet with such pods.
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