2 fein Vacuum system HG's

[stockbub]

OK, got vacuums hooked up and put vacuum gage on them. With Valves shut off completly, putt 7 HG. when I have all 9 valves open on 5 x 8 table, just surfacing with nothing else on it, I have a little over 5 HG. I know someone has told me but what exactly does this mean? What are ways to check for leaks?

[richards]

Martin,
The formula to convert inches of mercury to pounds per square inch is:

PSI = 0.491130 X inches of mercury

So, you're getting about 2.5 lbs of force per square inch. (At my altitude, I get 5-inches of mercury with the valves closed.)

Depending on the material, you might need as little as 100-lbs of pressure to hold a part secure, or upwards of 1,500 lbs. For instance, particle board, with its good friction, takes a lot less pressure to hold down than melamine coated particle board. (Just think how easy it is to slide a sheet of melamine onto your Shopbot. It acts as if there are ball-bearings under the sheet.)

[harryball]

Something doesn't sound right... with the valves closed my twin feins pull about 7" Hg, when the valves are open with nothing on the table, just pulling an open spoilboard I'm getting less than 2" of Hg. When I'm pulling on a full sheet I see about 5 to 5.5" of Hg, sometimes as much as 6. It sounds like your spoilboard is not porous enough. I can continue cutting and hold a sheet in place with as little as 2.5" Hg showing on the gauge.

If my math is right (I've not slept in a few days so it may be wrong)

So with a sheet loaded and pulling 5" of Hg, less the "tare vacuum" of 1.5" that leaves 3.5" available. Round down (cause I'm conservative) to 3" and that's about 1.5 lbs / sqin of force. So on an entire sheet that's almost 7000 lbs of force.

Interestingly enough... on one of my projects I have a small panel that is cut first when the vacuum is close to 5" Hg. The math shows that part has about 120 lbs of force on it, it cuts well and I can not casually move it. As the project progresses the vac drops to about 2.5" at the very end of the project at which time I can easily move this part on the table... the math tells me I should have about 35 lbs at the time. Seems about right.

Robert

[stockbub]

I hope that the perceieved problem of spoilboard not allowing enough air through is not a problem. I will have to see what is going on tomorrow to see how it works with a sheet of plywood on it.