Hi all,
I'm just starting to gather designs and materials for a medium sized (around 24"x36"x4" cutting area) cnc router.

I'd like to make the table with a vacuum clamp system. Does any one know how big a vacuum pump is needed to clamp? CFM, HP, watts anything will help. Will a large shopvac work?

That is very diffacult to answer. It depends do much on the smoothness of the table surface, the smoothness of the part, the size of the part relative to the table surface, the ratio of vacuum holes in the top relative to total top area, etc. And I suppose you also have to take into consideration the cutter size, rotation speed, feed rate and depth of cut since these will be related to the torque forces trying to rip the part off the table.

From what I've read, unless you are just routing foam, you will need a very, very large vacuum motor to build a vacuum table. You need something with enough suction power to hold the material down solidly while you drive a router through it. Come to think of it, foam might require more vacuum than a shopvac could deliver too since it's so pourous.

I'm definately no expert though - I can't figure out how vacuum tables are supposed to work unless you employ a perforated sacrifical sheet of some kind. Otherwise you'd just be holding down the sacrificial sheet and your workpiece would be free to slide around... :confused:

This may give you some idea of the relevant considerations to your question.

Techno_Isel Education (http://www.techno-isel.com/H840/PDF/004.pdf#page=10)

DC

It is possible to build a clamping system without a high volume pump. But, it requires a very good, tight seal.

Check out how Rob made his. http://www.cnczone.com/forums/showthread.php?t=17535&page=1

Some good info on vacuum clamping jigs can be found in Shop Notes magazine, issue #40.
And you can build your own pump with info at www.joewoodworker.com