Good afternoon. This is my first post here, although I’ve been lurking for some time now. I have a small hobbyist CNC (Next Wave Automation Piranha) purchased used from our son when he upgraded to a 4’X8’ CNC. The Piranha is a good starter/learning machine, and a larger CNC may be in my future, possibly an Open Builds Lead 1010 or similar. I use Vectric VCarve Destop and Fusion 360, depending on the job. My geographic location is close to sea-level.
I spent many long hours reading this and other forums about CNC vacuum tables. I read about Pascals, inches of HG vs water, Microns, PSI, etc. I know about leakage, and the difference between hold down force and lateral force. Gasketed pallets, venturi pumps, rotary vane pumps, diaphragm pumps, two-stage pumps, zoned systems, etc. It seems there is an infinite array of approaches, with costs ranging from $50 to hundreds or even thousands…and results across a similar spectrum. I studied until my brain was mush. So after a friend with a CNC Shark build a simple vac table and got good results, I did the same and my table works incredibly well for the jobs I do. My question is how/why it works because something doesn’t seem logical to me and I’d like to understand it before possibly making it the basis for my next, larger, set-up.
Specs: The spoilboard is 1/2" MDF, 12”x18”, with 199 quarter-inch holes. Therefore, each hole is 0.049 sq in, for total hole area of 9.75 sq in. The holes are spaced 1” apart on the x and y axes and there are a few extra holes around the side of the board for free air flow. Directly under the spoilboard is a gridboard of 3/4” MDF, with quarter-inch grooves in the X and Y directions intersecting under the spoilboard’s quarter-inch holes. On the bottom side of gridboard is a manifold to accept a 1” vac hose. The vac hose inserts into a plenum that is connected to the gridboard grooves. The two pieces of MDF are stacked, screwed together, and the edges are taped with blue tape for sealing. Otherwise, no sealing of the MDF surfaces. The whole contraption is permanently affixed to the Piranha’s aluminum t-slot table.
The $35, 3-gallon, 3 hp Shopvac I’m using for suction has a 1” hose. Therefore about 0.7854 sq in of area. I would guess about 1.5 PSI, but that's not important for my question today. My last one lasted about a year, and I just replaced it. I figure $35/year is acceptable for my hobby. Pros: cheap, effective. Cons: loud, loud, loud.
I’ve calculated that about 16 of my quarter-inch spoilboard holes will provide about the same area as the shopvac hose (0.784 sq in). Therefore, if I cover 183 holes, leaving 16 holes open…that should allow the shopvac to pull as much air through it as if the hose were completely unobstructed. So I do that…I place my workpiece on the table and cover all but 16 of the remaining open holes with various sized pieces of plexiglass. The suction force is great, well more than enough to hold the workpiece in place while milling. And the vac sound never changes…in other words, it never produces that sharp whine that you get when you start restricting the air flow.
So…if the vac is getting air equivalent to a completely unobstructed hose…how can it still provide strong suction to the other holes? That doesn’t seem logical to me…but it works! And if it's getting air equivalent to an unobstructed hose, shouldn't it last a long time without "burning up"?
Thx, rink.
P.S. I’m thinking about whether a zoned table with a cheap vacuum pump would be a better solution for my next set-up. I’ll have to ask some questions about that in another thread.